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Discussion Starter · #1 ·
Hello everyone,

So, i have been building and repairing 18650 batteries packs for ebikes for a few years now, and one thing that i noticed is that the most problematic part in these packs is usually the faulty BMS. This inspired my current project, which is a 14s 52V battery that i plan to build without a BMS. I have two low-alarm buzzers (each supports up to 8S) to eliminate the danger of over discharging and monitoring the discharge process, and a ISDT Q8 hobby charger to charge and balance the pack, that balances a maximum of 8 cells.

What i want to ask is, since i don't have a parallel charging board (for now), is it possible to charge this battery by charging first one half of it, and then the other? The battery is spot welded (as you can see in the picture), and i will put two sets of 8 sense wires (in each one - and 7 +) to the pack that will be compatible both with the charger and the low voltage buzzers. I would then like to split the battery in half (electrically, not physically) by soldering two extra wires in the middle of the pack, so that i actually get two 26 volt batteries that are permanently connected in series by nickel. Now my biggest concern is, when i am charging one half of the battery, will it in any way affect the other half of the battery since both halfs are connected in series with the nickel strip? And is the difference in voltage (when one part is charged and the other one is not) dangerous in any way when the battery is not discharging?

I am aware that this system is not very practical since it means that i have to charge two separate batteries every time and that i can't just charge the battery "a little bit", but always have to charge the both parts equally, but it's a experiment :)

I am not an electrician and most of my knowledge on this topic is from forums and the internet tutorials, so i would really like someone with broader knowledge to give me advice and tell me is the above described system safe.
 

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Yes, you can split charge each side as 7S or 6S+8S

but ONLY one side at a time.

If you designed two physically separate sub packs that were only connected for discharge

then with two chargers you could charge both at once.

Or maybe even parallel the two halves to be charged by a single charger.
 

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Discussion Starter · #3 ·
John thank you very much for your answer.

Yes, i am planning to charge only one side at a time. For now i only have one charger so this is the only way to go.
I thought about parallel charging, but i am not sure how to arrange the sense wires then.... so i gave up for now and decided on this electrically less complex solution, although it is also less user-friendly.
 

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Yes, you can split charge each side as 7S or 6S+8S

but ONLY one side at a time.

If you designed two physically separate sub packs that were only connected for discharge

then with two chargers you could charge both at once.

Or maybe even parallel the two halves to be charged by a single charger.
If one side of the output of each charger is grounded then two of them could not be connected to different points in the same battery at the same time (that would short those two points together), but if the outputs are floating the I don't see why the two sections need to be disconnected from each other to charge them simultaneously. Grounding one output doesn't make sense to me, but the iSDT specs and manual don't clearly say one way or the other - are the charger outputs not floating?
 

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You would want to be very sure the outputs are fully isolated, from right back to the input circuits.

For regular mere-mortal users best to advise in public only those two scenarios

1. if the two halves remain connected only charge one half at a time

2. design and wire the sub packs so they are fully disconnected from each other while charging
 

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I thought about parallel charging, but i am not sure how to arrange the sense wires then
Yes if you wanted to top-balance while parallel charging, that would need to be designed in from the beginning.

Personally I really like the idea of wires brought out from the pack, one pair per cell/group or even per cell

in sets of 12 say terminated in a waterproof plug like TE's DT series

into an interconnect harness block that creates the xPyS layout

where the BMS if any gets plugged in

could be a different xPyS layout for the charging process

or for balancing using active dedicated gear

and leaving the cells fully isolated when being stored

But I digress...
 
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