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**Spdracr39** · 04-19-2018, 10:53 AM

I was reading an old thread related to voltage drop on my 4 T-105's when running the microwave (1400 watts total load) with the inverter. Voltage drops from 12.6 to 11.5 and after a minute or so the inverter drops out due to the low voltage reading. The batteries immediately recover to 12.6 volts with no load. I want to avoid this but I don't want to drop down to the lowest voltage cutoff as it is 10.8 volts and way below my comfort level. I would like to use the microwave for 2-3 minutes a couple of times a day without starting the genny. I have a Xantrax HFS2055 inverter and the batteries are all new. Not that I want to but is there any way I would be able to use all 2000 watts available with this much voltage drop? What do you recommend ?

Sweetbriar · 04-19-2018, 11:36 AM

My first thought is to install larger gauge wire between the batteries, batteries to ground and batteries to inverter. Also if possible relocate the inverter as close as possible to the batteries to limit the length of the wire. Ensure all connections are tight, clean and free of corrosion.

Last summer I installed a medium sized residential fridge with a 2000 watt inverter. More then enough inverter capability and I figured 10' of 2 gauge wire wouldn't be a problem and it sure wasn't to operate the TV, recharge a laptop or other low wattage demand items. First time I started the fridge the inverter shut down due to low volts caused by the high power demand of the fridge compressor start up. Did the same for a coffee pot. The DC cables were to small and too long limiting system capabilities. Ended up relocating the inverter closer to the batteries and using 4/0 cable for DC power. I probably oversized the cable but I was running out of time to make the setup work before leaving on summer vacation.

mobilemike · 04-19-2018, 12:05 PM

All inverters instillation manuals have a page that lists the wire size required for length of travel and the specific size of the inverter.
Are you sure your batteries are wired correctly?
Are you sure your wire size is correct?
Are you sure you have the correct size fuse inline?

**Spdracr39** · 04-19-2018, 02:45 PM

Quote:

Originally Posted by mobilemike

All inverters instillation manuals have a page that lists the wire size required for length of travel and the specific size of the inverter.
Are you sure your batteries are wired correctly?
Are you sure your wire size is correct?
Are you sure you have the correct size fuse inline?

Yes, Yes, and Yes. I am using 2/0 battery cables. The cable from the inverter to the breaker is only 18" long and the cable from the breaker to the battery bank is maybe 24". 200 amp dc breaker and 30 amp a/c for input voltage and output voltage with 10-3 romex on that side. The ground wire is also 2/0 but is a single run of 36" from the battery to ground and 18" from the inverter to battery. None of the breakers tripped only the internal low voltage shutoff built into the inverter to protect excessive discharge on the batteries. I guess I need to check the voltage at the battery under load and compare it to the voltage at the dc input for the inverter while under load and see if it is internal battery resistance or the cables. I can go up to a 4/0 if it looks like the cables are an issue.

MisterT · 04-19-2018, 02:56 PM

the 'simplest' method, without any modifications to anything, including the Inverter LBCO setting, is to only run the microwave for maybe 30 seconds at the time to maintain the Volts.

here's why: most Inverters have the LBCO setting, to protect the batteries, which is a Low Battery Cut Out of the inverter, from undue depletion of the battery levels.
Why you see when you run the microwave, or coffee maker, etc., is the initial 'draw down' of Volts while in operation, but the actual 'real' level of the batteries aren't necessarily at that level, which is why you see the immediate return of a more 'normal' Voltage when the microwave stops.
The LBCO generally cuts out the inverter if the Volts stay lower than the setting for 30 seconds or more. Yours may wait a little longer, but testing it will tell you what that number is, and then you can break up your microwave runs into 'less than' the LBCO setting.
If your LBCO setting is something you can change, you can lower it substantially WHILE your are using the microwave, and then reset it back. Or, if you can turn it off temporarily, do that.

