Battery efficiency vs. temperature

[MIgardener]

  I have a small solar system that powers the light in my chicken coop. The light is on a timer and is on from 3am untill 7am.(It is used during the winter to suppliment sunlight and increase egg production) Usually the 80Ah battery is pleanty enough to power the 100 watt bulb for a just four hours. But sometimes when it's really cold(-30)it will only make it untill 5 o'clock before the inverter shuts down due to low voltage. My question is, how much does the cold effect the efficiency of batterys?

[DamonHD]

It's not particularly a matter of efficiency; the available capacity of batteries varies with temperature.

Rgds

Damon

[Flux]

Temperature does affect battery efficiency, the capacity is less at low temperatures but the life is longer.

I suspect the issue concerning your problem is one of terminal volts more so than efficiency.

For a fully charged battery left standing without charge or discharge it will settle to about 12.8v at 25 deg C.

This figure drops at lower temperatures and the charge and discharge voltages are similarly depressed in cold conditions.

For a reasonable load on the inverter the usual assumption is that the battery will be flat when it reaches about 10.5v. This will be the inverter low volt shut down designed to protect the battery.

With temperatures below 0C you will reach this low volt cut off before the battery is completely discharged.

I think what you ought to interpret this as having a battery that is too small for the job. You could discharge it further in cold weather if you could alter the inverter cut off, but to me it implies that you are discharging to the battery's limit under warm conditions and that will lead to a short life especially as your 100W load is not a large one.

Flux

[DamonHD]

Flux; thanks for the correction.

Can you summarise how the round-trip efficiency (for lead-acid) does vary with temperature, independent of changing capacity?

Rgds

Damon

[Flux]

No I can't give any exact figures but I seem to remember that the charge efficiency in Ah is fairly constant with temperature. That seems to be the usual quoted efficiency.

For us what really matters is the Wh efficiency which will be significantly lower as the voltage during charge is higher than nominal and during discharge it is lower.

If my suspicon that Ah efficiency is fairly constant then due to the lower voltage at low temperatures the Wh efficiency will be lower even alowing for the lower capacity.

Most figures are given at 25C and it is not easy to find what happens at low temperature. I do know that the internal resistance rises to the point that they won't start engines in arctic conditions and you have the seemingly crazy situation of a heater run from the battery to raise its temperature to drop the internal resistance. Must be a fine balancing act when you combine that with glow plugs on a diesel.

Flux

[hydrosun]

The original question was about running a light for chickens to increase egg production. If you don't need the heat but just want more light it would make more sense to use a more efficient light bulb. A led bulb(s) to put out the same lumens as a 100 watt incandescent would only use less than 25 watts.  Then your battery bank would be big enough at the lowr temps. A cfl might not start at the low temps but leds would work fine. Last winter I used a 12 volt led strip off a 12v timer and battery to try to increase egg production.  My birds were getting old and it didn't seem to make any difference. This year with young chickens and no light we are getting sporatic egg laying. Which is fine for just our own use. I did set up a small solar system for a farmer with 100  layers and he reported good success with a few led lights.
So like any system, the more efficiently the power is used the smaller the battery and solar array needs to be.
Chris

[Mary B]

Battery capacity versus temperature

[DamonHD]

Interesting, thanks!

Rgds

Damon

[Mary B]

Why I am building a sealed battery box disguised as a bench for one side of my office/radio room/ solar control point. Vent it outside through 2 inch pvc, small fan blowing fresh air in on the bottom to push hydrogen out. My porch where the bats are now can drop to zero f.

[kitestrings]

MAlana,

Thanks for sharing this chart.  Very interesting.  Do you know the source, and or if it is lead acid, gel, Li-ion or a melding of some combination?  ~ks

[Mary B]

Came from Rolls Battery I think, one of the bog battery manufacturers.  It was for lead acid.

[Bruce S]

Sorry for the small drift.
MaryAlana;
Will you be using solid wood to lessen any RF noise put out from the controller? Or have you had any of these issues?
I'm curious has our HQ guy is wanting me to ASSIST him with solar assisted backup, for the 2-ways and a base.
Thanks
Bruce S

[Mary B]

My controllers have been pretty RF quiet actually, a few ferrite bead on all the wires in and out and all I  have left is a couple of birdies here and there. My computer puts out way more noise. The inverters are the rf generators, I searched quite a bit to find a low cost RF quiet inverter and settled on Samlex PST series. So get yourself a handful of RF noise ferrites from Radio Shack and go at it. Morningstar MPPT 45 controllers, they are quietish

[Bruce S]

Thanks for the heads up!