Batteries - Determining Level of Charge

State of Charge and Capacity are quite different.

When a 100 amp-hour battery is new, then when it's at 50% state of charge, it has a capacity of 50 amp-hours.

After many years when it's at 50% state of charge, it may have 25 amp-hours capacity.

Both voltage and SG indicate state of charge - if you know more about the battery you can transate that to capacity.

thats why I didnt mention capacity-usually a figure provided by the manufacturer in perfect test conditions on a battery selected for test.
"condition" relates to it as it actually is?

Capacity is measured in amp-hours

State-of-charge is measured in %

Point taken, thanks. Basically, need to know 'how much longer will it run my fridge'.

Thanks Robin!

Gday Moose, Max (and all), hydromters may be great, but don't work too well on AGM or gel batteries!

Cheers Andrew

G'day Andrew

Yep, my problem exactly... I'm getting an AGM sealed unit.

Cheers, Max

Thanks Wiskers

I have a voltmeter, but it would definitely be more practical to permanently mount a meter on my camper trailer, since the battery is in an awkward location.Will check out Derek's eBay listings.

Cheers, Max

SG = specific gravity

Leroy

hmmm lately starting to sound a little like out banned mate BillS!!

Leroy

Nah, think Derek's just trying to help. Besides, he's not selling stuff with blatantly unprovable and unlikely claims, is he? To the uninformed like me, it's all useful info.

Cheers, Max

I have concerns about spending more on the battery monitor than on the battery.

Yeh, me too! If it wasn't for the weight and space, I'd just buy an extra couple of batteries....

Thanks Norm

I guess your voltages (eg 11.5V='charge soon') are lower than Robin suggested above because you have a load attached. As you point out, voltage readings are very imprecise, but doesn't matter once you 'learn' your battery/load setup, and get an idea of where 'full', 'half-full' and 'charge now!' occur.

Cheers, Max

Hi Hughsey

From the 'crash course' I've done in batteries over the last week or two, I think you would see only a short recovery time on a cranking battery, but possibly quite a long time with the deep-cycle batteries, due to their different construction and discharge/recharge time frames.

As an AGM deep-cycle battery can recharge relatively rapidly, it may display the 'recovery' characteristics of a 'cranking' battery.

Actually, the two reasons I'm prepared to spend a bit more on an AGM is:

1. They can recharge faster, whereas most deep-cycle batteries take a long time, which isn't very practical for me.

2. When not in use, they discharge quite slowly, around 1-3% per month, and then will readily recharge after quite some time out of service. Since I wouldn't remember to recharge a wet cell battery 'every 90 days', and so most likely bugger up the battery, this is an important feature for me - and no doubt save me $$$.

Cheers, Max

. . . but read the bold print "Here are NO-LOAD typical voltages vs state of charge". "These voltages are for batteries that have been at rest for 3 hours or more."

In the case of a wetcell deepcycle, not for 12 hours.

If you use these voltages while the battery is discharging you will be stopping discharge before you need to.

If you use these voltages with an AGM battery you will be stopping discharge before you need to. AGMs start at 13.0 volts.

I am improving the table of voltage vs capacity while discharging at various currents.

Hi Bill
I also use a similar chart from Collyn Rivers, and it works for me.
I have tried to apply the "KISS" factor to my system.
I use 90 a/h, 730cca sealed lead acid calcium batterys, 2 in the cruiser hard wired together, and 1 in the camper through an anderson plug. They are all identical and the 2 in the cruiser are both the same age.
My power needs are constant as I run a 110 L waeco fridge/freezer 24/7 and so have to be mindfull of it every day. The vehicle is used almost every day of the year, some days for 1 hr and others for 15 hrs.
I have a large LED digital volt meter readout built into the dash, directly wired to the battery's, so at anytime that I am in or walk past the vehicle I can see at a glance what the state of the battery's is.
I have a 6amp 3 stage jaycar charger hard wired into the system and I plug this in most nights in the summer and when I dont do many k's during the winter so that I start each day with around 13 volts so no matter where i am or what im doing it will see out 24 - 36 hrs plus, ok.
My fridge cuts out at about 11.5 volts and at this level the deisel cruiser will still start with so much capacity.
I know this isnt the best system, but its simle and it works, and if i get 2 years from the battery's I will be happy and will turf them for a pair of new ones and at $ 140 each are cheap compared to some others available.
I am going to upgrade to a 12 or 15 amp 3 stage charger because when camped for a few days and having to top the batteries up with the genny, it will charge faster so I dont have to run the genny as much. I also have all 3 batteries connected together while in camp.

Cheers Pesty