Enclosure for AGM batteries in ambient temps of -40 F to 80 F

UnevenKeel · February 2021

Hello from the great land! I just retired from one job and now I have another: purchasing and installing a PV system at our remote cabin property in Alaska.

I'm still at the design stage, working with local vendors. I've done several estimates of our future power needs, and the guesses seem to settle on 3.5kW of power with around 5 kWh per day of energy consumption. My first quote came back with 1.3 kW of PERC panels, 9.9 kWh of 24V AGM batteries, and a 4 kW inverter. I'm considering bumping up to 1.95 kW of panels. A 6kW diesel generator will provide AC when the panel output drops in November, and as needed in the summer.

The intended usage is for near full time occupancy Feb-April and June-September. And I have cell signal and will be installing a weather station and having some form of remote monitoring of the premises and system. So the system needs to be "hot" year round. The panels will be mounted with an adjustable tilt so that they can be at optimal angle for summer and near-vertical for winter. Latitude is 62 north.

Now to my issue, the battery enclosure. My thought is to house the batteries and electronics in an insulated enclosure, essentially a 4'x4'x4' insulated box. Actual dimensions will vary. According to basic heat loss calculations, an enclosure that size with 6" of fiberglass insulation (and no air infiltration) will be warmer than ambient by 1 F for every 1.1 watts of power radiated inside. So if I leave an incandescent 60W light bulb on inside it will raise the temperature by 53 F.

The AGM batteries have a designed operating temperature of 32 F to 104 F. I figure the heat from the electronics and the heat from the internal resistance of the batteries will go a long way toward generating that heat. If I need more I can put in a battery blanket, light bulb, or whatever. Controlled by a thermostat, ideally.

Ventilation will be essential in the summer. The low-tech solution I am considering is a couple vents, one low and one high. They can be open in the summer and be filled with a block of insulating foam in the winter. And/or have a vent fan controlled by a thermostat.

Does it sound reasonable? And does a Conext SW 4024 really have a standby draw of 40 watts? That seems pretty high but this is my first go with this technology, and there's an uncountable number of things I don't know. I'd appreciate feedback and suggestions, particularly from northern dwellers.

-UK

mike95490 · February 2021

With AGM, which is not harmed by freezing, if you calculate the capacity needed at your expected cold temp, you may not need to heat the enclosure . The cells will also have some self-heating from their recharging, about 5% of their input power.
They do have the capacity to emit hydrogen gas when overcharged and at end of life, so they need the same venting that flooded batteries need..

The size of your generator seems overly large, with a 4Kw inverter, a 6kw genset will only need to run a couple hours a day at most, to recharge the batteries, and diesels do not like being overcooled. The XW series inverters are a lot more flexible with generator support and load support, and I would suggest scaling back the generator and looking at higher end inverter. I also think a 48V bank would be better for a year round household, even if daily use is not that much

UnevenKeel · February 2021

mike95490 said:

With AGM, which is not harmed by freezing, if you calculate the capacity needed at your expected cold temp, you may not need to heat the enclosure . The cells will also have some self-heating from their recharging, about 5% of their input power.
They do have the capacity to emit hydrogen gas when overcharged and at end of life, so they need the same venting that flooded batteries need..

The size of your generator seems overly large, with a 4Kw inverter, a 6kw genset will only need to run a couple hours a day at most, to recharge the batteries, and diesels do not like being overcooled. The XW series inverters are a lot more flexible with generator support and load support, and I would suggest scaling back the generator and looking at higher end inverter. I also think a 48V bank would be better for a year round household, even if daily use is not that much

@mike95490, thanks for your input. I'm considering 48V, just not sure what the threshold is for deciding on the larger voltage.

The generator is a whole other set of trade-offs. The 6 kW was recommended for charging the bank and supplying household power.

The specs on a Rolls S6-460 AGM battery say it has a resistance of 1.6 milli-ohm. If the charging current is 60 amps then each battery will warm by 5.76 watts. That assumes of course I understand the spec correctly and the power equals I*I*R. So four batteries will be generating 23 W of heat during a charging cycle.

The other electronics will also generate heat as they do their work. It's not critical to know at this stage, more of a curiosity as to what the total heat output will be. If I get the system up and running this summer then I'll have plenty of time to monitor the heating and adjust my thermal management appropriately.

wellbuilt · February 2021

Here is a pic of my battery box , it sits in my garage .

I have 2” of insulation covered with fiber glass panels .
Flexible vinyl pan on the bottom .
There are 4” pvc pipes on the front , and caped off for winter with insulation in side .

The 2” vent pipe runs inside and can draw warm air from inside the house or utility room .

My 2” pipe has a power vent on the other side of the lid , its made to work with the out back fm80 charge controller
The fm80 powers it when the battery’s start to absorb .

In the summer time my 4” vents are open and I leave the lid to the box open 2” or so .

My garage stay 67o unless I have the garage door open the battery’s stay @ 75 to 77o max
Winter time the battery’s stay at 50o +- when it’s 0 out side just from charging .
We get Dow to -15 or 20 for a week or so every year .

The garage is insulated but the temp will be as cold as out side.

