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I am trying to build a system where I have a hub with a battery and a small robot with a battery. Both batteries are 4S Li-Po batteries.

In the system, I want to wire up the hub's Raspberry Pi Zero (could be normal Pi if impossible with Zero) and battery such that it keeps the robot's battery full as it tries to drain from idle power draw. I know that you can essentially just plug together two batteries and they will equalize but I want to keep one battery fully charged, which will lead to an imbalance, so I'll need a circuit of some kind.

I am a novice at all things circuit design, so any advice at all on how this could be achieved would be useful. I've heard stuff about buck-boost converters, but I'm unsure how they would fit into such a circuit.

I'm not even particularly looking to build an entire circuit myself, I'd rather use pre-built boards to do most of the work.

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    \$\begingroup\$ If you don't know what you're doing, trying to do your own LiPo battery charger is a very bad idea. Please don't burn your house down. \$\endgroup\$
    – Hearth
    Commented May 25, 2021 at 5:58
  • \$\begingroup\$ Ok, "novice at all things electrical engineering" is a bit of an overstatement, I have soldered before and put together a drone before. I just haven't done any circuit design beyond just turning on a lightbulb. I appreciate the concern for my safety, but I'm asking because I currently don't know how to do it, and have to know what I'm doing when I build it. Such questions are kinda the point of forums. \$\endgroup\$
    – Bubinga
    Commented May 25, 2021 at 6:09
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    \$\begingroup\$ I know that you can essentially just plug together two batteries and they will equalize as Hearth says, please don't burn your house down: connecting an empty to a fully charged LiPo battery will lead to a very high current, and this means you essentially must not do that. [Edited by a moderator.] \$\endgroup\$
    – mmmm
    Commented May 25, 2021 at 7:11
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    \$\begingroup\$ Starting guide - more available if nobody else answers - flag me if this is the case say a week from now. | You need a boost converter that will run from 4S LiIon in any state of charge that outputs enough voltage to operate a 4S charger. This will need about 18V or more. The 4S hub battery can be as low as about 12V (4 x 3V) and as high as about 16.8V. | 4S chargers and 12-17V in / 18V out boost convertres are avaiable commercially or you can roll your own. \$\endgroup\$
    – Russell McMahon ♦
    Commented May 25, 2021 at 7:19
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    \$\begingroup\$ @Bubinga What I mean by that is that LiPo batteries will burst into flames or even explode if you mistreat them. It is not what I would consider a beginner project. \$\endgroup\$
    – Hearth
    Commented May 25, 2021 at 13:28

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You need two (or three) modules. These can be DIY using input from elsewhere - application notes from IC manufacturers is usually a areasonable source of these. Input from You Tube videos and websites in some cases works well but can also be very unsatisfactory.

  • You need a boost converter that will run from 4S LiIon in any state of charge that outputs enough voltage to operate a 4S charger. The 4S hub battery can be as low as about 12V (4 x 3V) and as high as about 16.8V.

  • You need a 4S charger. This will need about 18V or more.

  • As a separate module or included in the charger you will need a 4 cell BMS.

4S chargers with or without BMS, separate 4 cell BMS units, and 12-17V in / 18V out boost convertrer are avaiable commercially or you can roll your own.

EXAMPLES ONLY

4 cell BMS here

enter image description here

or, an example of an IC that would allow you to "roll your own" here.

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