Battery/BMS Issues

[John Frum]

It never got above 13.7V with the full charge. When I woke up, it was 13.3V. That .17A is probably just the maintenance draw for the stereo, fridge, etc. It's got to be something else

Do you have a shunt based battery monitor?
Do you have appropriate means of isolation built into your system?

 

[PaulAtreides]

What were the cell voltages when you turned off the charger? What were the cell voltages when it said 58% remaining? Did the BMS ever show any current draw? What does the BMS show for "time left" and "remaining"?

The battery stopped drawing current at about 3.41V per cell, or 13.7V total.

No current draw on the BMS! That's why I'm confused.

Time left on my BMS (Overkill) refers to charging time remaining. It was at 0, meaning it was as fully charged as the converter would charge it.

 

[PaulAtreides]

Most BMS units break the negative side. So the positive side should go to your fuse and cutoff switch then to all of your loads. The negative side of the battery should go to the BMS B- lead, and noting else (besides the BMS balance lead). The output of the BMS on a common port would be the P- lead which then goes to your ground and - buss bar for all of the loads' negative sides. For all practical purposes, the BMS is part of the battery. I have seen one install where the user also ran a heavy negative cable from the battery negative to the frame ground. This basically eliminated any chance for the BMS to measure the current or interrupt the load or charge current. He thought the system was working fine until he left something on overnight and it killed the battery since the BMS had no way of shutting it down.

My negative is going to a bus bar, which then goes to the chassis ground. No negatives anywhere outside of the BMS.

What I think you need to do is isolate the battery system from the trailer and test to make sure the battery is performing correctly on it's own. If it is not, we need to trace that first. If it is working correctly, then we have to trace what is going on in the trailer. I would start with just the battery cells and BMS and nothing else connected. Connect just the charger and bring the battery to your full charge voltage. For a 4S LFP, I would suggest bringing it to 4 x 3.4 = 13.6 volts if you can adjust the absorb voltage on your charger. Once it is charge, what does the BMS read? If it did not go to full, is it at least 85% or above? If not, do the capacity reset. This should be your new full charge setting. Disconnect the charger, so it is just the battery and the BMS. The BMS will pull a small amount of current, but it should not be enough to pull the battery down for many days. Before my inverter/charger arrived, my battery bank sat a month with the BMS powered, and it only dropped about 1-2% out of my 360 amp hours. So let that sit overnight and see if the voltages drop. LFP cells will settle down a little, so you may see some drop, but the capacity remaining should not change much. If this is looking good, apply a small load. What do you have available? Would it be hard to just run a wire to the LED lights that you said pull 2 amps? That would be a good start. Enough to show on the BMS current meter, but not so much to kill it quick. Running the 2 amp of LEDs, the capacity meter should drop 1 amp hour every 30 minutes. In 84 minutes, it would drop 1%. Yes, 1.4 hours. At 2 amps, it would take 70 hours to use 50% of a 280 AH battery bank. That is 2.9 days of your 2 amp LED lighting. And that is just 50% of the capacity. So you should be able to just check it every few hours with the LEDs on and see if the BMS is even close to these numbers. Just check it every 3 hours or so and see if it is losing about 3%. If it is going faster, you have a problem.

Pre-install into the trailer, the battery was totally fine! Zero issues with any kind of draw. The problems only started once I installed it. But, very minor current on the positive cable so I'm like

 

[Freep]

Dropping 42% overnight is huge. How many hours would you say?

I had this happen to me one night with my 271ah battery when it was newly installed. We went to bed with a pretty full battery and woke up to a drained battery. I never was able to figure out what happened. It hasn't happened since. I've gone over everything since then at least a half dozen times with other mods and general maintenance.

I have always chalked it up to me leaving something on like the fridge set to DC. But I've never done that before or since. It remains a mystery to this day.

 

[PaulAtreides]

Do you have a shunt based battery monitor?
Do you have appropriate means of isolation built into your system?

I haven't installed the shunt because I didn't have the right terminal size ?‍. I've got a Victron. Got the terminals yesterday and I'm hoping to have some time tonight to install it.

I have a Blue Sea switch. Also, not installed yet.

