cmg_george
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Sounds like a very interesting project. That's the goal here? Sell the board? Open source?
18650 equaliser
- Thread starter Brian Drury
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not2bme
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Definitely following this thread! I assume the greater the voltage difference, the greater the current. Do you have any calculations on current vs voltage difference? I'm just wondering what happens if a pack gets too big that the tolerances will get wider?
Brian Drury
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neurocis said:
Hmmm, it is not a constant. The ratio of power consumed to power transferred is massive when there is a large imbalance. This drops as the terminal voltages become equal and eventually the consumed power becomes dominant. However, at that stage by far the largest amount of power will go in the LED's but you could always manage without them I suppose.
It all seems a bit academic to me as the absolute power level is tiny.
This short video shows the equaliser in action.
https://drive.google.com/file/d/1OPxCOyfpz4ntLbjlMxYZFXkKJ0jOpWri/view?usp=sharing
Better explanation:
The battery pack has 8 series connected static cells and 7 flying cells.
If we assume each cell has a capacity of 2Ah and a nominal 3.6V then each cell holds 7.2Wh of energy.
The flying cells are effectively in parallel with the static cells but with a transitioning time of a few nanoseconds every 2 seconds. For practical purposes we can ignore switching time and simply add the static and flying storage power together. This means the pack holds 15 X 7.Wh (108Wh) of energy.
From a conceptual point of view all the cells are in parallel using a time division multiplex technique. ** This is important to understand! **
For my application the packs will use higher powered 18650's and the packs will be parallel connected using diodes. This can be done because the pack voltage is high and the current is low. You cannot do that with low voltage high current packs.
I chose 8 cells because that gives me the correct voltage for my 250W inverters. Each inverter will probably have 10 packs and there will be up to 10 inverters. This approach allows a progressive build in a modular form.
The packs all communicate with a master arduino using serial communication.
Hope that helps
not2bme
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Let's see if I understand this flying capacitor correctly, this system has 15 cells in total. It has 8 cells connected in series as the 'static cells'. It also has 7 cells that are connected in parallel as the 'flying cells'. The 7 cells are basically charging and discharging each of the 8 cells to keep them in balance. Is that correct?
Brian Drury
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not2bme said:
Yes.
Is there any current limiting on the flying cells? Your test case is the extreme example, are you putting a 4.2V cell directly in parallel (through some low Rds resistance of the MOSFETs) with a 3.0V cell? Seems there would be a lot of current dumped into the low cell which might cause issues. Would be helpful to see a schematic or other diagram of how this works.
Brian Drury
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rev0 said:
Good questions Rev.
No, apart from fuses there is not an intentional current limiting device.
The fuses consist of specific thin traces on the PCB. An image is attached. I have yet to experiment and determine at what current the fuses will rupture. The idea is that should a fuse break it can be replaced with a short length of wire between the 1206 pads. I have not seen this done before so any comments are welcome.
So, what is the current likely to be in the extreme case of massively un-balanced cells?
Assume cell A = 4.2V cell B = 3.0V.
Assume each cell has an internal resistance of 0.1R
The PCB traces are about 0.05R for each leg so again 0.1R for each cell
The wiring is about 0.03R for each leg so 0.06 for each cell
The connectors I cannot measure
So, the resistance between the two cells is approximately 0.26R
The potential difference between the cells is 1.2V
Current is 1.2 / 0.26 = 4.6A
When I ran the extreme equalising test the PCB did get warm in the obvious area but no more than about 10 deg above ambient.
Some questions:
Why would this happen? Why would someone construct a pack using highly imbalanced cells?
Why would the equaliser allow such a huge imbalance to occur? If it did then it must be faulty.
You asked for the schematic. I have attached a copy. It looks unusual because the MOSFETs are in PowerPAK SO8 packages which have 11 connection pads that must all appear schematically to assure an accurate ERC.
The schematic is work in progress. This is not a finished design.
BrianDrury said:rev0 said:
Good questions Rev.
No, apart from fuses there is not an intentional current limiting device.
