question

walkman avatar image
walkman asked

Monitor State of Charge doesn't match actual SOC?

Hi

Im running the following

1500w solar into Morningstar MPPT into...

6x140ah 12v in series/parallel to get a 24v 420ah system...

Feed this through a Victron Phoenix Inverter (24/3000)...


I have been monitoring via a Victron BMV 712

Just had a low battery alarm on inverter.

The BMV was showing 68% but battery bar only one increment. Unfortunately we have had a tendency to glance at the SOC% as a guide but are finding this massive discrepancy and potential battery damage.

Batteries were new about 9 months ago.

The BMV shows it has synched 200 times since install.

I'm wondering if the settings are appropriate and/or I need to be more proactive in my monitoring of the monitor?!!!

Any suggestions as to how we have arrived at this place?

(We had just returned from a 10 day absence and whilst I didn't specifically check (looked at SOC%) I assumed the batteries would have been on float. We have not had any major power usage events in the last day since returning, yet here I am with heavily discharged batteries and a monitor that is inaccurate.)

Settings for BMV

Capacity 420ah

Charged Voltage 26.8v

Tail current 4%

Charged detection time 3min

Peukert 1.25

Charge eff factor 95%

Current thresh 0.10A

Time to go averaging period 3m


Thanks for any suggestions.

Cheers, Neil

battery
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1 Answer
Justin Cook avatar image
Justin Cook answered ·

Hi @Walkman,

This sounds to me like you either have high battery self-discharge (which seems unlikely if the batteries are new, but it's a possibility) or you have one or more loads that aren't running through the BMV shunt - which latter is, I think, the most likely cause.

If you have anything at all connected to the battery NEG other than the shunt, the BMV can't see that load and so cannot update the SOC display to account for it. As an example, if you have a chassis ground that is connected directly to the battery rather than to the "loads and chargers" side of the shunt, anything in your system that uses a chassis ground for its ground return is going to be a load that the BMV can't "see", and therefore it will not be reflected in your SOC display.

Your settings appear correct, so my first recommendation is to go check the installation; remember that nothing other than the "battery" side of the BMV shunt can be connected to your battery bank's NEG takeoff. Absolutely nothing. ALL system NEGs, including chassis grounds, charger NEGs, load NEGs, the inverter NEG, your charge controller NEG, etc - it all must be connected to the "loads and chargers" side of the BMV shunt.

Check that and report back!

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