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!!!!

Inverter is fine, you have inadequate battery.

2oldman wrote:Inverter is fine, you have inadequate battery.x2- remember, 4 amps @ 120 vac is ~44 amps @ 12 vdc. I am amazed you could run it that long.

x3 also don't (ever) use over 50% of capacity before recharging if you want the battery to last.

Do battery manufacturers use the same standards (allowable voltage drop and same load) to test batteries in order to establish amp-hour ratings.My understanding is that amp-hour ratings can be manipulated to achieve higher ratings by allowing for a greater drop in voltage during the test to establish the rating. If one manufacturer uses a 12.3V limit while another uses an 11.8V limit they may call them both 105 amp-hour batteries but you're not going to get the same performance. I would think there is an industry standard for setting amp-hour ratings but I'm not aware of it.The amperage used by the manufacturer to establish the amp-hour rating can also have an affect. You can establish a longer amp-hour rating by drawing a lower current over a longer time.

I would check your "shortened battery jumper cables" wire size and type of wire. They could be #6 aluminum. Could be poor connections. Both of which will increase volatage drop.1000 watts could draw around 80 amps.Refer should draw about 40 amps at 12 volts while running.

Inverter too small? | Good Sam Community

Almot
Explorer III
Aug 15, 2013
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.
pianotuna
Nomad III
Aug 15, 2013
Hi,

None of the Cobra inverters are PSW. Some are rated to run microwave ovens.
Almot
Explorer III
Aug 15, 2013
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.

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