DIY 280Amp LifePo4 With BMS voltage problem.

larrylwill

New Member
I built a 280Amp LifePo4 pack with Xiaoxiang 250 Amp BMS recently, I added a Inverter, charger, AC switch and installed it in my motorhome. Everything is hooked up correct and working. After a few weeks I found that I have a 0.5amp drain on the battery. Today I was trying to find the phantom load and disconnected the Neg B- battery wire. Only the B+ and C- wires still connected but the 12v system was still active. I found that if I disconnected the Balance wires the battery was no longer connected. So somehow the balance wires are feeding the 12v to the load. Is it possible that one or more of the balance circuits is bad? Right now I only have the B-. C- and balance wires connected. All readings with an ohm meter and the app are correct and everything seems to be working except for this problem. Anything I'm overlooking? I haven't found the 0.5 amp drain yet.
Ha1be8e9813a0447396fb7fadefe7be876.jpg
 

time2roll

Photon Sorcerer
If you disconnect the balance wires, the BMS will cut power as it cannot read the cell voltages.
Best to use a DC clamp-on ammeter to find the 500 milliamp draw.
That draw is fairly normal for an RV to power electronic controls, radio memory, detectors, etc.
 

larrylwill

New Member
I understand but I disconnected the Neg battery wire from the RV, The power has to be coming through the balance wires. There is nothing running especially when I have a DC cut off switch to disconnect the battery when plugged in. I have a clamp on Ammeter but I want to find out why I have house battery power unless I remove balance wires first. I will work on it today.
 

smoothJoey

Ooga Booga!
I understand but I disconnected the Neg battery wire from the RV, The power has to be coming through the balance wires. There is nothing running especially when I have a DC cut off switch to disconnect the battery when plugged in. I have a clamp on Ammeter but I want to find out why I have house battery power unless I remove balance wires first. I will work on it today.
Please post a picture
 

shadowsteve

Solar Enthusiast
Are you seeing the 0.5A as reported by the BMS? I have the same BMS (2x 150A version) on two 280Ah battery packs. My BMS reports about 1.5-1.8A draw overnight (no solar to cover it) but I have two BMS plus the RV DC loads including a wifi router and inverter (connected but not turned on in standby). If I recall it showed 0A draw when it was just the BMS & cells connected with nothing tied to them.

I also only have a small connection to the B+ on the BMS as I understood it was only needed to support the heaters load. Some docs show nothing connected to it and let the BMS power itself thru the balance wires 12v connection.
 

larrylwill

New Member
Where do you have your battery Pos and Neg connected? (also only have a small connection to the B+ on the BMS) Since the load is on the C- all the battery current to the system has to come from the B+ of the battery to the BMS so a small wire should get hot.

The 0.5A is read on my Trimetric battery monitor, Its a shunt on the Neg wire to the chassis. Its very accurate. Besides the battery goes from 100% to 60% in about 3 weeks. Some could be the readout of the monitor itself.
I have only the battery neg on the B- and only the Battery Pos on the B+ connector. I have the battery circuit disconnected at the MH service panel. I can turn it on or off so It cant be the Propane monitor its turned off also the Trimetric. It could be the Inverter even though its turned off. The 0.5a only shows up when the MH is powered which could be the Propane monitor and Trimetric leds, but when turned off the battery on my panel those are turned off. I keep the MH in my garage plugged in. What does your battery read after a couple weeks.
Im also having strangest with cell 2. When I got them it was low, about 130mv after balance it was ok, then it started reading higher then the rest, now its back to reading low. I find the app is not that accurate, Im using the CarploungeBMS app. Right now it reads 3.569, 3.508, 3.558 and 3.565
 

hankcurt

Solar Addict
@larrylwill
In the photo of your board in post #7, it appears that you might be using all of the balance leads. The two wiring method diagrams in Post #1 do not show all five of the balance leads being used.

In method #2, where they have no connection to B+, they are using the red and three white balance leads, but not the black. I found a similar diagram on the lithiumbatterypcb.com website. If you scroll down to the diagram, it specifically states not to attach the bc0 balance lead.

In method #1, where there is a connection to both B+ and B-, they show only three balance leads in use. but they show them connected to pads on the other side of the board. There are traces from those pads over to the balance lead connector, so I believe that those are the three white wires on the balance connector, which makes sense since they are called bc1, bc2, and bc3.

So what I concluded from looking at the diagrams is that the red balance lead has a direct connection to the B+ pad and the black balance lead has a direct connection to the B- pad. If you are using the black balance lead, this would be why you still had power with the B- pad disconnected.

They appear to be using the B- current carrying conductor as part of the balance/cell voltage detection circuit. This is a poor design for accuracy, as the voltage drop in the current carrying wire will affect voltage detection. For this reason, I think this is a not a good quality BMS.
 

larrylwill

New Member
hankcurt: Good catch. I looked at the diagram 100 times and never noticed what you saw. It came with no instructions, so I just connected it as in diagram 1 and hooked up all the balance wires. +, - and each cell. I think what you observed is correct. In your opinion which method would be the best. Thanks
 

hankcurt

Solar Addict
Since the BMS really only controls the connection between the B- and C- connections, and does not act on the current at the B+ pad, I would say that the best option is to use the red balance wire and not connect anything to the B+ pad. The advantage of not using the B+ pad is that voltage drop on the positive power cable won't interfere with voltage sensing on the positive most cell.

You still have to not use the black balance wire and just keep the connection between the battery and the B- pad as short and as low resistance as possible. Voltage drop on that power cable will affect the voltage reading on the most negative cell.
 

shadowsteve

Solar Enthusiast
So I think your saying is connect the battery to the inverter and leave out the BMS, also remove the Black balance wire.
I'm not using the black balance wire in my setup. I assume you mean to not use the B+ as a junction point.

If at some point you use the onboard heater controls you'll want a power feed back to the B+
 

hankcurt

Solar Addict
Yes, I would connect the positive battery cable to the load/charging device directly.

Also, once you are no longer using the black balance wire, I would be very cautious about removing the connection to B-. The installation instructions say to connect B- before connecting the balance leads. They don't say what bad thing happens if you don't connect B-.
 

shadowsteve

Solar Enthusiast
Yes, I would connect the positive battery cable to the load/charging device directly.

Also, once you are no longer using the black balance wire, I would be very cautious about removing the connection to B-. The installation instructions say to connect B- before connecting the balance leads. They don't say what bad thing happens if you don't connect B-.
Any time I'm disconnecting the BMS from the battery I pull the balance connector first. I also connect it last when putting it together.

I built my batteries as standalone packs. They connect to bus bars before connecting to loads. The connection to the inverter should have a suitable fuse inline
 

MisterSandals

Participation Medalist
1637956489423.jpeg

Is the the current BMS wiring?
Pin 1 of the balance lead needs to go to negative terminal in lower right (cell #1). Pin 2 to positive on cell1. Pin 3 to positive on cell 2. Pin 4 to positive on cell 4.
 

shadowsteve

Solar Enthusiast
View attachment 73588

Is the the current BMS wiring?
Pin 1 of the balance lead needs to go to negative terminal in lower right (cell #1). Pin 2 to positive on cell1. Pin 3 to positive on cell 2. Pin 4 to positive on cell 4.
Yes, that's essentially a current picture but no, the negative lead (black)in the balance wire connector is not required to be connected to the batteries. That connection is already made by the B- on the BMS. Some drawings show it connected, some don't. I chose to not connect it and the pack is running fine.

 
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