BMS + Charger help

Greetings skaters!

After reading a ton of information and seeing how people’s opinions divide on the basis of BMS to use (charge only / discharge), I’ve figured that I’m slightly stuck on that basis…

I’m currently building a 12s2p (Samsung 40T/4000 mah) battery, and when it came to choosing BMS for my build, everything became very complicated. On the one hand, I figured that bypassing BMS is safer in terms of the motors locking mid-ride; however, I’ve read that I would require an external fuse (I have no idea about it atm of writing this). However, I’m still stuck in terms amperage of the BMS, so can anyone help me out on that basis?

The second problem is the charger. I think I need to get the charger similar to this one, but once again, I’m not entirely sure if that may be too much for my battery. Would this type be the right choice ?

NZ$ 23.78 22% Off | 50.4V 3A Lithium Battery electric bike Charger For 12S 44.4V Li-ion battery pack e-bike Charger high-quality Plug EU/US/UK/AU

Thank you in advance, guys!

[ Additional info on my set up:

Motors: 6374 170 kv ESC: Focbox Tenka]

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External fuse… I’ve only heard that you put those on the negative side of your charging wire. If you have a loopkey with a fuse built in, you can do that on the negative side of the battery connecting to the esc as a power key. But other than that, my battery doesn’t have a fuse since I’m only doing a 2A charge (I recommend 2A charging battery chargers, but 3A is okay with a fuse). I’ll see in the future when making my own to add a fuse for a much higher charge rating.

Your bms must say 44.4v on it. Anything other than that isn’t for a li-ion. The charger is fine, but for safety reasons, you add a 10A fuse on the charge wire (negative side of course).


Your bms doesn’t have to say that on it to be li-ion. There are tons of different bms out there and some don’t even say a voltage on them. If you are worried about bypassing your discharge and in the event of a failure you want to try and save your esc, an anti spark should pop before your esc does. I bypass my batteries because I would rather have someone bust their board than their head. I typically try to keep my charge at 1a per parallel cell. But the 40t are capable of more so that charger should be fine.


Boils down to if you want a basic bms, go see my man the duck at if you want a smart I can help you out.


I agree to an extent. However, there are life04 type bms that will limit charging to life04 batteries. I’m not sure you can use those on li-ions. That’s what I meant when I said it must say a certain voltage for that battery.

An example would be 12s 36v life04 bms. Some doesn’t say that but rather 12s bms, but I would read carefully before buying.

Also, respects to duck battery. Love that guy.


Awesome! Thank you so much for sharing a link)))

P.S - it appears he run out of stock with 12s bms T_T, but oh well, I think I may find similar one since I now the specs!

I’ve heard that tenka doesn’t really like anti spark that much and it just doesn’t work when it’s there, but I’ll look into that :smile:


The only confusion I have now is the discharge current spec. From what I see, there are BMS 20-60A (12s li-ion), but I have a feeling that it doesn’t matter since I’m doing a bypass, right? I don’t really need to care which one should I take ? Or am I wrong? What would you recommend?

P.S - Sorry for sooo many (sometimes stupid) questions! :sob:

Get the lowest and cheapest discharge if you are bypassing. I use the llt 20a discharge for most of my builds. I have one if you need one.

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You don’t need to view discharge if bypassing. You’re good there.

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Ah, it is alright, thank you for your kind offer though! I just wanted to make sure I’m getting the right thing :smile: :blush:

Oki doki, thank you so much!! :smile:

Oh, I have one more question, I have an option of choosing a separate charge and discharge wires and joint; which one would you recommend?

Here’s the link to the one that I’ve found: | Li-ion 12S BMS 48V

That would work (I’m assuming here). It is specifically used for these batteries:

Ignore the 48v though. That is from a different perspective of nominal. Assuming it’s 4v nominal.

Thanks! I’ve got this BMS with a separate port (we’ll see how it goes), and while I was looking for something like this, I’ve also found this charger…I’m pretty sure I’ve seen other skaters use something similar in their videos, so do you think this would be a good choice? (It seems like it also has a built-in fuse)

Depends on the battery charge limit and max charge it can take. You just need to look up the specs of your battery pack.

Gosh, physics ain’t my best friend…

So I have a 12s battery. Which is 44,4 volts if I’m not wrong and if fully charged 50,4…

But how to calculate the Ah? Since I don’t know how much time I would spend on the board and what is the power drain…

I’ve seen one of the skaters building the same battery (with a different layout, though - his one is a bit thicker, and my one is slim, but both 12s2p using the same cells (40t)), and he said that his one is 344 Wh. So technically, that would mean 344/44,4= 7,8 (~8Ah), right?

So my battery spec would look something like this → 12s2p 8Ah 44,4 V (50,4)

So, knowing that - do you think this is a good choice for a charger?

If its the samsung 40T cells, then this would be the reference I was talking about:

6A max current charge.

So yes, you’re fine. When ever you need to know what is okay as a charge limit, just look up the specs of the battery you have (type of cell the battery brand is). Charge rate would be your key in this case.

Edit: thanks to batterymooch, I now know there’s a data sheet.

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I’m late to this party but I’m very surprised you didn’t follow your own advice. Your 1.5A/4A numbers are wrong.

It took several seconds to look up but here are the ratings from the 40T1 datasheet and the 40T3 Tech Info Presentation. At least your mistake was in the safer direction.


They’re not mine. It came from the website where you purchase the battery cells itself. You didn’t even check it. Crazy. But I’m glad they’ve added the data sheet. It’s easier to know.

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