Battery Balancing Discharge Problem in 24v system

[Oski]

With Will’s guidance I installed the following in my overland truck:

• 700 watts of solar
• Connected to EPEVER MPPT 40A 150v
• Charging (2) 100ah 12v Battle Born(BB) connected in series
• Monitored by Victron BMV-712

Here is the problem. After making sure all settings are to BB specs and that the batteries are fully charged …. I find that upon discharge I can only get to about 60ah of usage before the internal BMS in one of the batteries shuts down. When I separate the two batteries and measure what is going on … I will find #2 battery has been clearly shut down by the BMS … but the other (#1) will measure around 13 volts.

Having had this happen repeatedly, I sent #2 battery back to Battle Born. They said there was a problem with the BMS … fixed it and sent it back to me. I hooked everything back up …. Fully charged the bank and rechecked all MPPT settings. Needless to say I hoped I would actually get closer to a full discharge of 100ah from the two batteries.

Over the weekend while camping off grid …. It did the same thing again. When I went to bed, the Victron said we had expended 46 AH and we should have 40% more capacity. In the middle of the night with just a 3 amp draw from the refrigerator …. The system shut down .... just as it has in the past.

At this point all that I can think to do is return both batteries to BB and let them try and figure out why the two batteries are not discharging evenly. Before I take that action … I am wondered if the smart people on this forum could tell me if there is something I have done that is precluding these two from discharging evenly.

After spending close to $2K on these … I would really like to get the last 30% or so of their capacity.

Thanks!

 

[gnubie]

Are you doing the initial charge separately or in parallel rather than series? You want to bring them both to the same SOC before using (ie charging or discharging) them in series.

 

[Oski]

Are you doing the initial charge separately or in parallel? You want to bring them both to the same SOC before using them in series.

Thank you for the reply. I have done it both ways (per BB tech suggestion) I made sure they both had 14.4 for a sustained period of time.

That said, at one point the manufacturer suggested I purchase one of their approved charging units. My feeling was that is kind of defeating the purpose of the system I set up with the EPEVER charger.

 

[Craig]

How do you have them wired can you show A drawing or a picture

 

[Oski]

Here are the batteries ...

... and this is the MPPT

On the positive side ... between the MPPT and the batteries I have installed a 50amp circuit breaker and a 300amp fuse ... per Will's website.
On the negative side there is a shunt for the Victron 712.

 

[RCinFLA]

Did you setup all Victron parameters for your battery AH and charge efficiency number?

Any chance of temperature trip, bad temp sensor or its placement location subjecting it to higher or low temp.

Did not see your inverter type. Some sinewave inverters have quite high idle consumption.

If battery BMS is still tripping, try recharge just a little until it reactivates. Let it sit for 30 minutes and check open circuit voltage.

A 13v battery open circuit for BB is marginal area with an unreliable indication of remaining capacity. It might collapse quickly with moderate load.

Get yourself a DC capable clamp-on ampmeter. Make sure your DVM is accurate.

If I had to take a guess based on limited info I would say you are drawing a higher peak current somewhere along the timeline then you think you are. Something like a refrig defrost cycle can surprise you.

 

[Oski]

These are some great thoughts ... thank you so much! I will put replies below ....

Did you setup all Victron parameters for your battery AH and charge efficiency number? I DID PER BB TECH SUPPORT

Any chance of temperature trip, bad temp sensor or its placement location subjecting it to higher or low temp. THIS IS AN INTERESTING IDEA. I HAVE REPLICATED THE PROBLEM ON A NUMBER OF DIFFERENT OCCASIONS AT DIFFERENT TEMPERATURES .... BUT I WILL MORE CLOSELY MONITOR THAT.

