Significant difference in voltages between Charge controller and 712 battery monitor

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zaczaller asked Jan 15 2022 at 10:32 PM

Significant difference in voltages between Charge controller and 712 battery monitor

My solar charge controller is showing float and that my lithium 300ah battery is at 14.6 volts but my victron 712 battery monitor is showing 13.14 volts. I just setup this entire system and I’m not sure why the charge controller is entering this float when the battery is not fully charged.

Charge controller is victron 150/70

I also would like to know the appropriate settings for bulk/absorption, equalization if possible

(700 watts of solar on the roof)

MPPT Controllers voltage

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Answer 1 · 2022-01-16T04:01:45Z

bbmz answered · Jan 16 2022 at 4:01 AM

It‘s hard to answer this question without further details. You should probably comment on your cables / wire sizes and the situation this has happened in.

The most obvious answer would be a „voltage drop“ caused by the resistance of a contact point, connector or cable. 700W@12V are already nearly 60A …

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zaczaller commented · Jan 16 2022 at 3:40 PM

The wire from my bus bar to the batteries and the bus bar to my 2000 watt inverter be 4/0 gauge wire. could that be restricting it? All of the other wire between the solar and the controller and the controller to the bus bars are using 10 gauge wire. Lastly I did add a battle born 12volt battery heating pad straight to the positive and negative bus bars. It goes from the bus bar to a thermometer and then the heating pad so it only turns on when the meter reads below freezing. Could any of this be the reason my charge controller is showing much higher voltage then my battery monitor?

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bbmz zaczaller commented · Jan 16 2022 at 7:19 PM

Thanks for the additional informations. Time for some calculations …[calculations assume full load - in partial load voltage drop / losses are obviously greatly reduced]

Let‘s assume an inverter-efficiency of 95%: 2000W at 12V will result in 175A (with an voltage drop of 0.03V per meter on 4/0 cable). The battery and inverter cable in 4/0 is adequate!

Solar cable: 700W@35V -> 20A -> 10 gauge [0.67V/m] is fine - b.t.w. are your panels in series or in parallel?

charge controller: 700W@12V -> 58A -> 10 gauge [0.2V/m] is critical. My suggestion would be 6 gauge …

It‘s hard to tell if the voltage drop by your cables - please be aware that every connector/connection point will cause additional voltage drop - is the only reason for your problem. But from my point of view it is at least an essential part of it.

A 2m distance (-> 4m cable + connections) between battery-bar and charge controller should be sufficient to create your voltage difference of 1.4V (at full sun / 700W PV power).

Have you an Venus / Cerbo / … gateway installed? If the voltage drop is really the reason you should see a correlation between solar strength / power of your PV and the voltage-difference between charger and battery monitor.

At night the voltage between charger and battery monitor should be nearly identical. At sunny days the voltage difference should be maximal.

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zaczaller commented · Jan 17 2022 at 8:18 AM

The voltage is very similar when its night time.

4668DFC2-959B-49C3-9728-37FD6F5AF692.png

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4668dfc2-959b-49c3-9728-37fd6f5af692.png (304.9 KiB)

bbmz zaczaller commented · Jan 17 2022 at 11:57 PM

Thank you for the screenshots. These prove my already mentioned theory - see comment from yesterday - about a voltage drop of the insufficient cables between charge controller / MPPT and the battery bar.

The wire gauge 10 is causing your problems and should be changed to - at least - gauge 6 (like mentioned yesterday).

Doing so should greatly reduce your problem.

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Answer 2 · 2022-01-16T05:59:23Z

Ingo answered · Jan 16 2022 at 5:59 AM

Double check the BMV-712 fuse rating, it should be 1A. Some older units shipped with 100mA fuses that caused a voltage drop that could explain your findings.

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Answer 3 · 2022-01-17T06:19:20Z

kevgermany answered · Jan 17 2022 at 6:19 AM

You'll have to read it on the fuse.

Answer 4 · 2022-01-17T23:59:37Z

bbmz answered · Jan 17 2022 at 11:59 PM

The fuses are not the problem - otherwise the voltage difference should be indipendent from sunlight / charging current.

The reason is the insufficient cable and therefore resulting voltage drop between MPPT and bus-bar (see comments above).

An exchange to gauge 6 wire (or better) should solve the problem.

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Significant difference in voltages between Charge controller and 712 battery monitor

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Significant difference in voltages between Charge controller and 712 battery monitor

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