Cold tolerant inverter

NZRR · February 2011

I am looking for advice on a cold tolerant inverter. I am trying to supply power to a barn for heated water buckets and lights during the winter. Water buckets draw 250 watts total for 24 hours per day, and lights are 22 watts total for about 15 minutes per day. I have four 120-watt modules charging four 400 Ah batteries wired in series/parallel for 12 volts total at 800 Ah. I am using a Trace TS series inverter (TS512) to make AC for the loads. The problem I have is when it is cold (less than around 20 - 25 degrees F), the inverter shuts down with an overload error, even when there is no load on it. When it is above 25 degrees, it works fine. Has anyone had luck with operating inverters in cold environments? I thought about building an insulated box and putting an incandescent light in it for heat, but thought there might be a better solution. Thanks.

Cariboocoot · February 2011

Re: Cold tolerant inverter

The problem isn't the cold so much, it's the loads.
How many water buckets @ 250 Watts each? That adds up to a lot of power very quickly. A TS512 is only 500 Watts, so two 250 Watt loads and it is maxed out. Never mind the lights.

What's happening is that the batteries get cold and lose power. If the inverter has to supply full load, it is doing so from lower battery Voltage. It draws more current; it can't come up with the power; everything shuts down.

Want some more bad news? Your batteries aren't going to stand that kind of loading either. You have 800 Amp hours, but the most you can draw would be 400 (at 50% Depth Of Discharge). That's roughly 4800 Watt hours. Over 24 hours that's only 200 Watts available per hour. See the problem?

So how many of these water buckets do you have? Basically, you need a complete revamp of the system. Electricity is not a good way to heat things. Any other options like insulated water trough? http://forum.solar-electric.com/showthread.php?t=10478&highlight=water+trough

If you have to use electric, I'd start by upping the system Voltage to at least 24.

waynefromnscanada · February 2011

Re: Cold tolerant inverter

I've had 2 SureSine 300 inverters just going through their 3'rd winter in an outside, unheated shed here in Nova Scotia, with temps as low as -10F to -20 F. One operating continuously 24/7, the other as needed for a dedicated load, and no problems at all.
http://www.morningstarcorp.com/en/sure-sine
I've been VERY happy with them and very impressed with the quality of build.

My concern is what appears to be an extremely small PV settup you have for such a load, and neither do I expect the batteries will handle it.
From my experience, the PV wattage you have will never keep up with the demand of your electric heating load.

icarus · February 2011

Re: Cold tolerant inverter

Do a search on these boards for heated water trough ideas to reduce the loads for keeping the water ice free.

Tony

Ps Agree with 'Coot, too much load.

Cariboocoot · February 2011

Re: Cold tolerant inverter

Wayne makes another good point: that 480 Watt array is woefully inadequate for an 800 Amp hour battery bank. It should be about 3 times that size, and it would probably need two charge controllers (because you're looking for 80 Amps of charge current).

NZRR · February 2011

Re: Cold tolerant inverter

Thanks for the replies. To answer the load issues, the total load on the system is 250 watts (one bucket at 130 watts, and the other at 120 watts). Lights are on for a few minutes each day. So I am drawing about 2 to 2.5 amps per hour. With 400 Ah available, I should be able to run the heaters for 160 hours. During the day, the charger is making power for the system, so it is only on cloudy days or at night that I am actually drawing power from the batteries.

If I could get the inverter to stay on, I might be more concerned about the batteries, but the problem is the cold shutting the inverter down. I do have more modules I can add to the system, but I am not convinced I need them yet.

I'll check into the SureSine inverters.

Hope this helps clarify the situation.

Thanks.

BB. · February 2011

Re: Cold tolerant inverter

Be really careful about describing loads in Amps... That is missing the other 1/2 of the equation--Volts and Watts.

Power = Volts * Amps
Amps = Power / Volts
Amps = 250 Watts / 120 VAC = 2.08 Amps @ 120 volts
Amps = 250 Watts / 12 VAC = 20.8 Amps @ 12 volts
Amps at inverter really is = 250 watts * 1/0.85 Inverter eff * 1/12 VAC = 24.5 Amps

So, your battery would (roughly) last (assuming 12 volt bank, 400 AH):

400 AH / 24.5 Amps = 16.3 Hours (to 100% dead battery)

Normally, we recommend cycling a battery to 50% State of Charge for long life (and never below 20% state of charge)... So the battery should be cycled by 50% to ~8 hours.

-Bill

Cariboocoot · February 2011

Re: Cold tolerant inverter

Unless it is defective, the cold does not shut the inverter down. It does "shut down" the batteries. This would be a particular problem at night when there's no current coming from the panels.