Did not see your inverter type. Some sinewave inverters have quite high idle consumption. I HAVE PURPOSELY LEFT THE INVERTER OUT OF THE EQUATION TO ELIMINATE THAT VARIABLE. I DISCONNECTED IT DURING TESTING

If battery BMS is still tripping, try recharge just a little until it reactivates. Let it sit for 30 minutes and check open circuit voltage. INTERESTING THAT YOU MENTION THIS ... WHEN IT TRIPS, I WILL HOOK UP MY 24V BATTERY CHARGER FOR LESS THAN 30 SECONDS AND THE SYSTEM FIRES UP. I WILL BE SURE AND RECORD THE OPEN CIRCUIT VOLTAGE.

A 13v battery open circuit for BB is marginal area with an unreliable indication of remaining capacity. It might collapse quickly with moderate load. INTERESTING INSIGHT ...

Get yourself a DC capable clamp-on ampmeter. Make sure your DVM is accurate. HAVE ONE ... WILL DOUBLE CHECK ACCURACY

If I had to take a guess based on limited info I would say you are drawing a higher peak current somewhere along the timeline then you think you are. Something like a refrig defrost cycle can surprise you. WOW! YOU ARE CLAIRVOYANT! TO MOST RECENT FAILURE CAME AFTER I RAN THE AIR CONDITIONER FOR A COUPLE OF HOURS. ALSO WE USED OUR INDUCTION COOKTOP TO PREPARE DINNER. OBVIOUSLY THIS USED A GOOD NUMBER OF AH'S. THAT SAID, DO YOU THINK THE VICTRON 712 BATTERY MONITOR IS NOT ACCURATELY READING THE AMP HOURS LEFT IN THE BATTERIES?

 

[RCinFLA]

These are some great thoughts ... thank you so much! I will put replies below ....

If set up properly Victron would count the AH but AH may not indicate much if you are overloading with peak currents.

I assume you are sure you have the correct shunt for Victron. I am not familar with it particularly, but sometimes battery monitors have settings to work with different types of shunts with different maximum amperage and different mV per amp shunt resistances. Double check you have setup matched to your particular shunt.

You mentioned some big hitter appliances. 100 AH batteries are not very large capacity. I would not consider running an air conditioner with that amount of batteries.

It's not just about AH's. You have to watch your peak currents. An inverter operating near its maximum capability can drop to 80% or less efficiency. If your cabling is too small gauge the inverter gets less voltage and will draw even more current to supply needed output load. The BMS series switch in BB will get hot if asked to pass a sustained high load current. It will eventually cut out when it gets too hot.

 

[RCinFLA]

There is a point of sustained high currrent drain where you enter a thermal runaway condition. This can all happen within a few minutes or longer if current is somewhat lower.

High load drops inverter efficiency, requiring more current. More current means more wire drop requiring even more currrent. On BMS, sustained high current builds up heat in BMS series MOSFET switches which causes their resistance to go up by 50% which causes them to heat up more and drops voltage to inverter, which causes inverter to draw more current to supply output load, which causes more BMS switch heating.

Positive feedback collapse, all possible within a few minutes of drawing too much peak current. If you put your hand on the top hump of battery where BMS is and it is uncomfortably warm you are entering the danger zone.

 

[Oski]

There is a point of sustained high currrent drain where you enter a thermal runaway condition. This can all happen within a few minutes or longer if current is somewhat lower.

High load drops inverter efficiency, requiring more current. More current means more wire drop requiring even more currrent. On BMS, sustained high current builds up heat in BMS series MOSFET switches which causes their resistance to go up by 50% which causes them to heat up more and drops voltage to inverter, which causes inverter to draw more current to supply output load, which causes more BMS switch heating.

Positive feedback collapse, all possible within a few minutes of drawing too much peak current. If you put your hand on the top hump of battery where BMS is and it is uncomfortably warm you are entering the danger zone.

Thank you so much for this information. I didn't realize the limitations of what I have installed. I charged up the batteries over the last few days ... then today turned off the solar charging. I am going to let the system slowly discharge (with no big draws) to see how that goes.

I do plan on adding two more 100ah Battle Born 12v batteries in series ... then parallel with the other two. I wanted to get this sorted out before pulling the trigger.

You have been very helpful ... thanks!

