Help on preventing battery failure in extreme cold

Re: Help on preventing battery failure in extreme cold

It sounds like too much loads and too little power (solar panels/genset/etc.).

How cold are the batteries getting? How much power do you use? How much solar panel do you have?

Taking batteries below ~11.6 volts (resting voltage) runs the risk of permanently damaging the battery bank. Taking them to 10.5 or below probably guarantees that the batteries are toast.

You can install a Battery Monitor (Xantrex and Victron units. and probably some others, have a programmable output that you could set to "alarm" at 50% state of charge and turn the alarm off at 80% state of charge--that will save your batteries) that turns off the loads at 50% state of charge (or 20% if you really want to deep cycle).

Adding more batteries is probably not the answer (especially if you do not have enough solar panels/backup power sources to properly charge them).

AGM's may be a better option... They are ~10% more efficient and will not freeze damaged if taken well below freezing with low state of charge. But they are much more expensive batteries and still do not solve the issue of going to 10.5 volts or lower (toss batteries, buy new batteries).

And there is always the conservation angle... Make sure you have the most efficient hardware practical and that you turn off unneeded loads (night, between shots/communications cycles/etc.).

Using PVWatts for Muskegon (trying to find lake front site for local weather conditions), 1,000 watts of solar panels (round numbers), fixed array mounted at latitude and 0.52 system derating (end to end efficiency):

"Station Identification"
"City:","Muskegon"
"State:","Michigan"
"Lat (deg N):", 43.17
"Long (deg W):", 86.25
"Elev (m): ", 191
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 43.2"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.3 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 2.04, 32, 2.66
2, 3.74, 56, 4.65
3, 4.43, 71, 5.89
4, 5.23, 78, 6.47
5, 5.96, 88, 7.30
6, 6.07, 85, 7.05
7, 5.85, 83, 6.89
8, 5.64, 80, 6.64
9, 4.91, 70, 5.81
10, 3.78, 56, 4.65
11, 2.34, 34, 2.82
12, 1.78, 27, 2.24
"Year", 4.32, 762, 63.25

Between summer and winter, it would look like you need to either cut your winter power use by 1/2 or 2/3'rds -- Or increase your solar array by 2-3 times... (1.78 hours of average sun per day in December vs 5+ hours of sun per day in summer).

If you have stretches of cloudy weather, your options become:

• A lot more solar panels+batteries
• Small automatic or manual genset to recharge bank
• Cut power usage when battery/solar panel performance is low

I don't think heating the batteries would be an answer unless they are well under 32F (~20% loss in capacity). Many times an insulated box and/or burying batteries a few feet underground will address most cold weather issues (need to monitor that they don't overheat in summer).

-Bill

Re: Help on preventing battery failure in extreme cold

TimeScience wrote: »

Looking at out weather station, it looks like it has been consistently between 0 and -20C for the last month. cold! This hasn't been a warm winter around Chicago, that's for sure.

Insulated box/buried box?

I know we should switch to sealed batteries and will do so in the future. at the moment it isn't an option at these sites due to budget constraints. Likewise, we'd have more solar out there but these camera systems are in wildlife areas and we are trying to limit their footprint so one panel is probably the max for now.

Problem with solar--more sq.feet is more power.

Based on your suggestions and discussions with some other folks, we'll probably implement the following...
(1) Add 1-2 more batteries so the bank is large enough to withstand long period s of cold

Hate to say it--probably a bad idea. That plus using Marine batteries which are not usually very good at deep cycling.

Looking at using inexpensive "golf cart" batteries may be better (keep costs low, cycle them till they did, replace).

(2) Reduce the system hours to only run for an hour or two during mid-day

Running during daylight hours hours only saves battery losses (but during cloudy weather, there is nothing much else you can do).

(3) Put the batteries in heavy duty insulated coolers that are painted black.
(4) We may bury the batteries as well to keep the ambient temp more consistent. Might not be possible this year since the ground is frozen.

Take my suggestions with a grain of salt--San Francisco area is not known for much snow/frozen ground.

You might want to drive a hole in the ground and drop a thermal couple to get an idea of how deep you would need to go to find warmer/more stable temperatures.

(5) Update our software to turn of the PC and shut off the system if battery voltages are detected to be below 11 or .5V

Deep Cycle Battery FAQ

State of Charge/12 Volt battery
100% 12.7
90% 12.5
80% 12.42
70% 12.32
60% 12.20
50% 12.06
40% 11.9
30% 11.75
20% 11.58
10% 11.31
0 10.5

Cold batteries have higher resting voltage (at freezing, the resting 100% charged voltage ~1.2 volts higher:



So, your 11.9 volt (cutoff the night before) may be 13.1 volts or more.

A good, temperature corrected Battery Monitor will take much of this into account. Measuring resting voltage and temperature is a pain to determine actual capacity.

For a nicely balanced system, you would want to match the solar panels to battery capacity around 5-13% of battery AH Rating... Take 10%:

• 3x 100 AH * 14.5 volt charging * 0.10 rate of charge * 1/0.77 charge system derating = 565 Watts of solar panels

More batteries means you should have more solar panels for proper charging. (big/dead batteries take a long time to recharge with a small array).

A 565 Watt Array in December will produce, roughly:

• 1.78 hours of sun per day * 565 watts of panel * 0.52 derating = 523 WH per day December
• 523 WH / 12 volts = 44 AH per day

The above is assuming a 85% efficient inverter... Multiply the above numbers by 1/0.80 (x1.25) if your equipment is DC powered.

Also, if you cannot justify a Battery Monitor, for $60 there are some nice DC Amp*Hour / Watt*Hour meters that you can use to monitor your typical daily load (AH/WH usage). Flip the meter around and connect it to your solar charge controller, and you can log power generated.

You need to know your power needs and panel output... Without that--we are guessing.

-Bill