The fuses consist of specific thin traces on the PCB. An image is attached. I have yet to experiment and determine at what current the fuses will rupture. The idea is that should a fuse break it can be replaced with a short length of wire between the 1206 pads. I have not seen this done before so any comments are welcome.
So, what is the current likely to be in the extreme case of massively un-balanced cells?
Assume cell A = 4.2V cell B = 3.0V.
Assume each cell has an internal resistance of 0.1R
The PCB traces are about 0.05R for each leg so again 0.1R for each cell
The wiring is about 0.03R for each leg so 0.06 for each cell
The connectors I cannot measure
So, the resistance between the two cells is approximately 0.26R
The potential difference between the cells is 1.2V
Current is 1.2 / 0.26 = 4.6A
When I ran the extreme equalising test the PCB did get warm in the obvious area but no more than about 10 deg above ambient.
Some questions:
Why would this happen? Why would someone construct a pack using highly imbalanced cells?
Why would the equaliser allow such a huge imbalance to occur? If it did then it must be faulty.
You asked for the schematic. I have attached a copy. It looks unusual because the MOSFETs are in PowerPAK SO8 packages which have 11 connection pads that must all appear schematically to assure an accurate ERC.
The schematic is work in progress. This is not a finished design.
Thanks, sounds like the balancing current isn't too bad then. For the fuse, I have had some experience with this, I can say you should have thermal relief, mine had vias and large copper pads on a 1206 I believe which caused a lot of the heatto be wicked away from the fuse and thusincreased the breaking current considerably. You can also use a 0 ohm SMD resistor as a fuse, I think 0603 was around 8A, I have a video that I can double check for the value later.
Brian Drury
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To make a direct comparison between flying capacitor and flying cell equalisers you would require large super cap devices with a capacitance of 100s of Farads. These can be quite large and certainly comparable to the dimensions of 18650 cells.
Smaller capacitors can be used but the switching frequency must be much higher and consequently switching losses are greater.
Super caps are expensive and all they do for you is to provide an energy path between any two cells. The flying cell has a huge advantage because it becomes part of the pack. A pack of n cells will have n 1 flying cells so the pack capacity will be (2 X n) -1. The only cost overhead is the simple clockwork logic equaliser circuit.
Maybe I have missed something but as far as I can see this approach is far better than the dissipative top balance method.
Smaller capacitors can be used but the switching frequency must be much higher and consequently switching losses are greater.
Super caps are expensive and all they do for you is to provide an energy path between any two cells. The flying cell has a huge advantage because it becomes part of the pack. A pack of n cells will have n 1 flying cells so the pack capacity will be (2 X n) -1. The only cost overhead is the simple clockwork logic equaliser circuit.
Maybe I have missed something but as far as I can see this approach is far better than the dissipative top balance method.
I am very interested in this project, I followed you here from the diyBMS project over at openenergymonitor.org
I have a few questions:
1. How hard would it be to cut this down to a 4S system, from 8S? (ignoring the extra flying cells, which I understand there would be 3 of instead of 7)
2. Are you generating your cell voltage graphs via the balancing board plus Nano, with no other equipment? In other words, does this provide monitoring of each cell voltage in the pack? If not, what else is required?
3. Given a pack with 4 static cells and 3 flying cells, would this be capable of 4A continuous current (2000mAH cells @ 2C)? You mention the diodes being a limiting factor for current, but it has been quite a few years since I did any electrical engineering or circuit design.
4. Would it be possible to make a pack with each static cell instead be 2 cells in parallel? So there would be 4S2P (8) for the static cells, and 3 flying cells (or maybe 3S2P (6) for the flying cells?) (assuming that all parallel cells are very evenly matched)
5. Do you have an estimate for the BOM cost for each pack (balance board plus Nano board and connectors/wires)?
6. Is the Nano board powered by the cell it is connected to, or by a central power supply?
I have a few questions:
1. How hard would it be to cut this down to a 4S system, from 8S? (ignoring the extra flying cells, which I understand there would be 3 of instead of 7)
2. Are you generating your cell voltage graphs via the balancing board plus Nano, with no other equipment? In other words, does this provide monitoring of each cell voltage in the pack? If not, what else is required?
