Does anyone have recommendations for a 12V battery setup that can power a 1500W inverter?The background: my travel trailer is currently powered by two 6V golf cart batteries (Duracell SLIGC110 215Ah) connected in series. I recently installed a 1500W (3000W peak) pure sine wave inverter with the aim of being able to power my 1.35kW microwave while out in the woods. Unfortunately, when I turn on the microwave, the inverter trips out after a few seconds, and I'm pretty sure it's due to the low voltage protection (which shuts off the inverter when the battery voltage drops to 10.5V).I've measured the voltage at the battery terminals while running the microwave, and the voltage does indeed drop to 10.5V, so it looks like the battery's internal resistance is just too high to supply the 120A or so that the inverter is drawing with the microwave running. I did an experiment with a smaller 100W load, and the battery voltage dropped by about 0.2V, indicating an internal resistance of approx 0.02 ohms. Is that high for a battery of this type? Does the battery just need replacing? Or do I need a fundamentally different battery setup? Any thoughts welcome!

You honestly just will not be able to pack a large enough battery bank onto a TT to power the huge power draw things like microwaves and ACs can require. Get a generator if you really want to run them.

Looks like not enough inverter for that microwave. A typical "1000w" microwave wants 1500 watts input, and that 1500w will pull about 150 DC amps from the battery bank, needing say a set of #1 AWG wires between inverter and battery bank with the two wires being under ten feet long each.You could run the MW for a few minutes on the two 6s if they are over 75% full, but you will want to double that to four 6s if you want to run the microwave with the batts as low as 50% SOC.The battery voltage gets dragged down by the inverter running the microwave which is normal. the inverter will alarm and shut down at some low battery voltage. 11v for the alarm is common. So the idea is to get your MW job done before the alarm goes off. If the MW job is two minutes, you don't need as much battery as when the MW job is 10 minutes.12s instead of 6s will help a little, but not change the story.

donn0128 wrote:You honestly just will not be able to pack a large enough battery bank onto a TT to power the huge power draw things like microwaves and ACs can require. Get a generator if you really want to run them.Lots of trailers have four batteries, which is plenty to run an inverter running a microwave for a typical five minute job. You can do it with two batteries if they are AGMs.

That 1.35kW is the input power for the microwave - the actual cooking power is only 900W. I've measured the power being used by the microwave when supplied from shore power using a "kill-a-watt" plug, and it never draws more than 1200W. So, I think the 1500W inverter should be fine - as long as the batteries can stay above 10.5V while powering it.Seems like this should be a fairly common setup so I'd love to hear from someone who has done this in practice. I don't really have space for 4 batteries, but I'm assuming I could draw more amps with two 12V batteries in parallel (compared to two 6V in series). Do AGMs have lower internal resistance?Thanks!

Battery recommendations for 1500W inverter | Good Sam Community

furballs
Explorer
Oct 13, 2018
What sort of tow vehicle? My TT is not wired heavily enough for a large inverter, even with two Trojan 6 volt batts. My solution was to put the 3.5KW inverter/charger into the truck bed along with 6-6volt agm batts. Short double aught wiring. Fused at 300 amps and I blew several fuses during testing. None blown in normal use. The TT plugs into the inverter through normal 30 amp shore connection. Moving everything to the tow vehicle makes all DC cable runs very short. Cooking in the MW is no problem.
BFL13
Explorer II
Oct 13, 2018
Trackrig wrote:
I have a large microwave with a large inverter and four Trojan T125 batteries. When on just batteries, I'll use the microwave to heat a cup of water or a quick plate reheat, but I'd never use it try to actually cook something.

Bill

If your meal is BBQ steaks by your efforts outside, and DW is inside doing two "baked" spuds (10 minutes on MW) and frozen "California Style " frozen veg (5 minutes on MW) that is a total of 15 minutes at 150 amps or 37.5 AH

Is that a big deal or not? Depends on the scenario. With four 125s at 500AH, 37.5AH is peanuts usually. Three hours of solar at 13 amps.

You can afford to pig out on MW time! :)
Lwiddis
Explorer II
Oct 13, 2018
Needed battery capacity is properly determined by an energy use survey. Your energy use isn’t my energy use so my battery capacity probably won’t work for you.
Trackrig
Explorer II
Oct 13, 2018
I have a large microwave with a large inverter and four Trojan T125 batteries. When on just batteries, I'll use the microwave to heat a cup of water or a quick plate reheat, but I'd never use it try to actually cook something.

Bill
BFL13
Explorer II
Oct 13, 2018
Yes AGMs are the best with holding their voltage under heavy draws.

Ok so the culprit in your case now must be the inverter-battery wiring being too thin and long, and the two 6s being too low or some combination of all that.

You should be able to run the microwave for a few minutes with the two 6s as long as they are over 75% SOC and the inverter-battery wiring is sufficient.

For better results with two batteries you could go to 12s with as much AH as possible, or even better, use two AGMs with as many AH as you can, and go as short and fat as possible with the wiring.

EDIT-- with a TT it is necessary to put the inverter up front close to the batteries. Often the inverter will go in the front pass through and holes drilled through the floor so the wires can go down and under the front cap and up to the tongue battery position. Your shore power cable can reach the inverter up in there. Turn off the converter, put fridge and WH on gas, and you are on inverter "whole house".
beemerphile1
Explorer
Oct 13, 2018
What size cables and how long?
syperk
Explorer
Oct 13, 2018
That 1.35kW is the input power for the microwave - the actual cooking power is only 900W. I've measured the power being used by the microwave when supplied from shore power using a "kill-a-watt" plug, and it never draws more than 1200W. So, I think the 1500W inverter should be fine - as long as the batteries can stay above 10.5V while powering it.

Seems like this should be a fairly common setup so I'd love to hear from someone who has done this in practice. I don't really have space for 4 batteries, but I'm assuming I could draw more amps with two 12V batteries in parallel (compared to two 6V in series). Do AGMs have lower internal resistance?

Thanks!
BFL13
Explorer II
Oct 13, 2018
donn0128 wrote:
You honestly just will not be able to pack a large enough battery bank onto a TT to power the huge power draw things like microwaves and ACs can require. Get a generator if you really want to run them.

Lots of trailers have four batteries, which is plenty to run an inverter running a microwave for a typical five minute job. You can do it with two batteries if they are AGMs.
BFL13
Explorer II
Oct 13, 2018
Looks like not enough inverter for that microwave.

A typical "1000w" microwave wants 1500 watts input, and that 1500w will pull about 150 DC amps from the battery bank, needing say a set of #1 AWG wires between inverter and battery bank with the two wires being under ten feet long each.

You could run the MW for a few minutes on the two 6s if they are over 75% full, but you will want to double that to four 6s if you want to run the microwave with the batts as low as 50% SOC.

The battery voltage gets dragged down by the inverter running the microwave which is normal. the inverter will alarm and shut down at some low battery voltage. 11v for the alarm is common. So the idea is to get your MW job done before the alarm goes off. If the MW job is two minutes, you don't need as much battery as when the MW job is 10 minutes.

12s instead of 6s will help a little, but not change the story.
donn0128
Explorer II
Oct 13, 2018
You honestly just will not be able to pack a large enough battery bank onto a TT to power the huge power draw things like microwaves and ACs can require. Get a generator if you really want to run them.

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