BMV 712 soc full too early

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aaronmiller asked Oct 3, '19

BMV 712 soc full too early

Hi,
Setup is a Century Marine flooded lead acid under the bonnet of my 4wd, charged by a Redarc BCDC 1225D. I am working on the assumption that the redarc knows what its doing and when its showing greater than 13.6v and or 3 amps going into the battery, the battery is not full.
When using the default settings for Peukert and charge efficiency, soc is indicating that the battery is full whilst i am still seeing 10+ amps going into the battery so i know its nowhere near full.

The general advice from victron suggests that doing a sync will help ensure the accuracy, but it seems that unless the soc drops to below i think its 60% then a sync will not occur. The problem with that is the soc from day to day doesn't go below an indicated 85%.
The only usual thing running is the 12v 47l fridge.

Is the 712 behaving as usual, or should i be tweaking the Puekert and charge efficiency figures?

battery charging

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Answer 1 · 2019-10-03T08:56:19Z

klim8skeptic answered · Oct 03, 2019 at 08:56 AM

What charge profile (volt) have you set the redarc to?

What charged voltage have you set the bmv to?

Has the redarc done a "full" charge cycle? (boost>absorb>float)

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Answer 2 · 2019-10-03T11:08:48Z

aaronmiller answered · Oct 03, 2019 at 11:08 AM

Redarc is set to profile A (14.6)

bmv to 13.1

yes many times, this has been ongoing since installed 6+ months ago

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klim8skeptic ♦ commented · Oct 03, 2019 at 06:25 PM

Hi Aaron.

I would definitely look at bumping up the charged voltage a little, have a look at page 12 of the manual. https://www.victronenergy.com/upload/documents/Datasheet-BMV-712-Smart-EN.pdf

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aaronmiller klim8skeptic ♦ commented · Oct 03, 2019 at 08:42 PM

Hi,
Float on the bcdc is 13.3, it is suggested to set the charged voltage to 0.2 below float in the manual?

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klim8skeptic ♦ aaronmiller commented · Oct 04, 2019 at 08:16 PM

Hi Aaron.

Charged voltage should be set just below the absorption voltage.

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aaronmiller klim8skeptic ♦ commented · Oct 07, 2019 at 08:08 PM

Ok will do,
but I don’t think that will solve the problem. That will effect whether the bmv syncs or not, but as I typically only draw down the battery by around 10% each day and then it charges back up the battery doesn’t get low enough for a sync to occur.

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Answer 3 · 2019-10-03T16:00:56Z

Justin Cook answered · Oct 03, 2019 at 04:00 PM

@aaron.miller, a couple of thoughts:

1: Are you certain that no chargers or loads are bypassing the shunt? If a load (such as your fridge) is connected directly to the battery rather than the NEG attached to the "loads" side of the shunt, the BMV would be reporting 100% SOC much sooner than is actually the case because it can't "see" the outbound current of the load.

2: Is the Ah capacity of your battery set correctly in the BMV?

3: How long have you had the BMV connected? When you first connect it, the default option is for it to think that it's starting out at 100% SOC, so if you were actually at 60% when you connected it and you didn't set it accordingly, it will think that 60% is 100% until it learns better (usually 5-6 discharge/charge cycles).

I kinda think that option 1 or 3 are the most likely causes of this behavior, but if you check everything out and it's not any of the 3, definitely report back to us... I'm always super curious about unexpected behavior in these.

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aaronmiller commented · Oct 03, 2019 at 07:48 PM

Hi,
1. there is only 1 cable attached to the negative post which is the cable coming from the battery side of the shunt

2. battery has 100ah capacity and is set in the unit

3. I have had the unit for around 2 years, it has been in the current car for 6 weeks. I relocated the head unit, when I reconnected the data cable from the shunt, the battery symbol was flashing and soc was at 0%. The battery was charged by a 240v charger with the negative clamp on a earth point and the soc showed 100%
since then the battery has cycled daily, the fridge runs 24/7 and the car has been run long enough maybe 10 times to the point that the bcdc has gone into float.
With both the previous car and this car, the soc has always returned to 100% at least an hr before the bcdc goes into float.

the cabling in the car has the following setup.

negative cables from the bcdc and solar Anderson plug go to an earth point on the body inside the engine bay approx 10 cm from the cars main earth point

the negative wire from the fuse box in the rear of the car, goes to the negative post on the start battery. A negative wire goes from the negative post on the start battery to the load side of the shunt.

Aaron

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Justin Cook ♦♦ aaronmiller commented · Oct 04, 2019 at 05:58 PM

Hmmm well if your fridge is running constantly, I think it's a very high possibility that the BMV is registering 100% SOC but the Redarc is continuing bulk charge because it's basically powering the fridge... even with a full battery, the constant draw of the fridge is likely enough to make the charger think it's not full. Basically, the charger is trying to cross a certain voltage threshold before switching to float, and isn't being able to because the fridge is drawing down the voltage at the same time.

Not 100% confident in that, but I'm guessing that if you switched off the charger, let your fridge draw down the battery a bit, then switched off the fridge and let the charger run by itself without any loads on at the same time, I'd bet that your charger goes into float a lot sooner.

All that being said, it's also worth noting that the BMV's SOC calculation is based on a number of floating-point variables including its memory of how your battery has acted in the past... it's not based upon a particular voltage reading, while your Redarc settings likely are, so I don't find the discrepancy in timing all that alarming.

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aaronmiller Justin Cook ♦♦ commented · Oct 04, 2019 at 08:10 PM

Hi Justin,

To clarify the fridge is turned on 24/7, it cycles on for maybe 10 minutes per hr. The redarc does get to float regularly.
I don’t think having no loads would make much of a difference, It puts out 25 amps and once the battery gets above approx 80% soc the current going in is tapering off so their is plenty of reserve capacity.

At this point the soc is basically useless and I have to go on the number of amps going in to gauge how full the battery is

Aaron

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