So 250 Watts total load for the water buckets. The lights are an "incidental" load. But the inverter is sucking up a few Watts itself, even when doing nothing. Probably 20. Then we get math that looks like this:

270 Watts total load @ ~12VDC = 22.5 Amps. 200 available Amp hours divided by 22.5 Amps equals 8 hours of run time. Now that assumes the heaters are on all the time. Hopefully they're thermostatically controlled. But if the weather is cold, they may well be on all the time. It's amazing how fast heat dissipates when you don't want it to! If you then factor in the drop in available battery power from the cold (you're looking at a 25% drop in Amp hour rating if the batteries cool down to the mid 20's F as you describe) you end up with maybe 6 hours of run time in the cold. After that you have dead batteries. Pretty awful, isn't it?

Could you get your batteries inside an insulated box to reduce heat loss?
The array size is still small for the bank; it really does not have enough capacity to recharge batteries that have been used overnight and supply power for loads at the same time.

I really don't think the inverter is the problem here.

westbranch · February 2011

Re: Cold tolerant inverter

A question, why not use 24 V (or 12 V) heater elements and get the inverter out of the equation. then there would not be any inverter losses plus no low V cutout...

e

techntrek · February 2011

Re: Cold tolerant inverter

I would also look at how well the buckets are insulated. Adding a few inches of blue or pink foam board all around the bucket will cut the energy requirement. It doesn't absorb water so it should retain its R-value. Also consider the top - maybe its possible to add an insulated lid, which the animals can open by lifting the lid from an extended edge. With that on there you'll cut your energy requirements a bunch since you'll be cutting radiative heat loss, evaporative heat loss, and convective heat loss all in one shot. Wouldn't take much to engineer and the payback over the winter will be great.

NEOH · February 2011

Re: Cold tolerant inverter

nzrr,
What is the Battery Voltage when the Inverter refuses to start?
Is is below 11.5 Volts?
(I realize this is not the best way to tell if the battery is discharged but the Inverter cannot measure SG).

What is the SG?
Is it below 1.15 ?

IF the voltage is low then you have discharged your batteries. You do not have enough Solar Panels and/or Sunlight to operate at 20 Degree temps.

Sulfation of the batteries STARTS when the specific gravity falls below 1.225 or voltage measures less than 12.4 Volts. Continuous sulfation hardens on the battery plates, slowly reducing and eventually destroying the ability of the battery to generate Amps.

If you have habitually undercharged your batteries then sulphation, causing reduced capacity, may be your problem on these cold nights.

If the Voltage and SG is OK then maybe you have a faulty Inverter that is in fact failing during cold temps - it should not be.

Long shot - Are you getting FROST on the inverter?

Are your Batteries charging back up to Float every day?

westbranch,
I would be very careful about using a Heater without a Low Voltage Cut-Off. That is a good way to seriously damage your batteries especially if the sun doesn't shine tomorrow or the next day.

westbranch · February 2011

Re: Cold tolerant inverter

NEOH, sorry I did not mean without a low voltage disconnect.

I meant that the inverter would not stop working due to the voltage being too low for it to operate, hence cutting out.

I concur that the issue is probably too little input relative to the usage, and that the inverter is shutting down when it get below the setting for 'low voltage disconnect' which may be 'somewhat' influenced by low battery temps.

NZRR · February 2011

Re: Cold tolerant inverter

Hi All-

Thanks for the replies - I had forgotten about the DC side of the math, but I don't think the batteries are the problem. I can't even start the inverter with full batteries when it is cold, even in full sun. However, I will add to the array to make sure I am not ruining my batteries.

I appreciate the help.

westbranch · February 2011

Re: Cold tolerant inverter

NZRR, I went looking iin your posts for your charge controller but do not see it listed.

A list of all your system components make model etc would help us review the 'balance' of your input and loads fully.

also the type of battery may be affecting your low charge state, what are they?

From what you tell us you are just at the low limit of recommended charge rates

using my panels specs...
120 w panels =~ 6.8 amps @ '12v' x 4 = 27.2 amps

Minimum 3% amp rate recommended 800Ah x 3% = 24 so you are about 3A above the minimum. About 1/2% above minimum.....

Its better to be in 5 - 10% range, 40 to 80A (As Cariboocoot said)

ps my inverters both work well at zero F when we arrive at the cabin and for hours until it comes up to temp in the winter, they've never blinked once due to low temp... they do generate a bit of heat while running...

techntrek · February 2011

Re: Cold tolerant inverter

NZRR wrote: »

I will add to the array to make sure I am not ruining my batteries.

NZRR, make sure you read my last post on the last page. Adding more PV may be taking a sledgehammer to a problem that can be solved using a screwdriver (literally). Enclosing those buckets could cut your energy requirement in half - or more - and then your existing PV will be overkill. $75 in plywood and foam board vs. $1000 for 2 more panels.