I turn my system off when I’m not up there , I get snow every day from November to April and is messes with solar production , by the time the 1” of snow melts off its getting to dark to charge .
The solar guys don’t speak of the snow .

I have not seen my panels since Christmas 🎄 my place runs on 2or 300 watts day or night with frig and freezers I don’t use the freezer this time of year , I have plenty of ice .
running they are 125wats each but don’t run at the same time .

the Well is going to be solar pumped from 800 feet to a holding tank in the utility room
I run the place on a 2800 watt Honda .

If I can keep the panels clear of snow I can just run every 4 day to get to full charge .

This year I’ve been running every morning for a hour or 2 and a good absorb charge every 4 days .

I can go 2 full days with out charging but then I need a very long absorb time .

I’m using about5 gallons of fuel a week with no solar this year, last year I had 20 gallons @ Halloween and in April I still had a few gallons left

I’m running golf cart battery’s now but I will switch to I16 when these die .

As mike said 48vs is the way to go , and the reality is no matter what you numbers say you cant have to many solar panels . My first bank cost me 135 $ each then with the trump tariffs my second bank cost 165$ I pick them up at a supply house in my area .
I would buy a good charge controller and fill it with as many panels as it will take .
I’m in NY and DEC and JAN are dark with 4500 watts of panels I get 200/300watts sun up to sun down with a couple hours of 800 to 1200 watts if the sun pops out , this time of year I can get 3/400watts with a few good hours of 1700to 2800watt if i have no snow in the panels . In June I get 3700 watt for a good part of the day.

Image: https://us.v-cdn.net/6024911/uploads/editor/n4/ic2bhujw7ztz.jpeg

Dave Angelini · February 2021

About 27 watts for CSW 4048 idling. About 30 watts for XWP. They both can be used in search mode using a 11 watts and 8 watts.
Search mode when unattended can be hard to use as your loads need to be tested by you. Most do not use search as it can be a pain with refrigerator loads or anything that cycles. Search can also be a danger to small critical loads that need to be powered.

UnevenKeel · February 2021

wellbuilt said:

Here is a pic of my battery box , it sits in my garage . ...

Thanks for the info and the picture. Your battery temps in winter sound consistent with what I'm anticipating.

Winter will be when our place sees the most use, and I will be tilting to near-vertical too. So I should be able to squeeze a few watts out of the panels then.

UnevenKeel · February 2021

Dave Angelini said:

About 27 watts for CSW 4048 idling. About 30 watts for XWP. They both can be used in search mode using a 11 watts and 8 watts.
Search mode when unattended can be hard to use as your loads need to be tested by you. Most do not use search as it can be a pain with refrigerator loads or anything that cycles. Search can also be a danger to small critical loads that need to be powered.

Dave,

I appreciate the info on standby current. I also saw the recent forum post about the issue with search and newer refrigerators. I'm anticipating having a persistent load anyhow for telecomm so I don't think I'll have to worry about that.

Graham Parkinson · February 2021

Hi UK

If you are in a coastal area of Alaska you have many of the same concerns about winter always on loads that we have in cloudy BC winters. We can go 10 days or more easily with no real solar inputs.

PVWATTS incorporates historical weather data for your location and will tell you what you would get each month from 1.95 kW of PV panels, I suspect that 1.95 kW is nowhere near enough and you would need to at least double that for the loads you describe.

It sounds like you are on the right path of thinking about loads vs capacity but there is still a lot of design work needed to model if your system would meet expectations. Worth hiring a designer with local experience or paying someone knowledgeable in the business like Dave Angelini to review your needs and location before buying any gear!

If room for a larger PV array is the problem, if there is any way that you can design your telecom and refrigeration loads to avoid having to run the inverter 24/7 - you will be ahead of the game. Although in more benign climates having an inverter always on can work, in your area having an inverter always on and the likelihood of long periods without sunshine and is a guaranteed recipe for dead batteries unless you have a huge array and matching oversized battery bank. This is a really common situation in my area where people often ruin their batteries from leaving the inverter on. In areas with intermittent solar power, phantom loads really add up, not just inverters but from charge controllers, control panels, and all the network gadgets each drawing a few watts 24/7. Most recently my nearest neighbour had his system drained down (again) by the inverter and charge controller loads, the AGS did not succeed in starting the generator and the batteries may be toast (again).

Just an inverter load of 30 watts 24/7 represents 0.72 kWhrs/day. Telecom equipment usually runs off an AC wall that is making something like 12VDC. To avoid the inverter being on 24/7 you may find it better to use a quality 48VDC-12VDC converter off your 48 VDC battery bank to power always on loads like telecom equipment.

Refrigeration is another system defining load, if you are planning a smallish solar system located in an area much better to consider a DC fridge like a Unique brand fridge or even a propane fridge to use during shoulder season and winter. If you are planning a large PV system and are going to be around to monitor all the risks involved in backup generator function (and malfunction), then an efficient AC fridge relying on always on or sleep mode inverter operation might make sense.

Happy designing!

Enclosure for AGM batteries in ambient temps of -40 F to 80 F

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