3. Given a pack with 4 static cells and 3 flying cells, would this be capable of 4A continuous current (2000mAH cells @ 2C)? You mention the diodes being a limiting factor for current, but it has been quite a few years since I did any electrical engineering or circuit design.
4. Would it be possible to make a pack with each static cell instead be 2 cells in parallel? So there would be 4S2P (8) for the static cells, and 3 flying cells (or maybe 3S2P (6) for the flying cells?) (assuming that all parallel cells are very evenly matched)
5. Do you have an estimate for the BOM cost for each pack (balance board plus Nano board and connectors/wires)?
6. Is the Nano board powered by the cell it is connected to, or by a central power supply?
Brian Drury
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Hello brwainer.
Some answers:
1. The equaliser or balancing board can operate with 2, 3, 4, 5, 6, 7, or 8 static cells.
2. The equaliser is just clockwork logic. No software just one simple MC4047 to each static pair. The graphs are generated by the BMS board which includes the charger and is based on the nano. Yes it does monitor each cell in the pack.
3. If you wish to draw 4A from the pack (whatever the number of cells) you need to remember that the flying cells are effectively in parallel with the static cells therefore ignoring the small system losses the average current drawn from any cell will be 3A. The diodes are only required if you wish to parallel multiple packs.
4. The concept I am exploring here is scalable in either voltage or current.
5. The most expensive component is the nano which is about 2 GBP. This is designed to be a very low cost solution so I have selected low cost components. I anticipate the largest cost to be automated assembly which will be essential as the components are so small. However, there is little point in exploring build cost until a good number of people are interested and so far there are very few.
6. The nano (BMS) is powered from 12V DC which also provides power for the constant current charger.
It will be easier if you tell me what you wish to do.
Some answers:
1. The equaliser or balancing board can operate with 2, 3, 4, 5, 6, 7, or 8 static cells.
2. The equaliser is just clockwork logic. No software just one simple MC4047 to each static pair. The graphs are generated by the BMS board which includes the charger and is based on the nano. Yes it does monitor each cell in the pack.
3. If you wish to draw 4A from the pack (whatever the number of cells) you need to remember that the flying cells are effectively in parallel with the static cells therefore ignoring the small system losses the average current drawn from any cell will be 3A. The diodes are only required if you wish to parallel multiple packs.
4. The concept I am exploring here is scalable in either voltage or current.
5. The most expensive component is the nano which is about 2 GBP. This is designed to be a very low cost solution so I have selected low cost components. I anticipate the largest cost to be automated assembly which will be essential as the components are so small. However, there is little point in exploring build cost until a good number of people are interested and so far there are very few.
6. The nano (BMS) is powered from 12V DC which also provides power for the constant current charger.
It will be easier if you tell me what you wish to do.
Thanks for the responses Brian. I am trying to make 4S battery packs that can be used for a number of different projects. For example, in one project I would power a 4S 1300W Electronic Ducted Fan that is rated with a max draw of 79A. If each cell can provide 4A then that means 20 cells in parallel at a minimum. Im envisioning 20x 4S2P packs being in parallel for that project (talking about the static cells only) which would give max 2A current per cell. In a different project, the current draw would be more like 4-5A continuous so less cells are needed in parallel, but all-day runtime is desired, so either a lot of packs in parallel, or quickly swapping packs out between use, spare, and charging. Overall Im looking at a fleet of 40-60 4S2P packs.
The ideal battery pack would be 8 cells in a 4S2P configuration with a balancing/protecting/monitoring BMS, enclosed in a plastic case. The packs would use standard battery interconnects (like laptop batteries) for power and data. The interconnect and battery casing would interact with a hard mounting plate. There could be individual mounting plates on a belt, or several mounting plates joined into a long column to be used in a backpack or for charging. The project requiring 20+ cells in parallel would use 3 columns of 7 each. I know that in a large parallel deployment, resistance differences due to the inter-pack connections has to be managed.
