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hednout asked

Using BMV-712 to measure SOC of series connected batteries which are charged INDIVIDUALLY.


I am planning to use a BMV-712 to monitor the SOC of two Battle Born 12-volt Li batteries connected in series to run a 24-volt trolling motor. The batteries are charged individually by 12-volt chargers.

I can find multiple diagrams for using a BMV-712 to monitor:

12-volt batteries wired in parallel, and charged separately,

6 volts in series to make 12 volts and charged with one 12-volt charger.

6 volts in series to make 12 volts, with several connected in parallel to increase capacity.

This is what I have in my travel trailer, which is charged through the trailer system, or can be switched to be charged by a Sterling 60-amp battery to battery charger I can connect to the truck with an Anderson connector. This is monitored with BVB 712.

What I do not find is how to use one (or two) BMV-712’s to monitor series connected batteries being charged individually.

My thinking is this:

I have 12-volt batteries A and B connected in series to yield 24 volts.

Both batteries A and B are charged individually.

Connecting to measure both batteries:

If I connect the BMV shunt between the negative terminal of battery A and the trolling motor, the shunt will detect the energy supplied to the trolling motor by BOTH battery A and B, but will only detect the energy charging battery A. With each use and charge, the calculation of the SOC will be off by 50%. The only ways I can think of to compensate for this is to adjust the peukert exponent to be about .52, or the charging efficiency factor to about 200%. The ranges shown in the manual do not include these drastic settings so I doubt either is possible.

Connecting to measure ONE battery of the pair.

If I connect the BMV between battery A and B the shunt measures the current flowing from battery B to battery A. When charging, the shunt will measure the energy used to charge battery B. The SOC should be accurate for battery B. Batteries A and B are “matched” per Battle Born- and under the same load from the trolling motor, so the SOC of A should be very close to battery B.


Is there a way to measure the SOC of both batteries?

Is there a better way to measure the SOC of one of the batteries?


The rest of the story:

I have two Battle Born 12-volt 100 AH Li batteries connected in series to produce 24 volts to run a trolling motor. The batteries are charged individually by two different systems. The primary means of charging is a Dual Pro charger (15 amp per battery) The batteries will also be charged by a Minn Kota MK-2-DC onboard alternator charger. Its output is up to 10 amps per bank with a 25-amp alternator, but my outboard only puts out 20 amps.

It would have been simpler to have two 24-volt Li batteries in parallel charged by 24-volt chargers- but that is not what I have…


BMV Battery Monitor
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3 Answers
Kevin Windrem avatar image
Kevin Windrem answered ·

Your evaluation is correct. I see two solutions:

1) fit each battery with its own BWM and insure the charging of each battery is through it's own shunt

2) Charge both batteries in series using a DC-DC charger from your 12volt charging sources

If you charge each battery separately, there's no guarantee each will end up at 100% so I can't recommend a third possibility:

3) set up the BWM to reset to 100% SOC when powered up.

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hednout avatar image hednout commented ·

Kevin

Thank you for your response.

Solution 1

My understanding is:

If I set up a BMV on both batteries, the one connected to the trolling motor will still see the energy of both batteries, and the SOC will be inaccurate. If I had a 36 volt trolling motor, with 3 batteries and each charged individually and each having having a BMV the one closest to the trolling motor (A) would show the energy from itself and batteries B and C . Battery Bwould show its own energy and that of battery C. Only battery C would show its own load and charging energy.

Solution 2

I do not have room to mount another charger in the boat. It is also my understanding that with the Li batteries the charger will stop when the first battery is fully charged- regardless of the SOC of the other batteries.

Having onboard A/C chargers with 2 , 3 or 4 individual 12 volt banks is quite common with trolling motors.

Solution 3

This would help with one BMV on the B battery when using the A/C charger for a sufficient time.

When underway the the alternator charger will shut off with the motor- noty because the batteries are fully charged, so the SOC would reset to an incorrect value.

