AM3 Power supply - Which one to buy

[JW]

Hi all,

Another AM3 question to the gurus out there.

What power supply should I buy. Any solutions to think about. I will be on location and not have access to 240v.....

My thanks as always!

[AlexN]

This questions is not really AM3 specific, as MOST mounts these days are 12v and require around 3A (Max when doing a full speed slew... for just tracking, they use less than 1A most of the time... around 0.7A give or take).


So, if you want to run the mount off a battery you have a couple of questions to answer.

1 - Budget.
2 - Portability (how far do you have to carry it, what is your lifting capacity)
3 - How long do you want to be able to reliably run the mount?

Lets say you have a healthy budget, and portability is not too much of a concern.

Then, its simple math.

12V @ 0.7A is your load. (I'm assuming you're going to do one or two go-to slews, but for the most of the time, you'll just be tracking)

12V*0.7A = 8.4W
if you want to run that rig for, say, 6 hours through the night, lets say you get a 12v 10AH Lithium Iron Phosphate battery, (LiFePo4)

12V * 10Ah = 120Wh. and 120 watt hours / 8.4W of load = 14.28h of run time...

So 12V 10A, at first glance, looks like a good option. Its small, light, relatively cheap, and will give you 14.28h run time...

Well... sort of.

Batteries, even high quality LiFePo4's are only good until about 90% discharged... right.... so we take our 14.28hrs run time, and multiply that by 0.9.

12.86h - ok... still nearly 2 full nights.. pretty good...

Except... not really...
Because you don't want to just power the mount... you want to power all that gear in your other post too...

AsiAir, 2600MC, guide camera, autofocuser, dew heater bands etc... and lets say you're wanting to do 3 targets per night, so we'll take that initial calculation for the mount (Averaging 0.7A) and we'll up that to say, 1.1A to factor in a bit more slewing about.

So:
Mount: 13.2W on average

AsiAir 3W at idle, so lets call it 7W on average when its running and actually doing things.

2600MC really depends on how hard you're running the cooler, but lets say, worst case scenario, we're running at 75% cooler power... so probably looking at about 26w on average (will get peaks higher when its downloading images etc, and it will drop during the night as the ambient weather gets cooler... but - lets average it out at 26w.

dew heater bands (1 for the Esprit 100, 1 for a guide scope)
These are usually about 12V * 0.33A at 100% draw, the asiair is not smart enough to be controlling them based on operating conditions, so lets just say you run them at 50% capacity... 4W

So average load is something like 13.2 + 7 + 26 + 4 = 50.2W.

so back to the 12V 10Ah battery.
12V 10Ah = 120Wh
less the efficiency factor x0.9 = 108Wh
divided by average load - 108/50.2 = 2.15h of run time....

So, lets back-calculate.

Your rig is going to pull 50.2W, and you want to run for, lets say, 2 full nights (6.5hrs of run time per night) 13H, so 50.2W * 13H 652.6Wh

divide that by 12v to get the required amp hours.

54.38Ah * the efficiency factor = 59.82AH.

So, if I were you, I'd be looking at a minimum of a 12v 60Ah battery for your rig...

I run off mains power at home, however out in the field I have a 12v 120Ah battery to run my rig (similar to your prospective setup), and I have a small fold out solar panel I connect to the battery during the day to top it off... I've had this last about 7 nights in the past, and when I got home I threw it on the charger and it was at about 70% charge.