Any Recommendations for a Cheap 14s BMS?

I'm working on my ebike build right now and currently one of the last parts I need is a decent BMS. I don't know how fancy it needs to be as this is my first build and it's going to be a bit crusty (it's a cheap ebay conversion kit). Of course I want all the basics but I don't think I need bluetooth or any of the other toys yet. I might like it to have an on/off switch connection as a backup for the one on my controller but other than that I don't know of anything else I need on it.

I'd appreciate any recommendations, as I'm having trouble digging through aliexpress and telling if any BMS in particular is more likely to be any good.

Some questions you'll need answers to before you can pick one:

For what chemistry? meaning:

What voltage do you want it to balance the cells at?

What cell voltage do you want it to shut off charging at, to protect the cells? (HVC)

What cell voltage do you want it to shut off discharging at, to protect the cells? (LVC)

How many series cells? (this one you've answered in the title: 14)

How many amps does it need to supply continuously (and peak) to the controller?

What is your space limitation?

Do you want separate charge and discharge ports? (regen back into the battery is not controlled)

Or the same port for both? (allows controllable regen)

I'm using standard 18650 cells, so LiPo chemistry.
4.2V max
3.7V typ
I would assume ~3V min
14 cells in series
My controller is rated for 24Amps so I would think a 30+ Amp BMS would work fine
Space limitation is difficult to say exactly. But it should be less than 90mm wide and as short and thin as possible from there. I'm waiting for some parts before I test fit the case it'll go into so I can fully realize my pack size and how much room there is for the BMS with all the wires.
Separate port is probably better for me. I'd love to have same port in case I upgraded to a regen capable controller, however, the wiring inside my case will get a little more difficult. So I can make do with either.

Thanks for the response BTW, I'm happy to provide any more info that is needed. Can't wait to show off the build to you guys too, if all goes well it'll look amazing.

I think I've settled on the idea of a single charge/discharge connection. It'd make it easier if I decided to connect an external battery pack to the main pack through the charge connector.

I'm debating on how to properly protect the charge connector though. I was thinking of using an XT60 male connector on the main packs side and a female connector on the charger so that it would flush mount better. However, I realized that the charge connector would still be dangerous if shorted (the BMS would probably stop it but it still feels iffy). A 3d printed plug might be enough to protect it though. I was considering putting a Schottky diode on the port and that should prevent any shorts while still letting it charge.

Personally I recommend using female connectors on the bike itself for anything, even though that means the male pins on the charger would be exposed and could short to other things if not careful. (as long as the charger doesn't power up until connected to the battery, as many smart chargers do, then even that isnt' a risk).

If you use a male connector on the bike/battery, I highly recommend a fast-acting fuse inline with at least one of the leads to it.

The diode is going to waste power as heat, and drop the voltage to the pack. If the diode is inside the pack then the heat will go into the cells closest to it, and if it's significant it'll age those faster than the rest of the pack (albeit not very quickly). The voltage drop you'd have to raise the charger voltage to compensate for.

I'm using a female panelmount XLR (to match the Satiator) for charging the lighting pack on the trike, and was using one for charging the main pack too, until I mounted a new charger (meanwell LED PSU) to the trike and wired it in permanently.


It doesn't take much of a connector for a charger; you don't have to have high current capability. So you don't need xt's, if you prefer a panelmount solution.

Thanks for the help. Yeah, I think I'll look around for a different connector that can be panel mounted (so that it's flush) without the risks that using one of the XT line would bring. Changing the connector should've been my first instinct.

On the diode for short circuit protection. I found a better trick for this specific case, you can use a P-channel MOSFET as a one way switch that has far forward resistance and voltage drop. It won't prevent short circuits in all cases but for a charge port it might work.

Also I realized that most BMS have short circuit connection but I doubt it's best practices to rely on that for protection. So I'll do some research on some alternative connectors that would work better.

BTW, on the BMS. I might just go with this one. It's near the top of most orders, has a single charge/discharge connector which gives me the ability to parallel it with an external pack through the charge port, and it seems to have reasonable specifications.

My only concerns is the current limit, it has some headroom but I wonder if it's enough, the lack of an external switch and some more details on the short circuit protection.

Any ideas on if it seems like it'd work well?

Jacob's Ladder said:

On the diode for short circuit protection. I found a better trick for this specific case, you can use a P-channel MOSFET as a one way switch that has far forward resistance and voltage drop. It won't prevent short circuits in all cases but for a charge port it might work.

Click to expand...

Keep in mind P-channels generally have much higher resistance than N-channels, and they also still have a body diode that will pass current the other way, so a single FET won't prevent a backflow. It also uses up power, and creates heat.

There is a circuit called "ideal diode" that uses multiple FETs and some other parts to make a switch, though. This also uses up power, etc.

There are also some FET-based precharge circuits you can find here on ES and elsewhere, but again, they use up power and create heat.

The advantage to the FET-based circuits over a simple diode is that there is less voltage drop that you have to compensate for (by raising the charger voltage). But they do still all have disadvantages, too.

Also I realized that most BMS have short circuit connection but I doubt it's best practices to rely on that for protection.

Click to expand...

The same is true of other FET-based protections--when FETs fail, they usually fail shorted, so then they aren't any protection against current flow. (except under extreme failure conditions, where they explode into bits).

So I'll do some research on some alternative connectors that would work better.

Click to expand...

There are a number around that have pins so far recessed inside that they would have to be deliberately shorted, but generally these are larger and can be more expensive (as they are less common). I don't recall the names, but if you start up a chat on Mouser or Digikey's sales support page, you can ask what connectors they have that meet this need.

My only concerns is the current limit, it has some headroom but I wonder if it's enough, the lack of an external switch and some more details on the short circuit protection.

Click to expand...

Could'nt say if it'll be a good option or not, but if you look up Dnmun's posts, he has helped people figure out how to install a switch to enable or disable a BMS. He doesn't post anymore, but his existing stuff should help you find the spot on your BMS to do it.

As for the current limit, keep in mind the FETs do create heat, so the closer you run the BMS to it's design limit, the more of that there will be concentrated in a smaller area. You can heatsink it, ensure it has airflow, etc., to mitigate or prevent heat problems.

You think it might be better for me to get something like this? https://www.aliexpress.com/item/14S...h-android-Bluetooth-app-UART/32824233108.html

I think I saw it mentioned in the Bluetooth BMS thread but that thing is over 500 replies in and it's a bit difficult to parse.

We need what to know what you are going to us it for. And what cells match your needs ? So cells can take mega regen. Some will be a fire ball. Or high use plus high regen. High heat.

Well for the time being, I'm not planning on having regen braking, I don't have a controller that supports it yet. Also the cells I'm going to use are recycled cells that I'm testing and matching. I recognize this isn't ideal but a battery of the size I want is prohibitively expensive for me at the moment. I think all the cells that I'm going to use will be able to support 1A min. charging individually and I'm planning on running a 14s13p pack, so I feel relatively confident that they could take some high current charging in short bursts.