Power consumption & battery questions

[gu4800]

I am very green when it comes to batteries and what all the figures mean. I have a basic question (well, at least I think it is basic for someone who is "in the game").


I have a FullRiver AGM 90ah battery, and I am testing how much power my new Engel draws over given periods. What do I test for? I have a multi-meter, and tested the following:

Volts before test: 13.6
Volts after 24hrs: 12.4
Volts after 48 hrs: I'll know tomorrow

The car is not being started at all, so there is no chrge going in.

My specific question:
Does measuring the volts before, during and after the test provide any useful information? If so, can this information translate into amp hours or approx. useful timeframe for discharge?

I know the battery has 90ah, but from what I have read you don't actually use all of them (or, I think the fridge will not draw below a certain level). But what does this mean when it comes to a/hrs and volts? How do the two relate?

Should I test some different wat to get more meaningful inof?

Any comments/guideance appreciated.

[mickyd555]

the fridge will probably turn itself off once the supply voltage gets to about 11.8V or something like that. 90Ah is the rating on the battery, it means that it can supply 1 amp for 90 hours, or 90A for one hour. from that you should be able to work out a few things...........but you will need to start the car to give the battery a bit of charge, just dont leave it too late otherwise it could kill it and youll need a jump

[Cossie]

Disconnect the battery, switch the multi meter to Amps (and make sure the lead is in the right hole), touch one of the probes on the battery terminal and the other on the now removed battery clamp.

this will tell you how many amps are being drawn from the battery.
assuming there is nothing else drawing current then thats how much your fridge is using.

[gu4800]

Thanks for both of the above.

Mickyd - in simple terms, how does amps relate to volts? (I knew 90a/h means 1 amp for 90 hours, etc but it isn't much good knowing that if I can't measure amps properly - see below). And no worries about the battery going flat - it's a dual battery and the fridge is the only thing running off that battery.

Cossie - I know I can measure amps, but the fridge cycles off and on and I would need ot sit there with a stop watch and measure how much it draws when running, and then time how long it actually runs over an hour. I was hoping to use to volts reading before, during and after to get some stats.


The Engel book says "the cooling efficiency will be severly reduced if voltage falls below 10.7". I am reading this as meaning it probably will be ratshit under about 11volts. So i guess this means that I should simply do as I am doing and check the voltage after 48 hrs and see where I stand. If it is less than 11volts, then I'll need to run my car at least once a day to "top-up" the battery. If it is above 11volts, then I should be good for 2 days with no running. And finally, maybe I shouldn't bee too concerned about amp/hours and just look at volts.

[mickyd555]

you should probably be looking at ya alternator and find out how quikly it will charge ya battery up. cause i think you should give it a bit of a charge everyday, maybe 10 mins or so, it wont hurt and will keep the battery in good charge. otherwise a deep cycle would be the way to go, they can handle full discharge.

[MAVRK-4]

I found with my new 40L engel its rated current when running is anywhere between 0.5A and 2.5A.

Under normal operating conditions on fridge setting it cycles on and off say every 5 minutes, giving an average duty cycle of 50%, ie. running for 30mins every hour (listening to it for say 20 mins should give you an idea)

when tested with a multimeter whilst running it draws just over the 2.5A, ie @ 50% cycle will draw 1.25A//hr

Calculates it should under average conditions run about 10hours a day (50% of the day & cool of night it will run less), 10hrs @ 2.5A = 25Ah/day

Should be a rough enough calculation on how long it can run off your battery without charging... but you can't run your battery any less than 20% capacity.. ie. your 90Ah becomes less than 72Ah

[MAVRK-4]

Cut off voltage for most Low Level Cut-outs circuits is around 10.5V-11.0V any lower than this and you will cause permanent damage to most car batteries. This is usually the 20% margin.

[-Scott-]

Amp-hour ratings are typically based on what's referred to as a C20 rate. Divide the quoted A-hr rate by 20, and this is the current it is rated to deliver for 20 hours. (A C10 rating means divide by 10 etc...)

At higher current draws more energy is lost in the battery's internal resistance, so the effective capacity (what you get out) is reduced. At lower currents there is less internal loss, so you get more out.

Electricity 101:

Current is the flow of charge in the circuit. 1 Amp is one Coulomb every second.

Voltage is the amount of energy per unit of charge. 12V is 12 Joules of energy for every Coulomb of charge.

12V at 1A is 12 Joules per Coulomb and 1 Coulomb per second, so you have 12 Joules per second - which is coincidentally 12 Watts.

Hope this helps,

Scott

[RoldIT]

The debate rages about how low you can run a battery. AGM batteries should not go below 20% charge left which is 12.0v on the multimeter. But there will be plenty of people that diagree with this ...

Reference :

http://www.batterystuff.com/tutorial_battery.html

http://www.fridge-and-solar.net/agm.htm

http://www.uuhome.de/william.darden/index.htm

[Tojo]

has anyone ever tested how low their battery voltage can go and still start their car?

[-Scott-]

That's a bit like asking how warm can your beer get and still be drinkable...

[gu4800]

Don't know about how low a battery can before starting your car (I run a dual battery), but the fridge (60L Engel) ran for 48 hours straight with no charging and the volts went from 13.8 to about 11.8 after the 48hrs, That's about as low as I am prepared to go so I don't damage the battery, but pretty happy with that.