State of charge questions on new bmv712 monitor

jdobush

I've just added a victron bmv712 battery monitor to my system along with new batteries and a number of improvements.

Overall I'm very impressed with the abilities and accuracy of this monitor. But a couple things I just can't figure out. The state of charge calculation seems way off. 

Most of the battery settings come straight from the manufacturer (Northstar blue+ agm). I've been running with a minimal load to dial everything in.

So, SOC should be total capacity minus consumed ah correct? So 489 / 606 is 80.6% right?

I think by design the shunt's ah in and out should be spot on, but the SOC is just a calculation. The only setting I don't know is the peukert exponent - but is that going to reverse above a 20hr rate? I've been drawing quite a bit less than that but I don't believe for a second I have more magical battery capacity beyond that point (batteries list 100hr rate as the same as 20hr)

Any ideas?

jdobush

Well I answered my own question to a point. I found some online calculators to figure out a peukerts number from the battery's capacity chart and it pointed me to about 1.15. I tried that and numbers look more reasonable. 

I'm still not convinced I get appreciable "extra capacity" at lower draws - I know I've seen some FLA batteries that sell on their 100hr capacity because it is noticeably higher. But I think at some point the natural loss of the battery over time has to play in? 

BB.

When marketing gets a hold of a product, they can twist the numbers to almost anything--It seems.

Smart Gauge, besides selling a voltage based battery monitor--They have lots of White Papers on batteries and systems--Including Peukert.

http://www.smartgauge.co.uk/

The Peukert Factor appears to be a "curve fitting" number that takes several battery resistance/chemical activity/etc. factors and squishes them into one number...

Our Host, NAWS has a few numbers:

https://www.solar-electric.com/learning-center/deep-cycle-battery-faq.html/

Some Peukert Exponent values (not complete, just for info). We don't have a lot of data. Trojan T-105 = 1.25; Optima 750S = 1.109; US Battery 2200 = 1.20.

-Bill

jdobush

Thanks for the links and info. it sounds like I shouldn't worry more about it, if 1.15 sounds reasonable for an agm, which I've read in other places as well.

BB.

It is a good starting point. Adjust up or down as needed. 

 Bill