No BMS - Solar Charge Controller for Lithium Battery

Lawcomleo

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Jul 4, 2017
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I just got an idea that I think will end all this Battery Management problems for Lithium cells, especially for those of us building the packs ourselves.

I will use 24v system inthisexample:

STEP 1: The controller will take supply from the PV
STEP 2: The MPPT system will step down the voltage to 4.1V while boosting the current.
STEP 3: The 4.1v busbar will have 7 (or 14 for 48v systems) +ve and-veoutputs.
STEP 4: Connect the terminals to individuals cell packs.

I will need your suggestions and inputs. I believe this is a far cheaper way to use lithium cells without bothering about balancing.

The first park that gets to 4.1v automatically stops charging. The cells with the highest voltage will charge slower while those with lower voltage will charge faster due to the varying potential difference.
 
Maybe I misunderstood your idea, but you can't charge your packs in parallel whilst they are connected in series.
Well actually you can, sort of, but you would need 7 (or 14 for 48V) separate chargers with isolated outputs, one for each pack.
 
Its cheaper getting a propper BMS instead of building a system that can charge in paralell and then load in series :) As Watts-on said you need isolated DC/DC chargers.
 
All above and :
Do not forgetsome protection on the low voltage side :) ... a over discharge protection.

On a 48V system you need 14 chargers... not sure it is going to be cheap(er)
 
It is unfortunate. But there is no effective way to use Li-Ion batteries without some kind of BMS.

You need for each set in parallel under and over voltage protection. Plus the ability to balance. Over voltage protection must be able to shut the whole charge setup down (or there will be a fire), if whatever your using to balance cannot keep up. Under voltage protection must shut your load off, to save your cells. BMS usually have overload protection too, save your cells if your fuse or breaker isn't quick enough.

There are many cheap alternatives out there, Batrium is good for peace of mind. It is tried and tested. Many forum members use it, and I have yet to see any regrets.

However, for those of us looking to do things cheaply, while I haven't settled on an idea yet, I have been toying with Arduino. However one thing that does bother me, is I am putting a lot of faith in that little microprocessor, and computers being computers, what if one has a glitch or locks up.

There are also many 'smart' BMS with bluetooth ect, you can buy from Aliexpress or Bang good.
 
It could work. ONLY if you bring the string out of service and use relays to switch from series to parallel. But for this to work effectively, you would at least 2 strings to stay in operation. That is the only way I can see this working safely
 
Korishan said:
It could work. ONLY if you bring the string out of service and use relays to switch from series to parallel. But for this to work effectively, you would at least 2 strings to stay in operation. That is the only way I can see this working safely

That is a good idea. However if a bank was far out of balance, it would cause trouble when it switched to parallel.

Now that really has me thinking but. Having two, or even three strings would not be too bad anyway, suggest if you needed to service one. I was going to have two strings anyway, one from those blasted Sanyo heaters (if I can get them to co-operate!)

However, there would be some kind of interruption when you switch over as well. I don't know if you could use a big capacitor for this or not, as the difference in battery voltages may cause all kinds of issues.

I doubt such a system could be automated.
 
If the switching via relays was done say, once a week, the packs shouldn't be so far outta wack that it'd cause issues. They may drift by as much as .5V. If they are drifting further than that in a weeks time, you have some major issue that needs to be addressed; most likely a bad cell(s) that are self draining and taking the pack down with it.
There won't be any kind of interruption from the inverters perspective. The voltage will remain the same as far as it's concerned. The only issue would be is if you happen to pull more amps through the inverter than what 1 string could handle. If this is the case, you didn't size your string correctly, or didn't use appropriate wiring.

Let's say you do a weekly balancing using the switching method and have 3 strings.
Monday Evening/Night: Take String 1 offline and put it into balance mode. Bring back to production mode by early AM
Wednesday Evening/Night: Take String 2 offline and put it into balance mode. Bring back to production mode by early AM
Friday Evening/Night: Take String 3 offline and put it into balance mode. Bring back to production mode by early AM

The reason for offsetting the days is to make it easier on you. The reason for evening/night offline mode is because this is usually the least amount of power usage required. Thinking about 9pm/10pm or so, maybe 11pm. All cooking is done, the house doesn't need as much cooling at this time (summer time), water has been heated after showers, most lights are shut off.
The reason for coming online in the early AM, around 4am or 5am, is so the full system is available for morning routines of showers, cooking, bringing the heater online to warm up for wake up comfort (winter timer).
Granted, this doesn't cover everyones schedule nor does it cover all possibilities. Tailor to your needs.
But I was showing when you could do the maintenance periods so it gives the least possibility of impact to the users.

