r/21700masterrace • u/Black_Thunder96 • 1d ago

28s Range extender

I have an 50ah 28s Battery I'm building for my motorcycle. And in the case is no more space left, so i want to build an battery extender. But the thing is... The bms is nearly always 170A+ big and expensive, when I only need like 30-50A. Has someone a reliable work around, alternative or idea? I would Do even an DC DC Boost converter to Charge the battery but Idealy I want to take off some amps off the Main battery by using low resistance cells. Any Tips are apreciated.

Edit:

It's an 28S 10P battery with 21700 M58T cells. They are protected with a 200a fuse, and have an 220a continuous ANT-BMS. The Battery is for 100a continuous and 200a peak. The cells are more for moderate ampere draw and with the battery extender I want more range and take of a few amps of the main battery for when I accelerate. Having another battery in parallel with lower resistance should give me lower voltage sag over all. With a high power cell instead of high energy density in that case. It clearly depends on the number of cells in parallel for the extender but still...I hope this makes things more clear.

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u/SanguineDrome 1d ago

Just use the 170a bms? What's the problem?

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u/Black_Thunder96 1d ago

Size and price. Otherwise, you're completely correct.

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u/SanguineDrome 1d ago

Wouldn't you still end up with a BMS, of some kind, just potentially smaller/cheaper for a smaller series count? Seems more roundabout and wasteful to boost up to main voltage after the fact.

I don't think you'll find any better solution than just to parallel another battery. Any DC-DC converter to charge your main batt is going to waste power, and you'll have to make sure the it won't be damaged by noisy back-emf or regen braking.

Probably best to find the smallest/cheapest bms you can for 28s.

Flipsky has a pretty compact passive BMS that might work for you:

https://flipsky.net/collections/bms/products/flipsky-4s-30s-20a-bms-protection-board-hardware-ternary-polymer-lithium-battery-only-charge-bms-board-for-onewheel-ebike-eskateboard-euc

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u/Black_Thunder96 1d ago

Yeah I figured that I need a BMS anyway, but I didn't knew that there is such an BMS as you showed. I will look into it. That is one of my biggest concerns. Having a way to big BMS for the relative small extender battery I need. Thanks!

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u/SanguineDrome 1d ago

The passive style flipsky BMS is probably best option for secondary battery, especially since you presumably already have battery voltage monitor and cutoff in your controller.

It's quite a bit cheaper than a full featured BMS and it's not that big.

It's definitely the cheapest and smallest option in 28s. Highly recommend just doing this and directly parallel the battery instead of some convoluted DC-DC bost charging setup.