Battery Monitor SOC

[sailorboy1]

I'm really starting to question the Battery Monitor SOC (state of charge) function. Mine is programed to reset the monitor to 100% charge based on reaching a high enough voltage and a low current, which it considers charged because the acceptance is low (think I currently have mine set to 14.2V and 4 amps(1%)). But I can see ways of this being wrong.

Last year my monitor would say the batteries were 100% when I knew they weren't when I left the boat. I know they weren't charged sometimes because when I left the boat the monitor was telling me that I was still down -20 amp-hours and there wasn't any charging while I was gone, but when I came back 4 days later it was reading 100% SOC and 0-AH out. Later in the year I confirmed by gravities that the batteries were not fully charged regardless of what the monitor said.

I still don't know the real reason why the monitor would reset to 100%, but I think it may be due to a small trickle solar panel that is for the start battery. Even though the isolator should prevent the power from that panel going back to the house batteries I know in fact that there is voltage feeding from it to the house bank (because if I completely isolate the circuits the house voltage would go down compared to the start battery with the sun shining). So the house bank would see the trickle charge voltage but get no amps and would therefore reset to 100%.

I now have a 290W panel for the house batteries that should fully charge the bank during the week. But the question of the monitor really still exists as to the use of the SOC display.

Does it make sense to program the monitor in such a way that it would NEVER be able to see conditions for 100% SOC reset and then just manually reset it once in a while when I know the batteries are fully charged? I don't know if in doing this whether the monitor would ever get to 100% SOC in normal use or just stop at 99%. But it seems better to have have more charge than displayed than having the monitor saying SOC is more than what it is.

[s/v Jedi]

Let's start with what battery monitor you have... and pray it ain't Xantrex...

[sailorboy1]

I don't really know why it would matter, but I have a Victron

[goboatingnow]

not a Dutch Battery Monitor....

[s/v Jedi]

Originally Posted by Don L

I don't really know why it would matter, but I have a Victron

It matters for the manual I'll have to open to check it out. I have a Victron myself of-course, but wouldn't even know how to use it anymore so have to check it.

Originally Posted by goboatingnow

not a Dutch Battery Monitor....

Indeed, must be good solid Dutch I got a NMEA2000 interface for it that was as dead as Osama, so it's not all sunshine there

Got the manual, lemme have a look

[s/v Jedi]

Got it:

For a reliable readout, the state of charge as displayed by the battery monitor has to be synchronised regularly with the true state of charge of the battery. This is accomplished by fully charging the battery. In case of a 12 V battery, the BMV resets to “fully charged” when the following “charged parameters” are met: the voltage exceeds 13.2 V and simultaneously the (tail-) charge current is less than 4.0 % of the total battery capacity (e.g. 8 A for a 200 Ah battery) during 4 minutes.
The BMV can also be synchronised (i.e. set to “battery fully charged”) manually if required. This can be achieved in normal operating mode by holding the + and – buttons simultaneously for 3 seconds, or in setup mode by using the SYNC option (see sect. 3.4.1).

I agree with their default parameters so recommend you go back to those. Also, check is you entered the correct battery capacity. When that's all done and your batteries are fully charged, do the manual sync and check if it works better.
My unit is on defaults and always works well. But I normally never have a full battery either...

[s/v Jedi]

I have an idea: your batteries may be tired and in float charge, hooked up to a charger continuously? Did you give them a couple of deep cycles like I recommended last time (when people made fun of me calling them "lazy")? I was serious though..

If on a charger all the time, check your charger settings: it should now and then do a maintenance charge by switching to absorption mode. If it doesn't, switch it off, wait 5 minutes while using decent power from the batteries (inverter to waterheater or something) and then switch the charger back on. It should then charge the battery fully AND sync the monitor in the process.

[noelex 77]

If you have an extra solar panel feeding the house batteries that is not going through the battery monitor it will cause the monitor to reset the SOC to 100% prematurely, when using battery return amps.
It is not clear from your description why the solar panel designed for the start battery is giving current to the house battery, but if this is the case, and its not fed via the battery monitor the SOC will reset to 100% prematurely in some circumstances. and will result in a subsequent SOC reading that is higher than it should be.

The first step is to ensure all current entering or leaving the battery is fed via the house battery monitor shunt. Are there any wires attached between the shunt and battery?