The perfect BMS would be this one http://www.batteryspace.com/custom-...1-1-support-for-12-8v-60a-rated-lfp-pack.aspx but $62 is a lot when I am talking about 20 of them at a minimum. I can possibly order them wholesale as the same company does business through Alibaba as well, but I dont know how much cheaper they could sell it for: https://szsmartec.en.alibaba.com/pr..._smart_BMS_PCM_PCBwith_SMBUS_Communicate.html
It seems like AllPCB has an assembly service as well, is that the type of thing you are talking about given sufficient interest? I know you are talking about having ~100 or these, and even my 20+ wouldnt be fun to solder.
Would it be possible to design your balancing board such that if the center column isnt populated, the board could function as two seperate 4S boards? And possibly even be cut in half? This way there would only be one PCB and it is just a small assembly difference that makes it 2x4S or 8S. The challenge I see is in the connection to the nano, or nanos if cut in half.
The ideal battery pack would be 8 cells in a 4S2P configuration with a balancing/protecting/monitoring BMS, enclosed in a plastic case. The packs would use standard battery interconnects (like laptop batteries) for power and data. The interconnect and battery casing would interact with a hard mounting plate. There could be individual mounting plates on a belt, or several mounting plates joined into a long column to be used in a backpack or for charging. The project requiring 20+ cells in parallel would use 3 columns of 7 each. I know that in a large parallel deployment, resistance differences due to the inter-pack connections has to be managed.
The perfect BMS would be this one http://www.batteryspace.com/custom-...1-1-support-for-12-8v-60a-rated-lfp-pack.aspx but $62 is a lot when I am talking about 20 of them at a minimum. I can possibly order them wholesale as the same company does business through Alibaba as well, but I dont know how much cheaper they could sell it for: https://szsmartec.en.alibaba.com/pr..._smart_BMS_PCM_PCBwith_SMBUS_Communicate.html
It seems like AllPCB has an assembly service as well, is that the type of thing you are talking about given sufficient interest? I know you are talking about having ~100 or these, and even my 20+ wouldnt be fun to solder.
Would it be possible to design your balancing board such that if the center column isnt populated, the board could function as two seperate 4S boards? And possibly even be cut in half? This way there would only be one PCB and it is just a small assembly difference that makes it 2x4S or 8S. The challenge I see is in the connection to the nano, or nanos if cut in half.
Brian Drury
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Wow, I guess you mean this sort of thing:
Fan
I have attached a drawing showing the basics of my approach. The idea is to use parallel connections only at a higher battery voltage. This avoids the requirement for individual cell fusing and makes it easy to expand the system. Each fused battery in my simple diagram will of course have its own dedicated BMS and equaliser.
Your approach with only 4S cells means the diode voltage drop is a larger percentage of the total. I prefer to use a higher battery voltage.
The equaliser is scalable but with 20 cells in parallel and assuming 100 m Ohm internal resistance for each cell then the flying cells may try to shift as much as 200A. This will require much more powerful MOSFETs and significantly higher current rating for the PCB traces and wiring.
Currently the limitation I am seeing is that the sum of cell internal resistance plus PCB tracking and cables is limiting current sharing when the potential difference between cells becomes very small.
I plan to improve this by re-tracking the PCB to use much thicker traces. Cabling is tricky because I need a multi-way connector. The ribbon cable I used is neat but too resistive.
Your multi parallel approach increases this problem by a long way.
An alternative approach you may like to consider is to use the 8 cell battery with a high current buck converter to provide say 100A at 14.8V. I have not looked into this in depth but LT have a 50A design which you could double up to give 100A.
50A Buck
Fan
I have attached a drawing showing the basics of my approach. The idea is to use parallel connections only at a higher battery voltage. This avoids the requirement for individual cell fusing and makes it easy to expand the system. Each fused battery in my simple diagram will of course have its own dedicated BMS and equaliser.
Your approach with only 4S cells means the diode voltage drop is a larger percentage of the total. I prefer to use a higher battery voltage.
The equaliser is scalable but with 20 cells in parallel and assuming 100 m Ohm internal resistance for each cell then the flying cells may try to shift as much as 200A. This will require much more powerful MOSFETs and significantly higher current rating for the PCB traces and wiring.
Currently the limitation I am seeing is that the sum of cell internal resistance plus PCB tracking and cables is limiting current sharing when the potential difference between cells becomes very small.
