Battery pack size

[Gem]

Hi all

To run say a C11 on a CGEM mount plus, at times, an ATIK camera, and maybe a dew heater, for a 3 to 4 hours per session... What size battery pack would be sufficient?
I am not looking to do longer sessions.

[jenchris]

What's the wattage on the dew heater?
If you have a deep cycle battery, it need be no more than 18a/h I'd say, but if the dew heater is a whopper, it'll be touch and go.

P= IxV - so you've got 18 x 12 = 216watts for an hour, or 113 watts for two hours or 56 watts for four hours

[Gem]

Thanks. Not sure about the dew heater yet. I want to get the scope and see how much dew there is around here before seeing if a heater is needed or not. I was just trying to get a feel if an 18h was ok, or if I am dreaming. Comes from a dob background, I haven't had dew problems in the past (nor the need to power mounts or cameras!).

[joecool]

A few calculations...

A Kendrick C11 dew heater is 28W = 2.3A. 2.3A * 4hours = 9.2Ah. You do not want to go below 50% the rated Ah of a deep cycle battery if you want it to last, so the dew heater will use up an 18Ah in 4 hours... You may want 2 of them or go twice the size.

The rating of a deep cycle battery is to discharge it to 0% charge at 10.5volts (and some of your equipment may not work at such a low voltage). If you go below 10.3 volts it is toast. Going down to 20% is possible but not recommended. A battery will last twice as long discharging to 50% vs 20%. This is a trade off of price of battery and how often you will be doing this and want to replace the battery.

Using a camera means using a laptop. An average laptop will use about 60Watts. 60/12 = 5Amps * 4 = 20Ah which means 40Ah battery just for that. I do a bit of processing of images whilst it is clicking away, so I don't dim the screen and am cranking the hard drive and CPU, so probably go a bit higher than this. If you set the power controls to low CPU at idle and dim the screen then this can be reduced. A laptop with a 10" screen may only draw 25Watts = 2A = 8Ah over 4 hours (16Ah battery)...

The scope itself, well can't find measurements of your mount, but HEQ5 Pro apparently tracks at 1A with similar equipment and your mount shows max current a little high at 3.2A so this is probably right. Allow for 1A * 4h = 4Ah (8Ah off the batteries rating to 50%).

9.2Ah + 20Ah + 4Ah = 33Ah / 50% = 66Ah battery. And allow at least 20% for efficiency loss on top of these figures if going through an inverter to run things at 240V.

Just found this site with some peoples experiences http://cs.astronomy.com/asycs/forums/p/43492/431909.aspx

Here are a couple of quotes
"My system draws about 6 amp. I had an 80 amp hour deep cycle battery that would run the system for 6 or 7 hours before the dc to dc converter for the laptop dropped out from low voltage. I wanted to be able to image over two consecutive nights camping out at deep sky sites so a bought another 80 amp hour battery. One battery runs the laptop, the other battery runs everything else. With this set up both batteries will go about 21 hours before the voltage drops too low, and they both drop out at about the same time."

"I use the Optima D34M (55aH) with an 800 watt inverter (I have three 12 VDC outlets in addition to the one on the inverter). I can run my laptop, an LXD-75 SN-10 (w/EQ-G mount), a small LCD monitor (DVD player), and my Canon XTi for a night's worth of observing with this setup."

Mark.

[jenchris]

You have a laptop that runs at 50 watts!! wow!! What sort of internal battery does that have??
My laptop runs for two hours on a woeful internal battery (NICAD) of about 1.5 ah - at 17V. - 11 watts - I run APT the internet and maybe GHD and a webcam.
If I had a 50 watt laptop, I'd stick the output from the fan onto the corrector and the dew would boil off quick smart. To be honest I do have it on mains when in my garden as it has a brighter picture and doesn't keep going to screen save!

[joecool]

Yes, those are the trade offs. To clarify my numbers a bit more - My ASUS Pro50N (an older but basic system) has a 4400mAh 11.1V battery and might last 1.5 hours. That works out at 32W on battery with screen dimmed to 50% and processor barely ticking over. I also have an eeePC which draws about 10W, but only as a 7" screen and is dead slow! It works for downloading images though...
Here is a fellow who has tested the power draw of a similar laptop through the mains adapter at different settings. http://jimmyauw.com/2008/09/21/491/
At 50% brightness with CPU clocked slow it still draws 23W. He says that WLAN adds 1W to that (so I've got internet from the house to choose targets through the night). This = 2A (2A * 4hr / 50% = 16Ah battery just for the laptop) and goes up to 54W on single core processing images at max brightness. This = 4.5A (4.5A *4hr /50% = 36Ah battery).

Even running your laptop off the battery in low power mode (because we want it for 4 hours and not 2) = 1.5Ah/2 * 17V = 12.75W * 120% (for power adapter losses) = 15.3W.
15.3W / 12V = 1.275A * 4hr = 5.1Ah / 50%(safe battery discharge level) = 10.2Ah battery to run this laptop...

So, it all depends... How big is grant's laptop and what settings is he using?

Mark.

[joecool]

An update on power usage for the mount, a review of HEQ5-Pro (similar to CGEM mount?) by an iceinspace member
http://www.iceinspace.com.au/index.php?id=44,288,0,0,1,0

And I quote...
"the HEQ5-Pro mount only lasted one night on a 17 A/Hr battery before the on LED started blinking to indicate low battery power."

Mark.

[Gem]

Thanks for the info!

[RobF]

Its surprisingly difficult to get these sorts of figures even on the net, so I spent a night playing with my gear and a multimeter prior to heading off to a dark sky site recently. I needed to make sure I had sufficient battery power to get through 2 nights if possible. Some experiments with my gear:

- Laptop with a 4 port unpowered usb hub running everything: 1.8A average
(15" screen, DVD drive - not a baby atom processor lappy)
(USB hub running mount, focuser, QHY9 and autoguider)
- QHY9: 1.4A average (at about 80% cooling which is stacks on most nights)
- HEQ5Pro mount: 0.7A (0.35A parked but on, 1.2 A slewing 800x)

I don't have any dew heaters in my gear sorry. Judging by the time it took me to charge up again afterwards, the draw down figures in amp hours were very close to what the numbers above indicated. Hope it helps a little.

[joecool]

Thanks Rob, nice numbers. I needed some too as I am after a battery for my own rig to take to a dark site.

1.8 (laptop)+ 1.4(imager) + 0.7(mount guiding) + 2.3 (100% on dew heater) = 6.2A. So for 4 hours running the battery down to 50% requires 50Ah battery.

I'm figuring on doing this myself for 2 consecutive nights without having a generator to re-charge, so 100Ah AGM deep cycle(about the size of my 4WD battery) would be about right... hmmm perhaps a second one for all nighters :lol:

Thank you,
Mark.

[RobF]

And of course once you're talking 100ah batteries you're getting into the relm of perhaps buying a decent invertor generator if you'll be doing this too often. I was tempted to buy a cheapy, but behaved and bought another battery as I figured there was a greater chance of me using it regularly. Be interested to hear how you go on the adventure :)