Victron SOC % vs Voltage

[Dougcole]

Hi All,


I've been a bit suspicious of the readings I am getting on my Victropn BMV 700 SOC monitor. I have 6 6V FLA GC batteries for my house bank, wired in series and parallel. They are rated at 220 AH each (660 AH total for the bank), but I set my monitor at 600 AH when I replaced them to allow for a little leeway. The bank is 2.5 years old, so last year I adjusted the AH total on my SOC down to 550 AH.


I typically keep the monitor set on % of charge. If I get down to around 60% or so, I charge up the batteries by running the gen-set but I usually don't have to do this as we run the motors or use the gen-set for something else (cooking or AC) which keeps the bank up.



Lately though, I have been noticing the percentage drops really quickly. Last weekend we were on the hook for a few days and I was hitting the 60% mark in about 5 hours. I found this alarming, so I started watching my amp draw. Over the course of a few hours at no point did I see the draw exceed 8a, most of the time it was around 5a.


This was mid day, basically just running the two fridges and a few small things like the FW pump and the head. I'm sure there were spikes when the fridge compressors or the head pump kicked on, but those are pretty short lived.


So last night here at the dock I turned off the battery charger and left it off over night (8 hours). Fridges, head, fw pump on, some lights lights on for a little while. Got up this morning expecting to see 55% or so but nope; 96%. Put my multi meter (a good quality fluke) directly on the batteries and got a reading of 12.4 V. Check the SOC monitor and it is reading 12.39V.



I'm flummoxed. Isn't 12.39 V about a 50% charge? And if I was only drawing 5 or 6 amps for 5 hours how was I going through about 240 AH of battery use?


Should I pay more attention to the voltage charge and less to the % charge?

 

[SteveK]

Doug you are raising questions i am asking myself. Recently replaced FLA 5 year old bats as they were not lasting/holding voltage as shunt soc showed. Loads overnight drained batteries more than expected.

Now I have 4 of the old FLA at home hooked to the recently revived inverter/charger.
At start the 4-6v batteries that have sat unused and separate from each other read a surprising 6.1~ each. Wired up 12.6v.

With volt meter and amp meter hooked up started inverter charger. Volts climbed to 14.4 and absorb light came on monitor, followed later by float 13.5-6v. (No shunt monitor)
Turned off charger and added ~9.6A load. The volts went from 13.5 down to 12.05 as I watched before they became steady and test stopped. Time was not measured but estimated at 20 minutes.
My conclusion is the batteries needed to be replaced. They were draining faster than the calculated rate would suggest they should.

 

[Don L]

You cannot go be the voltage to compare to SOC unless you turn off all loads for a couple of hours. My experience with GC2 golf cart batteries is under just a 5-10 amp draw you need to add 0.1-0.15 V to BM reading. If you or that the OP batteries seem good

 

[jleonard]

This is the chart I have been using

 

[kchace]

Hi All,


I've been a bit suspicious of the readings I am getting on my Victropn BMV 700 SOC monitor. I have 6 6V FLA GC batteries for my house bank, wired in series and parallel. They are rated at 220 AH each (660 AH total for the bank), but I set my monitor at 600 AH when I replaced them to allow for a little leeway. The bank is 2.5 years old, so last year I adjusted the AH total on my SOC down to 550 AH.


I typically keep the monitor set on % of charge. If I get down to around 60% or so, I charge up the batteries by running the gen-set but I usually don't have to do this as we run the motors or use the gen-set for something else (cooking or AC) which keeps the bank up.



Lately though, I have been noticing the percentage drops really quickly. Last weekend we were on the hook for a few days and I was hitting the 60% mark in about 5 hours. I found this alarming, so I started watching my amp draw. Over the course of a few hours at no point did I see the draw exceed 8a, most of the time it was around 5a.


This was mid day, basically just running the two fridges and a few small things like the FW pump and the head. I'm sure there were spikes when the fridge compressors or the head pump kicked on, but those are pretty short lived.


