Two 12V LiFePo4 batteries in Series one shows Reverse Polarity

[Travis]

Do your BMS have any Bluetooth or anything? Can you see what's going on in there?

 

[Solgato]

Thanks for your input Travis. Unfortunately I can’t. However I may slowly be getting to the root of the problem. I just checked the battery and after 15minutes or so it wasn’t pulling amperage anymore and resting voltage was 15.3V. Put the LED dome light on it and it is holding steady with the load.

It would seem perhaps the BMS went into overcharge protection due to a runaway cell or something else.

It would make more sense than a LVD since I take it pretty easy on the bank the way I use it having solar onboard providing power at the same time, and most of the time the boat just sits daily in the yard with strong sun and full voltage on the panels.

When I look at the reported data using Victrons App, I always pay most attention to the bank resting voltage after a charge cycle and what the daily minimum and max is. It’s always consistently right at my target Float V (27V) dropping only slightly through the night as the BMV and shunt pull about .10A.

Now to get to the bottom of what’s happening inside the battery now that I have a good understand of why things were happening.

Hopefully documenting this issue of one battery going into open disconnect when wired in Series with another producing/resulting in a negative voltage reading will help others who may experience the same alarming and confusing thing.

Just checked voltage again. With Dome light still attached pulling a load for the last 5-10minutes and it’s down to 14.35V now under load.

Do your BMS have any Bluetooth or anything? Can you see what's going on in there?

 

[Solgato]

With regard to Travis’s comment about how some BMS systems work, for example Battleborne states that when their batteries go into HVD (>14.7V), the BMS will prevent a charge current from continuing, but the battery can still be discharged. So I take this to mean that the battery BMS would reset itself once enough power was drawn out to reduce the voltage to <14.7V.

If perhaps the BMS in my batteries works the same way, then one potential explanation for the behavior is that the battery went into HVD and remained because it was wired up in Series with the other battery and it never had enough of a load on it get the BMS to reset because the other battery was trying to charge it. Once removed from Series connection I could draw some power from it with the LED light because it’s a one-way design, and then maybe I didn’t leave the load on it long enough for the BMS to wake back up and reconnect. This would explain why it would not take a charge overnight. But it doesn’t really explain why the battery only measured 6.5V under load during a load test unless a few cells are so out of balance or bad that the BMS cut them off completely.

I’m going to leave a load on the battery and draw it down, then charge it back up and do a capacity test if voltage doesn’t get too scary. Not sure why it’s going to 15.2V, that’s pretty high.

 

[Travis]

With regard to Travis’s comment about how some BMS systems work, for example Battleborne states that when their batteries go into HVD (>14.7V), the BMS will prevent a charge current from continuing, but the battery can still be discharged. So I take this to mean that the battery BMS would reset itself once enough power was drawn out to reduce the voltage to <14.7V.

If perhaps the BMS in my batteries works the same way, then one potential explanation for the behavior is that the battery went into HVD and remained because it was wired up in Series with the other battery and it never had enough of a load on it get the BMS to reset because the other battery was trying to charge it. Once removed from Series connection I could draw some power from it with the LED light because it’s a one-way design, and then maybe I didn’t leave the load on it long enough for the BMS to wake back up and reconnect. This would explain why it would not take a charge overnight. But it doesn’t really explain why the battery only measured 6.5V under load during a load test unless a few cells are so out of balance or bad that the BMS cut them off completely.

I’m going to leave a load on the battery and draw it down, then charge it back up and do a capacity test if voltage doesn’t get too scary. Not sure why it’s going to 15.2V, that’s pretty high.

It measured low under load probably because the solar was on and you were just measuring the difference of the open circuit

 

[Travis]

You should hook them both up in parallel and charge them up to 14.5 volts and hold them there for an hour. Then put them back in series. Looks like you should also have an extra balancer similar to the one I have listed here but I don't know of any that work at the 12 volt level to keep the two 12 volts in series balanced but also shut themselves down when below top balancing current. Maybe you could connect one and then have the center tap wire be interrupted by an auxiliary control of your charge controller? That way whenever the charge voltage falls below top balancing voltage the charge controller will disconnect the balancer so that it doesn't throw it out of balance. These balancers are known to cause imbalances when you are not at top balance voltage. https://www.amazon.com/PowMr-Batter...6a-abf0-e79e5a34e1ea&pd_rd_i=B07L8XKZ71&psc=1

 

[Short_Shot]

Never heard of such a thing.

