Cell conditioning

[eq1]

On a side note, anyone know why a single cell might charge and discharge less than two cells in series? I'm using my SB989 to cycle some waning AA 2300mAh cells. I did single cells and was only able to charge on average 405mAh, and discharge on average 146mAh, over 4 cycles (2.3A charge, 1.2A discharge, .9V cutoff, 6mV deltaV). I put the two in series and I'm getting about 800mAh discharge thus far...

[Eli]

Originally Posted by eq1

On a side note, anyone know why a single cell might charge and discharge less than two cells in series? I'm using my SB989 to cycle some waning AA 2300mAh cells. I did single cells and was only able to charge on average 405mAh, and discharge on average 146mAh, over 4 cycles (2.3A charge, 1.2A discharge, .9V cutoff, 6mV deltaV). I put the two in series and I'm getting about 800mAh discharge thus far...

2.3A charge and 1.2A discharge is too much for AA cells overall. Even my newer 2700mAh Powerex cells would start to quiver at those rates, and older or unused cells need to be woken up gently.

Try 1A charge and 500mA discharge. I always charge my AA's at 1A, and I have some that are 12 years old (1700 and 2100mAh). Lately I've been trying to charge the older ones at only 800mAh....

If they're particularly weak, you might need to use even lower rates.

A few things to add to Ron's comments..

Discharge voltage nearly doesn't matter with six cells in series without monitoring at the cell level. 0.9 * 6 = 5.4V.

That could be five cells at 1.0V and one cell at 0.4V, or any combination thereof. With this in mind, it actually makes more sense to use a higher minimum voltage if you're truly worried about overdischarge. At the same time, you want to give the cells the maximum workout you can. I use 0.750V in the testing rig currently, which is rather arbitrary.

5-6mV is fine, I've always used 6. Better chargers also allow you to specify the time after deltaV is reached that charging stops.

I'm going to go out on a limb here and say that, unfortunately, testing sticks at 5-10A is perhaps slightly less than an exercise in futility. It will allow you to detect gross problems, but it doesn't matter if a stick gives 5Ah at 5A if it only gives 2Ah at 60A, or if it drops below 1.0V per cell under 80A of assist.

We're partially "lucky" that the car is self compensating to a large extent. It would be MUCH harder (than it already is) to rebuild a battery if the car didn't adjust charge/discharge rates based on the batteries condition. One of the reasons a brand new battery feels so powerful is because there are none of these voltage based restrictions that even a healthy stock pack faces, let alone a rebuild.

[Hybrid-Battery-Repair]

You can solder the power supply cord directly to the circuit board. The place I was talking about though was where the power comes out of the charger and goes to the battery. We replaced those with extension cord wires from a dollar store.

Originally Posted by wingin73

Thank you....

I will do exactly that. I am in fact melting the power supply to charger negative side banana clip thus ill redo that as well as put ring connectors on the battery connectors.

I'm not in a hurry and I don't care to damage the superbrain so ill just go with a 5a discharge / 5a charge.

[IamIan]

Originally Posted by eq1

On a side note, anyone know why a single cell might charge and discharge less than two cells in series?

In addition to the other comments from Eli about voltage and SoC fluctuation from multiple cells in series ... The chemical reactions to charge or discharge are about energy not charge.

Charge ... ie Amps and AmpHours are commonly used ... even dominantly used when people discuss batteries ... but Amps and AmpHours are only measuring the number of electrons ... as a rate passing a point with amps and as a total sum that has passed a point over a period of time with AmpHours.

But each chemical reaction of the molecules is about a specific amount of energy , not a specific amount of electrons... a specific amount of electrons can have different amounts of energy... and with different amounts of energy you will have different amounts of those chemical reactions.

For example ... if you look at the attached graph of Ah and Wh cycle efficiencies from my tests of my batch of A123 cells ... yes most of them are high efficiency ... in the mid to upper 90s ... but you can also see that there are some cells that are more efficient than others ... and not just in Wh of energy , but even Ah of electrons ... So if I took say one at 97% Ah cycle efficiency and connected it in series with one at say 93% Ah cycle efficiency ... sense they are series they will both see the same amps of current at the same time for charge rate or discharge rate ... when I put 10Ah into the 97% efficient one I get 9.7Ah back out ... but when I put the same 10Ah into the 93% Ah cycle efficient one I only get 9.3Ah back out ... even though both saw the same Amp rate and Ah going in to charge them and the same Amp rate coming back out to discharge them.

And this assumes that there is no capacity difference between the cells ... which can also cause a difference.

And this assumes there is no significant amount of time spent idle that a difference in Self Discharge could also create a difference.

[Subdewd2]

Well said.

[MN Driver]

Originally Posted by Eli

..clip..

A few things to add to Ron's comments..

Discharge voltage nearly doesn't matter with six cells in series without monitoring at the cell level. 0.9 * 6 = 5.4V.

