Modifying a Chinese 43.8V charger to output 42V

IdefixRC · Apr 26, 2017

Hi guys,

stumbled upon the forum when I was looking for advice on how to modify my - accidentally, stupidly, wrong click, purchased - Chinese Mebo 43.8V LiFePO4 charger to output 42V for the LiIon packs it was intended to charge.
While I understand that the BMS will protect the cells from overcharging in any case, I would rather spend some time to modify the charger to somewhat hit the 42V mark instead (currently it does 43.1V on idle).

First thing I naturally did was to look for potentiometer on the board.....no such luck.... :?
I'm attaching a few pictures (behold the awesome soldering job) of the output end (V+ and V- outputs and respective cables can be seen on the pictures) in the hope that someone can point me in the right direction to identify the voltage sensing circuit and resistor that does all the magic (I believe I found a LM324 on the back which might be just the guy...)

Thanks a bunch !!

falqo · Apr 26, 2017

Wouldn't it be easier to just place 1 or 2 diodes in between the V+ cable? The voltage drop (0.6V-0.7V on average, depending on the type of diode) of each diode takes down the V+ voltage.
If you would do so, DO pay attention to the current rating of the diode(s) you use (compared to the max charging current of your charger) and also check you do not exceed the maximum power dissipation (Wattage) of the diode(s)!

IdefixRC · Apr 26, 2017

Thanks for the reply.
Yes that would be an option to get it down but I think it would affect how the chargers switch off / charge cycle or not (how would it sense the voltage?)

amberwolf · Apr 26, 2017

Before you modify anything:

How many series cells is your battery pack?

If it is a 10s, and is a lithium chemistry other than LiFePO4, then you're better off leaving it where it is, so that there is sufficient charge voltage to force balancing if it's a cheap BMS (the most common kind).

If your charger is only exactly sufficient to fully charge every cell to 4.2v, then if there is ever an imbalance the BMS may not be able to fix it. (not in any reasonable time frame, if at all).

See this thread for an explanation:
https://endless-sphere.com/forums/viewtopic.php?f=14&t=87848

IdefixRC · Apr 26, 2017

Thanks. It is a 10S LiIon pack and a cheap china BMS.
So you suggest to charge at 43.8v instead?

amberwolf · Apr 26, 2017

If you want to be sure it's ok, you can measure the cells during charging, at the BMS connector, to verify they only go up to 4.2v.

If you still want to change ti down to 42v, you could put a switch on whatever mod you make to cut it in or out, so you can still go up to the original voltage when you need to balance it better/faster.

I don't have any specific help on the mod itself, though. Best I could say there would be to look up the terms

charge* mod*

in the first posts of threads, and display by topic, and see if anyone has done one like yours.

IdefixRC · Apr 29, 2017

Tested it today. Connected a RC balancer in parallel to the cells to monitor. The voltage at the cell goes to 4.26 and higher very quickly. Not that the cells reach that charge level but at least the voltage from the charger reaches that level at a few of the cells.
Now I'm not trusting the china bms very much and before I blow a 10s3p pack up I think I rather go with the lower voltage option instead.....

Played around a bit with the charger but did not yet manage to find the right circuit to adjust the voltage. The smd components in the back combined with the horid soldering does not make it easy to trace everything either :-(

Modifying a Chinese 43.8V charger to output 42V

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