Battery Testing, trying to save time

[katou]

I'm testing a boatload of cells, some A123, some Moli cells from toolpacks on my CBAIII

Here's my question. What C rate should I be testing at? I am currently testing at 1c so each cell takes one hour to test.

But, Moli cells are good to 2-5c depending on who you ask. If I tested at 5c, each cell would take 12 minutes...

So, I'm a bit in a muddle. Does it make a difference to test at a (safe for the cell) high C rate when attempting to determine cell capacity in Ah?

I know that cell capacity will APPEAR to be less when tested at a higher C rate.

Katou

 

[cell_man]

The cells with higher IR will be more effected by testing at higher C levels than those with a lower C rating. However, if you are still well within the capabilities of the cells I wouldn't have thought the differences would be so significant, assuming the cells under test are within the normal range for IR. By observing the voltage under load and the capacity I'm sure you would quickly get a good idea for what cells are well matched. AFAIK the CBA doesn't have separate sense wires for the battery terminal voltage. The battery terminal voltage is only measured through the load so especially when the load increases any additional resistance between the probes and the cell can appear as increased IR.

 

[AndyH]

The CBA isn't configured for 4-wire sensing, but can do three. Here's an image from West Mountain support that gives details for voltage sensing on the positive line. I've been using this with my CBA II and the 500W amplifier and it works well.

Andy
View attachment CBA_remoteV.jpg

 

[katou]

Okay, I just got mine, so I'm a little behind on the tech, but why would I want to do this 3 wire sensing thing?

Is this for measuring IR or something?

Katou

ps. glad to get your input on this Andy, I was hoping to get your 2c on this one.

 

[AndyH]

It's a way to allow for the voltage drop in the cables between the CBA and the battery.

If you're running a test and there's a voltage difference between a meter on the battery and what's on the computer screen, and you can't calibrate it out with the software, the third sense wire should help with accuracy.

If you're comparing a bunch of cells and really don't care if the actual numbers are off a bit, then don't worry about it.

My pleasure!
Andy

 

[Doctorbass]

3 wire sensing method... CURIOUS?? :?

it's like solving the problem by HALF....

or assuming that the V drop loss of the positive and negative lead are the same so we multiply the error by two..?

Doc

 

[cell_man]

I didn't realise the CBA could do that, but I think it's always better to have remote voltage sensing cables. Battery terminals with a bit more oxidization on them will appear to have higher IR. Even if you can accurately calibrate for the cables, there is still gonna be some cell to cell variation in the losses between the tabs and the leads. At low loads and/or for relatively high IR cells, this is insignificant, but for higher loads with very low IR cells it can have an effect IMO. Not that you should really be getting too concerned with this if you are just checking cells as the OP is planning.

Going back to the original question, I can't see a problem with checking cells at 2 or 3C to speed up the testing process. Cells with higher IR will be more effected by discharging into at a higher load, but I wouldn't get too hung on it. Paralleling cells will tend to offset some capacity variation when it come to building the pack. I'd be looking to just find the runt cells and get rid of them. Then try to group the cells together so that they have approximately the same capacity per parallel group. That's about the best compromise if trying to make the best of what you've got.

 

[tailwind]

I tested sixty of the A123, ie 6 of the 36v toolpacks.

Heres the procedure I followed to test ten at a time.

Bulk charge a pack to 34.5v.
Single cell charge each cell to 3.60v with CC/CV ending at 0.1amps.
Let them sit for an hour.

Try and test at the same room temperature and using the same wires for each test.

Hook up pack to CBAII and set load to 2.3Amps, 1c rate,
Setup a camera with movie mode and have your multimeter ready
When the AHrs used approaches 2.0AHr,
Set the camera running and quickly record the voltage for each cell, calling out the cell number as you go,
Rerecord cell 1 to see how much the voltage has dropped.
If you are quick it wont have dropped much if any,
Stop recording

Monitor the voltage of the lowest voltage cell until it reaches 2.00v
Stop the CBAII and record the AHrs used.

This will give you the AHr capacity for this one cell.
There is a relationship between the voltage of the cell and its capacity. It is not a straight linear relationship, but the higher the voltage the more capacity it has left.

My cells with 2.3Ahr capacity were reading 3.00v at 2.0AHr used, whereas a cell reading 2.75v only had 2.06AHr capacity.

This will weed out the dud cells if you have any.
You can always go back and test the cells individually to see what capacity they have.

Greg

 

[cell_man]

You can get a very good idea of the capacity of each cell in a series string by discharging it into a resistive load. Keep the resistive load well within it's max rating and it should remain reasonably constant. Record the voltage with a cell log 8 and use the alarm to remove the load when the first cell hits LVC. You can add up all the data points on the recorded data and assuming the resistor is a perfect resistor, determine the current for each data point and calculate the total Ah for each cell and observe the voltage each cell had dropped to whilst delivering that Ah. When the cells start to drop voltage rapidly towards the end of the discharge, they do so in a very predictable way IME so it is fairly straightforward to predict what the cells which still had some remaining capacity would likely have achieved. Alternatively just match them so they have an equal capacity for a given final voltage, say 2.5 or 2.7V.

Having a constant current load is only important if you are trying to determine absolute capacity values if doing the above method IMO. If you only need relative and approximate values, a resistive load will do the job just fine and is definitely more than capable of weeding out any dud cells. Assuming the terminations between the series cells are good, it also gives you a good idea of the relative IR of the cells under test.

 

[AndyH]

3 wire sensing method... CURIOUS?? :?

it's like solving the problem by HALF....

or assuming that the V drop loss of the positive and negative lead are the same so we multiply the error by two..?

Doc

Hey Doc - it's a step in the right direction. :wink:

I'm using a CBAII, a 500W amp, a pair of 2AWG cables to the cell, the positive sense wire, and the Pro software and it's working very well. For what I'm doing, ±.01V is fine.
The CBAIII is supposed to be much more accurate at higher discharge rates, but I haven't used a -III.

Folks that want the best explanation of why and when to use 4-wire testing, just search Doc's testing posts.

 

[katou]

If only it was summarized somewhere. It's a drag finding it, then reading all of the extraneous stuff, then getting the info.

I'll probably still do it sometime, but it's a pain. I keep saying we need a FAQ.

Katou