Small fridge - how long (ballpark) will it last with a 100ah battery (or will it)

MarkNewYork

OK this is the beginners corner so I don't feel too bad about asking such a basic question, I've tried to figure it out with all the formulas and theoretical charge curves and all that but I'm not powering a space satellite it's a small refrigerator. So looking for a ballpark real world estimate/opinion.

Say I have:
A fully charged 100ah AGM battery (Vmaxtanks) 
A 12v 2000 watt PSW inverter (4000 watt peak) mfr name I won't mention but it begins with an A and rhymes with "shame"

I want to keep a small 120v fridge powered overnight during an outage so I can turn off the generator. OR just keep it going for an shorter outage so I don't have to drag the generator out at all.

I've monitored the fridge's wattage and it uses 20 watts for 8 min and then 140 watts for 8 minutes. It's very consistent.

My amateur calculations are as follows:
Average wattage draw: 74w - add 20% for motor startups 15w - so a total avg wattage of 89 watts.
So I'm thinking it draws 9 amps from the battery?
So a 100ah battery will keep that going for 11 hours.

Is that way too optimistic? Or foolish to tax the battery like that?
I'm only planning on doing it occasionally, during an outage.

Many thanks for any advice.

mike95490

You should stop consuming the battery at 50% discharged.  Going deeper into discharge really hurts the cycle life, and it's likely the battery is only going to supply that load for a short while.

MarkNewYork

mike95490 thanks. So ballpark I should use only half the ah of a battery in calculating its usable capacity? Is that where most inverters would cutoff? At 50% discharged? Or would you suggest I add another voltage cutoff device and set it to cutoff at, say 12v (assuming it's delivering a 8-9 amp load?)

Photowhit

MarkNewYork said:

Average wattage draw: 74w - add 20% for motor startups 15w - so a total avg wattage of 89 watts.

The start up isn't a huge energy use, but a spike.

MarkNewYork said:
A 12v 2000 watt PSW inverter (4000 watt peak) mfr name I won't mention but it begins with an A and rhymes with "shame"

...and it should!

So I looked at the specs and they don't make it easy, there are several 2000 watt pure sine, I choose the cheapest here's the manual Link;

https://www.aimscorp.net/documents/PWRI200012120S%20May%205%202018.pdf

There's this information;

They don't say it but it's their maximum efficiency that is over 90%, There is no curve to see what a 80 watt load would be (roughly 20 watthours at 12 volts = 1.666 x 140 watthours at 12 v = 11.666 or 13.333 amps per 2 hour period half of that = 6.666 amps per hour, 6.666 x 12 volts = 80 watts), So I'd use at least 1.15 as a multiplier for the energy used by the fridge and the inverter, over the .9 amps it uses it's self (No load current) So 80 x 1.15, = 92 watts + 10 watts for inverter use (the No Load energy use) so figure a 102 watt per hour total load minimum to the battery.

Convert to amps at 12 volts, 102/12= 8.5 amps per hour. 

100 amp battery if it is in good shape, I would take down below 20% SOC and likely wouldn't trust it that low. So figure 80 amps available So somewhere around 80/8.5= 9.5 hours, Then you would want to look at the batteries power curve if available. It doesn't look like one is available, the amp hour rating is based one discharge over 20 hours, you will be discharging over 10 hours so the capacity will be less;

https://www.vmaxtanks.com/MR127-100-Deep-Cycle-High-Performance-AGM-Battery_p_24.html

So here's the 5, 20 and 100 hour rating for a similar Trojan battery;



So figure the actually capacity at around 92 amps at that discharge rate...

MarkNewYork said:

 So looking for a ballpark real world estimate/opinion.

So I'd want to shut it down after around 8 - 8.5 hours based on your observations of the fridge and assuming a battery in good condition (they lose capacity over time)

MarkNewYork said:

Is that way too optimistic? Or foolish to tax the battery like that?
I'm only planning on doing it occasionally, during an outage.

"Foolish" Only you can determine that, it will shorten the life of your battery, If your battery does get taken down below 20% it could kill it. Likely the voltage drop while running with a minimum charge on the battery, the inverter will shut down around 20-30% at a working voltage of 9.5volts.

So one man's educated guess based on what has been presented....

softdown

Just another idea here. Might make more sense to just buy ice bags if you have a nice cooler. Many Costco stores sell ice bags really cheap. Many do not sell ice. Not sure what Sams or Walmart needs - other than profit. I'm often startled by the price of ice when on the road. You can freeze one gallon water jugs at home maybe. I freeze milk so I have cold milk on the road for a week or so. Doubles as chiller for cooler pretty nicely.

All kinds of problems with the small fridge really. If it is warm it will take a lot of energy to cool. Very few small fridges are very energy efficient. Batteries are fairly easy to wound - even mortally. Loss of efficiencies everywhere - from the battery to the inverter to the fridge.

How do you charge the battery? The battery gets weaker every year - no matter what. Many never see their 4th year.

You have received a lot of good tips above. As for me I have a bad feeling about powering a small fridge with one such battery.

MarkNewYork

Photowhit said:

 one man's educated guess based on what has been presented....

Genius figures and many thanks for digging up the additional info - you did indeed find my shameful inverter. (I have the one with the built-in transfer switch)
I think I will just set this idea aside for the moment and plan on running the generator overnight. It does indeed sound like a stretch for the battery. Thanks again for the help.

softdown said:

buy ice bags if you have a nice cooler. 

I actually hadn't considered using ice! Although this isn't a top loader it's a small kitchen fridge/freezer but I think ice could work in a pinch. Certainly cheaper than replacing my battery!

BB.

A Honda eu2200i will run ~24 hours per day on 2-3 gallons of gasoline pretty nicely. I powered my in-laws home for ~3 days using the older eu2000i and an extended run tank (basically a sealed fuel cap with tubing that can draw fuel from a 5 gallon gas can or outboard motor tank--Note: not sure if an "extended" run fuel cap is available for the eu2200i--May take a different size cap). And they had 2x full size refrigerators and a full size freezer (probably about the maximum load I would suggest).

Put the genset out in the back yard, and could not hear it running...

-Bill

softdown

Honda became the worlds largest engine manufacturer because of their reputation for engine quality it seems. Honda cars did not used to be quite as reliable as Toyota but their engines were generally considered to be a little better than even Toyota - I believe. Their transmissions? Not so much.

InCogKneeToe

My small fridge runs 4, 105ah Batteries down to 12.5-12.7V from Float overnight. 10pm to 6am