I am building a li-ion battery pack in a 7-series (3.6V) cell configuration. That gives 25.2V nominal voltage and 29.4V charging voltage.

  • I have a BMS with balancing charging capabilities. This BMS has high current limits, 45A for discharge and 8A for charge.
  • I also have a buck converter what for lowering the voltage from my 42V/2A DC power supply to the 29.4V required for charging.

And here cometh my problem on account of my lack of abilities to understand anything more complicated than a toothpick: since the BMS can chooch at up to 8A when charging, what's keeping it from overloading my 84W power supply or the buck converter betwixt her and said BMS?

What I want to achieve is keep the charging current at around 2A max, as opposed to the 8A max of the BMS. This would ensure that no more current is drawn from the PSU than her rated 2A.

Am I right in suspecting that my lame buck converter is not up to the task as I need a CC/CV converter and not a CV only? That is, a converter that allows me to set the current as well and not only the voltage?

Or, if my power supply says she's regulated and protected against over-current, does that help me here? Can I count on the BMS not being able to pull too much current THROUGH the buck converter FROM the supply because of her being regulated? Would that limit the whole system to not more than 84W? Or will the supply simply cut off electrocity when overloaded, so that no angry pixies come out of it at all, aborting my charging?

The supply is made for charging my kickscooter with li-ion batteries inside. Is that kikkskooter limiting the charging current within its brain box, or is it relying on the supply as described above?

Any response is appreciated. Sorry if the answers can be found somewhere around here, I couldn't find anything that would help me.


1 Answer 1


You'd need to post a datasheet for the DC power supply. The DC power supply, like you said, can only put out 2A. The 8A on the BMS is just a limit, it'll take 2A just fine. The balancing usually comes in the form of a switched resistor. Active balancing is quite expensive and bulky. The most likely scenario is that the BMS is just passing through your input to charge and taking care of the balancing/cut-offs.

When a DC power supply says "Over Current Protection" that is usually in the form of a hiccup-mode where the converter will turn on at full power, realize it is still overloaded, and then shut back down. Sometimes this happens indefinitely, sometimes only a few hiccups before latching off.

CC is an additional circuit so not all power supplies will have one, and if they do, they usually specify it. Without a datasheet/more info for every part involved there is no "right" answer.

Keep your scooter in a vice.

  • \$\begingroup\$ Thanks for the response. Unfortunately I don't have any datasheet on the power supply so I'm thinking about getting an adjustable current converter that will with its feedback loop vary the output voltage to ensure that the current stays constant. \$\endgroup\$
    – Surmoka
    Commented May 12, 2021 at 15:25

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