Help Please! How do I test all 48 LiFePo4 cells on my my boat? (12v system, 3.2v 100Ah CALB)

[TommyHolly]

I think the tester V meter is wrong, maybe needs to be calibrated ?

I used another brand new multimeter and that one shows almost the identical voltage as my yellow multimeter and both don’t match the capacity tester?

 

[wholybee]

I tried my first capacity test using that cheap $40 capacity tester that will had on his channel. For some reason I can’t get it to work, but it might be because of the battery?? Maybe I discovered my first bad battery?

- I too balanced the battery at 3.65v until the charger showed 0 amps going in.
- When I hooked up the battery to the tester, it showed 3.52v
- I watched the video again and set up the tester so it shuts off at 2.60v as a Low Cutoff Voltage like he recommended.
- Then I adjusted down the current to 20A, which on my battery is 2C, just like in the video.

**Almost immediately, the Low Voltage alarm went off and the test stopped immediately. The screen flashed “2.60!” I couldn’t resume the test because the battery would instantly drop down under 2.6v during the test??

I grabbed a multimeter and it was actually displaying 3.17v directly on the battery. So maybe the capacity tester is bad??

I was forced to turn down the current to 15A and the voltage on the Capacity Tester says 2.7v which is dangerously low to shutting off the test again. The multimeter directly on the cell showed 3.102v. So maybe I have to turn it down all the way to 1C 10A???

Im thinking either the cheap tester is bad or the battery is bad. I may make a new thread about this tester thing to see if anyone has experience using it?

Get rid of the alligator clips. Alligator clips to a bolt are going to provide a very poor connection and will cause voltage drop while under load. Replace the alligator clips with ring connectors, and properly torque them to the terminals. The connection needs to be directly to the terminal, not through a bolt.

 

[TommyHolly]

Get rid of the alligator clips. Alligator clips to a bolt are going to provide a very poor connection and will cause voltage drop while under load. Replace the alligator clips with ring connectors, and properly torque them to the terminals. The connection needs to be directly to the terminal, not through a bolt.

I did exactly that.

I bought 1' foot of 10 gauge tinned copper wire, installed ring terminals and a 30A in-line fuse, sanded them down lightly to remove any corrosion from Panama's humid air, and bolted them down. That fixed the problem and now I only see a 0.1v difference between the Capacity Tester display and a voltmeter directly on the battery.

My question for you is, what Ah capacity am I looking for? What do I consider "good"? 89Ah?95Ah? Nothing less than 100Ah?

Once I separate the good from the bad cells, I was told I match them together with the best to the worst so it evens out...

 

[MisterSandals]

I was told I match them together with the best to the worst so it evens out...

This may have been my recommendation if you are combining in 2P before series. Its a solid strategy.

 

[TommyHolly]

This may have been my recommendation if you are combining in 2P before series. Its a solid strategy.

What Ah capacity am I looking for?
What do I consider "good"? 89Ah? 95Ah? Nothing less than 100Ah?

 

[MisterSandals]

What Ah capacity am I looking for?
What do I consider "good"? 89Ah? 95Ah? Nothing less than 100Ah?

Yes, its the capacity that will cause cells to diverge at the knees in particular. The lower capacity cells will reach the knees and accelerate away from the norm. A strong partner cell will keep a weaker cell's voltage more inline with the rest. So the theory goes anyway.

So combined capacity of the pairs being equal to the other pairs would be optimal. If you have individual cell capacities and great notes, perhaps you can add some real numbers and results.

You do have your cells labeled and are taking notes right?

 

[MisterSandals]

What do I consider "good"? 89Ah? 95Ah? Nothing less than 100Ah?

I'd be happiest with them all the same capacity (and at least 80% or more of rated capacity). Once you get cells of varying capacity, the capacity of the pack is dependent on the weakest cell (or cell pair in your case).

Think of the cells as being links in a chain. If each link is rated for 100 lbs but some are only capable of 70 lbs, it does not matter what the stronger cells can handle. But if you can pair your 70 lbs link with a 130 lbs link, it should be as good as a 2x 100lbs links if you could somehow double up chain links... (poor metaphor?)

 

[TommyHolly]

Yes, its the capacity that will cause cells to diverge at the knees in particular. The lower capacity cells will reach the knees and accelerate away from the norm. A strong partner cell will keep a weaker cell's voltage more inline with the rest. So the theory goes anyway.

