question

MarkG avatar image

BMV702 state of charge is not synchronised

Hello, I have recently installed the BMV702 in an Off-grid solar system in rural DR Congo. But up to now no synchronization has taken place.

I am using a MPPT150/70 to charge 6 12V 220Ah Deep cycle gel batteries, in a 24V system.

So I put Battery capacity at 660Ah. I changed Charged voltage to 28.5 (according to recommendations elsewhere for pure solar systems). Tail current still 4%, at 3 minutes (both unchanged at default values).

I have seen that the voltage reaches and remains at 28.6 no problem. So what could be hindering the synchronization? And what should I do to improve the settings?

Some ideas: Could it be that running charges prevent the sync time or tail current to be optimal? Could it be that the MPPT changes to early from absorption to float hence reducing the voltage?


Kind regards,

Mark

BMV Battery Monitor
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2 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hi @MarkG,

To clarify,

Q1. Looking for an initial synchronisation, as you know the batteries are full, but this is not reflected on the BMV?

Or

Q2. Are you concerned about regular automatic synchronisation not occurring?

A1 - If it is just an initial synchronisation, then you can achieve this by either using the VictronConnect app (if you have the bluetooth or USB dongle).

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Or, if you do not have the dongle, you can also press and hold the up and down buttons simultaneously for 3 seconds

A2 - If regular synchronisation under normal operation is not occurring, the first recommended setting to adjust is increasing the tail current %. You did not specify how much current was still flowing into the the battery to hold it at 28.6V.

Typically a battery that is still absorbing more than 4% of its capacity is not yet full, and still charging, but not in all cases, so increase this until you are happy with the results. In your system of 660 Ah, 4% is about ~26A.

It would also be very useful if you could post the history of the MPPT from VictronConnect, this will give a lot more information about the charging status of the system and batteries and might present some clues as to why it isn't happening, and what to do next.


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Thanks. I bought the dongle some weeks ago and I'll probably get it soon so that will give some more information about the history.

I have done a manual sync once, just before the MPPT charger switched from absorb to float, but I heard that this does not necessarily mean that the BMV is measuring well. That's why I waited for the automatic sync. I will try to be there to read the tail current.

To be continued.

@Guy Stewart (Victron Community Manager)

1. The tail current is often around 16-20 amp in absorption mode which is ok, but

2. Because of loads (eg washing machine doing cold wash small moves) voltage sometimes shortly (seconds) drops drops below the set charged voltage of 28.5v or

3. Sometimes voltage doesn't fully reach 28.5v and absorption ends at 28.4v because of fixed absortion time of charge controller (1h if batteries haven't been discharged much).


What would you use as charged voltage for victron deep cycle gel 220ah batteries? The manual puzzles me talking about 0.2 of 0.3v below float voltage. That would be 26.4v according to the manual. But the controller gives 27.6v as float voltage. And wouldn't the monitor think batteries are full too early (eg while charging on a cloudy day being above 26.4 v but not having more than 20amp because of the clouds)? That's why I put 28.5v at first. But sync happens if I put charged voltage at 27.4 for exemple.

Thanks, Mark

Justin Cook - Bay Marine Supply USA avatar image
Justin Cook - Bay Marine Supply USA answered ·

@MarkG, what is the depth of discharge that you're pulling from your batteries? The BMV-7xx will only automatically synchronize at the end of a full charge cycle, which means letting the batteries pull down to ~70% and then charging back up to 100%. Top off charge cycling, like 90%-100%, doesn't count as a "cycle" to the BMV's algorithms, so it won't synchronize under those conditions.

If you're not getting the batteries down to ~70% (good for you, that means your charge sources are keeping up with your loads) but you really want the BMV to complete an automatic synchronization, I recommend that you either temporarily disconnect one of your charge sources and let the batteries pull down, or throw some heavy loads on the system at night when they're not charging, or something of that nature to get a full cycle out of them; the BMV will automatically synchronize at the top of that cycle (when you've charged back up to 100%).

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@Justin Cook, Thanks for replying. I think indeed that the depth of discharge might be insufficient. After a night I'm usually at 25.30, it won't get much lower. What do you use as a 70% in the batteries I use? And how often would I have to do a forced discharge like that? Can't I just do a manual synchronization?

@Justin Cook: I tried some loads but still no synchronization. What next?