[monte]

Just a quick note to those using battery monitors that will be obvious to some more experienced sailors but oblivious to others (like myself)
When I installed my battery monitor I expected the percentage to show the actual percentage of battery remaining, and decided this is the simplest way for me to monitor the state of my batteries.
However, as the percentage is a calculated guesstimate of amps used/amps charged it can become out by up to 20% over time. It is in fact fairly close within a few days after the batteries are fully charged but after a couple of weeks of not plugging in it could be well out because amps out are actually higher than those read by the monitor (due to Pookets law or whatever his name is). Sure the monitor applies an adjustable value for this, but its not always accurate. So after a couple of weeks of saing and anchoring, relying on solar and wind to charge the batteries, the monitor might say batteries are at 65%, but, the voltage is at 12.2V. The voltage indicates the batteries are actually at 50%. This reading will be most accurate after the batteries have rested for a while (probably first thing in the morning)

These are approximate voltages/% of charge
12v - 40%
12.2v-50%
12.4v-65%
12.6v-80%
12.8v-100%

Hopefully this will help someone (and their batteries) as it has taken me a while to figure it out!

The monitor is a good piece of equipment, it will tell the amps out, and amps in, which can be really useful, and the % state of charge is fine to use if you fully charge regularly, just don't rely on it if its weeks between fully charging your batteries.

Corrections or clarifications are welcome, I just wanted to state my views in case it helps others. BTW, mine is a victron bm 100 as well
Cheers
Monte

[sailorboy1]

Originally Posted by noelex 77

If you have an extra solar panel feeding the house batteries that is not going through the battery monitor it will cause the monitor to reset the SOC to 100% prematurely, when using battery return amps. The little 5W (I'm guessing) trickle charger panel than comes standard on my boat only goes to the start battery. The 290W panel I just installed goes to the house battery and also to the start via the isolator.
It is not clear from your description why the solar panel designed for the start battery is giving current to the house battery, but if this is the case, and its not fed via the battery monitor the SOC will reset to 100% prematurely in some circumstances. and will result in a subsequent SOC reading that is higher than it should be. That is my theory based on the last 2 years of what really happens. It only would happen if no-one was on the boat and therefore all house loads were off. I never have seen a current into the house batteries from the start battery solar panel, but it does bring the voltage in the house bank up if everything is off (doesn't really matter now as the 290W panel is supplying the batteries anyway).

The first step is to ensure all current entering or leaving the battery is fed via the house battery monitor shunt. Are there any wires attached between the shunt and battery? All ground feeds for the house bank go though the shunt! It is possible to feed the start battery via the house though the isolator, but no current should pass from the start to the house.

The real question I'm asking on the thread is whether it makes sense to basically disable the the automatic 100% SOC and only reset it to 100% manually.

I can easily see a cloudy day dropping the solar charge current down enough while in absorption that the monitor could reset early.

[noelex 77]

Originally Posted by Don L

The real question I'm asking on the thread is whether it makes sense to basically disable the the automatic 100% SOC and only reset it to 100% manually.

The 100% reset on the battery return amps should be the most accurate measure of battery charge if it working correctly. It can be disabled,but battery monitors do not maintain long term accuracy without this feature.

Originally Posted by Don L

I can easily see a cloudy day dropping the solar charge current down enough while in absorption that the monitor could reset early.

To reset to 100% SOC it requires a low current and a high voltage. There should be time delay to allow for short term fluctuations.

Only a battery that is close to 100% will sit a high voltage with a low input current (with an appropriate time delay to allow for a sudden drop in solar output, before the surface charge drops). Set correctly it the most foolproof, electronic method, of determined a charged battery ( note most battery monitors list this as 100% in reality it is more like 95%)

Cloudy days should not result in a premature reset as when the solar output drops the battery voltage will also drop (with a slight delay that should be incorporated into the algorithm ). If the battery voltage is high with a low charge current the battery is in a very high SOC.

My own meter is set for 1.25% with a 4 min delay. If these parameters are met the controller drops down to a float voltage and the amp hour counter resets to 100%

[Timd_chapman]

See this; www.smartgauge.co.uk

[sailorboy1]

so maybe just change the time delay, I have it currently set for 5 minutes but don't know what I may have had set at last year

[JayH]

I have a Victron BMV. My own experience is that when plugged into the dock, the automatic sync feature works fine. But away from the dock and charging from solar panels, the sync happens prematurely.

I think this is because the solar has limited capacity and when there are loads (refrigeration, etc) that match the charge current, the BMV sees high voltage but low net current, which triggers the sync.

Perhaps the solution is to raise the "Vc" setting. The Victron documentation says “Make sure the voltage-charged parameter [Vc] is always slightly below the voltage at which the charger finishes charging the battery (usually 0.1V or 0.2V below the ‘float’ stage voltage of the charger).”

In the meantime I have disabled the automatic sync feature on my BMV. Waiting for a better answer...

[s/v Jedi]

Did you miss my posts Don?