best 12v fridge battery

[scott/gqfun]

just after a new 2nd battery or maybe a 3rd to run a fridge (40 engel) n lights while camping. i would like to get 2 days/3 nights with out needing to start the car. i was thinking the blue top optima but i feel it has a low amp/hour (66aph) 4 the price. would i b better off with a agm 140 amp/hour 4 the same price. maybe its a big ask but space is not a drama it's in the back.

thanks scott

[Green as Grass]

Optima's are AGM. Look at their marine range, they go up to 900amp hours. I bought an Orbital and have to say that it's indestructible. What brand is the 140 amp hour ?
How many amps per hour does your fridge draw ?
My advice would be to stick with a major brand AGM.

[scott/gqfun]

id say it chews about 2 amps a hour. thats 900cca not 900 amp/hour but if it was it would b the size of my fridge

[DamTriton]

Optima's are AGM. Look at their marine range, they go up to 900amp hours. I bought an Orbital and have to say that it's indestructible. What brand is the 140 amp hour ?
How many amps per hour does your fridge draw ?
My advice would be to stick with a major brand AGM.

Wrong.

That refers to the cold cranking (CCA) amps NOT amp hours (AH). Two totally different things.

OP - go for the 140AH or two if you can afford it (you will need it)

A few calcs for you:
Demand...
Allow an average 2.5 amps per hour for the fridge at 60 hrs is going to be 150 AH
4hrs of light at 12 watt (flouro) per night x 3 nights is 12 AH

Total of 162 AH needed.

Supply...
Batteries can only be discharged about 60-70% of their capacity (depth of discharge ie from 100% down to 30-40% charged) if you want them to last in the long term so your 140 AH batteries are only worth a "real" 90 AH. To cover your 162 AH requirements you will need two fully charged 140 AH batteries (ie., 180 "real" AH) to cover yourself with a bit of leeway.

Total 2 x 140 AH batteries needed for a "real" supply of 180 AH

[scott/gqfun]

any brands u recommend

[DamTriton]

any brands u recommend

What brand was the 140 AH you were looking at? I'll check it out for you to see if it suits...

[-Scott-]

If a 40L Engel uses 60Ah a day there is something dramatically wrong. Try less than 30Ah, if it's used properly. Allow 5Ah per night for a few hours of lights. 2 days @ 30Ah (generous) + 3 nights @ 5Ah and you need a nominal 75Ah. Taking a 140Ah a little below 50%, you'll be fine.

[DamTriton]

Not arguing with you Scott, but most people think that their Engels run on about 1-1.5 AH because that is what they calculate it to be sitting in their lounge room on the 240 volt supply, looking at the duty cycle alone (12 colt current draw is about 2.5 amp @ about 40% duty cycle in a reasonably steady 20-25 degrees temp and good ventilation). This unfortunately does not take into consideration the fact that the fridge will be stuck in a car during the midday heat/sun with limited ventilation for the heat exchanger coils for 10 hrs a day when it would be working hard to maintain its temp, let alone being opened and losing the cool air several times a day.

I would also hazard a guess that the manufacturers do not test their fridges in the same sort of environment as we tend to put them in (hot car, sealed volume of air in car, exposure to midday sun). My guess is it would be rated similarly to the "lounge room test" to give the best possible results for marketing (low running AH's). He's also up in Townsville where it is difficult for anything to keep cool any time of the year (your choice - hot, or hot and wet).

Being safe and taking the 2.5 AH or even 2 AH worst case would be reasonable. Even at 1.5 AH you would still be asking too much of a "real" 90 AH battery (listed 140 AH). He is also going away for 2 1/2 days (2 days/3 nights), not 2 days.

The two battery solution will lengthen the service life of the two batteries compared with one battery being worked hard (repeated high depth of discharge). It will also allow the batteries to derate over time with little change in usefulness, using more of their service life before needing to be replaced due to limited output. In essence "do it once, do it right, and enjoy it for 5-7 yrs". Two batteries also has the advantage of a bit of redundancy if one battery decides to cark it, or you need to borrow one for a temporary starting battery until you get home.

[-Scott-]

How much current does an Engel draw? Engel claim "DC Power Consumption: Variable from 0.5 to 2.5 AMPS Maximum" Are you seriously suggesting that the compressor will run 24 hours per day?

Also have another think about "cold air escaping". This isn't a household fridge. The Engel has its door at the top, not the front. As you know, cold air settles to the bottom - how will it all be lost every time the door is opened?

I have a 39L AutoFridge that I run on an 85Ah AGM battery. I have low battery cutouts on both battery and fridge, and still get two days of use without hitting a low battery cutout, even after 4 years of cooking the AGM under the bonnet - so the battery should be somewhere below its nominal capacity.

Yes, extra batteries are always nice to have. But to advocate 280Ah of capacity to run a 40 litre fridge for 2 days / 3 nights is ludicrous.

[DamTriton]

How much current does an Engel draw? Engel claim "DC Power Consumption: Variable from 0.5 to 2.5 AMPS Maximum" Are you seriously suggesting that the compressor will run 24 hours per day?

I am "seriously suggesting" that it will not be at the lower end of the current draw in a closed vehicle in the tropics. I would say in the order of 1.5-2 AH would be realistic and safe, based on your estimates of 1.25 AH (30 AH/day) in a more temperate climate. Tropics rarely gets below 20 degrees and is usually 30+ degrees before 10 am until about 9-10 pm, and the high humidity can markedly reduce the efficiency of the refrigeration system.

Also have another think about "cold air escaping". This isn't a household fridge. The Engel has its door at the top, not the front. As you know, cold air settles to the bottom - how will it all be lost every time the door is opened?

Air is displaced every time you (a) put you hands in the fridge, (b) remove articles from the fridge, and (c) place items back into fridge.

Yes, extra batteries are always nice to have. But to advocate 280Ah of capacity to run a 40 litre fridge for 2 days / 3 nights is ludicrous.

I was advocating more than 90 AH would be needed given his environmental circumstances. He implied he could get a decent price on a 140 AH battery and that is what I ran with. He could use 2 x 100 AH batteries with the same DoD safety margin at the 1.5-2 AH fridge consumption level for 60 hrs plus his lighting and other needs.

[-Scott-]

I was advocating more than 90 AH would be needed given his environmental circumstances. He implied he could get a decent price on a 140 AH battery and that is what I ran with. He could use 2 x 100 AH batteries with the same DoD safety margin at the 1.5-2 AH fridge consumption level for 60 hrs plus his lighting and other needs.

Fair enough. I read that as 90Ah being the "useable" portion of a 140Ah battery, and thought you were then advocating two of them.