This is a long post but is worth it for those wanting to power their systems remotely.
I've done abit of research on batteries to run the Obsessionator and have found a few points that might be useful here.
Usually these power packs use a standard sealed SLI (starter,lighter,ignition) lead acid battery (car and UPS batteries etc), if so, I wouldn't use this battery pack on your telescope system.
These SLI lead acid batteries they are not designed for long duration use but are used for high power short interval applications such as starting a car engine.
If you discharge more then 20% of its capacity, you can permanently damage the battery, if you go alot further you will definitely damage it, do this a number of times times and what you'll find is that the time it runs your scope in the field will become shorter and shorter and a time will soon come when it might not work at all.
In this case you'll need to replace the battery in the pack.
I used to go through a car battery every 8 months or so when I was using it to run my LX200, I couldn't figure out why, but now I know that I was draining them way beyond their ability to come back to life after a recharge attempt.
If you have a low power system like say an ETX, then get your self a cheap voltmeter and make sure that your SLI battery never runs below 12.5 volts (a 12volt battery is actually 12.65-12.7 volts), when it gets to that point turn the scope off.
For a higher power system, what you want is a reserve capacity battery such as a deep cycle battery a leisure battery or a marine battery (preference in that order). Its preferable to get one that isn't sealed so you can top up the electrolyte when needed. These batteries can discharge 70% and be recharged with no ill side effects.
One rule with all batteries is to recharge them ASAP after use, do not leave them lying around discharged.!!
LEISURE/DEEP-CYCLE BATTERIES
All the below applies to deepcycle/leisure/marine batteries only.
A word of advice!
Take care to calculate all your 12 volt items and their requirements before the initial purchase as a lightly used battery will eventually be the most economic.
Leisure batteries are designed to give a steady low amp discharge rather than meet the sudden high power demand required from a car battery to start an engine.
The life of a leisure battery is largely determined by the depth of the discharge cycle. We will say as an example that a leisure battery that is discharged each time by 15% of it's capacity may last for ten years. The same battery discharged by 30% of it's capacity will last for 8 years, 50% for 5 years, 75% for 3 years and possibly being discharged by 100% lasting only 1 year.
A 100 amp hour battery theoretically delivers 100 amps for 1 hour. Or 10 amps for 10 hours etc in theory, but in reality you should refrain from draining the battery more then half of its capacity for longevity reasons.
Power is amp x volts, so 100amps at 12v = 1200 watts, or 1,2kW 1.2kW for 1 hour is 1.2kWh.
Any form of heating element will cause a rapid drain on a battery, so know in advance its power requirements,
All batteries self discharge when not in use and leisure batteries will self discharge 7 to 10% monthly.
How to kill your battery dead!
1/. Uncontrolled Charging causes overheating and the releasing of hydrogen gas, dont recharge in a small confined room in the house (and here's looking at you Frank) unless you want to blow your house up
2/. Used too little (memory effects, looses amphour capacity, so cycle every 2 months)
3/. Stored discharged (sulphating, any flat battery left flat will be scrap within 3 months)
4/. Do not store on a cold surface as this rapidly increases the self discharge rate.
5/. Too much deep cycling (a good rule of thumb is not to go beyond 50% of the battery's endurance at worst)
Volts - State of charge
12.7V or over — 100%
12.5V — 75%
12.4V — 50%
12.2V — 25%
12V or under — Discharged
When battery manufacturers state that a battery has a capacity of 110 Ah they presume the battery will be operating where the ambient temperature is 25 degrees centigrade. For every 1 degree centigrade drop in temperature the battery capacity falls by 1%
The time required to recharge a leisure battery can be calculated as follows. The recharge time in hours equals the battery capacity in Ah, multiplied by the Depth of Discharge in %, multiplied by 0.8, multiplied by 1.5, divided by 100 times the charger current rating in Amps, plus one hour. For example, a 110 Ah battery, discharged to 50%, on a 10 Amp charger, would take about 7.6 Hours.
This is what I use to power the obsessionator.
http://www.irishastronomy.org/boards/vi ... 88&start=0