That could be five cells at 1.0V and one cell at 0.4V, or any combination thereof. With this in mind, it actually makes more sense to use a higher minimum voltage if you're truly worried about overdischarge. At the same time, you want to give the cells the maximum workout you can. I use 0.750V in the testing rig currently, which is rather arbitrary.

You'll know when the one cell drops to .4v if you are watching the voltage. A graph makes it obvious if you've got a cell dropping out. I can connect my load and pull 30 amps, 50 amps, or whatever I want(to the point of their destruction if I go too high), I can watch the bad cell(s) drop before the others.

[Eli]

Originally Posted by MN Driver

You'll know when the one cell drops to .4v if you are watching the voltage. A graph makes it obvious if you've got a cell dropping out. I can connect my load and pull 30 amps, 50 amps, or whatever I want(to the point of their destruction if I go too high), I can watch the bad cell(s) drop before the others.

I agree, but the Superbrain and most other chargers don't watch the rate of voltage change, they just wait for the absolute voltage to reach the min voltage per cell * number of cells setpoint.

[Eli]

I thought this deserved discussion in the forums rather than in PMs, as its pertinent to the discussion.. and incorrect.

Originally Posted by Hybrid-Battery-Repair

"One stick shot up to (over) 1.665V per cell under only 40A of charge"

It is physically impossible for NiMH cells to exceed 1.45V per cell. What you were measuring was the V potential of the charger.

Yes, that's a crappy stick, but it never got over 1.45V and probably not over 1.4V

Ron, where on Earth did you hear that? That's not true at all, and it's disturbing that you would even say that. I am guessing something must have gotten lost in translation, because you've been around these things long enough to know that isn't true.

V=Ir. Higher internal resistance means the cells voltage spikes higher under any given current. If I hit them with a 70A charge, they probably would have spiked to over 2V per cell; the cell brings the charger voltage down to its level. The voltage difference between the cell's voltage and the charger's open circuit voltage is why current flows into the cell.

The cells reach over 1.45V per cell every single time you drive your car. The limit is hard coded in the BCM at 192V or 1.6V per cell, 19.2V per voltage tap.

If you'll note, the specs on the new cells specify a 1.6V cut off. This is typical for a NiMH cell under high rates.

Resting voltage for a fully charged NiMH cell is 1.4V, but during charge and discharge they will follow Ohms law.

Stock stick charge/discharge curve graph:



During the IR tests(large dips at the beginning of discharge), you can see that cell 3 drops the lowest, meaning it also has the highest internal resistance.

Cell 3 also has the lowest voltage while discharging, and the highest voltage while charging. Coincidence? I think not.

[IamIan]

Originally Posted by Eli

Resting voltage for a fully charged NiMH cell is 1.4V, but during charge and discharge they will follow Ohms law.

My only add on for additional detail ... would be that there can be conditions for the cell itself to be over 1.4V without the externally applied charge current creating a dV from Ohmic reaction.

It is short lived and is sometimes refereed to as a 'skin effect' ... Sense the total voltage between the + and - terminal is the combination of all the voltages between the two sides ... sense the chemical reactions and distribution inside the cell happens slower than the electronics flow of electrons ... it is easy to see in the short time right after the charge current is removed ... it takes additional time for the chemicals in the cell to 'normalize' to distribute the charge that was applied... it happens with a -dV from a discharge and a +dV from a charge.

In an application like the Insight where one can go from 50A of Charging to 100A of discharging or vise versa in like a second or so ... the chemicals in the battery will not distribute and 'normaize' the in the cell medium in that short time period and thus the electronics monitoring it need to keep that kind of short term effect in mind.

This is even seen in the graph Eli posted ... once the discharge ends ... but before the charge starts ... initially the cell voltage jumps up the first instant is from loosing the -dV from the discharge ... but after that Ohmic effect is done ... it continues to rise at a logarithmically decreasing rate of increase , moving closer and closer to a zero slope until eventually the charging current starts and once again you see the +dV effect as well.

As for the 1.45v of the cell itself ... in a I1 with 120 cells ... any time one sees over 174V for the whole pack there must be cells over 1.45v... I've seen this many many times even in the OEM IMA behavior.

[Eli]

Yep.. and that "surface charge" as it is sometimes called leaves a NiMH cell quickly, and actually needs to be counted if you expect to get near the rated capacity of the cell. Discharge should be started at >=1.400/cell.

You can see that the cell voltage was ~1.45V just before the discharge started in the above graph, the stick had just come off the trickle charge a few minutes prior. Here's a closeup of what we're talking about here:



Trickle charge stops when the voltage starts falling. If the discharge wouldn't have started, the cells would have slowly ticked down to about 1.4V and then even more slowly down to about 1.375V, representing the roughly 5-10% SoC loss experienced during the first few hours after charge stops...the rate of loss being greatly dependent on temperature, actual cell SoC and the position of the moon, of course.