SOC drops consistantly with no drain.

[IRMartini]

I'm new to all this after having a full system installed Oct last year and although I'm coming up to speed with understanding whats going on and making sense of the data I have noticed something that looks a bit odd to me?

After the battery has reached its lower cut off value so my inverter reports that I'm only using energy from the gird the battery SOC drops 1% roughly every 2 hours very consistently.
(I currently have the cut off set higher than normal due to the lack of daylight hours I didn't want the SOC to speed months at very low levels that's bad isn't it!? normally the cut off is set at 10%)



Is this expected behaviour?

Thanks for any help or advice.

 

[Grakat]

Do you have anything connected between the shunt and the battery. If so you will have a battery drain that is not captured by the shunt as a load and will not adjust the SOC.

 

[400bird]

It may be the idle draw on your inverter. In the same state, my inverter pulls a nearly constant 30 watts. This is normal. Depending on the inverter(s) it could be 100 watts or more.

 

[rpdom]

Some inverters run off battery power internally. They will stop using battery for your load at the cut off point, but keep using it to power themselves. Mine takes around 30W in that state. There may be a lower cut off point when the inverter will top the battery up a little from the grid, just to prevent it going too low. I have my minimum SOC set to 12% and force charge SOC set to 10%.

 

[IRMartini]

Thank you for the replies its very much appreciated.

Grakat,
I'm not familiar with the term 'shunt' I'm assuming its the inverter or maybe a specific part of the inverter, either way I do not believe I have anything between the battery and my inverters so I don't think this is the reason especially considering what I have learned below.

400bird & rpdom,
This sounds like what I'm seeing!
My battery is 9.4kWh with 8.93kWh useable
I assume I can roughly calculate 1% like this 8930/100 = 89w
I have two inverters so approx. 45w each.
Not as efficient as yours but certainly plausible explanation of the constant drain on the battery I'm seeing, yes?


I have some follow on questions if you don't mind
Originally my minimum SOC was 10% and with the lack of daylight hours I was reaching this earlier and earlier after the sun goes down and so was spending more and more time with the constant drain on the battery. By the next day I was seeing very low SOC levels 2-3% before the sun comes up the next day.

Is running the battery down to very low levels 2-3% not ideal or down right bad?

How about the opposite my max SOC is 100% would it be wise to set this as 95% or lower?

I'm quite willing to trade off some usable charge if it helps extend the life or reduce the stress on the battery.

Thanks again for everyone's help and advice it really is a help.

 

[rodrick]

Stay out of the top 10% and bottom 10% and your batteries will thank you with a long service life

 

[IRMartini]

Stay out of the top 10% and bottom 10% and your batteries will thank you with a long service life

Thank you rodrick I think I will do just that!

 

[rpdom]

I don't have any option to limit my max SOC, so that always goes up to 100%. I'd like to keep it to under 98% ideally. In theory my batteries are good down to 5%, but I've got that set higher as they tend to get out of balance at the low end and I can end up with one at 6% while the other is alarming at 4%.

 

[IRMartini]

I only have 1 battery to worry about at the moment but I have wondered if the additional cost would be worth the extra capacity?
Question for another day I think

 

[rpdom]

I only have 1 battery to worry about at the moment but I have wondered if the additional cost would be worth the extra capacity?
Question for another day I think

I've only got two 2.4kWh batteries. When I can afford it I'm definitely going to add at least another 4.8kWh to that. It'll add less than a year to the payback time on my system.