 

[tictag]

If you've charged them up fully, they should both then discharge equally; you should never reach where a point where one has 'emergency' disconnected whilst the the other still reads 13V. I don't believe there is any user configuration required for BattleBorns so I can't right now think of any external (to the batteries) root cause for this issue.

It would be interesting to know what you find out.

 

[Oski]

If you've charged them up fully, they should both then discharge equally; you should never reach where a point where one has 'emergency' disconnected whilst the the other still reads 13V. I don't believe there is any user configuration required for BattleBorns so I can't right now think of any external (to the batteries) root cause for this issue.

It would be interesting to know what you find out.

Thanks for the note. I did another test of the system to see if my using big amp draw appliances were the root of the problem. Here is what I did:

• Left the system out in the sun for two days with nothing on (except for the little fan keeping the composting toilet happy ... just .23 amps)
• Disconnected and tested each battery .... 13.65v and 13.67v
• Reconnected them and left them alone for another afternoon.
• At the end of the day I flipped the breaker to disconnect the solar system
• I turned on the 12volt refrigerator (3 amps when running) and periodically all the LED lights in the cab (2 amps)
• For the next two days I monitored the system with the Victron 712.
• I got to 65AH and once again the system shut down
• Once again when I disconnected the two batteries I got one at 13v and one shut down ... 9volt

From there I connected my battery charger to the system and withing 30 seconds it comes to life and started accepting a charge from the panels.

So to remove another variable I went ahead and purchased a Victron Smart MPPT to replace the EPEVER. Honestly I have wanted to have the bluetooth capability of the Victron ... along with some of the historical data that apparently it will record for me. So in the next day or two ... I will hook it up and see if it has a different charging algorithm for lithium batteries that will take care of this problem. I will keep you posted ....

I have to say ... this is dissappointing. I really like the Battle Born product .... but the inability to see what is going on with the built in BMS is really frustrating.

 

[RCinFLA]

There is going to be some variances between batteries, in BMS MOSFET resistance, temp sense thermistors, and even placement of battery resulting in variation in ability to dissipate heating. The weakest link in the chain will go first.

 

[RCinFLA]

At the end of the day I flipped the breaker to disconnect the solar system

• I turned on the 12volt refrigerator (3 amps when running) and periodically all the LED lights in the cab (2 amps)
• I got to 65AH and once again the system shut down

You never specified inverter and the fact you are running so many high draw appliances I jumped to assumption the two batteries were for a 24v inverter. You also talked in terms of 100 AH expectation on monitor. If is a 24v system how do you get the 12v for refrig. I see title, just making sure you are not tieing refrig to one of the two series batteries.

It is hard to make out the wires in your pictue but does not look like anything near enough gauge to support the type of load currents you are getting with the appliances you mentioned, especially on a 12v inverter.

12 v systems should be avoided for inverters greater then about 1500 watts, maybe 2kW on short peaks, because the battery/cable amperage is so high. 12v systems are tough to keep path resistances to acceptable values. Inverters will almost always be less efficient on lower voltage supply. Batteries, even closely matched, especially adding in their own BMS variations, will not split current equally when placed in parallel.

 

[Oski]

Again ... thank you for taking time to comment .... I really appreciate being educated about this. I will put answers below.

You never specified inverter and the fact you are running so many high draw appliances I jumped to assumption the two batteries were for a 24v inverter. (THE INVERTER IS A 3000W - 24V AIMS UNIT) You also talked in terms of 100 AH expectation on monitor. If is a 24v system how do you get the 12v for refrig.(THE 12V SYSTEM COMES FROM A 24V TO 12V CONVERTER RECOMMENDED IN ONE OF WILL'S VIDEOS) I see title, just making sure you are not tieing refrig to one of the two series batteries.(NO ... ALL 12V GOES THROUGH A BLUE SEAS CIRCUIT BREAKER BLOCK)

It is hard to make out the wires in your pictue but does not look like anything near enough gauge to support the type of load currents you are getting with the appliances you mentioned, especially on a 12v inverter. (ALL OF THE WIRES BETWEEN THE BATTERIES THEMSELVES AND THE INVERTER ARE 4 GUAGE)

12 v systems should be avoided for inverters greater then about 1500 watts, maybe 2kW on short peaks, because the battery/cable amperage is so high. 12v systems are tough to keep path resistances to acceptable values. Inverters will almost always be less efficient on lower voltage supply. Batteries, even closely matched, especially adding in their own BMS variations, will not split current equally when placed in parallel.

