Two Wire BMS

Been a lurker for longer than I want to admit - now I'm a poster.

I'm new to BMS's and I don't understand the difference between the kind that have two wires and the other kind. The two wire ones seem to only require one port that is shared for both charge and discharge. The other kind has a dedicated discharge port.

Is one better than the other? Seems like most batteries use the seperate discharge and charge ports, but other batteries like sunthing28 on ebay has the two wire bms. Is it ok to have the charge and discharge ports shared?

Thank you

yes, having the charge and discharge through the same port on the drains of the output mosfets is ok.

very common

Can this be done with any BMS. For example, can you just not connect the C- pad and instead run it straight from the B- to the charger? Or do you need a specific BMS that comes without a C- pad and only has P- and B-?

nope, if you have a separate C- spot on the BMS with a set of mosfets for charging then you will lose the ability to stop charging at HVC if you charge through the P- lead.

3 wire BMS is what is generally used for most ebike packs, in my experience. I'm not sure why, but when a BMS is configured as 2-wire, it can't deliver the same current levels, from the same number/type of fets (at least that is the case for the BMS maker we use). This means an equivalent rated 2-wire BMS costs more than the 3-wire equivalent (this may not apply to all makers).

A 2-wire BMS is best if you are planning to connect packs (each with a 2-wire BMS) in series/parellel. This is why we keep some 2 wire 7S BMS, built with 100V fets, so we could series up to 3 of them, but they'd still work like a single pack.

3-wire BMS in such circumstances can leave you open to possible issues WRT charging, and also means you couldn't connect 2 packs in series and charge them. This is because a pack could receive charge current through the discharge lead, after the charge circuit has gone OC.

For a single pack, with single charger, 2 or 3 wire will work just fine.

dnmun said:

nope, if you have a separate C- spot on the BMS with a set of mosfets for charging then you will lose the ability to stop charging at HVC if you charge through the P- lead.

Click to expand...

As rightfully stated by dnmun, Only the Charge circuit will cut at HVC. You can charge through the P lead in an emergency (or maybe if you were wanting a quick charge, plug in a 2nd charger, something I've done before), but you'd want to ensure the cells were reasonably balanced and charge to less than a full charge, when charging through the Discharge lead.

Thanks for the great info, people! Very helpful community!

I am looking for a quality two wire BMS for 10s 15/20A. This is for a 36v li ion battery back. Any ideas?

I have a question about charging through the discharge port of a three wire BMS. People have said that you can charge at higher currents and the HVC wont work, but what about the cell balancing? Will the BMS still balance the cells when charging through the discharge port?

Balance, yes, in that each cell that reaches HVC will still activate it's drainage resistor.

But since the HVC can't cut off the discharge port, the charger will still be feeding current thru all the cells, still charging them, possibly at a faster rate than the balancer can drain the high ones down, so they will continue to get higher and higher, albeit at a slower rate than before.

As long as the cells are never very far out of balance, that's not a big deal. But if you have cells that are a lot farther out of balance than others, you could end up with a problem, perhaps even damaged cells.

Thanks, that is good to know. I have been charging through the discharge port without realising it as my phase runner controller came with a charging connector and I was using that. This of course uses the dishcharge port as I realised the other day.

I will keep using it with my high power charger but only charge to say 4.1V per cell and when I want to do a full charge I will use the batteries charge port.

One option is to install a DPDT switch, with the common poles on teh controller +/-, and the switched poles on the battery's +/- charge and discharge ports. The switch does need to be beefy enough to handle whatever current the system pulls from the battery in operation, with DC (not AC) specs.

Then simply switch to charge port for charging, and to discharge port for discharging. You'll still use the Phaserunner port for charging so you can use it's shunt to monitor if you like (with the CA, etc).


If you use an On-Off-On type instead, then the center position will disconnect the pack entirely from the controller/etc., to prevent any drainage of the pack if left sitting connected accidentally. The only catch with this is that there will be a spark when swithcing back on, which will wear the switch (slowly, usually).

But if only using an On-On type, it only switches between charge and discharge ports, then the capacitors in teh controller won't discharge so there won't be a spark.

cell_man said:

3 wire BMS is what is generally used for most ebike packs, in my experience. I'm not sure why, but when a BMS is configured as 2-wire, it can't deliver the same current levels, from the same number/type of fets (at least that is the case for the BMS maker we use). This means an equivalent rated 2-wire BMS costs more than the 3-wire equivalent (this may not apply to all makers).

A 2-wire BMS is best if you are planning to connect packs (each with a 2-wire BMS) in series/parellel. This is why we keep some 2 wire 7S BMS, built with 100V fets, so we could series up to 3 of them, but they'd still work like a single pack.

3-wire BMS in such circumstances can leave you open to possible issues WRT charging, and also means you couldn't connect 2 packs in series and charge them. This is because a pack could receive charge current through the discharge lead, after the charge circuit has gone OC.

For a single pack, with single charger, 2 or 3 wire will work just fine.

Click to expand...

What about the current rating of the BMS? do they add up as well when you put 2 2-wire BMS's in series?