Running a dewstrap on a 9V battery

[LewisM]

I know about these http://www.tech2000astronomy.com/zap/ and am considering to order considering I am tired of cables going everywhere, but was wondering if it would be feasible to convert a standard dew strap (built for 12V DC) to a battery operated one using a 9V alkaline (simple 9V clip and a female RCA plug). Or maybe make it a 12V pack like Vixen used to make (8 x 1.5V D cells in series)

Or is there going to be some odd impedence issues etc, or maybe damage the strap - I believe under-voltage can be as bad for an item as over-voltage.

Appreciate the tech help on this one.

[AndrewJ]

Gday Lewis

What type of strap is it???
If it is a simple resistor string, or nichrome wire, it wont hurt anything, it will just put out a lot less heat.
Ie many dew "controllers" use PWM to effectively reduce the operating voltage as required to control the heating power out.
Ohms law gives V = I x R
and power out P = V x I
Mixing these 2 you can get P = V^2 / R
Thus at 12V you get 144/R watts
and at 9V you get 81/R watts
so about 56% of what you would get at 12V

Andrew

[LewisM]

Thanks Andrew.

I wonder how long an A23 12V battery (the small ones) would run a dew strap (they are apparently merely 8 x 1.5V button batteries in series) . Some claim their 9V straps run a year on one battery.

Failing all this, I may just buy a small 12V and run it off that. I got 4 years out of 1 jump start pack that uses a similar battery, so 4 years is pretty decent odds. I just want the compactness of the battery alone.

[AndrewJ]

Gday Lewis

Some claim their 9V straps run a year on one battery.

That would probably work if they weren't ever turned on

Can i ask a simple question???
How long does a ubiquitous 12V 7.2AHr battery last with a dewheater????
A 9V Alkaline battery is rated at between 400-700mAhr
ref one datasheet i found http://data.energizer.com/PDFs/522.pdf
If you drop from 12V to 9V on a set resistance strip, your current draw will go up by about 30%
Sooo
12V with 7200 mAhr vs 9V with 700mAhr

No contest. unless the strip is only for a red dot finder etc with a very low heat requirement
You are better off looking for the new light weight LiPo battery packs etc that are used by the model plane nuts. Very high energy density and light weight, but costly.
( Or, just run one 12V wire to yr mount and have a distribution box on the mount to supply 12V as reqd. )


Andrew