Making a battery pack to charge another battery

I'm new here, excuse me if this is completely misguided.

I have an Evolve Bamboo GT electric skateboard. It's great, but the battery sucks. I have been using this thing for errands and fun, and I'm starting to really feel the restriction.

I don't want to touch the internal battery of this thing, though, because I might end up replacing it, and I want it to be resellable. Instead, I want to build a battery pack that effectively replicates the charger for this skateboard. This is the charger in the image: https://i.imgur.com/znJVxA4.jpg, it provides 42V and 4A to the skateboard.

What I was thinking is, I could build a 10s4p pack, which would provide, based on the cell type, something in the range of 44v to 36v. In front of this, I would attach a DC-to-DC step-up (like this one) to fix the output to 42V. I would then somehow fix the maximum output to 4A, assuming such a thing is possible. Then I would cut off the charging cable from a sacrifical spare Evolve charger and bolt it into the discharge cable, plug it into the skateboard charging slot, and essentially use the skateboard while it's also being charged by this thing. (I have done a prototype of this with a battery pack that can output 110V AC and the 42v4A charger, and the board seems to be able to handle being charged while being in use just fine.)

My expectation is that:

a) Skateboard doesn't blow up
b)This external battery charges the internal battery of the board to 100% normally, even if the external has 5% battery and the internal has 95% battery.

Is there a better pack for this than 10s4p? Do I really need the step-up?

most guys just put a battery in parallel, and when they want to extend range, carry it in a backpack.
it is best if both packs are at the same exact voltage, but a 1 volt difference will work fine.
1 will charge the other when they are not equal. if 1 is way down, it should pop your fuse as the amps will be too high, so check with a DVM before hooking up.
If you want to experiment with a BOOST, get 1 with a constant current pot. Then it will keep the amps safe all the time. I worry that vibration, or rain will kill it. and it wastes 5-15% of the power. it will need a low voltage warning, or it will over discharge!

I think it might be best to connect parallel, at the same voltage, then run the board too.

Or find places to plug in a charger, topping off as you can during the day.

As for keeping the battery intact, its value is already zero dollars. You used it.

If you're going to build a battery pack to extend the range, just build one the same voltage as the built-in pack.

Then to use it, charge both packs, and then plug in the extender pack to the charge port, and ride with both packs paralleled.

If there's a chance the charge port wont' securely hold the plug in, you'll want to do something about that before riding, because if it does come loose and it's not plugged immediately back in, it's possible the voltage difference that will eventually occur will cause too high a current flow once plugged back in (beyond whatever the charging limit is for the built-in pack).


It's a lot simpler, smaller, and more efficient (meaning more range) than using any conversion electronics (which will maybe have an 80-85% efficiency, and often worse than that).

The only drawback to it is both packs have to be at the same voltage before plugging in the extender.

If you're going to build a battery pack to extend the range, just build one the same voltage as the built-in pack.

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Does charging port count as a proper parallel, though? The charging port connects to BMS, not to the battery inside directly.

Another problem is that the battery inside skateboard is simply crap. It's around 200Wh - the pack I want to build it slightly more than 500Wh. 200+200 (paralleling) would not be acceptable range for me.

It seems that I need two additional electronics: one to generate constant voltage (a buck/boost) and another to act as a Li-ion charger that is capable of doing Constant Current / Constant Voltage charging. For the former there are many options, for the latter, I couldn't find anything. Am I missing something, or looking for something that simply doesn't exist?

If you want to experiment with a BOOST, get 1 with a constant current pot. Then it will keep the amps safe all the time. I worry that vibration, or rain will kill it. and it wastes 5-15% of the power. it will need a low voltage warning, or it will over discharge!

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For over-discharge, my topology will be Battery > BMS > Boost. The BMS will cut off the voltage when the battery is suitably empty.

As for keeping the battery intact, its value is already zero dollars. You used it.

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It's less about keeping the battery intact and more keeping the board intact.

steamknotty said:

It's around 200Wh - the pack I want to build it slightly more than 500Wh. 200+200 (paralleling) would not be acceptable range for me.

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Paralleling hasn't anything to do with pack capacity limitations, only taht they be the same voltage (and chemistry so the HVC and LVC are the same). You can parallel a 10 terawatthour pack with a 1 watthour pack, as long as they are the same voltage when you plug them in, and chemistries are teh same. They will discharge in parallel, and if you leave them hooked up when charging, they will also charge in parallel, and act as a single pack.

It seems that I need two additional electronics: one to generate constant voltage (a buck/boost) and another to act as a Li-ion charger that is capable of doing Constant Current / Constant Voltage charging. For the former there are many options, for the latter, I couldn't find anything. Am I missing something, or looking for something that simply doesn't exist?

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You can eliminate all that if you just use the same number of series cells and the same chemistry. Then just ensure they are the same voltage when you plug them in (or very close), so the inrush current isn't too much for the wiring/connectors/BMS.

I'm sure what you want already exists, and is probably already being used here on ES by someone for something--I just don't know which thread it's in. :/ I think there is one by Cowardlyduck that has what you want, if you look thru his posts, but I might be misremembering.

A few thoughts:

1) Paralleling is by far the easiest way to go
2) If for some reason you really, really want to add some cost to this thing, then use a buck converter instead of a boost. It's safer and cheaper.