Turns out its a thing. I had no idea either until just now and went and looked into it. There's no mechanical or chemical difference between positive and negative ends of a lead acid battery when fully discharged. Both plates in the cell are identical in composition at that point and the polarity can be reversed.
Its not until a charge is applied that the two plates become different in composition.
Very interesting, and obvious when you think about it.


Lifepo4 on the other hand uses a different anode and cathode material so this shouldn't be possible, but I am no expert here.

 

[Solgato]

It measured low under load probably because the solar was on and you were just measuring the difference of the open circuit

That measurement was with the battery removed from the boat. Showed 13V something then as soon as I hooked up the LED light it dropped to 6.5V under load.

Wouldn’t take a charge and only delivered enough power to light a few LED’s in a 12V dome light.

After I jumped it off the other battery it started behaving normally except that the voltage rises quickly to 15V+ with a charger on it, pulling just under 2A and then it tapers off to zero once it reaches 15.3V and stops pulling Amps. I haven’t left it on any longer to see if the voltage rises any further.

Applying the same load of the light for a while then draws it down to 13.4V where it rests and continues to power the light without sagging.

There is talk from others who have opened these batteries up and discovered that although they didn’t request or pay for it, the BMS had a Bluetooth board and was all setup to report cellular data, however no one seems to have been successful in communicating with it using common Apps.

I am suspicious about the issue being related to the battery closest to the shunt with the batteries being far apart and if the shunt doesn’t create more resistance or something along those lines. It does introduce a few more connection points, wire terminals, etc.. and a big block of brass of course.

Otherwise I find it hard to believe the batteries could get so far out of sync with the way I use the boat in that although I do pull max power out of them quickly at times (100A+), I rarely discharge them past 50% and if I do, by the time I pull the boat out of the water and get home they’ve already recovered a fair amount of capacity before the chargers kick off and the system wakes the next day to perform a full charge cycle including an absorption period to allow the cells to internally balance. That typically happens by mid day and then the system just floats until sunset then sees an insignificant load through the night until it wakes for its next charge cycle.

Anyway, if I can get this battery back to a confident performance level after charging, I will reinstall and monitor each battery individually for a while to see if they drift. A couple of Smart Senses would let me see individual voltages, then if I notice some drifting I’ll install a Balancer which will be a little tricky due to the need for it to be placed from a mid pickup point between batteries.

 

[Solgato]

Turns out its a thing. I had no idea either until just now and went and looked into it. There's no mechanical or chemical difference between positive and negative ends of a lead acid battery when fully discharged. Both plates in the cell are identical in composition at that point and the polarity can be reversed.
Its not until a charge is applied that the two plates become different in composition.
Very interesting, and obvious when you think about it.


Lifepo4 on the other hand uses a different anode and cathode material so this shouldn't be possible, but I am no expert here.

Glad you researched it. Didn’t want to explain it in detail. I learned about it the “hard” and frustrating way.

I don’t know if it applies to Lithium either, but I have found some info about batteries/cells in series connection that are so far out of balance the strong cell can draw the weak cell down as it forces current through it potentially reversing its polarity, only found one article that discussed it, but maybe it’s a rare possibility. I’ll see if I can remember where that info was from.

 

[Solgato]

I changed the title of the thread to something that relates more to the symptom for others who may come along seeking advice about a polarity issue.

 

[Texas-Mark]

After I jumped it off the other battery it started behaving normally except that the voltage rises quickly to 15V+ with a charger on it, pulling just under 2A and then it tapers off to zero once it reaches 15.3V and stops pulling Amps.

Assuming your meter is reading correctly, and you are measuring directly on the battery terminals, that is too high for LiFePo4.

 

[Solgato]

Assuming your meter is reading correctly, and you are measuring directly on the battery terminals, that is too high for LiFePo4.

Agreed.

Here’s a thread about polarity reversal in multi-cell NiMh.

