7S6P Custom Offroad Battery topic


So let me start off with the background story. One of my good friends contacted me for issues with an electric wheelchair. His girlfriend is coping with some strange muscle disease, which they are still trying to diagnose. She luckily can still walk, but it is exhausting and painful for long distances. They already bought that wheelchair secondhand. Because trust me, these wheelchair prices are out of this world. These manufacturers know they can rely on social security and that is visible in their prices. Just getting this thing diagnosed and worst-case scenario, replacing the battery would’ve already set them back 700 euros. The cheapest 1-on-1 LifePo4 replacement I could find was 450 euros without shipping. I promised them I could find a better solution by replacing it with a custom-made Li-on pack, which would be lighter, more waterproof, more capacity and a faster charger, their original charger was 0.75A. All this with a maximum budget of 400 euro.

Just to give an idea, here was the original setup:


Esc, motors and joystick control (this clicks on the back with the motors pushing directly on the wheels)

I don’t want you to be bored reading to death, so I will post the building process pictures here below with some captions. I believe pictures say more than words

Build Process:

BMS diagram (I’ll save you my sketches :smiley:)

Refurbished cells to save some money (Samsung 40T from Nkon.nl) Had to “Dremel” the copper remains from previous bus terminals

Checking voltage of every individual cell (mucho importanté)

Aligning and glueing the S groups (6parallel)

Figuring out the spacing/orientation inside the case

Preparing the nickel strips, removing any sharp edges

First welds done

Packing the cells once more in kapton and double sided tape for assembly

30Q (my test battery) vs the 40T’s

–I seem to have no pictures of the series connections, but they are visible after the BMS was mounted–

Packing the outside in fishtape to prepare the BMS mounting

Soldering the BMS, power and charge leads in place and wrapping the thing up

I always fuse my batteries, 60A on the powerleads, 10A on the chargeleads

Adding some extra fluff for convenience, 2 QC3.0 chargers for when they hit a festival

Et voila:

Comparison: Old LifePo4. 25.6V 15 Ah 384Wh New Lion 29.4V 24Ah 604.8Wh

She already told me her wheelchair seems faster and she probably only has to charge it every 2 weeks under normal circumstances

Parts list and cost breakdown to be followed …

Feel free to give me feedback, it’s a rough draft right now, but I hope it is helpful and some inspiration for some. This is not an esk8 battery, but it certainly can be and it’s not harder to figure out 10s or 12s if that’s more convenient for your setup.


Thats a lot of P. Cant wait to see the enclosure you’ve got picked out for this.

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Just a suggestion and maybe you’ve already done this. Add some kapton tape inbetween the balance leads anywhere they cross. That way they cant rub together and eventually short out. I know its a bit of overkill but its something I saw on a battery build video.

So what brand of spot welder is that and hows it working out for you?


Will keep that in mind, good feedback. I agree 100%

That is one of the first portable spotwelders, you can solder with it too. I had to email a guy in Korea I believe, send him some money through paypal and 3 weeks later that thing would show up.

I’ve found him back:

100% satisfied with the thing, but you need to add a fan to the side for it not to overheat if you are doing big batteries. I run it on a 3S 50C Lipo.

Edit: found this, translate with chrome browser https://blog.naver.com/aulakiria/220992039512

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Thr LiFePO4 packs have a far longer cycle life than li-ion and unless weight or size is an issue (like on esk8) I probably would have stuck with that chemistry.

Why the switch to li-ion 4.2V?


You might be right, but I figured since this esc is only pulling around 50-60Amps max. She wanted some more range and not more weight, this could be a good solution. Time will tell about the cycles. getting a 24AH LifePo4 would’ve been too huge. I also believe these batteries were chosen because there were no alternatives back in the time. I might be totally off.

Some of my lion boards are going into their 4th year almost, I see almost no degradation yet.

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