So combined capacity of the pairs being equal to the other pairs would be optimal. If you have individual cell capacities and great notes, perhaps you can add some real numbers and results.

You do have your cells labeled and are taking notes right?

Yes, all cells are labeled. I performed a resting voltage test for internal resistance taking notes twice a day for 12 days. And now I’ve started the capacity tests…

What I don’t know is, what Capacity Ah is considered “GOOD”??

Is 93Ah ok? Is 95Ah? Or since they are rated at 100Ah, are only the ones over 100Ah considered good?

 

[MisterSandals]

What I don’t know is, what Capacity Ah is considered “GOOD”??

I don't know either. I did not capacity test my cells, i built 3x 4S batteries and put them to work (running a green house with lights, fans, seed incubators...).

I started moving the weak cells all to one battery. Then the next weakest cells to another battery. I had 3x 4S batteries and i know which cells were strongest and weakest in each because my notes showed which charged highest/fastest (and also discharged lowest/fastest). From the 2 better 4S batteries, i made my pairs: strongest (from best battery) with weakest (from second best battery). Second strongest, paired with second weakest. This was how i made my RV's 2P4S battery, which was my most important battery.

This was my best pairing i could think of. I had absolutely no numbers of capacity which i could not control anyway.

So, i dunno, i didn't have much for recourse with my cells and they all seems reasonable so i didn't worry about their capacities. I bought more battery than i needed mostly so i didn't have to beat the crap out of them everyday when camping.

Sorry for such a weak answer (and so many words).

 

[TommyHolly]

I don't know either. I did not capacity test my cells, i built 3x 4S batteries and put them to work (running a green house with lights, fans, seed incubators...).

I started moving the weak cells all to one battery. Then the next weakest cells to another battery. I had 3x 4S batteries and i know which cells were strongest and weakest in each because my notes showed which charged highest/fastest (and also discharged lowest/fastest). From the 2 better 4S batteries, i made my pairs: strongest (from best battery) with weakest (from second best battery). Second strongest, paired with second weakest. This was how i made my RV's 2P4S battery, which was my most important battery.

This was my best pairing i could think of. I had absolutely no numbers of capacity which i could not control anyway.

So, i dunno, i didn't have much for recourse with my cells and they all seems reasonable so i didn't worry about their capacities. I bought more battery than i needed mostly so i didn't have to beat the crap out of them everyday when camping.

Sorry for such a weak answer (and so many words).

Great info! Thanks!
Ok I saw some videos and talked to some people here who are now saying 80% capacity or lower is considered a bad battery even though it’s still useable.

Also, thanks for the description of how you connected your batteries. I needed to hear that so I know I’m doing it right.

 

[wholybee]

For, now, just mark them all. If most test at 100Ah, and 4 test at 90Ah, I would not use the 90Ah cells. But if they all test at 80Ah, then I'd call them all "ok." Look for cells that are different. 80Ah is probably the minimum I would accept. When a battery is rated for x number of cycles, it would be at 80% at its end of life.

As a baseline, those 100Ah calb cells test at 110-115Ah when new. Or at least all of mine did.

 

[TommyHolly]

For, now, just mark them all. If most test at 100Ah, and 4 test at 90Ah, I would not use the 90Ah cells. But if they all test at 80Ah, then I'd call them all "ok." Look for cells that are different. 80Ah is probably the minimum I would accept. When a battery is rated for x number of cycles, it would be at 80% at its end of life.

As a baseline, those 100Ah calb cells test at 110-115Ah when new. Or at least all of mine did.

Ok cool.
So far the two worst cells are testing at 93Ah and 94Ah (with the old tester which cuts off too early). The rest are all close to 100Ah. So even the two extra battery cells I’m not using are testing close to decent.

I know the CALB cells test higher for the first 20-30 cycles at around 115Ah… the original owner tried claiming I had a 1,400Ah system and couldn’t figure out how he came to that number until I realized he was probably claiming the original capacity…rrrrgh

Thank you so much for your reply. Trying to figure out a system that someone else designed poorly and gave you bad numbers has been extremely frustrating. So everyone’s comments here have helped me a lot!!

 

[Honuz]

Why can't we remove post .. it's an ooolld threadh .. my bad.