Outer Limits 4x4 Board

Looking at a fridge/freezer that draws up to 4.5amps/hr.... My car battery is about 8 months old, NRMA one, says its 880cca, RC 180. And EXTRA Heavy Duty . Assuming the battery is fully or close to fully charged, is there any way I can work out how long I can run the fridge without hurting the battery and/or running it flat?

RockyF70 wrote:Looking at a fridge/freezer that draws up to 4.5amps/hr.... My car battery is about 8 months old, NRMA one, says its 880cca, RC 180. And EXTRA Heavy Duty . Assuming the battery is fully or close to fully charged, is there any way I can work out how long I can run the fridge without hurting the battery and/or running it flat?

What you NEED to know is the AH rating of the battery. From what you have given, there is no way to work out how long it should last, but most batterys of that size (13" along its longest side??) are about 80 AH and you should only discharge a sloppie to about 50% (40 AH), meaning you should be right for about 9 hrs running at 4.5A/H average consumption (40/4.5).

If the fridge draws 4.5 A only when the compressor is working and zero when it is off (ie and average of less than 4.5 A/H) then you can run it for longer.

As a guide, most fridges that use the Danfoss 35 compressor (= most fridges on the market usually up to ~80 litres or so) AVERAGE about 2.5 A/H. Engels use about the same. (40/2.5 = 16 hrs). Danfoss 50 uses about 3 A/H (mainly used in BIG fridges and some caravan setups).

A lot of course depends on whether you are using it as a fridge or a freezer, and if you are using it in the tropics in summer or Victorian High Country in winter.

As a side note, most wet cell starting batteries do not cope well with long deep cycle drains, and deep cycle wet cells do not cope well with repeated starting currents. They are OK for a one off or very occasional use of their opposite type, but not generally as a regular thing. All to do with the thickness of plates(--> deep cycle), and the number of plates (--> starting)

We really need an amp-hour (Ah) rating for any accuracy, although somebody cleverer than me may be able to work with the RC figure.

880cca I would (wildly) guess is somewhere in the region of 60-80Ah.

Presuming 60AH (at a C20 rate) that would run your compressor continuously for about 13 hours - to dead flat. To 50% depth of discharge, you're talking 6 to 7 hours continuous operation.

Operated intelligently, presuming consumption of 24Ah a day, 24 hours should be OK, and you should be able to start your Rocky after even longer running. A full recharge every second day should be sufficient, but that would be exceeding 50% DoD, so nominally shortening the life of your battery.

If battery is actually 80 Ah, you can either go longer (add approx. 1/3 to above guesstimated times) or work with a higher "safety margin."

Confused yet?

Edit: just read Gary's response - no arguments. My figure of 24Ah/day was based on specs for my 39 litre Autofridge.

Hmm . Thanks guys, cant find AH anywhere on the batt but it is exactly 13" long, and a maintainence free one if that makes any difference.
The guy said it draws between 2-4.5 a/hr, and its only a 25L. I might just leave it on a lower setting overnight just in case, atleast till I get a second batt.

RockyF70 wrote:Hmm . Thanks guys, cant find AH anywhere on the batt but it is exactly 13" long, and a maintainence free one if that makes any difference.
The guy said it draws between 2-4.5 a/hr, and its only a 25L. I might just leave it on a lower setting overnight just in case, atleast till I get a second batt.

What brand of fridge???

DAMKIA wrote:

RockyF70 wrote:Hmm . Thanks guys, cant find AH anywhere on the batt but it is exactly 13" long, and a maintainence free one if that makes any difference.
The guy said it draws between 2-4.5 a/hr, and its only a 25L. I might just leave it on a lower setting overnight just in case, atleast till I get a second batt.

What brand of fridge???

Wierd story . Went to the 'outlet' for a major camping retailer, and they have 2 fridges from US that they were trialing with the view of putting their name on it and selling em. They decided that the fridge/freezer market is too 'competitive' and changed their minds. So they have the 2 trial ones for sale for $400ea with 2 year warranty, no 'brand' per se. He told me what brand kompressor they have but I forgot, will ring up 2morrow and find out.

Update

Haggled the fridge down to $300 ea ( got the 2nd one for a mate ).... an they are sweet as

Have had it runnin all day in the car a few times and car starts fine at the end of the day, but i'm a little apprehensive about leaving it on overnight. Makes warm water COLD in less tha 2hrs

And accoring to the online manual it draws 48w or 4A. Do you reckon they would be giving its max draw, min draw, or average draw??

And this is a link to a store that sells them in the US.. note the price http://www.coleman.com/coleman/colemanc ... oryid=8595

All looks good. Except! "Cooler" is typically only applied to thermoelectric devices, not compressor fridges. No mention of a compressor, specifically mentions "no CFCs or (harmful) refrigerants." This suggests non refrigerated. But the current draw they're claiming, and the experience you're reporting, does NOT sound like a thermo-electric device.

I'll be quiet now.

"Environmentally friendly - no CFCs or harmful refrigerants"

Definitely Peltier effect.

That 4.8 amps would be the average (10 amp at 48% duty cycle???)

Just an update.... this Fridge is not a peltier effect cooler.

It uses a free piston Stirling engine.

http://www.globalcooling.com/howitworks.html
http://www.avxcel.com/free_piston_stirling_cooler.html

No peltier I know of can go to -18deg C

I think it is more efficient than a traditional refrigerant cycle involving a compressor due to the lack of wasted energy and highest conversion efficiency between mechanical and thermal energy.

The lecturer brought a Stirling cycle into class one day and ran it off a cup of warm water... one of the best things I've seen


Have you tried running it for any longer F70?

I'm looking into dual batteries for my Rocky... I'll give you the info when I'm done

Got yourself a bargain and looking at the specs of the striling cooler, it should be a very reliable and low power usage unit.
I have never heard of this type of cooler before ?