Power for dew heaters

[Darth Wader (Wade)]

Howdy guys

Well what do you know, I grab the scope for the first time in months and it dews over within half an hour of viewing... so now I'm looking at some kendrick firefly heaters. I'm keeping it basic, just the eyepiece heater and one for the primary, controlled by this little ripper:

http://www.kendrickastro.com/astro/d...icroController

Now I read in Chris's excellent recommendations thread http://www.iceinspace.com.au/forum/s...ad.php?t=74670 that a jump start power pack from Supercheap can be used to power GOTO scopes. What about dew controllers? The one above has a standard 12v cigarette lighter plug and runs at 7 amps (says it's capable of 10). I have no idea what this means, I'm hopeless with electrics. Help!

Cheers
Wade

[joecool (Mark)]

Hello Wade, here are some calculations...
Kendrick Firefly heaters - 12" optic (will it fit your dob?) draws 1.6A plus 0.3A for 2" eyepiece heater = 1.9Amps. Multiply this by how many hours viewing you do and then multiply this by 2 because you do not want to be running deep cycle batteries below 50% their rating or their life expectancy will be halved. Lets say leaving it out for 6 hours as an example. 1.9A * 6hrs / 50% = 23Ah battery. Kendrick recommends running at 60% to 80% once the optics are cleared of dew, so you may get away with a slightly smaller battery, but only by 20% at most. Add size for longer observing time too of course.

The Supercheap 600A jumpstart pack is 12Ah, so that will run the dew heaters for 3 hours. I think the 1200 pack is 17Ah (but don't quote me on that). That makes about 4.5 hours.

Goto scopes draw about 1Amp when tracking and 2A when slewing, so running the scope alone you can get away with fairly small batteries. A 17Ah might just barely run one for one night without dew heaters.

The battery rating of 1Ah means you can draw 1Amp for 1 hour, or multiples thereof (e.g. 0.5A for 2 hours or 2A for 0.5hours). But, that is measured as drawing the battery flat to 10.5Volts, which will reduce the batteries effective rating pretty quick each time you do this. Deep cycle batteries do not like going below 50% of their charge rating if you want them to last, so halve the Ah rating of any battery you look at.

Sorry, I can't help much more than this as I'm just looking for something myself. I run a power hungry laptop and LX200 scope with dew heaters and I'm looking at something to do remote observing - 100Ah (about the size of a 4WD landcruiser battery) may be too small, but in AGM (glass mat) design so they can be used as deep cycle. Then I will need a special charger. It needs to charge in constant current, then constant voltage, then go into trickle mode. Without one of these chargers the batteries do not come up to full charge (like when using a car charger), so you can halve their effective Ah rating again...

Mark.

[The_bluester (Paul)]

I have one of the 1200A jump starters and before the charge light started flashing it ran my goto scope for about 20 hours all up.

I would hope to get one full night out of it with dew heaters thrown in. Time will tell I suppose but there is nothing to stop me from buying another one just to run the heaters if needed.

[Darth Wader (Wade)]

Thanks guys. I'll probably run the heaters for max. 4 hours before I chicken out and head for a nice hot shower. Was just worried about possibly damaging the equipment if I did it wrong.

Cheers
Wade

[planet paul (Paul)]

where did you get the heater from?

Ive just been using a home made dew cap thats been working ok in the cold of ballarat

[sasup (Stacey)]

im running a Waeco raps 36 battery pack. This pack powers three dew heater cells,Argo Navis,servo cat and 2 fans , I have never run out.