Cold Weather Help

Philip C · December 2011

Need help with Low Voltage Cut Off for an Off Grid System.

The system consist of about 20kW of PV on (6) Xantrex 60 CC's and (3) Xantrex 6048 inverters with (8) Deka Maintenance Saver 1579 Ahr batteries (4 series 2 parallel for 3158 Ahr/24hr rate) We had a cold (38F at the lowest) and cloudy morning, the customer was running a dishwasher, water pump, oven, and geothermal heater (no heat elements). With everything running I've never seen loads get over 16-18kW so very unlikely to overload inverters. They experienced an F48 (battery under voltage), but Trimeteric meter was reading 81% full.

During these colder months, do I need to adjust the LBCO or insulate the batteries better. They are currently in an outside shed inside a wooden box. Its pretty rare for us to drop below 40F but we have seen 20F in the past. How do you deal with the occasional cold spell?

Thank You,
Philip

westbranch · December 2011

Re: Cold Weather Help

how long have they had the system?
Were there multiple start-ups occurring simultaneously?
with the DW running that water pump will be on/off. What type of pump? depth of static level?
Geo thermal should be a constant draw... but could have cut in.

hth

ps wish we had those warm temps

Cariboocoot · December 2011

Re: Cold Weather Help

Overloading the inverters is one thing, overloading the batteries is another. It is entirely possible that the cold temps combined with large loads all wanting power at once resulted in a drop of apparent input Voltage causing the F48 fault.

That much battery should handle up to about 14kW at once, but sudden current increases are murder; deep cycles don't respond well to them. You don't say what the LBCO is set at, but it shouldn't be below nominal 48 Volts in my opinion. I don't know what the wiring is either, but that and the connections can add resistance which ends up dropping the Voltage more while under load than when not. The Trimetric will keep track of current going in and out, but that has no bearing on the Voltage at the inverters being too low when the loads are on.

You probably can't duplicate the situation and check for V-drop across the battery wiring, but you should be able to calculate roughly what it might be under heavily loaded conditions.

Where Westbranch and I live, 38F is balmy. We don't worry about cold conditions affecting battery function until it gets below freezing. Around here it can get really below freezing: -40

niel · December 2011

Re: Cold Weather Help

that battery bank is being reduced in its capacity to about 83% by going to about 38 degrees f.
http://www.windsun.com/pictures/Batt_temperature1.gif
add to that you are using it at about a c/6 rate when about 16.5kw are being drawn and it is no wonder why it went low in voltage.
http://www.dcpower-systems.com/product_detail.aspx?gid=644&pid=26213
a readjustment in the low battery cutoff is one thing, but you need to curb your power usage at any one time, especially when those batteries are colder. i would advise to not exceed a 13kw draw under your present circumstances and keep in mind that at that rate your batteries would be good for about 4hrs without drawing past 50% depth of discharge (dod).

Blackcherry04 · December 2011

Re: Cold Weather Help

I'd also check the amp draw on the Dishwasher, they can be major power hogs if not run in the econo mode. If in doubt, unplug the element and let them air dry, users usually never know the difference. With a oven and some combination of burners you have a lot of resistive loads.

My 20 KW genny would be grunting with all that.

RoySalisbury · December 2011

Re: Cold Weather Help

I was about to ask an almost similar question and noticed this thread. Hope you don't mind me getting some "cold weather help" as well.

My system is a bit different.. 4-6v 420ah Trojans and 4 210watt panels. I currently draw about 120 watts of load. This time of year, the temperature gets down to about 32F/0C at night (sometimes a bit lower), and 45F/7C during the day. I figure my capaticy is reduced to about 65-75% on average. Over night my SOC goes to to 85%, so I am using about 15% of my "optimal" capacity.

Now, if my numbers are right, due to the cold weather, this could be a real-world SOC of ~75% ... But I could be wrong.. I'm just doing quick "best guess" math here...