My goal this past weekend was to get the battery hooked up and get to the shunt and switch when I had time since they aren't critical......unless you have massive draw problems lol

 

[PaulAtreides]

I had this happen to me one night with my 271ah battery when it was newly installed. We went to bed with a pretty full battery and woke up to a drained battery. I never was able to figure out what happened. It hasn't happened since. I've gone over everything since then at least a half dozen times with other mods and general maintenance.

I have always chalked it up to me leaving something on like the fridge set to DC. But I've never done that before or since. It remains a mystery to this day.

Maybe I just need to let it drain down ?

I've checked all of my connections. Everything's tight. Crimps are good. The only thing that stand out is the chassis ground, but it doesn't sound like it's a big deal - the previous (and current) ground is a terminal screwed to a painted chassis. My plan is to sand the paint so I get a good metal to metal connection. But I'm having my doubts this is the issue.

 

[PaulAtreides]

What I think you need to do is isolate the battery system from the trailer and test to make sure the battery is performing correctly on it's own. If it is not, we need to trace that first. If it is working correctly, then we have to trace what is going on in the trailer. I would start with just the battery cells and BMS and nothing else connected. Connect just the charger and bring the battery to your full charge voltage. For a 4S LFP, I would suggest bringing it to 4 x 3.4 = 13.6 volts if you can adjust the absorb voltage on your charger. Once it is charge, what does the BMS read? If it did not go to full, is it at least 85% or above? If not, do the capacity reset. This should be your new full charge setting. Disconnect the charger, so it is just the battery and the BMS. The BMS will pull a small amount of current, but it should not be enough to pull the battery down for many days. Before my inverter/charger arrived, my battery bank sat a month with the BMS powered, and it only dropped about 1-2% out of my 360 amp hours. So let that sit overnight and see if the voltages drop. LFP cells will settle down a little, so you may see some drop, but the capacity remaining should not change much. If this is looking good, apply a small load. What do you have available? Would it be hard to just run a wire to the LED lights that you said pull 2 amps? That would be a good start. Enough to show on the BMS current meter, but not so much to kill it quick. Running the 2 amp of LEDs, the capacity meter should drop 1 amp hour every 30 minutes. In 84 minutes, it would drop 1%. Yes, 1.4 hours. At 2 amps, it would take 70 hours to use 50% of a 280 AH battery bank. That is 2.9 days of your 2 amp LED lighting. And that is just 50% of the capacity. So you should be able to just check it every few hours with the LEDs on and see if the BMS is even close to these numbers. Just check it every 3 hours or so and see if it is losing about 3%. If it is going faster, you have a problem.

Yes, I've been thinking about removing the battery and recharging, then letting it sit to see what happens. As I mentioned above, the battery was totally fine before I installed it in the trailer, so I imagine it will be totally fine again if I pull it out.

 

[MisterSandals]

When I woke up, it was 13.3V.

Is that 13.30v or could it be 13.39v?

At this flat part of the SoC curve, it makes a HUGE difference.

the battery was totally fine before I installed

What kind of voltages or behavior is "totally fine"?

 

[PaulAtreides]

Is that 13.30v or could it be 13.39v?

At this flat part of the SoC curve, it makes a HUGE difference.

What kind of voltages or behavior is "totally fine"?

13.32V

Totally fine to me means holding steady at 14.1V, or 3.51V per cell, with no change in voltage over time. Right now, it's a continual, gradual drop that doesn't correlate with what my clamp meter is saying.

 

[John Frum]

13.32V

Totally fine to me means holding steady at 14.1V, or 3.51V per cell, with no change in voltage over time. Right now, it's a continual, gradual drop that doesn't correlate with what my clamp meter is saying.

If your meter off by a factor of 10 perhaps?

 

[PaulAtreides]

If your meter off by a factor of 10 perhaps?

Will confirm once I get the shunt installed

 

[GXMnow]

Clamp type DC current meters are not accurate for a low level current, and they need to be cleared and zeroed before a measurement. If I just clamp on my Fluke and take a reading, it could easily be off by 4 amps in either direction. On this one, I turn it on and off about 5 times to demagnetize the core, then latch it on and zero out the reading. Then clamp it around the cable. But even doing that, it could still be a little off if it was zeroed while the jaw is facing north/south. Clamp on is great for measuring 20 amps or more, but for an amp or two, it is just not that accurate. But in this case, it seems like you have a few amps of parasitic draw happening. That should be easy to find. Did you confirm that the battery negative is only going to the BMS, with nothing else going to negative battery terminal? And the load negative and ground connection is all on the BMS output?