I don't see why the system couldn't be automated. With the schedule above as an example, an arduino could simply flip all the relays over. Maybe you could even use a large contactor that has 7 or more poles on it so you could flip all the connections at once. A little over kill, but they'd last longer than the relays and you wouldn't have to worry about a single throw getting stuck/failing and causing a short (this, would probably be the worse thing that could really happen. a relay not throwing and 2 packs are still connected in series, and then the rest get connected in parallel. You'd have a mad rush of voltage and current across the whole string and possibly even melt something). So, in light of that, I'd use a contactor now that I'm thinkin about it as I write the up :p


Something like this one, though, I don't know how you'd wire it up. daromer could probably help ya out ;)
https://www.ebay.com/itm/1PCS-New-MITSUBISHI-CONTACTOR-SWITCH-SD-QR12/232286253093
 
ThatKorishan, is exactly what I am going to do. I cannot see any disadvantage whatsoever. MPPT at 4.2v will mean even a tiny amount of sun is going to see you being able to charge :cool:

Still need an under voltage protect, but given that you can be certain that your packs are balanced, you can do this at a system level.
 
Again, you could use the arduino to keep tract of the voltage of the string, and when it sees the voltage hit the cutoff point, trigger the main contractor to that string(s)


btw, thanks goes to daromer for introducing me/us to contactors :D
 
Korishan said:
Again, you could use the arduino to keep tract of the voltage of the string, and when it sees the voltage hit the cutoff point, trigger the main contractor to that string(s)


btw, thanks goes to daromer for introducing me/us to contactors :D



Yest indeed contactors

However the amps could be an issue. To charge a 24v system at 10a will translate (assume 7p) to 70a. So if you had 4 250w panels that would be 210a :exclamation:

However contactors and 'flying capacitor' style balancing. That is more though provoking (and probably a lot cheaper). I don't imagine a 200+ amp buck converter or MPPT setup is going to be efficient or cheap!

I am a big fan of 'analogue' balance systems. Microcontrollers leave too much room for computer error (if in the unlikely event that it does 'freeze' or glitch, it will burn you house down).
 
if the arduino froze, it'd be stuck either in parallel or series. it couldnt stick in both. it'd be either one or the other or off completely. So I think it'd be safe. An analog timer could work, too. Kinda like the ones you'd use to start a coffee maker at X time and run for Y minutes. Or an irrigation timer
 
Korishan said:
if the arduino froze, it'd be stuck either in parallel or series. it couldnt stick in both. it'd be either one or the other or off completely. So I think it'd be safe. An analog timer could work, too. Kinda like the ones you'd use to start a coffee maker at X time and run for Y minutes. Or an irrigation timer

Still - oh the amps. Look inside a welder - your going to need some heavy cable and good connections. I can almost smell the hot metal.

However flying capacitor balancing could be very effective.
 
Watchdog timer on the arduino :)
 
daromer said:
Watchdog timer on the arduino :)
I shall refrain from making any comments. My coding is amateur at best.

Still, I perfer fail safe.
 
watts-on said:
Maybe I misunderstood your idea, but you can't charge your packs in parallel whilst they are connected in series.
Well actually you can, sort of, but you would need 7 (or 14 for 48V) separate chargers with isolated outputs, one for each pack.

Exactly. The isolation to each output will be done within the device. The only challenge I suspect will be the component sizing bcoz of the large current requirement.
 
Geek :) Yeah and watchdog timer is cruicial but many people tend to miss that. Ie its a kind of timer that if the machine hangs just reboots it kind of.

Its not something that fixes every issue but it saves you from some issues atleast.
 
wim said:
All above and :
Do not forgetsome protection on the low voltage side :) ... a over discharge protection.

On a 48V system you need 14 chargers... not sure it is going to be cheap(er)

Looking at it closely,I now agree that it won't be cheaper than a BMS system.

An over-discharge protection is easy to implement using relays.

However I don't think BMSes are as effective as they ought to be. Even with BMS some packs willstill go astray.

If they were that effective we won't be building powerwalls from laptop batteries in the first place.
 
I know there is one BMS system that is very effective inkeeping my packs neatlyin line...
You canfind it in the right top corner of this page ;)
 
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