Help with building a DIY 18650 pack UK

So I need to get a BMS for my first build, I am really not sure what maximum discharge I need, I am going dual drive with these motors

And using two VESCs as the ESCs, my pack which I’m building is 10s2p Can someone please help me

1 Like

The BMS is meant to protect the batteries, and so I’d base the BMS rating on what batteries you’re looking at. For example, if you were going to use Samsung 25R 18650 cells (I used them, they can do 20A max discharge) in your 10s2p pack, I’d probably look for a 20-30 amp BMS. Those cells are happiest at 10-15A discharge. You could get a 40 amp BMS if you want to push the cells (get more power out of them but decrease their life). Then on your VESC I’d match or go a little lower than that amp rating (divided by 2 in your case).

You could also get a 40A BMS and then set your VESCs to 30A (15A each since you have 2) so you have a little headroom all the way around.

TLDR: Base it on the batteries. For an 10s2p 18650, I’d say 40A BMS at most (depending on the cell chosen)

So I’m using the Samsung ICR cells and on the spec sheet I found it said they can do 5600mA max discharge, I’m not the best at electronics so which bms would I need for that? Also when you say that it could shorten the life span of the cells, how much of a decrease are we roughly talking here?

1 Like

When I drive my batteries hardcore 18 amps continuous I get like 300 cycles and then the battery life noticeably reduced to the point where I want to get new ones. If you’re running 10A or less on a 20A battery you should get more like 600 charges with only %20 loss in capacity. I’m no expert on Samsung batteries but I think you want Samsung INR18650-25R not ICR’s

Oh so it is quite significant then, do you have any recommendation on what bms to go for?

1 Like

Well to be frank you should probably choose different cells. In my opinion 5.6 amps isn’t going to be enough. Here’s why: If we don’t care about lifespan and you run them at their max, with 2 in parallel you’ll get 11.2 amps max. With that I’d be shocked if you got up any hills and you’re acceleration and start from stops are going to be pretty lame. I don’t have a good gauge for how much you’d be able to do, but your board will feel very limited.

You really want to shoot for at least a 20A available discharge rate. If you put 4 in parallel (a 10s4p in your case) then you could get 22.4 amps, which is a lot better (and enjoyable).

So considering cell life, @smurf is correct. You’ll probably see double the cycle count for only going to half the battery’s rated max discharge. There’s other things as well, but we’ll just consider discharge rate vs rated discharge for now.

If you’re bent on using the INR cells and you don’t care about cell life too much, I’d recommend at least a 4p setup. If you’re bent on a 2p setup, you need a different cell.

1 Like

Okay, this makes sense, when I bought the cells I had a very limited knowledge in cells and wasn’t sure which ones to get, the reason I bought these were because they were brand new and cheap on ebay (not fake either) I’m of course trying to keep this pack as cheap as I can but still getting good performance, When I get home I’ll see if I can buy any more for a reasonable price but if not, what cells could you recommend which aren’t too on the pricy side? I’m flexible on changing the set up as well, I just don’t want it to bankrupt me

1 Like

The Sony’s would be good for 2P

PS someone around here might have a discount code or something

thanks man, I’m going to be really difficult here and mention that I’m in the UK so if you know any UK dealers then that would help and which do you think would give me the best bang for my buck taking into account the pack setup, really appreciate this

1 Like is in Europe

1 Like

As one answer naturally segues into the next question maybe start a build thread or just rename this one.

Good point, I’ll do that now The main thing which is currently confusing me is how do I determine what battery I need? How do I determine what my system is drawing and match that with a suitable set of cells? I mean I’m looking at pages of these 18650s and after a while they all look the same lol, what makes one more expensive than the other?

1 motor or 2? hills or flat land ? How much do you weigh? How many kilometers range? What’s the budget?

2 motors, mostly flat, 72kg, I am willing to spend about £120 on the cells, with regards to range, the more the better at this point, it wont be used for commuting so I don’t need a huge range

How about 10S3P Samsung 25Rs ([email protected]€3.2 each

What makes them a good cell? Sorry for my lack of knowledge, I just want to know what makes them better than others?

That’s only 5.6 amps. That’s not gonna cut it for an E-board. The minimum usable 18650’s would be 20a discharge and you would need 30 of those for a 10s3p Imo, a battery with 60a discharge is the bare minimum and even with that your probably gonna have plenty of voltage sag.

Okay so should I aim for a little higher than 60A just to give it some headroom to counter the voltage sag? Maybe 10s4p? Or go for a higher discharge cell I guess