Post #6 has some interesting info. Even speaks about it applying to Lithium.

The statement about not pulling from a midpoint is confusing. Maybe it refers specifically to a series string of 3 cells.

Battery Polarity Reversal

Opinions vary. I try to keep the voltage under load from going below 3.0 - 3.3 volts. I try to have the recovery voltage be > 3.6 volts. If a battery is significantly below 3.7 volts per cell, I balance charge it to 3.7 at 100 mA (gently). I charge to 4.17 volts, there isn’t much in the 4.1 -...

forum.arduino.cc

 

[Solgato]

Here’s the logged data from one of my MPPT chargers.

Unfortunately I forgot to look at the BMV’s logged data before I disconnected the system and removed the battery, but since my MPPT’s are networked with my BMV, they are getting info from the Shunt.

Looking at the MPPT daily history, you can see the last time I took the boat out (10 days ago) I drew down the batteries pretty good with the MPPT reporting a minimum voltage of 23.81V which would have been after the evening just before a new charge cycle.

The next day the chargers woke up and hit their Max V absorption setting then the batteries floated (27V) through the night with a minimum voltage of 26.6V recorded after the night.

After that everything was normal as the boat sat in the yard until 3 days ago where you see a Minimum Voltage reporting of 2.32V.

Now looking at the logged data with Battery Voltage and Solar Power selected as graphed variables, you can see when the Voltage drops the first time. The gray block represents that the BMV and shunt stopped reporting data and/or it was nighttime and the MPPT’s lost power due to the battery voltage issue.

Then you see it report again, I suspect because the MPPT’s woke and provided voltage to the system regardless of the battery condition, and some power was harvested. Then it drops out again, and so on.

Since the drop outs seem to occur at the low voltage recording, I would have to assume that the battery/BMS disconnected due to a LVD.

Make sense?

Have to see if I can pinpoint the actual time, but if it was during the evening with chargers off, it obviously wasn’t due to over voltage (although the battery seems to be having issues with that now) and since I have a small load on the bank throughout the night, it looks like it happened more toward just before the chargers woke than after they went to sleep the night before, further pointing toward a LVD issue.

The sharp rise and drop and large gray block is from me troubleshooting the system.

 

[Solgato]

Testing continues…

Yesterday I charged both batteries to equal voltages and applied a load to make sure each would deliver power under load, and they did.

I then reconnected them in Series and monitored their individual and combined voltages while the MPPT kicked on and Bulk charged for a bit before going to Absorption (changed to 28.5V for 1/2 hour) and then into to float at 27V.

Everything measured fine individually and together, with each battery being within acceptable balance of one another.

Chargers then kicked off at sunset, and today I woke and checked the App around 8am to see what the system was doing and noticed there was nothing happening, no charge cycle.

I assumed the reverse polarity issue had occurred again during the night, so I turned off the chargers which were supplying voltage to the system showing 27V, and immediately the Voltage dropped and eventually at about 6.5V the BMV shut down since that’s about the minimum the BMV and Shunt need to operate.

I then measured the individual battery voltages while wired in Series with solar charging off, and one was 13.90V, and the other -8.67V. The same battery is showing a negative voltage value again.

So I break the Serial connection between the two batteries, and the -8.67V now shows +13.18V.

I then launch the Victron App to review the logged data, and it seems the the “phenomena” occurred around 9PM last night. Looking at the Voltage reporting from the MPPT which gets its reading from the Shunt, you can see that the Voltage drops rapidly as of a switch was flipped. Then it starts to rise a bit, before the graph goes gray which means the Chargers no longer had power from the bank to log data.

Then the sun rises and the MPPT’s wake, the system sees full voltage and the BMV and Shunt come back on line thinking the battery bank is good, but the MPPT’s don’t go into a charge cycle, they just remain in Float at 27V.

So I now know that the issue is definitely happening at night when the chargers are off with nothing more than a .10A continuous draw on the bank which was full before the system went to sleep.

Next easiest thing to test for piece of mind is to remove the Shunt from the equation. It shouldn’t matter or make any difference, but I’m going to rule it out anyways before proceeding with opening up the battery in question.