What is the positive battery output going to? You should have a fuse, a switch, and a distribution system, either fuses or breakers. How many loads are coming from there? Do you know what every lead is feeding?

 

[Gazoo]

No current draw on the BMS! That's why I'm confused.

If I recall correctly, my BMS doesn't register any current if the current is less than 200ma's. Maybe someone else will chime in about this.

 

[John Frum]

If I recall correctly, my BMS doesn't register any current if the current is less than 200ma's. Maybe someone else will chime in about this.

I seem to remember someone else saying that too.

 

[PaulAtreides]

Clamp type DC current meters are not accurate for a low level current, and they need to be cleared and zeroed before a measurement. If I just clamp on my Fluke and take a reading, it could easily be off by 4 amps in either direction. On this one, I turn it on and off about 5 times to demagnetize the core, then latch it on and zero out the reading. Then clamp it around the cable. But even doing that, it could still be a little off if it was zeroed while the jaw is facing north/south. Clamp on is great for measuring 20 amps or more, but for an amp or two, it is just not that accurate. But in this case, it seems like you have a few amps of parasitic draw happening. That should be easy to find. Did you confirm that the battery negative is only going to the BMS, with nothing else going to negative battery terminal? And the load negative and ground connection is all on the BMS output?

My Klein has a zero button, which I used before testing. Yes, the B- is the only wire(s) connected to the battery negative (except for the BMS balance wire negative). The B- is connected to a busbar, which then connects via another wire to the *painted* chassis. (I'm going to fix this connection so it's metal to metal).

What is the positive battery output going to? You should have a fuse, a switch, and a distribution system, either fuses or breakers. How many loads are coming from there? Do you know what every lead is feeding?

The positive is going to a bus bar, then connects to the original bus bar that came with the trailer, located on the tongue. There's several wires connected to that bus bar and I have no idea where they go. I imagine at least one goes to the 12V fused panel in the trailer.

 

[GXMnow]

The positive is going to a bus bar, then connects to the original bus bar that came with the trailer, located on the tongue. There's several wires connected to that bus bar and I have no idea where they go. I imagine at least one goes to the 12V fused panel in the trailer.

This os probably a good place to start. If there are wires that you are not sure of, you need to trace them and make sure it is not a problem. If you can't find where it goes, disconnect it from the buss bar. If something stops working, then you found the destination. If it does not appear to change anything, leave it off. No reason to have power feeding a dead wire that could short out later.

 

[Zwy]

The battery stopped drawing current at about 3.41V per cell, or 13.7V total.

No current draw on the BMS! That's why I'm confused.

Time left on my BMS (Overkill) refers to charging time remaining. It was at 0, meaning it was as fully charged as the converter would charge it.

The battery just settled at that resting voltage. Mine has 97% left at 3.41v per cell.

You really need to draw the bank down, also install a shunt to be accurate. Best to discharge bank, then recharge to full again.

 

[PaulAtreides]

The battery just settled at that resting voltage. Mine has 97% left at 3.41v per cell.

You really need to draw the bank down, also install a shunt to be accurate. Best to discharge bank, then recharge to full again.

Thanks. You're probably right as my last theory of a parasitic draw is now out the window ?. I got home fully expecting a dead battery. Nope. Holding pretty steady at 56% and about 3.325V per cell.

 

[Zwy]

Thanks. You're probably right as my last theory of a parasitic draw is now out the window ?. I got home fully expecting a dead battery. Nope. Holding pretty steady at 56% and about 3.325V per cell.

Forget the percentage you're currently seeing as it is inaccurate until you complete a full discharge and recharge cycle.

 

[PaulAtreides]

UPDATE!: I did a capacity test and the results show a perfectly functional 280Ah battery. The BMS cutoff was 2.6V, so I probably could have squeezed a little more out of it to get the full 280Ah. I still can't wrap my head around the resting voltage of 3.325V per cell (down from a charged voltage of 3.51V). But, if that's where she sits, then... OK. I'll adjust the BMS and charge controller accordingly.

Thanks to all for your help with this!

Edit: ignore "Ene". I think I reset it by accident during the test.
Edit #2: the test was run at 10A, hence the 27+ hours duration