Gary RVRoamer · 04-19-2018, 03:52 PM

A fridge compressor doesn't have that great a demand, even at start-up, so 4x T105's in good condition and 2/0 cables should be able to supply enough amps to avoid the low voltage cutoff. Since it didn't, I'm thinking you have either weak battery(s) or some poor connections.

Even if the compressor start requires a momentary 6-8 amps @120v, that's only about an 80A draw (@12v) from the batteries. Their rated capacity is around 420-450 amp-hours, so they should deliver that handily.

Your complaint isn't an unusual one - it's common for a weak cell in a battery or a crappy wire connection to go unnoticed until a serious load is applied.

Old Scout · 04-19-2018, 04:04 PM

...Gary--he's talking microwave here, nor fridge....cable requirement for 2000 watt inverter is 2/0; 4/0 for 3000 watts. What you describe is pretty common--to me anyway. I have 2000watt inverter and 6 T-105 batteries. I can run my microwave [1800 watts] for 30 seconds, maybe a minute to zap some coffee, or a sandwich but not any longer or bats draw down [PS-- new T105, about a year old]....

twinboat · 04-19-2018, 05:43 PM

You don't say what kind of breaker you used, but most inverters are installed with large fuses. The breaker may be heating up and becoming a choke point for the high amps.

As a test, you could eliminate the breaker and run it.

You do have a direct run negative cable from the inverter to the battery or shunt, yes ?

Sudsy · 04-19-2018, 07:18 PM

Reduce the power setting (level) on the microwave. This will cycle the magnetron and allow the system to recover a little. Worth a try. Instead of 1 min on high do 2 min on 50% power.

Sudsy

vsheetz · 04-19-2018, 07:37 PM

A clamp on amp meter is a handy thing to have. With one you can check actual current draw at the inverter to see if reasonable. A Kill-a-Watt meter us a easy way to check if the microwave is pulling reasonable current. If current draws are within expectations then about the only thing you can do is set the cutoff voltage on the inverter lower.

nothermark · 04-19-2018, 07:51 PM

The first question I would have is where the voltage is dropping. I'd monitor the actual battery terminals and see if the batteries are dropping due to internal resistance. If that is the problem then it's a battery issue.
If the batteries are not dropping then look for a connection problem. The easiest first test might be to just touch each connection after a session of high current draws. You are looking for heat as any significant resistance will produce heat from the high current going to the Inverter. If that does not work then look for differences in voltage at each connection between physical batteries and the battery bank to inverter circuit.

bluepill · 04-19-2018, 07:56 PM

Quote:

Originally Posted by spdracr39

. I guess I need to check the voltage at the battery under load and compare it to the voltage at the dc input for the inverter while under load and see if it is internal battery resistance or the cables. I can go up to a 4/0 if it looks like the cables are an issue.

This ^^^

Also remember that having the battery bank drop as low as 10.5 volts for a few minutes under heavy load is not a killer, as long it bounces back above 12.0 volts within a few minutes of removing the load. Flooded lead acid batteries handle this service well.

If you find that you have little voltage drop in the wiring and the batteries are healthy, I see no problem setting the cutout voltage as low as 11.0 volts, as long as you are aware that this is not appropriate for running a smaller load for many hours.

Lastly, using a small counter top micro that draws only 600-750 watts may be an answer.

**Spdracr39** · 04-19-2018, 08:18 PM

Quote:

Originally Posted by twinboat

You don't say what kind of breaker you used, but most inverters are installed with large fuses. The breaker may be heating up and becoming a choke point for the high amps.

As a test, you could eliminate the breaker and run it.

You do have a direct run negative cable from the inverter to the battery or shunt, yes ?

Yes I do.

mobilemike · 04-19-2018, 08:19 PM

Yes as amps go up volts go down when drawing. If the volts recover to 12.6 after shut off the the batts are fine.
I think you have a faulty inverter. It is shutting down for low voltage before the low volt limit has been reached.
Its time for a call to the manufacture.