 

[Texas-Mark]

It sounds line the one BMS is cutting off for some reason.

Where is the shunt? Not between the batteries right?

 

[Solgato]

It sounds line the one BMS is cutting off for some reason.

Where is the shunt? Not between the batteries right?

Shunt is between one batteries negative and all +/- loads as it should be.

Only unconventional part is the distance between the batteries.

Earlier back I posted a basic wiring component drawing of the system.

The battery bank supplies a DPDT master switch where all the loads come together (MPPT, DC to DC converters, Wireless Controllers, etc..). On the other side of the switch are the power leads for the motors.

The switch provides charging and discharging of the bank at all times, and is used to power up the motor system.

Agree about the BMS. Wanted to try to determine if it was likely a HVD or LVD. Since it happens when the batteries are resting at night, I think it may just be a faulty BMS in general and not necessarily cell issues.

One thing I need to check is temp. Perhaps a bad thermistor is causing the issue?

Battery is not hot or bulging when charged,

These batteries do live in a challenging environment for electronics and contacts. Basically lots of humidity and condensation, coupled with salt in the air. This is one of the reasons I chose “sealed” batteries, however I can tell you from experience anything that is “sealed” develops lots of condensation, and galvanic corrosion due to electrolysis of dissimilar metals is a big problem, add that to DC power and you have the making for lots of headaches if you don’t limit your connection points and use proper materials.

 

[Texas-Mark]

Since it happens when the batteries are resting at night

No load at all? Could it just be going to sleep?

 

[Solgato]

No load at all? Could it just be going to sleep?

The BMV and Shunt and my Wireless Linear Actuator controller pull about .10A throughout the night.

The “phenomena” was recorded as happening after a charge cycle after the MPPT’s shut down and as you can see from the attached graph that also has the Amerage plotted, the batteries were resting and a very small amount of current was being drawn from those devices. No load spikes or shorts recorded before something “pulled the plug”.

Then it all came back online this morning and MPPT chargers woke and went to float and current rose for a bit, but it didn’t really take any for long before it just went to float, then you can see when I shut the chargers down, the voltage hit the floor, the Shunt then powers down but the MPPT’s remain online recording data with the charging disabled since they have sun power.

 

[Travis]

+13.18V seems lower than it should be. What was the other batteries reading at that time? Seems like you're on the right track. Like you're going to have to open these batteries up and measure the cells without the BMS interfering.

 

[Travis]

If you open the battery is it going to be a hack job that's hard to seal? Any warranty?

 

[Solgato]

+13.18V seems lower than it should be. What was the other batteries reading at that time? Seems like you're on the right track. Like you're going to have to open these batteries up and measure the cells without the BMS interfering.

That’s one of the things I’m trying to figure out with my posts -what should the voltage measurement be across a battery with a BMS that has “disconnected”?

I assumed it would read 0, but I’m finding info online that suggests some could read as high as +6/7 Volts, and regardless of what it reads, some have suggested it shouldn’t be trusted and the value is just arbitrary.

Remember once I put a load on the battery that has that low resting Voltage of +13.18 it will drop down to +6.5V or so, but strangely it will continue to let me pull power from it with an 12V LED dome light.

Strange indeed.

Regarding the cases, they are quite a bit nicer than most I have seen in videos of tear downs, very similar to the Valence XP except the boards aren’t separately accessible, but I think I can get into it without too much carnage. Hoping they used a butyl tape instead of glue.

I have been in contact with the manufacturer. I bought them direct and had them built to order with the BMS configured for high discharge rates.

I let them know the first time this happened some months ago, and contacted them last week to let them know it has happened again, and asked some specific info about the BMS and how it should behave, Waiting to hear back now.

I think I am approaching my Warranty, but you know living in Hawaii we are kind of on our own, so I may have to see if I could at least get a replacement BMS if in the end I suspect it to be at fault.

Hoping if/when I get it opened up I can identify the brand of BMS, and with any luck there might be a Bluetooth module already built in that I can try to communicate with. Other buyers of EWT have opened them up and discovered a full featured BMS.

I wouldn’t be surprised if in the end I find a fried Gecko or a colony of those pesky red ants inside ?