BMS Wiring for LiFePO4

[redpoint5]

I bought some cheap Ebay BMS boards for my 4s LiFePO4 batteries that I intend to put into motorcycles. The model number is HX-4S-F100A, but I cannot find any instructions on how to use these other than the graphic printed on the label.



C- is supposed to be connected to the charger, and P- is supposed to be connected to the load. My question is, can these be connected together, since in a vehicle both the load (starter/accessories) and charge (alternator) use the same connection? I haven't found a schematic or anyone else showing how they connected the board, so I don't know if those connections can be shared.

Finally, would this board provide balancing if only the balance leads are connected?

[oil pan 4]

Let me know how it works.
I spent about $600 on cells for my 60ah 13v LiFePO4 batt. A BMS wouldn't be a bad idea.

It won't work unless you external charge it. You need at least 14.8v a vehicle alt will typically make 13.8 to 14.4v most make around 14 to 14.2v I have found.
I say external charge it.

I would hook the battery straight up to the vehicle charging system like normal, since you are only supposed to run 10 amps through the charger circuit. An lithium battery will take full alternator output, which will be a lot more than 10 amps, bad for the charger circuitry.

[redpoint5]

Yeah, I'll probably just experiment to see how it goes, and measure current flow given difference scenarios.

My motorcycle probably wouldn't put out too much current for the BMS to handle, but as you mention, it probably won't charge at all if it's not getting more than 14.8v. If that's the case, I'll see if it still serves a balance function. Was really hoping it could provide low voltage disconnect ability.

Maybe I'll put one of these batteries in a car, and use a small solar panel as the charge input. I'm assuming it's ok to use the charge connection and leave the load connection unused.

[Stubby79]

I don’t see it working; they make BMSs that charge through the same port as the discharge. That’s probably the right way to go.

Otherwise you could run your alternator straight to the charge port, and let the discharge port run the bike, or else throw in Diodes to speed are the one from the other....

[teoman]

I recently watched a video on this.

It has a serial port and you can buy a bluetooth dongle for it.

Then there is an app for it ( or computer program and usb to uart converter) where you can calibrate it and define a whole bunch of parameters like do you want balancing while discharging or charging. It also had a input voltage cutoff if i remember correctly.

May not be ghe same one.

[redpoint5]

I don't think this device has the smarts to be programmed. The diode idea sounds interesting, but would represent yet another drop in voltage that would need to be overcome to charge the battery, and they would have to be rated for large power.

I'll try to play with the BMS tonight with a solar panel and see what it can do. The thing I liked about the specs is the extremely low working power draw, so it wouldn't run down a battery that was sitting for long periods of time.

[sendler]

Large LiFe starter batteries are very prone to stay balanced and don't necessarily need a bms if you can manually balance a couple times per year. They also have extremely low self discharge so can sit for a year without going dead.

[redpoint5]

I hate when I Google something to get more info, and the top hit is my own post from years ago.

[serialk11r]

Haha, happens to me too.

Looking at the diagram, it's a charge balancing only circuit, probably has a passive means of dissipating small amounts of energy, so I think you're fine if you connect the load to the charging directly. Effect is probably identical to putting diodes that burn off excess voltage, except in a nicer package that can be connected to a trickle charger.

[Stubby79]

(Dead thread, but suggestions might help future ppl)

My suggestion to anyone when picking a BMS...stick to "Two wire" BMS's unless you have a specific reason to want a separate charge line.

Two wire will behave like you're connecting directly to the battery...nothing weird screwing things up, unless you trigger the protection...

So if you're replacing a lead acid battery on, say, an e-bike, your regen will still work and not try and fry your BMS or behave like there's a diode blocking the regen. Or in a car, where the altenator is going to kick in at some point. Or anything else...

I really can't see much point to a three-wire, unless it's in a device which will only ever drain it and you need to be able to hook a charger up...but why you couldn't have it hooked up in parallel with the load/battery...I have no idea. Protection from dummies short-circuiting the charge port comes to mind, but I couldn't guarantee it wouldn't short out that way anyway.

Stick to two wire. Especially if you're not sure.