That said ... what would be the proper configuration if I do want to get a safe and reliable system that allows for the occasional use of an induction cooktop, microwave and even the air condition ... without the use of the generator? Am I crazy to think I can set up four 12v lithium batteries in 24v/200AH system to run these things?

Again ... thank you for taking time to comment on this problem.

 

[Silentpower]

Interesting thread. I have a system based on a 48v 100ah battery. I think I might struggle with what you are proposing.

 

[RCinFLA]

Induction cooking (one heater) about 1200-1500 watts.
microwave oven, size dependent, but assume 1200 watts (wall power is greater then microwaving power)
air conditioner, wide range depending on BTU rating. Small 5K btu window unit about 480 watts when compressor running.

Lets take a 1500 watt induction cooking heater for the example.

1500 watts / inverter effic of 90% = 1667 watts DC power consumed.
26 volts battery, no cable loss assumed

1667 watts / 26 v = 64 amps from battery just for cooker.
At 0.64C discharge rate on LFP battery of 100 AH rated capacity, it will deliver about 90 AH's of capacity before inverter shuts down for low voltage.

So roughly there is enough battery on present two series battery system for about 1 1/2 hour of nothing but induction cooking heater running.

Battle Born is limited to 100 amps, primarily by BMS heating, so that one device is getting you 64% of max continuous current.
If you turn on microwave also, you sink. If you turn on Air Conditioner also, instead of microwave, maybe depending on its size, but likely also sink because of compressor start up current.

Your original complaint sounds like pulling too much short term high current causing shutdown of BMS due to heating, not directly caused by AH consumption issue although that will be your next issue.

Air conditioners are hogs for cumulative AH because they run for such a long period of time. Their compressor also requires high start up current. If you make it to your set temp and compressor shuts off that cuts things down significantly but on RV's that may rarely happen. For RV/campers that want air conditioning they usually get a 3 kW quiet <60 dBA inverter-generator.

 

[Sunny Days]

You only have 2400 watts of battery! 24V x 100ah
that wont last enough with an induction cook top a microwave and a air conditioner
also the fridge my be defrosting and that alone can be 1000 watts. The microwave and aircon will pull at least 1000 watts at startup each. this is probably your problem.

 

[Oski]

Thanks for chiming in ... your note raises some questions for me (see below)

You only have 2400 watts of battery! 24V x 100ah (I AM STILL WORKING ON A FULL UNDERSTANDING OF AH AND WATTS. AS I UNDERSTAND IT WHAT YOU ARE INDICATING IS THAT I HAVE 2400 WATT/HOURS. SO THEORETICALLY MY TWO BATTERY 24V SYSTEM HAS 2400 WATTS OF POTENTIAL ENERGY EVERY HOUR ... UNTIL THE BANK IS DEPLETED... RIGHT?)
that wont last enough with an induction cook top a microwave and a air conditioner (TO BE CLEAR I NEVER USE THESE DEVICES IN UNISON. IN FACT I HAVE ONLY USED THE MICROWAVE A COUPLE OF TIMES. USE OF THE COOK TOP IS LARGELY USED TO MAKE COFFEE. WITH SETTINGS FROM ONE TO TEN ... I USE #7 WHICH DRAWS 1000 WATTS. USUALLY I HAVE USED THIS WHILE THE CHARGER IS ADDING 10-13 AMPS FROM THE SOLAR PANELS. AS FOR THE AC ... IT HAS A SOFT START FEATURE ... BUT I DO THINK I WILL AVOID TRYING TO RUN IT OFF THE BATTERY GOING FORWARD. I DO HAVE A QUIET GENERATOR THAT CAN DEAL WITH THIS USE WHEN IT IS ABSOLUTELY NECESSARY)
also the fridge my be defrosting and that alone can be 1000 watts.( THE FRIDGE (UNIQUE SOLAR FRIDGE) HAS BEEN AN AMAZING SURPRISE IN ITS EFFICIENCY. AT MOST IT DRAWS 3 AMPS ... AND THAT IS SEEMS TO NEED POWER PRETTY INFREQUENTLY) The microwave and aircon will pull at least 1000 watts at startup each. this is probably your problem. (AS MENTIONED ABOVE ... I NEVER HAVE RUN BOTH APPLIANCES AT ONCE ... NOR WILL I IN THE FUTURE)

