So how much usable power do I have left?

Hi Kevin, according to a 12 volt state of charge chart, your batteries still were at 80% SOC. Hope this helps, Chris

You still have a fair bit of power left.....just go camping and try it out.....let the batt get as low as 12V ( after the fridge has been off for approx 10mins before testing ) then recharge it...its the only way you'll know how it suits your way of camping....some times of year the fridge will use heaps of power and cooler times of year it will use heaps less, so real world testing is the way to go..


By the way I think you have the batts in parallel , not series !!

I know what your getting at Kevin but unfortunately it isn't a 5 second calculation and it isn't easy to come up with a reliable figure.

There is actually a better way , but it is much more complex and requires measuring the current flow, and rate of current flow out of the battery (there are instruments that do this).

Back to your simpler approach

Issues are - what was the real battery voltage ?

You can't just measure a batteries voltage in isolation - if you read it after
charging or just after (or when) it is discharging more than an amp or so you
get a false result.
(AS per Ctaplin's chart above)

Fridge use is also a real wildcard - its duty cycle changes a lot with temperature etc.


So what can you do in the context of a simple approach out bush.

My suggestion is that you have a voltmeter permanently connected that rings an alarm when battery gets to below 12v.

(In our case our Garmin 276c GPS has all these functions inbuilt).


This will work well enough for small current loads.
You can go below 12v , indeed some batteries are still 1/2 full then - and my car will start with 11v but this is an approach that won't leave you stuck in the bush.

There are too many variables to get an accurate answer.

Battery voltage is not a reliably accurate indication of battery charge.

I would estimate your power usage as:-

3 flouros for 4 hours, assuming 15 watt flouros, - 15 Amphours
35L fridge as freezer for 24 hours, 30 Amphours. This is very variable, depends on outside temperature, how often the fridge is opened, contents, ventilation etc.

Total 45 Amphours.

Assuming your batteries are FULLY charged then 140Amphours gives about 70 Amphour usable capacity depending how hard you want to work the batteries.

So my GUESS is you have about 25 Amphours usable capacity left.

Without monitoring equipment, it is a guess.

Thanks for that. I think I have the information that I was looking for although not in the precise form that I was looking for. But I spent most of my life as an accountant so I like precise numbers.

I have a multi-stage battery charger that checks the battery out in stage 1, does 80% of the charging in stage 2, finishes it off in stage 3 and goes on to maintenance in stage 4. When I turned the charger back on it flicked through 1 & 2 in about 5 seconds and settled in stage 3. That suggested to me that the charger (Ctek M200) thought the battery was at least 80% charged.

I am now fairly confident that I can stop roadside for 2 nights and if I want to stay somewhere a bit longer that a 120W solar panel will probably do the job.

I am also fairly confident that battery storage technology is not about to make coal fired redundant.

Kevin

Trying to follow all this has given me a headache

Cheers

Brendon

Battery voltage is not a reliably accurate indication of battery charge.



In a science lab, yes I would agree, but out in the scrub camping, with only a multimeter at hand....it sure beats anything else !!

If you know roughly how much voltage drop your fridge creates when running, you can test for voltage anytime you like and get a pretty reliable figure as to state of charge of your batt(s)..

You don't need to know if its got 50, 40 or 30% charge left......if the voltage reading is approx 12V, you know you need to start thinking about getting some charge into them..

If the reading is somewhere between 12 and 12.5V ( and you have been camping enough times to roughly know your power drain ), then you will have a fair idea whether you have 1, 2 or 3 days left before charging..

I use a small digital lcd volt meter connected to my batts, and usually check it a couple of times a day...not hard....

A practical solution. You shouldn't get a "headache" with this one.

When we got the Toyota it had a couple of 100 amp hour batteries in parallel under the bonnet.

Wondering about camping and how long would the meat be frozen I need to test the system. You can have yout theory. Maths do not apologise when the bridge collapses because the figures were wrong.

So, one Friday after a reasonably long drive assuming (!!!) that the batteries would have been fairly well charged, I turned the fridge (40L Engel), a couple of incandescent work lights, the cabin lights and the FM radio (quietly) on as well as the UHF being left on. Just as if we were camped.Well that's not quite true because we generally hit the sack early when camped and "up with the sun". However more importantly they were all still running quite happily on Sunday evening and the car (diesel) started perfectly.

Recently we added a second fridge and since one battery had failed a pair of identical 107AH (Allrounder) batteries. I left the same kit plus the small 21L fridge as a freezer running from Monday morning to Wednesday evening. All fine again.

That will do me. The beer will still be cold and the meat still frozen and the car will still start easily.

Phil