Matching cells asap with minimal tools

nonoceb

Solar Enthusiast
Joined
Jun 10, 2021
Messages
56
Hi i would like to share a method to test and identify matched cells when a "big" order is coming.
This time i received bulk 37cells from alibaba then i would like to pairing 4 cells by capacity and IR to paralleled setup like 4S3P.

Testing cell can be very time consuming and energy consuming so my method tend to optimize this!
I need for that :
2 BMS
1 Active balancer 2A like Jikong brand
1 MPPT victron 100V/50A

The fact is these cells already contain 40% soc so i will use it to my test cycle.
I separate these 36cells in 2 groups, the testing cells and the bulk feed cells :
I install 1 BMS on 8S cells testing group
I install 1 BMS on 24S cells bulk feed group

then i need to dump first the 8S testing cells to 4S cells in bulk group, for that i use the MPPT Victron because power conversion it's efficient and well suited for lifepo4 charging, limiting charging current is easy to setup with victron connect app.
So i use 8S cell as solar input to dump energy to 1->4S on bulk group; BMS will cut off when these 8S are dumped. Bottom balancing if needed.

Now testing 8S testing can began, clear history in victron mppt then connect 24S bulk cells as solar input and 8S testing cells as batterie and GO for charging at 40A.
When the battery 8S is charged in float mode you know precisely what is the battery capacity (kwh) thankful to victron history so you can put information on lowest cell capacity that can trigger the bms first. you can continue charging in 4S mode by removing 4highest voltage cells. For finish testing slow charging can be done at 3.65V and give you the residual capacity to charge in order to write informations on each tested cells.

Now we have 8S tested, characterized and fully charged we need to dump energy to bulk cells, the point is to charge always in priority the bottom of the 24S battery because we want the active equalizer "bottom" balance in priority the 17->24S (next cells for testing).
The mppt is wired to use tested and full cells to charging 0->4S then 5->8S then 9S->12S, just keep in mind the botton cell need to be more charged that top cell. BMS can be set to discharge at standardized 30% SOC for uniform cell compression later.

mppt effiency this config is about 96% so no need to add external energy!
Repeat the procedure as long you have bulk cells then give a feedback to your alibaba provider before timeout 😁
 
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