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Capacity check of 18650 batteries
Capacity check of 18650 batteries
There are several capacity testers available but I found this model to be very accurate and very consistent for it's price. It's important to use the same boards and same procedure for all your cells to create packs with the same capacity. This board needs 12V but power consumption is very low, so a single powersupply from your junkbox can power many of these boards. With the first 2 red buttons you can select the discharge current, use 1A. The thirth button allows you to toggle between mAh, wh and voltage.
The unit itself uses only a few mahs, so a small 12V powersupply can power easely more then 10 of these units. Capacity testing is a very important step in the process, if not the most important one. Don't make mistakes here.
Capacity test station
Capacity test station
Capacity testing unit
Capacity testing unit
After charging the batteries I let them sit idle for at least 10 days to check if there's any self discharge. If there is very limited self discharge (between 4.05-4.09, some TP4056 cutt-off early) I tend to charge them once more, if they drop below 4V it's straight to the bin. If they passed all these saftey tests the next step is the capacity test. This unit will discharge the battery from 4.2V to 3V. Once finished a buzzer will alert you. It's important to use the same testers and same procedure for all your cells. This will allow you to create packs with the same capacity later in the process. After the test I write the capacity on the cell and put them in 18650 4x5 holders. Every stack is 100mah difference. 1800 - 1900 - 2000 - .... This will make life much easier to make the packs later. The 4x5 18650 holders will later be used to built the battery packs.
Afther the capacity test make sure to charge the battery again to 3.7V or 3.8V. They don't like to sit idle for long time at low voltages.
Proces(sing) batteries
Proces(sing) batteries
It's not the most time consuming work, but it's by far the most important step in the proces. Decent capacity testing and afterwards making sure the cells get well distrubuted over the packs is key for you powerwall project.
When cells get hot, that's a no brainer, bin the cells. I mentioned this in the charger tab, but that's the main reason why I prefer DIY charger iso commercial chargers. With DIY chargers you feel that the cell gets hot and they wont slip into the powerwall. If the cell is charged, let it sit idle for more then a week to check for self discharge. If voltage is > 4.10V, proceed to discharge test. if voltage is between 4.05 and 4.09 charge it again (only 1 loop).....some 18650 chargers cut off a little early. You will notice, most of these "second chance" batteries will end up in the "spare batch" with capacity below 1900mah. If capacity is > 1850mah, batteries are good to go and will end up in a powerwall.
detailed discharge module
detailed discharge module
I've installed mine of a piece of wood. In the middle of the ZB206 module you will find a small hole. With a small screw you can mount it to the wood. Same story for the 18650 holder. Solder some thin wire, I used strands from old UTP cable, to the 18650 holder. The other side connects to the ZB206 module. If you reverse polarity....no stress, the unit is well protected and will not fail.
Discharge done
Discharge done
Once the test is done it's a good idea to sort the cells by capacity in the 4x5 holders. This makes it easier to count the number of cells and you have an overview on the general capacity of your cells.This way of working will help you to make the final packs. But that's for another topic: SORTING
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