I plan to improve this by re-tracking the PCB to use much thicker traces. Cabling is tricky because I need a multi-way connector. The ribbon cable I used is neat but too resistive.
Your multi parallel approach increases this problem by a long way.
An alternative approach you may like to consider is to use the 8 cell battery with a high current buck converter to provide say 100A at 14.8V. I have not looked into this in depth but LT have a 50A design which you could double up to give 100A.
50A Buck
Yes, that is the right type of fan. I am specifically targeting the 70mm size, and for overall compatibility with other projects I have selected a 4S motor. At this size there is also 3S or 6S ones. For my other projects, 4S (sometimes regulated to 12V) is the best solution, and I want to be able to share batteries. Think about interchangeable batteries for cordless power tools, but for more unusual loads. The connector/attachment mechanism will be a bit like this battery: https://www.amazon.com/Biswaye-Cord...514497738&sr=8-18&keywords=power+tool+battery except with as many blades as are needed for power plus communication between the Nano and central unit.
I think you've misinterpreted my plans a bit. At the level of the equalizer board, there will only be max 2 cells in parallel, aside from the flying cells. Each 4S2P pack will have a BMS, which for your setup means an equalizer board plus a Nano. If I had 20 cells in parallel that the equalizer board had to balance, then yes that would be a lot of current - but that is not the case. Overall its a lot like your plan to have 10 inverters each with 10 batteries, except I won't have inverters and my batteries have half the voltage.
Thank you for mentioning the diode voltage drop, I actually hadn't looked into that before. I'm seeing it as around 0.7V for silicon diodes, which is acceptable for my purposes. Also, while I understand the safety aspect, I didn't plan on having a diode between the parallel battery packs, I planned on them keeping each other at the same voltage during use despite small differences in capacity. This is how the EV people design their batteries.
The 8 cell plus buck converter idea is interesting, but adds a layer of complexity that I don't think I'm ready for.
My major questions remaining are:
I think you've misinterpreted my plans a bit. At the level of the equalizer board, there will only be max 2 cells in parallel, aside from the flying cells. Each 4S2P pack will have a BMS, which for your setup means an equalizer board plus a Nano. If I had 20 cells in parallel that the equalizer board had to balance, then yes that would be a lot of current - but that is not the case. Overall its a lot like your plan to have 10 inverters each with 10 batteries, except I won't have inverters and my batteries have half the voltage.
Thank you for mentioning the diode voltage drop, I actually hadn't looked into that before. I'm seeing it as around 0.7V for silicon diodes, which is acceptable for my purposes. Also, while I understand the safety aspect, I didn't plan on having a diode between the parallel battery packs, I planned on them keeping each other at the same voltage during use despite small differences in capacity. This is how the EV people design their batteries.
The 8 cell plus buck converter idea is interesting, but adds a layer of complexity that I don't think I'm ready for.
My major questions remaining are:
- Would it be possible to design your balancing board such that if the center column isnt populated, the board could function as two separate 4S boards? And possibly even be cut in half? This way there would only be one PCB and it is just a small assembly difference that makes it 2x4S or 8S. The challenge I see is in the connection to the nano, or nanos if cut in half.
- How many overall boards do you think are needed before automated assembly makes sense?
- If there was a diode preventing the batteries from keeping in balance with their parallel neighbors, what would that mean as the packs drain and the voltages drop at different rates? Would the output voltage be the average of the available supply? Even with well matched new batteries this is going to happen. In your example of the 8S16P, this would be differences between the voltages of each chain.
Brian Drury
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I shall address your question shortly but now is probably a good time to step back and explore the basic theory behind all of this. Hopefully some of the more experienced folk here will also comment.
(Q) Why do we consider it necessary to equalise or balance the level of charge in a battery of series connected cells?
(A) Because if we dont then a deep discharge may produce a below limit or even reverse polarity on any cell with a lower state of charge. Likewise when charging, if the cells do not have equal charge then an overcharge on some cells will be required to reach full battery terminal voltage.
(Q) Why not simply rely on a top voltage monitor to remove the load when the voltage reaches a cut off level?
