How long should a 12v 17amp Powertank last?

[John K]

Hi everyone,

for those that have been using portable power units for a while and are in the know, how long should a Celestron Powertank 17amp 12v unit last when using a laptop computer drawing 15 volts and 5 amps?

I am getting 3 hrs using my laptop computer. Does this sound about right?

If I was to upgrade to a 34amp 12 volt unit, does this mean I would get 6 hrs?

[rogerg]

That sounds about right. Have you measured the actual amp draw of the laptop while running?

The division is straight forward - 17 amp hr battery pack will last 3.4 hours in theory. I say in theory, because all batteries are designed to run at a certain amp draw, above that and they will not last as long. Your laptop draw is probably greater than the rate draw of the SLA inside the 'Powertank', so you'd be losing some time because of that.

Better quality batteries can handle high current draw rates, I believe. Typically 'powertank' type off-the-shelf things have cheap batteries in them.

You should however find that your laptop doesn't draw 5 amp all the time, that's quite high. My Toshiba draws about 2.5 amps while doing autoguiding through my webcam with the screen off, about 3 with the screen on.

See this thread for more info that might be helpful:
http://www.iceinspace.com.au/forum/showthread.php?t=14245

[janoskiss]

I guess you mean 17 amp-hour unit??
Capacity of battery is 12 V x 17 Ah = 204 Wh
Energy used by laptop is 15V x 5 A x 3h = 225 Wh

so if your figures are correct, you are doing very well with that powerpack. You are getting an extra 21 Wh of energy for free! :) If you want the battery to last though (i.e. last many charge cycles) you should probably recharge it sooner, say before you drained 2/3 of the capacity, i.e. use laptop for 2 hours.

Capacity is proportional to amp-hour (Ah) rating, so you are correct that a 34 Ah battery will give you twice the energy as a 17 Ah one. Radio Parts have lead-acid batteries at very good prices in Melbourne.

[Doug]

G'day John,
I'm not sure why you would want to run a 15v laptop on 12V, but anyway, what the ratings mean is that a 7amphour battery from full charge should deliver 7 amps for 1 hour, or 1 amp for 7 hours. So, if you were drawing 2.5amps it should last for about 2h 48mins before the power would fall off.
A 17 amp hour battery at the same 2.5 Amp load would last for around 6 hours 48 minutes. useful life per charge = Amp hour rating/current drain.
HTH
Doug

[rogerg]

Oops, I missed the bit about the 15v so didn't factor that in to my calculations. My laptop, while it operates from 15v runs through a converter (transformer? inverter?) from 12v. The amp draw is measured right back at the battery, so when I say my laptop draws about 2.5v, that's at 12v.

[Starkler]

Usually the A/h ratings are given for a 20hour discharge rate, ie a 20 A/h battery will supply 1amp for 20 hours but it certainly wont give 20 amps for 1hour, or 10amp for 2hours.
If you want to draw more than the 20 hour discharge rate , you have to take this derating into account.

To answer the original question, Id say you should be happy to get 3 hours. If you double the capacity , you should get a bit more than double the time from it due to the comparitive lighter loading.

[Doug]

Yup no probs, not many people would try to drag full current for a prolonged period anyway...................would they?

[John K]

Thanks for all the comments and suggestions guys, great info.

I have just checked the adaptor that I have for my laptop and in fact I think that it draws 13.8V at 10 amps and converts this for the laptop to 15 volts 5 amps.

In any case it sounds like 3 hrs is not too bad.

My JMI NGT scope has a Losmandy drive which I think draws 5 amps, so will now try both together.

Would say that if I get into full blown astrophotography that I will need a 34 amp unit or better, or an additional 17 hr amp unit.

Any other suppliers that you know of in Melbourne? or is there a way to put together a 60+ amp rechargable battery unit?

[rogerg]

If your laptop is going to be drawing 5 amps and your telescope is going to be drawing 5 amps, I'd be seriously considering running them off independent batteries so the one battery doesn't get power sucked out of it at 10 amp.

I use a 40 amp hour battery bought from http://www.12volt.com.au/

[Doug]

An alternative that you might like to consider would be a small 2 stroke 240v generator to power some mains operated power supplies.
It would depend a lot on your exact situation. If you are only going to use the batteries occasionally, you might not get as much service life out of them as you would wish. On the other hand frequent use would relieve you of the problem of running a long extension lead; on the other hand yet again, the generator could charge batteries on long astrocamps if that was what you are able to do. Just an alternative to consider against your intended usage.
Cheers,
Doug

[Don Pensack]

When I used a laptop in the field, my inverter lost as much power as the laptop took. I ended up using a 108 amp-hour dry cell deep-cycle battery so it would last an entire night (and it did, but barely!).
You have not only the self-protection aspects to most batteries that keep you from draining them completely, but you also have cold to deal with. My charged battery measured 13.1V at room temperature, but the same battery when cold was barely 12V. The cold significantly reduced the capacity of the battery.
So a word of advice--keep the battery in an insulated box when it's in the field and it will last much longer.