Yet another battery question
Submitted: Friday, Jul 03, 2009 at 17:25
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Member - Daniel M (QLD)
Gday
Ok here's what i've got. Waeco cab 50 fridge running off 105A/h battery charged by car with the help of a sidewinder dual battery system.
Now here's the problem. the fridge draws 8.33 amps running off 12v. This by my rough calculations would give me about 6hours of run time ( draining the batt to 50%, correct me if i'm wrong with that...), which isn't much good for running all night.
Now here's my idea that i'm not quite sure about....
The current draw on 240v is only .43A. Would this mean that if i invested in an inverter and run the fridge off the battery via the inverter i would be able to get more run time out of my battery? Having no experience with
inverters i'm not to sure wether it will work.
Also if it will work are there any particular size inverter i should look at?
Thanks
Dan
Reply By: tim_c - Friday, Jul 03, 2009 at 17:37
Friday, Jul 03, 2009 at 17:37
Dan,
Your fridge will only draw that sort of current while the compressor is running. The compressor shouldn't be running all night so your 105AHr battery should be more than adequate overnight (it should only be 'running' apx 30% of the time ie. assuming 12 hours overnight, the fridge would only be running about 4 hours in total - depending on ambient temperature, how often you open/close it etc.).
As for running off the inverter, they are never 100% efficient (at best, around 90%) so you will have losses running it this way. Not to mention that your 240v adaptor most likely reduces it back to 12/24v for your fridge (another inefficiency).
To explain the different currents simply, current is not the only factor that determines the POWER that your fridge is using... Power is determined by multiplying volts and amps, so at 12v and 8.33A, you are using around ~100W (watts). Similarly, at 240v and 0.43A, you are using ~100W so there is no real difference.
If still confused, run plug the fridge into the inverter as you've suggested, then measure the current that your inverter is drawing from your battery - the situation should become clear!
AnswerID:
372947
Follow Up By: Member - Daniel M (QLD) - Friday, Jul 03, 2009 at 17:46
Friday, Jul 03, 2009 at 17:46
Hey Tim
Thanks for that. Yeh i was told that my battery would run it all night but tried it recently and battery was dead flat in the morning. I did
check it the night before and it was fully charged. I can't understand why i can't keep charge there so was trying to find an alternative:)
Dan
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Follow Up By: tim_c - Friday, Jul 03, 2009 at 18:53
Friday, Jul 03, 2009 at 18:53
That sounds odd Dan. I have a 60AHr which is a tad small but even that it will run the fridge overnight (for one night). Maybe the battery is sus? or you loaded it up with warm beer and expected it all to be cold in the morning? or you left the lid off the fridge? or it was 40 deg. all night? or you had other stuff running off the battery? (just the obvious ones!)
Dunno mate, but you'll run the battery down quicker if you try running through an inverter! Maybe get the whole setup checked by a good Auto Electrician (including battery load
test - not just voltage).
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Follow Up By: tim_c - Friday, Jul 03, 2009 at 19:05
Friday, Jul 03, 2009 at 19:05
Sorry Dan, I just read your discussions with Derek (ABR) below - for some reason I'd assumed you were running a compressor fridge which should last a couple of days on a 105AHr battery (depending on conditions). Yes, the 3-way fridges use a lot of power on 12v so you might need to consider running from mains (240v powered site) or gas while stopped.
If you're planning to do a lot of car-based short-stay travel in remote areas (ie. no powered sites), I'd suggest a compressor driven fridge might be more suitable.
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Reply By: MrBitchi (QLD) - Friday, Jul 03, 2009 at 17:39
Friday, Jul 03, 2009 at 17:39
Couple of things, that would be the absolute maximum current draw, it would not pull that all the time.
I have a Waeco CF50 and it runs fine for a couple of days on a 100AH battery.
Hooking up an inverter will only make it worse. The inverter itself takes a certain amount of juice to run so you'll actually pull more from the battery.
With electronics there's no free lunches. The fridge takes a certain amount of Watts to run, using your figures about 100. (12x8.33=99.. or 240x.43=103)
Watts = volts x amps. Doesn't matter if the volts are 240 or 12, you will need the appropriate amount of amps to achieve the same Watts.
Hope that makes sense ;-)
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372948