jbakuwel avatar image
jbakuwel asked



I'm using a MP (firmware 2624-413) with two BMV-702's (firmware v3.08), one for each parallel battery bank. There's a pretty steady ~50W DC load on the system. The MP is the only charger (using in combination with a AC coupled PV inverter). The SOC as recorded by the MP is significantly lagging behind the SOC as recorded by the BMV702's, see these graphs:

If anything, I would expect the MP to be ahead of the BMV-702's, since the MP doesn't know about the DC loads. The BMV-702's also jump from 95% to 100% while the MP's SOC increases only very gradually.

The option "Synchronise VE.Bus SOC with battery" is greyed out (and reads Off) if I select the MP on VE.Bus as the Battery monitor. When I set Battery monitor to Automatic (or explicitly one of the BMV-702's), the option "Synchronise VE.Bus SOC with battery" switches to On. Am I correct assuming that when a BMV-702 is selected as the Battery Monitor, VenusOS will update/sync the MP's SOC with the SOC as measured by the BMV-702?

My first question is: what is the effect of that?

The batteries were still accepting approx 20A (~10A for each bank) when the BMV-702's decided to jump SOC from 95% to 100% which would indicate that the MP's opinion re SOC would be more accurate.

My second question is: why do the BMV-702's jump their SOC instead of showing a gradual increase?


2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

2 Answers
graham avatar image
graham answered ·

I do not know where you are in the world, but here in the UK I have frequently come across problems with the BMVs re-Syncing falsely when Solar is one of the charging sources.

When the BMV Charge parameters are left at default I have found that with Solar in the UK it is very easy to have a situation arrive where the BMV is presented with an input voltage and current level from the panels triggers the default Charge settings and the BMV re-syncs. This leaves the BMV saying that the batteries are fully charged when they are not. Hence your divergence

For number of years I have been setting the Charge Voltage to 14.1V, the current to about 1% of the Bank capacity and the time to a minimum of 15 minutes. This has the effect that a dull day or even just a short dull period will not cause the BMV to re-sync.

This means that where solar is the main source of charging that even when the battery is fully charged the BMV will not re-sync. But it has the advantage that the failure to re-sync can be seen by looking at the Ahs used which on a re-sync which will not have zeroed as they do on a re-sync.

From the manual

The BMV synchronizes too early

In solar systems or other applications with fluctuating charge currents, the following measures can be taken to reduce the probability for the BMV to reset prematurely to 100% state of charge:

a) Increase the “charged” voltage to only slightly below the absorption charge voltage (for

example: 14,2V in case of 14,4V absorption voltage).

b) Increase the “charged” detection time and/or decrease the tail current to prevent an early

reset due to to passing clouds.

See section 4.2.1 for set up instructions.

02. Charged Voltage

The battery voltage must be above this voltage level to consider the battery as fully


The charged-voltage-parameter should always be slightly below the end of charge voltage of the

charger (usually 0.2V or 0.3V below the ‘float’ voltage of the charger).

See section 3.7 for recommended settings.

BMV-700 / BMV-702 / BMV-712 Smart

Default Range Step size

See table, sect 3.7 0 – 95V 0,1V


Default Range Step size

158,4V 0 – 384V 0,1V


03. Tail current

Once the charge current has dropped to less than the set tail current (expressed as

percentage of the battery capacity), the battery is considered as fully charged.


Some battery chargers stop charging when the current drops below a set threshold. The tail current must

be set higher than this threshold.

Default Range Step size

4% 0,5 – 10% 0,1%



04. Charged detection time

This is the time the charged-parameters (Charged Voltage and Tail Current) must be met

in order to consider the battery fully charged.

Default Range Step size

3 minutes 1 – 50 minutes 1 minute

2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

jbakuwel avatar image
jbakuwel answered ·

Hi Graham,

I'm in New Zealand.

Thanks for pointing out the obvious - I should have had a better look at the default values.

I've set the values for the 48V/600Ah battery bank here to Charged V=53.68V and Tail current=1% and will see how that's going and will adjust if necessary.

thanks for your reply, it's much appreciated.


2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Related Resources

Additional resources still need to be added for this topic