Yet another battery question

Submitted: Friday, Jul 03, 2009 at 17:25
ThreadID: 70364 Views:2445 Replies:8 FollowUps:10
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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?

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Reply By: tim_c - Friday, Jul 03, 2009 at 17:37

Friday, Jul 03, 2009 at 17:37

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:)
FollowupID: 640182

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).
FollowupID: 640190

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.
FollowupID: 640193

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 ;-)
AnswerID: 372948

Reply By: Member - Ian H (NSW) - Friday, Jul 03, 2009 at 17:43

Friday, Jul 03, 2009 at 17:43
Yep, what they said.
AnswerID: 372949

Reply By: ABR - SIDEWINDER - Friday, Jul 03, 2009 at 17:47

Friday, Jul 03, 2009 at 17:47
Hi Dan

Is it a CAB-55 3 way gas fridge ?

They are better on gas when away from 240V.


AnswerID: 372950

Follow Up By: Member - Daniel M (QLD) - Friday, Jul 03, 2009 at 17:53

Friday, Jul 03, 2009 at 17:53
Yeh it is the CAb -55. ( oopps i think i called it a cab 50?). Gas line is all set to go for the next trip away. Just like to have that extra option though.

FollowupID: 640184

Follow Up By: ABR - SIDEWINDER - Friday, Jul 03, 2009 at 18:03

Friday, Jul 03, 2009 at 18:03
The inverter and associated losses will make power usage on 12V very similar.

I recommend you level the fridge and put it on gas when you stop. (Outside the vehicle)


FollowupID: 640186

Reply By: kiwicol - Friday, Jul 03, 2009 at 18:59

Friday, Jul 03, 2009 at 18:59
Hi Dan, another thing to take on board is that your car alternator will not charge your battery to 100% no matter how long you run the vehicle, so the more you use and drain the battery the less efficient it becomes. The battery may read fully charged on a multi meter but will not have the 100% charge to keep it at full capacity, for a longer time. The only chargers that will fully charge the battery to 100% are a smart charger, and a solar panel with a regulator. Many people on this site will disagree, but it is proven science. Also if the fridge is 3 way the 12 volt side is very power hungry and only really designed to be used while driving, better to use gas while parked. Col
AnswerID: 372960

Follow Up By: Gronk - Saturday, Jul 04, 2009 at 09:49

Saturday, Jul 04, 2009 at 09:49
Can you supply a website that supports the theory that an alternator won't fully charge a battery ??
FollowupID: 640246

Follow Up By: kiwicol - Saturday, Jul 04, 2009 at 18:49

Saturday, Jul 04, 2009 at 18:49
Hi Gronk, cant supply a web site, but an electrical engineer, whom uses this site by the name of Colin sorry dont know last name, has several books out on the subject. After reading his science i bought a smart charger and experiented with my 4x4 set up. I found his knowledge so accurate i have been able to extend usable life from a standard batteries.
I have a 92 diesal patrol with a standard 55 amp alt charging 2 overlander batteries wired together with only a anderson plug to be manually controlled by me to fully charge both batteries, and all power to be taken from the factory fitted battery. The second battery gets used for nothing except starting if main is flattened, and if needed for longer periods of camping. I have a christy generator for longer periods of camping.
For science i put the smart charger on the twin battery set up and fully charge both batterys. I go for a drive remebering my vehicle has no computor or big power drain, except power windows. When i get home i put on smart charger and watch how long it takes to bring back battery to full charge, remebering power usage has been mainly starter drain. Now i figure with minamal power drain ie fridge and lights while camping the alt should have kept batterys fully charged. Wrong i found the smart charger 25 amps per hour takes about an hour plus to bring back the batteries to full charge.
I have used the manual battery set up for the past 25- 30 years and have never been let down with a flat battery. The best buy has been the chrsty generator.
Regards Col
FollowupID: 640290

Reply By: obee1212 - Friday, Jul 03, 2009 at 20:48

Friday, Jul 03, 2009 at 20:48
Know too that the capacity of your battery reduces as it gets older and cycles through too many charges and recharges. Batteries that show all the signs of vitality may not look so good after a good try out without the backup of an alternater/charger. That's why we suddenly find out the motor wont start when we need it the most! (Gee, it worked yesterday dammit)

AnswerID: 372979

Reply By: RV Powerstream P/L - Saturday, Jul 04, 2009 at 09:48

Saturday, Jul 04, 2009 at 09:48
Read the blog here on smart charging explained just to get a better idea on what has been indicated by others.
AnswerID: 373037

Follow Up By: Member - Kiwi Kia - Saturday, Jul 04, 2009 at 14:41

Saturday, Jul 04, 2009 at 14:41
I read the commercial pamphlet and don't agree with some of it. The grammer could also be improved - does it come from China ?
FollowupID: 640271

Follow Up By: Maîneÿ . . .- Saturday, Jul 04, 2009 at 17:26

Saturday, Jul 04, 2009 at 17:26
can I ask you what "some of it" you don't agree with ??

We can ALL learn by (you) asking questions
as they say: there's no such thing as a silly question

Maîneÿ . . .
FollowupID: 640285

Follow Up By: Member - Kiwi Kia - Saturday, Jul 04, 2009 at 20:19

Saturday, Jul 04, 2009 at 20:19
The care and feeding of lead acid batteries gets very complicated as you know. What works on the laboratory bench does not always prove to be the same in the real world. Try a Google search on split charge diodes for some interesting points from a competitor. I would also like to see how they get around the heat problems when charging at high amperages as (in my opinion) that is the easiest way to damage the plates. It's a bit like tyre brands and tyre pressures and I would rather not get into an argument but simply say I have reservations about some of the claims being made regarding high current charging.

FollowupID: 640304

Reply By: RV Powerstream P/L - Saturday, Jul 04, 2009 at 15:34

Saturday, Jul 04, 2009 at 15:34
You have every right to have your own opinion but youn should state your theory for debate and I do not think the grammar will have any bearing on the units operation and it is designed by Sterling and made in Taiwan under supervision of Sterling Power Products.
Some people are illiterate and are geniuses some people are educated and are idiots.
AnswerID: 373074

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