BMV-712 Does not show charging current

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mbutts asked Mar 3, '20

BMV-712 Does not show charging current

Hello,

I have a motorhome with (2) 12 V AGM batteries and a Victron 712 to monitor them. Everything was working correctly until I let the batteries become discharged.

I removed the batteries & charged them fully outside of the vehicle. I also had my battery dealer test them (the batteries are less than two years old). When I installed the charged batteries in the motorhome, I ran the on-board generator to top off the charge.

With the generator running, the voltage shows 14.2, but the current wavers between zero & 0.2A. I get the same readings if I run the motorhome's engine. The manufacturers of the converter/charger and the battery isolater had me check a few things & they both appear to be working. When the generator or motorhome engine isn't running, the 712 shows a negative current amount.

Any suggestions on why the current reading is essentially nonexistent?

battery charging BMV Battery Monitor input current

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Justin Cook answered · Mar 03, 2020 at 06:15 PM

@mbutts, how are you charging the batteries from your generator? Obviously you have an AC-DC charger installed, but what make/model and what is its charge profile set to?

I'll note that if you charged the batteries fully outside the vehicle as stated, then the batteries are fully charged and running your generator and an additional charger won't do anything... if the batteries are full, they won't take any more current and so the BMV-712 may just be showing that there is, indeed, no current going into your already-full batteries.

I will also note that the BMV reports net current into the batteries, so if your charger is (just for instance) putting 2A into the batteries but you have some small DC loads that total 2A out of the batteries, the BMV will report the net current, which is 0A. The fact that you show a negative current from the batteries when your charge sources aren't running indicates that you have parasitic loads on the batteries that, when the generator or engine is running and you're charging your house bank, are being offset by the charge sources.

So, bottom line: my suggestion for why your current reading is essentially nonexistent is that it's because your batteries are full and, other than compensating for your parasitic loads, there is essentially no current going into your batteries.

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Answer 1 · 2020-03-03T18:58:04Z

mbutts answered · Mar 03, 2020 at 06:58 PM

@Justin Cook - Bay Marine Supply USA Thanks for the quick reply!

The converter/charger in the RV is a Progressive Dynamics PD4560K18LS8. It's a smart charger with 3 levels (14.4V, 13.6V, and 13.2V with occasional 14.4 V to reduce sulfanation). Charge profile is set to LA for lead acid. The generator is an Onan that's built-in to the RV.

I had fully charged the batteries, let them rest for a couple of days, then had them tested at the battery dealer. I assumed between the resting & testing, they would have lost some charge. Since the Progressive was putting out 14.22V, it looked like the charger thought the batteries needed some juice, too.

I will let the batteries sit for a few days with the battery "disconnect" in the off position (still has a draw of about 0.4 A). I'll give it another try then.

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Justin Cook ♦♦ commented · Mar 03, 2020 at 07:05 PM

@mbutts, the other important thing to check is to make sure that all NEGs are connected to the "loads and chargers" side of the BMV's shunt - so, your PD output NEG needs to be connected to the "loads and chargers" side of the shunt, not the battery, as does the NEG from your (I'm assuming) battery-to-battery charger that charges your house bank while the engine is running.

You're right, from letting the batteries sit for a couple of days and being tested, I would expect some current to be flowing into them, which is now why I'm suspecting that the charge source NEGs aren't connected to the shunt properly. Do you have a photo of the battery bank/shunt connections so that we can visually inspect the installation?

If not, the thing to check is: there should be absolutely nothing connected to your battery NEGs other than the "battery" side of the BMV shunt. No chassis ground connections, no connections from any of your chargers, nothing at all.

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mbutts Justin Cook ♦♦ commented · Mar 03, 2020 at 07:55 PM

@Justin Cook - Bay Marine Supply USA I'll shoot over to the storage yard tomorrow & get a photo (or draw a sketch) to make sure I replaced all the cables correctly.

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mbutts mbutts commented · Mar 05, 2020 at 08:01 PM

@Justin Cook - Bay Marine Supply USA I popped over to the storage place yesterday. I had left a draw running for just over a day and the batteries showed 12.01 V on the Victron. I started the generator to see what would happen & it was putting in 35+ A into the bank. I ran it for 20 minutes & the charge went from 80% to 92%.

If I ever have to remove the batteries for external charging, I will be sure to run a load for several hours on them before trying to charge with the built-in systems.

Thanks again for the support!

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Justin Cook ♦♦ mbutts commented · Mar 05, 2020 at 08:03 PM

@mbutts excellent, glad to hear it!

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