BMV-712 resetting to 100% SOC by Ah discharge/charged when charged parameters have not been met

DIY
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1

Hello, a quick question if I may?

I have had a BMV-712 Smart in my motorhome for several years, in a system monitoring 2x 95 Ah SLA batteries. It has worked very well for me and I have been very happy with it.

Recently I changed my batteries for a single 260 Ah LiFePO4 battery. I understand the relationship between the Charged Voltage, Tail Current and Charged Detection time settings, and these always worked well for me with the two SLA batteries.

Since fitting the lithium battery however, I have noticed that the BMV also resets to 100% SOC by a straightforward Coulomb calculation. I never noticed this with the SLA batteries and wonder if it is normal expected behaviour.

I have the BMV Charged Voltage set to 14.0 V (absorption voltage is 14.2 V), Tail Current set to 0.8% and Charged Detection Time to 5 minutes. These settings work fine to trigger a resync to 100% with the mains and B2B chargers.

But with solar, I am noticing that the BMV also seems to reset to 100% SOC based on Coulomb counting alone. For example, yesterday the battery started charging by solar at an indicated 88% SOC on the BMV - around 31 Ah discharged. Due to the weather conditions, intermittent periods of sunshine with passing thunderstorms and dark cloud cover that caused the solar charger to switch repeatedly to standby/night mode, it never achieved a charging voltage greater than 13.5 volts, but at round 3 p.m. the solar charger had, according to the BMV, replenished the 31 Ah discharge shown at the start of the day, at which point the BMV showed the battery to be 100% charged, and the time since last full charge on the history screen of the Connect app continued to show as 0 seconds until the lack of available sunlight resulted in the SOC depleting overnight because of normal discharge from the house battery, such as the alarm, tracker, and small maintenance charging current diverted to the vehicle battery by the solar and B2B chargers when they are in standby.

At the time the BMV switched to an indicated 100% SOC, the solar controller was indicating that it was still in bulk charging phase, albeit the voltage and current limited by the available solar.

This morning the BMV started at an indicated 2 Ah discharged, and a short time later was showing 100% SOC due to a similar charge having been returned to the battery, although the charging voltage had not risen above 13.42 V at the time. The time since last full charge again showed as 0 seconds continuously in the app, until a couple of hours later, when the solar charger had switched to float (13.6 V) from its full absorption voltage of 14.2 V, with a tail current of 0.3 A, when it reset as expected due to the charged detection parameters having been met, and the app began to show time elapsed since the last full charge.

Thinking about it, it does mathematically make sense that if the Ah returned to the battery matches or exceeds the maximum discharge since the last full charge, that the BMV would show 100% SOC irrespective of the charged detection parameters having been met. But it seems that this indication occurs too early in relation to the actual SOC of the battery, and as I understand the instruction manual, a resync should only occur when the charged detection parameters (charged voltage, tail current and detection time) have been met.

So I’m just wondering if this is expected behaviour, or whether I need to investigate changing other parameters of the BMV settings such as the Peukert exponent or CEF, to avoid a reset by Coulomb counting alone? It’s something that I don’t recall having seen with the SLA batteries in the system.

The app for the new battery’s own BMS displays an indicated 99% SOC irrespective of the charge returned to the battery exceeding its previous discharge, unless its own charge detection parameters have been met (>14.0V, tail current <1.0 A for 10 seconds).

I don’t intend leaving the new battery in a constant near 100% SOC state due to solar charging when not in use, but it’s one of the things I’ve been doing to check the battery vs BMS as it’s only recently been fitted.

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the Over__Voltage_Protection is 3.6 V per cell. That makes 14.4 V total voltage. If the battery is slightly out of balance, one cell will have a much higher voltage than the others. This then causes the charging to be switched off. With all the consequences of overvoltage, because an MPPT cannot reduce the current quickly enough. The manufacturers assume that you discharge a battery and then charge it with a charger while it is disconnected. If the battery is not well balanced, the BMS will switch off the charging current, and the BMS will attempt to reduce the charge of the overvoltage cell. The BMS then switches on again, and so on. However, this is not practical if loads are connected to the battery. Some users have reported faulty Truma heaters…

Physically, a voltage of 3.4 V per cell (13.6 V) is enough to fully charge a battery. A good compromise is 3.45 V per cell. An Over_Voltage is then very unlikely. I therefore recommend not setting the charging voltage above 13.6 V and continuing to charge the battery with the charger about once a month.

You have understood how the BMV works correctly. During consumption, a counter counts down, and during charging it counts up. This would work perfectly if the battery had no losses and the current sensor had a much higher resolution, were much faster, and its zero point were exactly correct. It is almost perfect, but not quite enough.

The SOC can be determined at two depths of discharge using the voltage and current: when the battery is almost empty and when it is almost full. This is used to synchronise to 100%. When the voltage at the battery is 13.6 V, the current can only drop below C/100 if the battery is actually full. It is a shame that the BMV cannot indicate that it has counted above 100%. That would make setting the correct values considerably easier. So you have to work your way towards the correct values. I set the BMV so that it synchronises too early. I can then tell from the jump in the SOC that it has synchronised:

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I can tell from the jump in the red circle that the SOC will be reliable the following day

It will never work perfectly. The longer the period during which it has not been synchronised, the further the displayed SOC deviates from reality.

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Thank you for your reply, which confirms my assumption regarding the counting down and up.

I also searched for any relevant topics and must have read through about twenty or so before starting this thread, but as soon as I posted this, the topic “BMV 712 Smart 100% state of charge too early” appeared at the top of the related topics list beneath my post, and in that thread @pwfarnell confirmed the same thing:

I think the reason that I never experienced the BMV resetting to 100% outside of the conditions determined by the charged voltage/tail current/charged detection time settings with the old batteries was because it overstated the amount of discharge, as I had deliberately set the battery capacity 10Ah lower then their stated capacities, so it probably always resynced on the synchronisation settings before the Coulomb counter returned the calculated net deficit to the batteries.

With the lithium battery, I found that the BMV was quite markedly understating the SOC compared against the counted Ah deficit, and I have had to disable the Peukert setting completely to bring it anywhere near to the %SOC and Ah discharged figures agreeing with each other as calculated against the battery capacity.

I think I will experiment with reducing the CEF to see if that will reduce the early 100% reading from the Coulomb counter calculation.

As for reducing the charging voltage to 13.6 V, I understand what you are saying, but my B2B and Solar chargers, neither of which are Victron units, only have options for Lithium charging settings of 14.4V/13.8V and 14.2V/13.6V. The battery manufacturer recommends 14.4V and charging until one cell hits OVP, but I already decided against that and settled for the 14.2V/13.6V algorithm. I have my IP22 charger set to 14.2V bulk/absorption, with no float and 13.25V storage, as I don’t anticipate leaving it on continuously, only to fully charge the battery periodically.

The battery has both active and passive cell balancing, both of which are only active above 3.4V per cell, so as I see it, only charging to a maximum of 13.6V would mean that balancing will never happen.