I found an old computer UPS that will be put to good use as a backup power source for the chook incubator.
I did a few tests to determine how long it would run for on the battery. Also to see how worn out the battery is.
Battery 7.2AH 12 volt battery.
UPS is rated to 500VA 0.6 PF) (300watts).
The rating of the heating element is 40watts, but measured 47 when it was running and full (ie warming up).
I measured the DC current from the battery terminals, 4.6amps. 55 watts.
Assuming that the UPS shuts down when the voltage goes below 12volts this will run for about 20mins (from the discharge curves 12v_7.2ah). The AC power measured was 47 watts. So the UPS isn’t terribly efficient, about 85 percent.
When the incubator is at operating temperature I expect that the current drawn will be about 1/3 or about 1.5amps. So runtime should approach 2 hours. I will measure the current again when we warm the incubator up next time. It may use even less energy to keep the temperature stable.
To estimate how old the battery is, I plugged in a 9watt lamp and let it run out. It ran for 90 mins. The lamp was drawing 1.25 amps DC. It should run for about 150mins (2.5 hours). So the battery is down to about 60 percent capacity.
A replacement SLA battery is about $30.
I could buy a larger battery also to extend the runtime. It won’t fit inside the UPS but I could attach it quite easily, externally.
Estimated runtimes for some larger batteries :
18AH $75 7 hours
26AH $129 10 hours
100AH $429 48 hours