My plan was to manually reset the SOC after charging with the A/C charger.

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hednout avatar image hednout commented ·

hednout avatar image
hednout answered · 2 secs ago

Kevin

Thank you for your response.

Solution 1

My understanding is:

If I set up a BMV on both batteries, the one connected to the trolling motor will still see the energy of both batteries, and the SOC will be inaccurate. If I had a 36 volt trolling motor, with 3 batteries and each charged individually and each having having a BMV the one closest to the trolling motor (A) would show the energy from itself and batteries B and C . Battery Bwould show its own energy and that of battery C. Only battery C would show its own load and charging energy.

Solution 2

I do not have room to mount another charger in the boat. It is also my understanding that with the Li batteries the charger will stop when the first battery is fully charged- regardless of the SOC of the other batteries.

Having onboard A/C chargers with 2 , 3 or 4 individual 12 volt banks is quite common with trolling motors.

Solution 3

This would help with one BMV on the B battery when using the A/C charger for a sufficient time.

When underway the the alternator charger will shut off with the motor- noty because the batteries are fully charged, so the SOC would reset to an incorrect value.

My plan was to manually reset the SOC after charging with the A/C charger.

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Kevin Windrem avatar image Kevin Windrem commented ·

It's true that during discharge, all BMVs would see equal currents but since the positive lead is on that battery's positive terminal the energy measured would be for that one battery.

During charging, you'd connect the charger between one battery's positive and the load terminal if that battery's shunt so only that BMV would see an energy increase, which is correct.


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David avatar image
David answered ·

I don't have batteries connected in series, but I do have two battery banks - one AGM Bank and one Lithium that are connected in Parallel and for that I use three BMVs - one 700 for the AGM, one 700 for the Lithium, and a final 712 that sees all the batteries as a single bank.

Configured each of the 700s to suit the specifics of the battery technology it is monitoring (Charge effeciency, Puekert, size, etc), and the 712 setup is a matter of coming up with a suitable compromise setting based on the combination of the two. So far it seems fairly good for the 712. Not 100% spot on, but accurate enough. On a discharge/charge cycle, tend to end up with the SOC out in the region of 10ths of a percent, which is ok by me.

You can avoid early SOC resets by tuning the tail current, current threshold and detection time.

This info might help a bit?

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marc-r answered ·

@hednout did you get this working? I want to do the same, and wondered how it would measure charging due to the typically setup of a 24v trolling bank that charges each battery seperatly.


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hednout avatar image hednout commented ·
What I have done is hooked up the 712 to monitor one battery and its state of charge, on the assumption that it will be the same for both batteries, as they are "matched" per the factory.


This is also assuming they are both being charged to 100%- and I am unsure if they are.

The charger I have charges each battery independently using 3 transformers. It is supposed to be set up with banks 1 and 2 for Li, and bank 3 for AGM. The lead for bank 1 is marked as Lithium, the others are just 2 and 3. Bank 2 consistently takes longer to charge its battery than bank 1. I was also having times it was not charging. It turned out the fuse holder was defective, and some times making poor or no contact. I replaced it- but that bank still takes longer to charge. I suspect this is heat related. If the internal construction is as I believe, Banks 1 and 3 have their transformers at the ends of the charger, and are better cooled than bank 2, which is between them. If they pause charging to cool down. this would occur more with bank two. I have been using a fan to increase cooling- but that cools all the banks, noty use bank 2. In addition, the Battle Born batterys them selves can limit their charging based on heat. The two Li batteries are mounted side by side with "long sides" together. For the battery charged by bank 2 the other long side is next to a bulkhead, so it does not cool well from that side either. I may try remounting them to leave a some space, but it will still be tight.

I ought to confirm with the company that it is in fact, a Li charging profile.

I have been having early SOC resets, and have adjusted the parameters mentioned by wildebus, and this seems to have resolved.



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