Battery SoC

Ger08 · Senior Member

Battery SoC

Peter_n_Margaret · Guru

There is really no such thing. A rough idea can be had from the voltage of the battery after it has been left to sit unused for a while.
The best way is to get an instrument that accurately measures power in and power out but even those need to be used with some caution.
Cheers,
Peter

STRETCH ARMSTRONG · Senior Member

Hi Ger08,

I've been reading lots of stuff put out by major AGM makers.

All l can say is Peter makes a good point. Everyones data seems to be different.

The best educated guess l can make is 12.8v. Determined by data from manufacturers l read.

They recommend a test like Peter said. Leave a battery that is fully charged and on float disconnected/unused for least 3 hours to settle then test the voltage, mine came in at 12.87volts.

Hope this helps,

Stretch.

Thought l would add my solar controller bulk is 14.4v and float is 13.6v.

-- Edited by STRETCH ARMSTRONG on Saturday 4th of July 2020 12:22:57 AM

Ger08 · Senior Member

Thanks for that. So what would 12.6v be?

STRETCH ARMSTRONG · Senior Member

This is a simple guide for AGM's that l found. I have seen others for AGM's that are different. Note that other types of batteries also vary.

AGM Volts

100% 12.80+

75% 12.60

50% 12.30

The reason l posted my solar controller charge voltages is there may be a difference depending on battery brand and/or the controllers charging voltages.Someone else will know more than me about this, l'm trying to learn about it also.

A friend of mine just had a new system installed on their bus. His controller is charging his AGM's to 14.8v bulk, 13.8v float and 13v at 100%

Cheers Stretch.

Whenarewethere · Guru

You need to let the battery rest for 24 hours & then see what voltage it is.

You could get a Victron BMV-702 battery monitor or similar.

If the battery is getting on a bit & it is critical for the fridge, wifey will not be impressed if it fails while you are remote camping. A new battery/s amortised over a few years is cheap relative to a third world war!

Ger08 · Senior Member

Thanks for all the responses

Warren-Pat_01 · Guru

Hi Ger08,
I keep a copy of this in my van's paperwork - www.energymatters.com.au/components/battery-voltage-discharge/ .

Note relying purely on voltage readings can give you the wrong idea particularly if the battery is approaching the end of its life. You need to use charge/discharge cycles to see how good it is. A sudden drop in voltage can show that all is not good. I use an H4 QH headlight globe for a load (low beam = 4.5A draw, high beam = just over 5A).

At work in the early 90s, one of our techs found a dud cell in a battery at a site just south of Ayr (they were 2v cells in a 48v battery). With cells you could see through, it was easy to see what the status was. The voltage was ok but the joiner between the post & the plates was eaten through & was showing signs of starting to arc - that battery was isolated immediately!! Much better than an explosion.

Muskat55 · Veteran Member

This article is quite technical and examines SOC under load and relationship to voltage. Points to ponder at the end is a good summary. https://marinehowto.com/under-load-battery-voltage-vs-soc/#

Jaahn · Guru

Hi smile

I do not know if there is much more to say about battery voltage verses state of charge confuse But I will anyway. If you are using your 12V electrical system every day, have a fridge and solar for charging then the voltage goes up and down up to 2 Volts depending on whether the solar is charging or the fridge is on or indeed any other item. At night there is no solar so the Voltage is down much lower but still goes down and up as the items turn on and off. If a big user is on it drops again another Volt or more but recovers when it is turned off. wink

So my point is, looking at an instantaneous voltage is useless. But if you get to look often at it while knowing what is happening, re charging and discharging you can get an approximate idea of how the battery is going. If you need to be surer then just switch things off for an hour to get a resting voltage which is close enough for normal use. I have a good easy to read meter inside and glance at it as I go about my day. If the voltage is not where I expect it should be then I look for the cause. EG left the bus lights on, a shadow on the solar or similar. Get to know your gear ! aww

Jaahn

-- Edited by Jaahn on Friday 10th of July 2020 01:25:36 PM