(SO .... CIRCLING BACK TO YOUR ORIGINAL CALCULATION .... I BELIEVE I HAVE BEEN OPERATING SOMEWHERE AROUND 50% OF THE 2400 WATT HOURS CAPACITY)

PLEASE ... I APPRECIATE GETTING SCHOOLED IN THIS SUBJECT. IF I AM MISSING SOMETHING LET ME KNOW ... IT WON'T HURT MY FEELINGS!

 

[Oski]

Good Morning RCinFLA ... thanks again for taking time to help me with this problem. By way of update .... I installed the new Victron Smart Solar MPPT last night. Hopefully this eliminates any concerns regarding the lithium specific charging algorithm being used. The Victron unit has been recommended by BattleBorn .... plus I think I can now gather historic data on the power generated by the panels. That said ... let me address your questions below ....

Induction cooking (one heater) about 1200-1500 watts.
microwave oven, size dependent, but assume 1200 watts (wall power is greater then microwaving power)
air conditioner, wide range depending on BTU rating. Small 5K btu window unit about 480 watts when compressor running.

Lets take a 1500 watt induction cooking heater for the example.

1500 watts / inverter effic of 90% = 1667 watts DC power consumed.
26 volts battery, no cable loss assumed

1667 watts / 26 v = 64 amps from battery just for cooker.
At 0.64C discharge rate on LFP battery of 100 AH rated capacity, it will deliver about 90 AH's of capacity before inverter shuts down for low voltage.

So roughly there is enough battery on present two series battery system for about 1 1/2 hour of nothing but induction cooking heater running. (These calculations are really interesting. One thing I have avoided is doing in my due diligence is this sort of number crunching .... so thank you for showing your process.)

Battle Born is limited to 100 amps, primarily by BMS heating, so that one device is getting you 64% of max continuous current.
If you turn on microwave also, you sink (when you say "you sink" ... does that mean the system fails?) . If you turn on Air Conditioner also, instead of microwave, maybe depending on its size, but likely also sink (same question as above) because of compressor start up current.

Your original complaint sounds like pulling too much short term high current causing shutdown of BMS due to heating, not directly caused by AH consumption issue although that will be your next issue. (This is certainly a concern. When I do talk to BB ... I will ask them if that might have been an issue for the BMS. That said, when I did the most recent test I did monitor the battery temperature. During the two days of testing I never saw a spike in temperature. (This is easy to monitor with the Victron 712 app) Non the less I am wondering if when I first tested the warranty repaired battery, I did damage to the BMS. I am hoping that this can be rectified by a thorough topping off of the charge into both batteries with the new Victron charger)

Air conditioners are hogs for cumulative AH because they run for such a long period of time. Their compressor also requires high start up current. If you make it to your set temp and compressor shuts off that cuts things down significantly but on RV's that may rarely happen. For RV/campers that want air conditioning they usually get a 3 kW quiet <60 dBA inverter-generator.(I think you have nailed it ... the AC gets turned on with the generator. I am pretty sure when that is removed from the equation, I can easily run the cook top and microwave (when needed) without seriously depleting the battery bank)

Again .... thank your for your assistance with getting this sorted out.