Inverter too small?

[patchedup]

I have a 1000w cobra inverter i bought to power a small 5 cubic foot freezer that pulls 4 amps when running. The battery i use for the inverter is 105 amp marine deep cycle group 29.The inverter is connected to the battery with shortened battery jumper cables, at about 3 feet long, from the battery to the inverter. After about 5 hours the alarm on the inverter sounds and indicates low input voltage of less than 11 volts. The battery at startup was charged up showing 13 volts on my meter. Is the inverter too small or is the battery to small? I thought i could run the freezer with this setup for at least 18 hours. With this info i can forget running my cpap off this setup for overnight. Thanks!!!!

[Almot]

If it is not PSW, this not good for a motor.

Also, 44A @12V (or 4A @120V) is too high, even for a freezer.
I suggest you get a 12V model, they run on less than 8A @12V. Like this Sundanzer DCF165 (DCR 165 is a fridge).

Or German Steca PF166 - this one can work either in fridge or freezer mode, and costs about the same.

Either model has 5.7 cu.ft volume, and draws 50AH per 24 hours at 90F ambient temperature with "average household use", according to their data sheets. This is roughly 25 Ah overnight, so if your goal is live through the night without running a generator, this is what you need.

I didn't look into latest 120V freezers, they are getting better and better these days, though you can't beat 12V models in terms of energy efficiency.

[pianotuna]

Hi,

None of the Cobra inverters are PSW. Some are rated to run microwave ovens.

[Almot]

patchedup wrote:
Thanks. Sounds like battery is not enough or not charged enough.

There is no "or" - the battery is not enough, period.

Simple math. 12V draw is 10 times 120V draw, plus inverter losses. You draw 44A from battery. Moreover, at such a high current the battery capacity is less than nominal - too long to explain why. So in 1 hour it is down to 50% even it was fully charged. If fridge cycle is 25% on/75% off, 1 hour of running = 4 hours time.

[12thgenusa]

If the 4 amps is from the tag, it is likely a maximum. Running amperage is probably no more than half that. If you use the SmartGauge Peukert calculator and set it to 105 AH bank size and the amp draw to 22 (which is probably a closer estimation of the actual draw) it gives an Amp Hours Available of 67.47 for a time of 3.07 hours. However that would be continuous duty cycle and to 100% discharge. If you multiply 3.07 by 3 for 33% duty cycle and divide by 2 for a discharge to 50% (approx. alarm trigger), you get 4.6 hrs.

IF the battery is fully charged and healthy
IF the draw it 22 amps
IF the duty cycle is 33%

In any event, need more battery bank.

[time2roll]

I am not sure if 4a is the start surge, running amps, or the average amps over the duty cycle.

[atreis]

The Cobra inverter is most likely not a PSW inverter, so the freezer wouldn't have been running terribly efficiently. (It would have pulled more than the rated current, whatever it is.)

MNtundraRet wrote:

The problem is that the OP was claiming about 5 hours out of 105 AH battery. I understand your calculation (I use similar) but the OP wasn't close with his 4 amp estimate for usage. That why I use a clamp-meter to check negative flow out of the battery for an hourly estimate on total use.

If the freezer was already cold it wouldn't have run constantly for those 5 hours. It would only have had to run enough to maintain the temperature. I figured that's how he got 5 hours out of it - 5 hours of wall-clock time doesn't equal 5 hours of actual load. (The OP probably doesn't know the actual amount of run time.)

[MNtundraRet]

Chris Bryant wrote:
The 44 was based on 90% efficiency with a 4 amp 120 volt draw- a common ROT estimate of 11 to 1.

The problem is that the OP was claiming about 5 hours out of 105 AH battery. I understand your calculation (I use similar) but the OP wasn't close with his 4 amp estimate for usage. That why I use a clamp-meter to check negative flow out of the battery for an hourly estimate on total use.

Also; a 44 amp draw on one 105AH (when new) 12 volt would have a good size voltage drop while running. Low voltage from the inverter is based on voltage reading "under load" meaning the inverter cutoff could come before the battery even reached 50%soc. His battery is a few years old.

[Chris_Bryant]

The 44 was based on 90% efficiency with a 4 amp 120 volt draw- a common ROT estimate of 11 to 1.

[2oldman]

MNtundraRet wrote:
As for the 44 amp guess, the inverter would have shut down in an hour or two..

Depends on the duty cycle of the freezer and the accuracy of 44(?)

[MNtundraRet]

Your estimate for amps is way off. How did you come up with it.

As for the 44 amp guess, the inverter would have shut down in an hour or two.. You can't get 2 gallons of milk out of a 1 gallon bottle.

[Old___Slow]

Not a Tech on the subject but my freezer does not run continuously.

[pianotuna]

Hi,

You could add enough solar to balance out the fridge. I'd suggest using a kill-a-watt meter to investigate with the real draw is from the fridge. You do need more battery bank.

watt meter

[2oldman]

patchedup wrote:
I was figuring at 4 amp draw, i was drawing 4 amp per hr from the battery that had 105 amp to start out with.

That 4a freezer is at 120v, which draws TEN times that (40a) from the battery. As someone else said, I'm surprised you could run it that long.

[patchedup]

Thanks. Sounds like battery is not enough or not charged enough. I was not on shore power at all. Battery was charged with a 15 amp charger pluged into the 115v ac port of a honda 2000i genny. I was figuring at 4 amp draw, i was drawing 4 amp per hr from the battery that had 105 amp to start out with. I know now i don't know how to figure the amp-12v-watt how much is how much.Thanks so much