Now my questions are:

1. Is my logic correct? Am I thinking about this the right way?
2. Is my math "in the ball park"?

I have already had got to a point of "inverter cutoff" at the beginning of the month due to a few bad days of weather, and even though the battery monitor told me I was only at 58% SOC, they system died (voltage took a SHARP drop off all of the sudden), so I am assuming that I needed to compensate for the temperature in the SOC. Is there a quick "ballpark" calculation that I can apply to the reported SOC using the temperatire that will give me a more real-world value?

I'm thinking that I also need to set my generator SOC cut in values different in the winter months vs the summer. When I get it hooked up I was thinking of an auto cut in of 75%.. but in the winter that may need to be higher (80-85%).

Roy

Cariboocoot · December 2011

Re: Cold Weather Help

Keep in mind that ambient temperature is not battery temperature. With current flowing in and out batteries will have an internal heat "source" and will never be as cold as the air around them. This is not true of car batteries that just sit for hours or days before you try to start your vehicle and discover the horrible effect really cold weather has on battery capacity.

With an OG system your worst-case scenario is batteries sitting outdoors, uninsulated, while no current is drawn over-night. They will be at their lowest potential just before morning charging begins. With even as little as R5 insulation around them they will perform much better.

I just put a brand new battery in the 4Runner for winter. The thing sits for days on end outside. It still needs a boost to get going when the temp drops into the Cariboo cold temps. Let the temp come up on its own and it will start fine (Christmas day it was 5C here!)

vtmaps · December 2011

Re: Cold Weather Help

Cariboocoot wrote: »

That much battery should handle up to about 14kW at once, but sudden current increases are murder; deep cycles don't respond well to them.

Please explain more... are you saying that 14kw is OK as long as it is not sudden? What happens (physic and chemistry) to the deep cycle battery if the load is sudden?
--vtMaps

RoySalisbury · December 2011

Re: Cold Weather Help

OK.. I guess for starters I will insulate the battery box. Its protected from the elements (wind, water, direct sunlight, ect), but not insulated.

Roy

Blackcherry04 · December 2011

Re: Cold Weather Help

vtmaps wrote: »

Please explain more... are you saying that 14kw is OK as long as it is not sudden? What happens (physic and chemistry) to the deep cycle battery if the load is sudden?
--vtMaps

Coup De Fouet Effect, it's French for Dip and Recovery, he means that if you cut a transfer switch, say off a generator and drop a sudden load on the Battery Bank. Like a in rush in reverse. I have a 12v bank and I see it all the time, it's usually .03 to .06 volt or so. I have seen it last as long as 10 minutes on my GC=2's. It probably takes time to ripple through the bank. Another reason to have higher voltage and less paralleled strings. I once thought it was a Refrigerator compressor that would drop out and short cycle and caused a abnormal drain. I eliminated it and it still does it. I have 4/0 cables so they are not a factor with say 1500 watts or so.

www.battcon.com/PapersFinal2010/BielerPaper2010AfterFinal_14.pdf

Philip C · December 2011

Re: Cold Weather Help

Thanks everyone for your replies

Let me try to answer the questions
The system has been up and running for about 8 months, the small system is down right now, so every load is running of the big system described. Normally the small system handles the water well, two refrigerators, and some lighting.
I don't know the dept of the water table, but I'm guessing the well could be 150ft deep or less, but I do know the water well surges at 6000W and drops down to about 3kW, but does not run long as it fills a tank.

Thanks for the heads up on the dishwasher, I will ask the customer to set that on Eco Mode, this could be a deal breaker alone!

Roy,
Don't mind at all!

I was thinking about insulating the battery box, but here in the South, it gets cold for about 3 days a year

OK maybe 14days, but not much more LOL So my guess is that the Insulation may hurt more than help?

Sounds like it may be best for the customer to warm up the batteries first in the morning through gradual loads. Let the sun come out and charge the batteries a bit also should warm up the internal temps also.

Thanks for your help.
Philip

Cold Weather Help

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