So last night here at the dock I turned off the battery charger and left it off over night (8 hours). Fridges, head, fw pump on, some lights lights on for a little while. Got up this morning expecting to see 55% or so but nope; 96%. Put my multi meter (a good quality fluke) directly on the batteries and got a reading of 12.4 V. Check the SOC monitor and it is reading 12.39V.



I'm flummoxed. Isn't 12.39 V about a 50% charge? And if I was only drawing 5 or 6 amps for 5 hours how was I going through about 240 AH of battery use?


Should I pay more attention to the voltage charge and less to the % charge?

Using the battery voltage as an indication of charge level is not very precise. To get even a reasonable reading, there must be NO load on the batteries for several hours at least and some specs say 24hrs.

Ken

 

[Portage_Bay]

If you are using voltage to determine SOC it may worth your time to read this.
https://marinehowto.com/under-load-battery-voltage-vs-soc/

 

[SteveK]

OK GURUS
How should we measure state of charge without disconnecting and letting batteries sit for a day or two. Real time measure soc.

 

[O C Diver]

OK GURUS
How should we measure state of charge without disconnecting and letting batteries sit for a day or two. Real time measure soc.

One of the problems you can have with a battery bank (with 6 volt batteries) is a bad battery, cell or pair. After charging the batteries you can test the individual pairs by removing the cable between the other pairs. Using an inverter with a constant load and an accurate volt meter over time should allow you to compare pairs of batteries. Ideally it would be best to accomplish this test after sitting a couple of days on shore power and starting the test after a battery charging cycle finishes.

Ted

 

[ksanders]

First, we need to stop using the published charts as they are for resting batteries which never actually happens on a real boat.

The Victron SOC meters are simply amp hour counters. They measure charge and discharge current to determine SOC

Here is a real world example with my FLA batteries, and this is confirmed by the manufacturers published data.

At 50% SOC while being discharged at a 20 hour rate my battery voltage is 11.6 volts. This is for a new bank.

How do you know when to replace your FLA bank???

Thats easy, when the voltage starts to decay below a level you find acceptable for a given SOC. For example wnen my batteries are at around 11 volts at 50% it will be time to replace them.

 

[SteveK]

Kevin, old school 11 volts. I remember that when I had an inverter only and instructions came with 11v equals 50% discharge, but safe to go to 10.5v. Even the current inverter default shut down is 10V. Now that was probably not resting voltage.

Volts can be used as a guide to your own system once you know how to read and compare to when the batteries were new.

I will have to read again the link to marinehowto as I do not think the voltage reported was at rest after a day or two. It is a guide. Yes the graphs on net say 12v is 50% at rest, but it is not really 12v if you let it rest, maybe 12.4-12.6

What did we do before SOC became the catch phrase and devices created to tell you at a glance what state your battery is in.

 

[ksanders]

Kevin, old school 11 volts. I remember that when I had an inverter only and instructions came with 11v equals 50% discharge, but safe to go to 10.5v. Even the current inverter default shut down is 10V. Now that was probably not resting voltage.

Volts can be used as a guide to your own system once you know how to read and compare to when the batteries were new.

I will have to read again the link to marinehowto as I do not think the voltage reported was at rest after a day or two. It is a guide. Yes the graphs on net say 12v is 50% at rest, but it is not really 12v if you let it rest, maybe 12.4-12.6

What did we do before SOC became the catch phrase and devices created to tell you at a glance what state your battery is in.

In the loaded test he tried to demonstrate what I was describing.

Part of the difference might be he used AGM batteries, I'm not sure.

 

[SteveK]

In the loaded test he tried to demonstrate what I was describing.

Part of the difference might be he used AGM batteries, I'm not sure.

I wondered that as well, but still a wet battery, same rules. Even my LFP has a default 10V low volt cut out but does not mention any other volts to stop inverting since LFP can totally discharge. At 10V pretty darn total discharge for FLA. That is 10V under load BTW.
So I am saying you can use volt gauge to guesstimate SOC once you understand that 12v on graph is resting SOC, under load at 11V is ~50% discharged.

