Battery Longevity Calculation Help

[MattL]

I don't know much about electrical systems and would like some help understanding the power usage of my fridge.

63qt ARB fridge
85Ahr AGM battery
Direct connection via ARB plug/wiring harness
Fridge temp set to 32 degrees F and had pre-cooled food/drinks and one frozen 2lb piece of meat, filling up the fridge about 80% of the capacity.

Recent test on a road, solid 24 hour "on" time (meaning the fridge was plugged in and powered on) and the battery dropped to 12.8V and my Pro-Logix charger stated 95% charge remaining when plugged into the wall at home.

Obviously being winter, temperatures were low but not THAT cold. I would say ambient temp was nothing below 50 degrees F or above 80 degrees F.

I have little to zero knowledge with power systems but is this about right? Would this mean that in this temperature range I'm running at about 0.178amps per hour on average?

My math:

85Ahr * 95% = 80.75Ahr -- so only 4.25Ahr used
4.25Ahr / 24 hr = 0.178 average Ahr (fridge cooling down and cycling etc) -- lets round that up to 0.2

So with an actual lets say 40Ahr until the battery gets too low I'd supposedly get 8 full days or 192 hours out of this setup -- lets round that down to 6-7 days.

Obviously only a long trip would be the real test here but I won't be doing one until late Spring 2017.

Am I way off base here?

 

[dwh]

You got the math right.

It's the assumptions that are skewed.

First, loaded with precooled and frozen foodstuffs, I wouldn't expect the fridge to really do anything the first 24 hours.

Second, if you were driving, then what power the fridge used didn't come from the battery, at least while the engine was running anyway. And what power was taken from the battery was replenished.

And finally, exactly how does that charger know the battery's capacity? Did you tell it? If not, it's guessing.

 

[MattL]

You got the math right.

It's the assumptions that are skewed.

First, loaded with precooled and frozen foodstuffs, I wouldn't expect the fridge to really do anything the first 24 hours.

Second, if you were driving, then what power the fridge used didn't come from the battery, at least while the engine was running anyway. And what power was taken from the battery was replenished.

And finally, exactly how does that charger know the battery's capacity? Did you tell it? If not, it's guessing.

For sure there are some serious assumptions here.

Sorry I forgot to add that the battery is not being recharged by the alternator, it's a stand alone simply powering the fridge.

No clue how it gets such a % capacity. I'm not even sure voltage is accurate as a measurement but both my multi-meter and charging unit state 12.8V. Is there a better way to somewhat know the battery level outside of voltage?

 

[dwh]

Okay, so let's say for the sake of argument that you really did only use 4.25ah. Again, precooled stuff, so don't expect the fridge to have to actually do much work in the beginning. It's a pretty well insulated cooler even when unplugged.

After a few days though, the stored energy (well actually, lack of energy ), from the precooled stuff would get used up and the fridge would have to work harder. So only using 4ah the first day doesn't mean you'll use that little on the subsequent days. So the 6-8 days guesstimate is probably somewhat optimistic.

Add to that the higher ambient temps when you do your actual trip and shave some more time off.



As for knowing what's actually been used from the battery, you'd have to use a meter that can count amp*hours.

Something like this would work:

https://www.amazon.com/Watts-Meter-Analyzer-WU100-Version/dp/B001B6N2WK

There are cheaper ones on Amazon as well.

 

[AndrewP]

My summer time 24 hour use with a 45 quart Engel is in the 20 amphour range. So with winter use, starting with frozen food, I could easily see your total consumption being low. But you don't actually know because you didn't measure it. Get a cumulative watt meter, measure the power used and report back!

Those cheap power meters are great for that. Something like this:

https://www.amazon.com/RGBZONE-Anal...1&keywords=watt+meter+power+analyzer+12+volts

 

[MattL]

Good info guys thanks, I'll pick up a meter and report back.

I agree with the effectiveness trailing off as the time goes by since all the stuff in the beginning is prechilled or frozen. Not to mention as food is consumed the air volume increases unless refilled with something.

My main goal is to understand how much I can get out of the battery before it's "spent" so I can see what I need for recharging in the future (direct atl. hookup/solar etc.).

 

[ajmaudio]

You have probably figured this out already but just in case:
Dont forget that you dont want to bring your battery below 50% state of charge often, or preferably at all. Doing so will shorten its life span, and decrease the amount of amp hours it can give you.

 

[Joe917]

Neither one of those meters appear to keep track of total ah used, only immediate current draw.

 

[dwh]

Neither one of those meters appear to keep track of total ah used, only immediate current draw.

The cheaper one that Andrew linked to says this in the item description under specifications:

"This device accumulates Charge [Ah] Amphour and Energy [Wh] Watthour readings till it reset or shut off."


The WattsUp that I linked says:

"Measures energy (Wh), charge (Ah), power (W), current (A) and voltage (V)"

 

[andytruck]

Is this for a car-camping style trip? If so, then solar is the way to go. One 100 watt panel and some sun should keep you topped off every day. Two 100 watt panels and you are self-sufficient as long as there is some sun. Panels are a little over $1 per watt.
They are easy to install too. red to +, black to -. Buy a solar controller to go between the panel and the vehicle battery and you are set to go. You will power your appliances off the controller.