when is a battery flat?

I'm with you on this one Roachie...I consider the battery flat at 12 volts.

Yes,
will second that too
(technically it's just ½ flat, but that's as far as it should go anyway)

Mainey . . .

Hi Patrol,

Yes I do have a charging situation when travelling. But it's not the worlds best.

. . . and if you want this graph to be meaningful, you have to read the not-so-fine print at the top "battery voltage at rest - no load" - and that means waiting after charging too.

If you are using a wetcell deepcycle, you'll have to wait many hours to get an accurate voltage.

If you're measuring voltage while connected to a load and you disconnect at 11.6 volts, you might find the battery still has 90% of its charge - it depends on the discharge current.

And it really comes down to the situation you are in at the time .....

How much value you place on your batts versus the priority of cold beer ???

I have 6 AGM's in the CT and don't like to see them drop below 12V...........but if they were 12V and I was going home the next day and there was beer to be drunk, then they would have to suffer a bit..

But getting to know how long your batt(s) lasts, usually prevents them going flat..

Measuring with a volt meter should be done when under no load....and although the battery needs time to fully settle after being used...it will give you a pretty good indication as to SOC..

If it shows 12.6V then its good....if it shows under 12V then charging at the earliest opportunity is the go ..

For most people , it doesn't have to be a super accurate science....just a general guide as to when the battery is getting a bit low and may need a charge..

I have a cheap LCD voltmeter ( off e bay ) that I know is between .1 and .2V out ( reads a bit low...which isn't a bad thing ) that lets me ( when I can be bothered to look ) keep an eye on SOC...

You can buy an A/H capacity meter that lets you know how many A/H's are left, etc,etc,...but they are approx $200 plus..

Thats a good question Mandrake because the answer isn't readily apparent and is a base reason for lots of wrong impressions.
This isn't helped by the compromises with using an AH estimator.

I like Gonks explanation to.

First, a battery is actually flat at about 10.5 volts as this is when the electrolyte seperates out leaving no acid left just water (using standard wet cell for ease of explanation).

But I'm with the group here in that 12v is a good and safe figure to use.
I take mine to the limits and use 11.7v as it does vary a bit with temperature and battery type.

The practical way to measure the battery is with a good multimeter but mostly importantly the battery should not have been charged or discharged heavily within roughly 1/2 hour
of when you measure it, and not subject to excessive heat/cold.

I.E. if your battery is running a fridge and drawing up to 6 amps, it may measure 11.8v but if you turn the fridge off and wait 1/2 hour the battery voltage will read closer to 12v , its true value, and this gives you a better state of charge reading which can be used with Sidewinders graph above.

You can use a bit of commonsense hear to get you closer though. E.G. if you read 12v with the fridge running then
you can assume its actually a bit higher and still ok to discharge a bit more.