(A) Because this will limit the battery capacity and the situation will worsen as time goes by.
(Q) If the cells are matched and all fully charged at the start why should they become un-balanced over time?
(A) Because the important parameter of internal leakage (self-discharge) is rarely measured and this is what creates the imbalance. Also, internal leakage is not a constant. It varies with temperature and time. See http://batteryuniversity.com/learn/article/elevating_self_discharge
My project seeks to explore the possibility of adding a simple clockwork logic circuit that can negate the detrimental effect of internal leakage variation to allow series connected cells to be operated over the full capacity range at all times.
(Q) Does this really happen?
(A) My personal experience is limited to fully charging a bunch of reclaimed cells and leaving them for a few weeks then measuring the terminal voltage drop. Yes I do see a significant variation, sometimes as much as 0.5V. However, the collective experience of the people reading this thread will be far greater than anything I have done so I hope to hear from them.
(Q) What is wrong with using resistive dump loads to effectively increase self-discharge of the good cells to match the bad cells?
(A) It is wasteful and the heat generated needs to be removed somehow. Also, the cost of the mechanism serves no additional purpose.
(Q) So is the flying cell equaliser the answer?
(A) Maybe. So far the results look encouraging. The technique definitely works but there are limitations. The obvious problem is interconnection resistance when differential voltages are very small. The solution to this is improved wiring, wider PCB traces and different connectors. The big plus with the flying cell is that it not only equalises charge distribution but also increases the battery capacity.
(Q) What next?
(A) This is a proof of concept exercise. I am hoping that the army of experts who gather here will review and comment on the approach. If there are flaws in the idea then lets hear about them.
(Q) Why do we consider it necessary to equalise or balance the level of charge in a battery of series connected cells?
(A) Because if we dont then a deep discharge may produce a below limit or even reverse polarity on any cell with a lower state of charge. Likewise when charging, if the cells do not have equal charge then an overcharge on some cells will be required to reach full battery terminal voltage.
(Q) Why not simply rely on a top voltage monitor to remove the load when the voltage reaches a cut off level?
(A) Because this will limit the battery capacity and the situation will worsen as time goes by.
(Q) If the cells are matched and all fully charged at the start why should they become un-balanced over time?
(A) Because the important parameter of internal leakage (self-discharge) is rarely measured and this is what creates the imbalance. Also, internal leakage is not a constant. It varies with temperature and time. See http://batteryuniversity.com/learn/article/elevating_self_discharge
My project seeks to explore the possibility of adding a simple clockwork logic circuit that can negate the detrimental effect of internal leakage variation to allow series connected cells to be operated over the full capacity range at all times.
(Q) Does this really happen?
(A) My personal experience is limited to fully charging a bunch of reclaimed cells and leaving them for a few weeks then measuring the terminal voltage drop. Yes I do see a significant variation, sometimes as much as 0.5V. However, the collective experience of the people reading this thread will be far greater than anything I have done so I hope to hear from them.
(Q) What is wrong with using resistive dump loads to effectively increase self-discharge of the good cells to match the bad cells?
(A) It is wasteful and the heat generated needs to be removed somehow. Also, the cost of the mechanism serves no additional purpose.
(Q) So is the flying cell equaliser the answer?
(A) Maybe. So far the results look encouraging. The technique definitely works but there are limitations. The obvious problem is interconnection resistance when differential voltages are very small. The solution to this is improved wiring, wider PCB traces and different connectors. The big plus with the flying cell is that it not only equalises charge distribution but also increases the battery capacity.
(Q) What next?
(A) This is a proof of concept exercise. I am hoping that the army of experts who gather here will review and comment on the approach. If there are flaws in the idea then lets hear about them.
Brian Drury
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Would it be possible to design your balancing board such that if the center column isnt populated, the board could function as two separate 4S boards? And possibly even be cut in half? This way there would only be one PCB and it is just a small assembly difference that makes it 2x4S or 8S. The challenge I see is in the connection to the nano, or nanos if cut in half.
Yes indeed a customised equaliser is clearly possible
How many overall boards do you think are needed before automated assembly makes sense?
