Hi everyone, I am building a massive 16-18kwh lithium battery pack using old modem batteries for a solar array and am looking for a good charger to spot test some batteries. I bought a 16 bay cell docter/megacell charger but I am not sure it is accurate and I am a little concerned that I wasted my money. Some cells test at low capacity and then I stick it in my xstarVC8 and they test somewhat normal, then I stick them in my hobby charger and they test even lower. So I am looking to gauge accuracy here, any suggestions?
Do all of those charges use similar discharge currents and the same termination voltage? Is the discharge current close to the one you’ll see in your setup? Or is it low enough to not cause significant voltage drop?
It’s configurable on the megacell charger, not sure what the profile is on the VC8. but the difference is like a full amp hour a lot of the time, I had it set to 1A discharge but now it’s set to 500mA. Also, the megacell charger seems to have IR/ESR readings all over the place, but it seems to pick out certain brands of cells more than others, so IDK. The VC8 shows them all around 50-150mOhms with no correlation in relation to the megacell charger.
The hobby charger (EV-PEAK-CQ2) has been set to 500mA discharge and is usually closer to the megacell charger though still 100-200mah off.
Discharge current is a MAJOR factor in capacity measurement, especially with old cells that may have developed high internal resistance.
For example: 2 cells with ‘original’ 2000mAh capacity: #1 that has been ‘hammered’ for 5 years in a laptop, #2 used occasionally from new in a flashlight and never charged beyond 4.0v, or discharge below 3.2v.
If you discharge them both @ 0.5 amps you might get : #1 = 1500mAh, #2 = 1800mAh. Different, but not a major discrepancy.
If you discharged at 1.0A you might get: #1 = 500mAh, and #2 = 1500mAh. Noticeable difference.
If you tried to discharge at 2.0A; #1 might fail immediately, and #2 might go 1400mAh. Ah, now the difference is stark.
Then, if you checked the voltage on #1 it might still be 4.1v on rebound. #1 simply could not deal with the required draw anymore.
Any charger using sliders to measure internal resistance is inaccurate due to the resistance of the sliders. It’s like trying to do precision measuring using an old wooden stick with hash marks on it. It’ll do some kind of a job but it’s pretty crude.
Internal resistance/Impedence
If you are going to put that much work into this project, I think you need to minimally use the likely discharge current that the cells will have to deal with on maximum draw. I’d recommend trying to do at least 2x that if not more to weed out weak cells. Cells that are already old and weak, making the grade….barely……may come back to bite you.
I'm currently test a couple Thousand 18650 cells ---- I have 6 OPUS BT 3100 that I use to discharge only (@ 1000 ma ) ---- I bulk charge cells 24 at a time in a homemade holder with a QMax hobby charger -- I had to figure correction factors for my OPUS chargers --they all test a little different ( 2 @ 1.00 2 @ .99 2 @ 1.01 ) I use the discharge mode because I recharge the cells in the jig to 3.83 volt for storage
What type of powerwall are you building --hopefully at least a 7S ---- 24 volt and 48 volt are the two best ways to go with Li-ion
I got lucky I purchased 1000 Ring battery packs for $ .65 a cell --- they are never used packs -- cells test 3050- 3200 mah
Looks like the Xtar VC4 Plus Dragon would do the trick. Measures Internal resistance with gold plated probes. Little spendy. Henrk gives it 3 smiley faces. His review: https://lygte-info.dk/review/Review%20Charger%20Xtar%20VP4%20Plus%20Dragon%20UK.html