The first rule in designing an off-grid system is to reduce your loads to the minimum possible. Adding more PV is much farther down the list.

ggunn · February 2011

Re: Cold tolerant inverter

NZRR wrote: »

Hi All-

Thanks for the replies - I had forgotten about the DC side of the math, but I don't think the batteries are the problem. I can't even start the inverter with full batteries when it is cold, even in full sun. However, I will add to the array to make sure I am not ruining my batteries.

I appreciate the help.

Can you start the inverter when it's cold with the load disconnected? In general, semiconductors (an inverter is made up largely of semiconductor devices) like it cold, and the colder the better.

NZRR · February 2011

Re: Cold tolerant inverter

Thanks again for your replies. I appreciate your time and expertise.

Complete system specs:
4 Rolls S560 6-volt 400 Ah batteries wired for 12v
Trace TS512 inverter
Xantrex C40 charge controller
4 Astropower 120 watt modules wired in series for 48 volts (putting out 7 amps)

Loads:
2 11 watt CFLs - 15 minutes per day, max.
2 heated horse buckets (120 and 130 watts, each) - 24 hours per day when really cold (I think; not sure if they are thermally controlled)

I have two more sets of panels from my house that I can add at no additional cost, so I am not concerned about that aspect. If it were a trade-off between insulating the water and buying more PV, insulation would be the way to go. In this case, not so much at this time.

I cannot start the inverter when it is cold with no load, and with fully charged batteries. It goes through its startup routine, then gives me a overload error. If I take it to a warm spot, it starts right up.

Hope this helps clarify things.

waynefromnscanada · February 2011

Re: Cold tolerant inverter

One note of interest: You have a 12 volt system and are supplying your controller with 48 volts. From my experience (others may have other opinions) the wider the voltage spread, the lower the efficiency of the voltage conversion with MPPT controllers. If the distance to the panels is not too far, and if wire size in your situation is not an issue, you might consider rewiring your panels for 24 volts, it should give you a bit more power, and in your situation every watt generated, lost through system inefficiencies, or wasted, counts big time and is very important.
And, as mentioned by others, insulating the buckets and confirming thermostatic control are extremely important in your situation.
It also sounds like you have a serious inverter problem. For whatever reason your inverter doesn't like the cold, and that's the very time you need it most, when it's cold.
Even if the bucket heaters are not thermostatically controlled, and if you cannot for whatever reason add that control, if they are (by you) well insulated, you could insert a diode (half wave rectifier) in series with the heaters, which would basically cut their consumption (and heat output) in half. That would be a HUGE reduction of the load on your system if indeed there is no thermostat control already. And if they are already thermostatically controlled, properly insulating them will result in a HUGE reduction in consumption.
Good luck.
OMG! Didn't realize your controller wasn't MPPT until I read BB's post above! Man you've been throwing away 3/4 of the power your panels have been putting out!
I had made the MAJOR mistake of ASSUMING, from the way you have your panels wired, that it was an MPPT controller

BB. · February 2011

Re: Cold tolerant inverter

The C40 controller is actually a PWM solar charge controller... If you wired the panels in series for 48 volts, they are only outputting 1/4 of their rated power. You need to put the 4 panels in parallel OR get a MPPT type charge controller.

Regarding the inverter--is it an overload alarm or an over voltage alarm (if it can tell you the difference). It is possible that if the batteries and charge controller are very cold, you are charging the batteries to >15 VDC (this is normal, very cold batteries and a temperature correcting charge controller will need to raise the output voltage to properly charge a cold battery bank with higher voltage).

What is the voltage reading of the battery bank when the inverter refuses to turn on (it is possible that the cold inverter over voltage set point drifts a bit when the inverter is very cold).

-Bill

Photowhit · February 2011

Re: Cold tolerant inverter

Yep the C40 is a set voltage PWM charge controller. It can charge 12 from 12 or 48v but it'll waste the extra voltage. It will (Ithink) charge 48 volt bank from 48v(nominal) array...

If you have a tractor and can find a hole digger and have a dirt floor... Dig down into the ground below the frost buy the biggest sewer pipe for the diamiter hole you can dig 12" for a 14" hole... Buy an end cap or if you have more time than money a flat piece of PVC and seal the bottom (caps of this diameter are expensive 6x the cost of 10' pipe (very old cost ration from making huge prints in B&W darkroom)

Place a cirulating motor/pump and run it on timer or try a disipating metal aluminum stuck into the water to equalize the temps top and bottom.

My Prosine inverter worked fine down to -13 this year...

Hope this helps.

Cold tolerant inverter

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