As you know the Chinese http://www.allpcb.com/ are amazing. My equaliser PCBs cost 0.46 each in a quantity of 10. Shipping to the UK was free and they made the boards the same day as I ordered them. So, one wonders how little they will charge for component procurement and assembly?
If there was a diode preventing the batteries from keeping in balance with their parallel neighbors, what would that mean as the packs drain and the voltages drop at different rates? Would the output voltage be the average of the available supply? Even with well-matched new batteries this is going to happen. In your example of the 8S16P, this would be differences between the voltages of each chain.
The diodes do not prevent parallel balancing in the way you may imagine.
Lets say you have a number of 8 cell packs wired directly in parallel without diodes. Natural balancing or equalising will occur if there is any difference in the battery terminal voltages. After a time this will settle to a nominal level. Now if you place a load on this large battery the current draw for each 8 cell packs will be equal but if one pack has a lower capacity than any of its neighbours then the current flowing out of that pack will reduce accordingly.
Adding diodes does not change the above situation except that reverse flow from pack-to-pack will be prevented.
So why have the diodes?
If a cell in one of the 8 cell packs were to go short circuit and there are no diodes then there will be some serious reverse current flowing into the defective pack and multiple cell damage will occur as well as the danger of combustion.
Diodes prevent any further damage should one cell die.
Yes indeed a customised equaliser is clearly possible
How many overall boards do you think are needed before automated assembly makes sense?
As you know the Chinese http://www.allpcb.com/ are amazing. My equaliser PCBs cost 0.46 each in a quantity of 10. Shipping to the UK was free and they made the boards the same day as I ordered them. So, one wonders how little they will charge for component procurement and assembly?
If there was a diode preventing the batteries from keeping in balance with their parallel neighbors, what would that mean as the packs drain and the voltages drop at different rates? Would the output voltage be the average of the available supply? Even with well-matched new batteries this is going to happen. In your example of the 8S16P, this would be differences between the voltages of each chain.
The diodes do not prevent parallel balancing in the way you may imagine.
Lets say you have a number of 8 cell packs wired directly in parallel without diodes. Natural balancing or equalising will occur if there is any difference in the battery terminal voltages. After a time this will settle to a nominal level. Now if you place a load on this large battery the current draw for each 8 cell packs will be equal but if one pack has a lower capacity than any of its neighbours then the current flowing out of that pack will reduce accordingly.
Adding diodes does not change the above situation except that reverse flow from pack-to-pack will be prevented.
So why have the diodes?
If a cell in one of the 8 cell packs were to go short circuit and there are no diodes then there will be some serious reverse current flowing into the defective pack and multiple cell damage will occur as well as the danger of combustion.
Diodes prevent any further damage should one cell die.
I think i understand now about the diodes. And I agree that based on the example of the PCB production, it would seem that companies like AllPCB can probably also do assembly at an amazing price. I know youre waiting for feedback on the circuit and such, and also to see if there is other interest, but when you do an assembly batch Ill be interested.
Brian Drury,
I did not completely understand the schematic because I was not sure what the chip does, and not all of the header connectors have a description.
I have been looking for something like this for a while, I would like to balance a 14S pack. I am hoping to eventually get 50 KWH so I can run my home for several days assuming I charge the pack with solar or a generator during the day. I too hate the idea of dumping the power and not using the whole capability of the the cell packs.
The questions I have are:
Can you put two of these boards in series and use 14 of the 16 cells?
OR can you just use the flying cells for the load and use 8 cells as the flying sells instead?
Can you hook the 7 flying cells in series?
I did not completely understand the schematic because I was not sure what the chip does, and not all of the header connectors have a description.
I have been looking for something like this for a while, I would like to balance a 14S pack. I am hoping to eventually get 50 KWH so I can run my home for several days assuming I charge the pack with solar or a generator during the day. I too hate the idea of dumping the power and not using the whole capability of the the cell packs.
The questions I have are:
Can you put two of these boards in series and use 14 of the 16 cells?
OR can you just use the flying cells for the load and use 8 cells as the flying sells instead?
Can you hook the 7 flying cells in series?