 

[ksanders]

I wondered that as well, but still a wet battery, same rules. Even my LFP has a default 10V low volt cut out but does not mention any other volts to stop inverting since LFP can totally discharge. At 10V pretty darn total discharge for FLA. That is 10V under load BTW.
So I am saying you can use volt gauge to guesstimate SOC once you understand that 12v on graph is resting SOC, under load at 11V is ~50% discharged.

Yep, you are entirely correct!

 

[BruceK]

Letting batteries sit no load no charge for an extended period before testing isn`t always convenient. As these are open FLAs a test using an inexpensive hydrometer is easy and fairly conclusive. If there`s one dud(defective) cell in the 6 batts, it will find it.

 

[kchace]

The best and most accurate way to test a battery is to measure the specific gravity. Of course only possible on Flooded batteries with removable cell caps.

Ken

 

[heysteve]

The Victron SOC meters are simply amp hour counters. They measure charge and discharge current to determine SOC

Just to expand on this a bit, the Victron SmartShunt defaults to automatically determining 100% SOC but also allows manually setting it. Here's their description of how the automatic mode works:

5.7.1. Automatic synchronisation
Synchronisation is an automatic process and will occur when the battery has been fully charged. The battery monitor will look at
a few parameters to ascertain that the battery has been fully charged. It will consider the battery to be fully charged when the
voltage has reached a certain value and the current has dropped below a certain value for a certain amount of time.
These parameters are called:

• Charged voltage - the float voltage of battery charger.
• Tail current - a percentage of the battery capacity.
• Charged detection time - the time in minutes.
​

As soon as these 3 parameters have been met, the battery monitor will set the state of charge value to 100%, thus synchronising
the state of charge.
Example:
In case of a 12V battery, the battery monitor will reset the battery’s state of charge to 100% when all these parameters have been
met:

• The voltage exceeds 13.2V,
• the charge current is less than 4.0% of the total battery capacity (e.g. 8A for a 200Ah battery) and,
• 3 minutes have passed while both the voltage and current conditions are met.​

If the battery monitor does not perform a regular synchronisation, the state of charge value will start to drift over time. This is due
to the small inaccuracies of the battery monitor and because of the estimation of the Peukert exponent. Once a battery has been
fully charged, and the charger has gone to float stage, the battery is full and the battery monitor will automatically synchronise by
setting the state of charge value to 100%.​

 

[firehoser75]

Doug,
Just some general comments on battery charging/treatment. Most house bank batteries (FLA, AGM, etc.) don't die on their own, they are "murdered" by their owners (the way they are actually used).
While these battery types are great for house banks and are probably? the most cost effective, they absolutely do not like to be operated regularly in a partial state of charge (PSOC). Flooded lead acid batteries (to have the best length of service) need to regularly be recharged to a full 100%. This is next to impossible to achieve using a gen set, main engine, etc. These batteries, as they become close to 100% (say approx. 90% and above) start to "resist" charging, and it takes longer and longer to impart smaller and smaller amounts of charge (amps). It takes hours of run time to fully recharge (which is what they need), not to mention that most engine alternators are not setup to properly (effectively) recharge a (large) house bank. Stock alternators are designed and installed to recharge the start battery and then "taper off" output drastically and quite quickly to ensure they don't overheat and burnout. If you have modified your alternator and setup, (eg. larger alternator capacity, external monitoring, temperature compensated for both batteries and alternator) then this PART of the issue becomes much less of an concern. However, this alternator system improvement does not "fix" the issue of charge resistance and the necessary long run times required for a full recharge.

The only ways I know of to achieve a regular full 100% recharge of FLA or AGM (wet type batteries) is overnight (or at least hours) on shore power (and reasonably large capacity shore charger) or an adequately sized solar setup in good weather (or maybe a combination of gen set to bulk charge followed by "top up" using hours of solar).
The best situation (for battery longevity) is fully recharging after every discharge cycle (however, not likely in the real world), but a full recharge at least weekly or twice weekly is a minimum for good performance over the longer term. However going back and forth between 60% and 90% SOC over the longer term will not be good for your batteries long term health Depth of discharge is also a factor in battery life, but a slightly different topic.