Is it bad to run out of power?

Stonius (Markus) · #1 11-05-2020, 02:09 PM

I've normally had access to mains power when observing, but I'm wanting to investigate a battery setup.

I know a deep cycle batter of around 140AH is ideal (and heavy!).

But I have a car battery and another lead acid battery, which I'm thinking of connecting in parrallel to see how long this can go for before it dies. But I have a sneaking suspicion that it's not a test you can safely do.

At least, I know cutting the power on the camera would mean the sensor would be in danger of damage from thermal shock as the cooler shuts off with no 'warm up slope'.

But even then, if I were to run it until it hit a certain level of 'flat' before failure, what would that be?

Is there an experimental solution?

Thanks for any help.

Markus

Outcast (Carlton) · #2 11-05-2020, 05:27 PM

What are you running & what is their cumulative power draw?

I run off mains power at home, pretty much for the same reason, to avoid the whole 'what if I run out of power' scenario...

I'm sure this will be howled down by those of a more technical bent but, here's a quick, back of the fag packet way to figure out your worst case...

Add up the max Amp draw of all your kit, for anything that's not straight 12V, I do this.. 5V = Amps x 0.5, 8V = Amps x 0.75. (I did say it was rough & dirty)... For my mount (AZEQ6) I figure about 2.5 - 3A when slewing..

Anyways, add it all up & then I use that figure as the absolute worst case A/hr figure... then I have a bit of a think about how it all really works... so, my dew heaters are on a controller & on around half the time.. camera.. pretty much full time, mount.. lot's less once it's slewed & just tracking..

This should give you a rough idea of what size battery will accomplish what; I don't want to carry a huge/heavy battery so, I use a 24 A/hr deep cycle battery (that I already owned) & I figure maybe 3 - 4hrs tops before I start to get appreciable voltage drop so, if running on battery.. that's about how long I would allow for my session...

Dunno about doing the testing regime you have asked about... because I'm a technical numpty, I probably wouldn't be doing it myself...

Don't know if this helps your thinking at all?? The other way you could go would be a LiFePo battery but, these are still pretty pricey for lots of A/hrs... but, stupidly light when compared to Lead Acid..

Cheers

Stonius (Markus) · #3 11-05-2020, 06:18 PM

Thanks Carlton,

I know you can calculate it, but I kinda dread finding the specs for everything. I don't even know if my dew straps will even have published data (cheap chinese brand).

Besides the mount (gemini 2), the dew strap, moonlite focuser, USB hub, Laptop, Hard Drive, Polestar camera, ASI 1600 and 290 plus Temp control - I wonder how accurate it can all be when it depends on how far below ambient you want to cool things, how much dew there is, how heavy your scope is and how it's weighted, blah blah.

I think it may be more accurate to just determine it experimentally.

I'm thinking maybe I could just run it for a few hours till it drops below a critical level. What *is that level though?

It's been a while and I tend to forget things I previously learned. If you let the voltage on a battery run down, the problem for components connected to it is, what? I seem to have a vague memory that it draws more current, and damages capacitors?

So for devices designed to run at 12V, and given a car battery is 13.7V nominally, What voltage is 'flat'?

Cheers
Markus

Outcast (Carlton) · #4 11-05-2020, 07:35 PM

Quote:

Originally Posted by Stonius

So for devices designed to run at 12V, and given a car battery is 13.7V nominally, What voltage is 'flat'?

Cheers
Markus

Can't comment specifically on what damage might be done however, if it's any help, my AZEQ6 flashes it's light at me (this is in indication of under voltage) at about 11.6V...

This is a fairly consistent figure with most 12V equipment in my experience... whilst some things will let you go lower without too much risk..

The question is... as you say... what can our gear safely tolerate... I'd suggest you don't want to go below about 11.8V as a margin of safety.. but, that's just my 'best guess'

For most of the 'brand name' gear, you should be able to find current draw on the manufacturer's site.. or sometimes, if it came with a 240v power supply, it might be on the power supply...

I have the cheap 5v Chinese dew heaters too, from memory, they work on about 2.5A @ 5v... so, I use approx 1A draw in my 12V calculation, which allows for the fact they aren't on all the time too...

For your camera, I know the ZWO's call for 12v, 3A supply for the cooling system so.. that might be your starting point?

Experimenting could be expensive... as you say, depends what happens to your gear if you undervolt...

redbeard (Damien) · #5 11-05-2020, 07:53 PM

Never let the lead acid battery go below 12v, you will damage it.

Did I mention.......

You can get a voltage meter and see what is going on and gently shut things down when the volts get to 12.0
There is also a voltage cutout you can add that will stop power when it has reached 12v. That's to protect the battery only.

Stonius (Markus) · #6 11-05-2020, 08:06 PM

Quote:

Originally Posted by redbeard

Never let the lead acid battery go below 12v, you will damage it.

Did I mention.......

So...does that mean 11.6v is okay?