Nuts & Volts - June 2007

SHOWCASE

READER FEEDBACK

Continued from Page 9

electronics. The inverter you mentioned in your email most likely has plenty of power to start and run your furnace. Some considerations about inverters are:

1. Zero or no-load power consumption. In other words, how much power does this inverter draw from a battery when it is simply "on" and not carrying a load. Less expensive inverters will draw anywhere from one-half to one amp of standby current, so if you leave it permanently connected to a battery, it will eventually draw the battery down to nothing unless you are charging it with a charger capable of supplying more than that current. Of course, you can simply disconnect the inverter from the battery when it is not in use to eliminate this problem.

2. Sine wave or modified square wave output. Less expensive inverters employ a digitally-generated approximation of a sine wave at the output. Most AC fan motors are inductive motors and may be particular about the "quality" of the power they receive, especially when starting up. My general experience is that inverters made today are good enough to start even the most picky inductive motors, provided you have enough headroom in the power rating of the inverter you choose. A general rule-of-thumb is if you choose an inverter rated for 200% or more of the motor's power rating, it will probably start reliably. Fan motors have a fairly limited load on them (usually a squirrel cage fan) and are easier to start than say, a refrigerator or air-conditioner motor.

3. Battery life. You want your battery

to be available and up-to-snuff when you eventually need it. This requires putting a quality charger on it — one with a fully automatic cutoff — and you will need to occasionally check on it to make sure everything is working properly and the water levels are correct, etc.

So, in the end, you should find yourself a decent battery charger, deep-cycle battery, and inverter, and with the breakout box I described in the article, you should be able to switch over to battery power on your furnace in less than 10 minutes. I would not recommend leaving the furnace on the inverter as a permanent installation, as this may be hard on your battery, inverter, and possibly your furnace fan motor(s).

I will consider a separate article on

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