Need some help regarding charging and discharging nimh cells, old folks please share..

[100kwh-hunter]

I am just 52, but com on....nimh.
I was there when cad cells were replaced with nimh chemistry.

With the latest testing (today!!!yes i got some time on my hands!) i did: i got per cell and per zb a 200-300 mah out of a cell.
With charging i could put in a 300mah per cell per charger, healthy cells, with good IR

But this was PER charger and PER discharger....

Now when i combine for a single cell! some chargers, the cell wants to have it multiplied by the number of chargers...
The same for the number of dischargers.....
So a cell can take 1A for charge and discharge.

So if i charge TEN cells with one charger they will take 200mah, per celll, but the charger can do 5A
Could be bad cells?!?!?! no i dont think so, i think it is the cells chemistry...

That is what i am going to find out this week.
(must be honest i did not do a good pretesting!!!! just throw them in blind!!)
But if i charge ONE cel with ONE charger it will take 200mah
If I charge the same cell with 10x a charger that can do 5A(50A total the cell will take 2000mah for charging?!?!?!?!?
What the heck?!?!?!?!
Same for discharging.
A nimh cell will discharge at ~200mah but when i connect more zb's it will give 2A
But when i charge or discharge at a single psu or zb it will take no more then 200mah...

maybe it is the charger and discharger?!?!?!?!while the resistors can discharge over 600mah.

Really strange chemistrie..

With best regards Igor

 

[rebelrider.mike]

I still have NiMH cells too. Some things just need 1.5V, and others have to have the AA and AAA form factor. Ever since I started doing capacity tests on Li-Ion cells, I've started doing capacity tests on my old NiMH cells too, to see if they are really going bad or if it's just my imagination.

One thing to keep in mind is that NiMH chemistry has a higher internal resistance than Li-Ion. So I think when they are put in series the resistance adds up. Which I think may mean more energy will be needed to charge them than if you try to charge them individually or in parallel. I might be wrong about that, but It's something I would pay attention to if I were charging multiple cells at once.

Also, because of the higher internal resistance, It takes a lot more energy to charge the cell than the cell will give back. A fair amount of energy is lost to waste heat just to push current through the higher resistance chemistry. This is why NiMH cells normally get warm during charging, but Li-Ion cells normally don't get warm during charging.

NiMH still has its place as a useful chemistry for sure, but I sure do like to use Li-Ion or LiFePO4 when I can.

 

[100kwh-hunter]

One thing to keep in mind is that NiMH chemistry has a higher internal resistance than Li-Ion.

My tests reveal the exact opposite.
Good NiMh cells (80 to 100%SOH) ranging in the IR from 4mohm to 10mohm, after that the SOH goes rapidly down, after 15mohm faster than a brick that is falling.
Li ion has basically two groups in IR, one ranging from 20 to 30mohm and one from 70 to 90Mohm.
Lifepo around 20Mohm.

The cells i use are D format.
And what i figured out with charging they can not/will not take more than ~300mah when empty, when near full at 1.35v they take ~35mah.
Now we are going to add more v (1.45v0)to speed up the charging process, what NiMh needs.
Hences the heating of the cell, called the delta curve. after this the v will settle again back to 1.350v.
That is the reason you need to quit charging, to long in a warm state can damage them.
To "revive" any sleepers it is good to wack them with 2.0v(but this is not tested by me jet).

The major issue with NiMh is the slow charging and slow discharging ~200mah max.
They also tend to go to sleep and need to be waking up every 6 months to keep them healthy.
I thought they could take and give more than 1Ah---->nope

NiMH still has its place as a useful chemistry for sure

yes. but only for storage or low v applications.
I thought it was prius that had some nimh cells in there cars, up to 450v
I think also that the internal assembly and IR is different than the bicycle batteries i use.

But hey, i connect 42 cells to get 48v nominal, indeed this adds up to 8ah, times 9 in a subsection = 75ah
Total storage for one battery 4.5 kwh for free, 3 added atm, 9 more will come.
So i have to except the fact that testing them is just a very slow process and that there is now quick way, bummer.