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I thought I'd give my old e-bike an upgrade, and I've build a 8S4P battery with charge controller (BMS). The thing is the electronics of the ebike quite working @ 31.5 V - a fully charged 8S battery is at 33.6V.

At first I thought I could throw away the idea, but when I found out it works as long as the battery stays below 31.5V I thought let's take an old laptop adapter (19V) and a 200W step-up convertor and make custom charger.

It didn't work out well. It works a couple of times, but then the laptop adapter breaks. Also the step-up convertor makes weird noises while charging.

I read somewhere here even with BMS you can't use a laptop adapter because they aren't CC-CV.. So I need some suggestions on how to make a charger that stops charging @ 31.5V.

What if I buy a step-up convertor with adjustable voltage and adjustable current, would that solve the problem?

Or what if I buy a 10S 42V lithium charger and use a step-down to get down to the desired voltage?

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    \$\begingroup\$ Welcome to EE.SE! Please draw a circuit diagram or block diagram of what you are doing. What is your charging current? How do you limit the current? \$\endgroup\$ – winny Feb 7 at 9:18
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    \$\begingroup\$ How do you balance the cells? Look, I'll give it to you straight: If you don't charge your bike lithium batteries in the proper way, your ass will literally be on fire before you know it. Do you have a medical insurance? You need either that, or a dedicated 8S Lithium battery balancing charger with the correct charge profile for your particular battery. \$\endgroup\$ – Dampmaskin Feb 7 at 9:24
  • \$\begingroup\$ Remember that a laptop adapter is simply a power supply. The actual charging circuits are built into the laptop. \$\endgroup\$ – Finbarr Feb 7 at 10:00
  • \$\begingroup\$ are you overloading the adapter? remember to multiply your amps by the voltage increase; 36v@2A causes 4a at 19v (minus inefficiency as well) \$\endgroup\$ – dandavis Feb 7 at 18:42
  • \$\begingroup\$ I thought the current would be limited by the laptop adapter. These adapters are rated around 4 amps at 19 volt so I think they probably get overloaded by not using a CC convertor. The cells are balanced by the BMS board.. \$\endgroup\$ – Sebversive Feb 7 at 19:59
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First you need to get the voltage up high enough, and then you need a power supply with a CC-CV mode. That is enough to supply the BMS, which will actually do the charge management.

For simplicity, you can just buy 33.6V 2A lithium chargers. However you can also buy step-up modules that claim CC-CV mode - set up correctly, in principle they should work but I would want to test that behaviour thoroughly (for example, with dummy loads). In particular, I don't know whether cheap BMSes can protect against all failure modes.

Given the fire risks of getting it wrong, I would tread extremely carefully here.

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  • \$\begingroup\$ Tnx, that's useful to me that you say step-up modules with CC-CV mode should work in principle - as it's the only way I see to make the charge stop at a non-standard 31.5V.. I ordered this one to try.. I do wear safety glasses and gloves ;) \$\endgroup\$ – Sebversive Feb 7 at 21:41

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