Telescope Reviews: 12volt DC Power Supply Amp/hrs Needed

I'm going to grossly oversimplify here...

Calculate the projected load you're likely to be drawing, then multiply that by the amount of time you expect to be drawing that load. Now double the result, because you will have underestimated. Everybody always underestimates their projected loads.

For this example's sake, let's say the figure you came up with was 30. Maybe 3 amps x 5 hours observing x 2, which is not unrealistic when you consider dew heaters and other accessories. So what capacity battery do you need? 30 Ah?

No. Because if you draw 30 Ah out of a 30 Ah battery, you've irreversibly damaged it. Oh, you'll still get some juice out of it for a while longer, but batteries aren't supposed to be deep discharged and won't react well. You'll find it just won't hold a charge for very long and become useless.

Ideally, you shouldn't discharge the battery by more than 20% of its capacity. Therefore in our example you'd need a 150 Ah battery to prevent discharging below 20%, quite a hefty weight to lug around with you.

If all you're powering is an ETX, without optional accessories, then 3 Amps average is likely quite an overestimate in most cases, and a 150 Ah battery is therefore overkill. Also, you can repeatedly discharge modern AGM-type cells to 50% and generally get away with it unscathed.

The revised example may then become 1 Amp avg x 5 hours x 2, a total of 10 Amps over a night's observing run. Discharging to 50% means you only need a 20 Ah battery, not too huge at all.

Another important factor is the discharge rate. A battery will last much longer if you suck amps out of it slowly. The faster you discharge it, the lower the capacity you'll get.

Look on the battery label, there'll be a "C" rating. That's the discharge rate at which the battery will supply the stated Ah capacity. If it says "150Ah, C10 @ 20 °C" then you're supposed to get 150Ah over 10 hours if the ambient temperature is around 20 °C. You'll get more amps if you discharge the battery at a slower rate, much less if you do so at a faster rate.

The temperature aspect is quite important too: battery lifetimes are prolonged in the cold, but their capacity diminishes very rapidly as the temperature drops. (In high temps, a battery will deliver amps above its rated capacity, but the heat ruins batteries, whereas cold generally won't.)

Sorry for the Battery 101 lecture, especially if you already know this stuff. But most people ruin good batteries very quickly because they're unaware of the damage they're doing by using one with insufficient capacity.

I say get the biggest battery still practical to haul around and you'll be fine. Just make sure you recharge it properly as soon as you can: batteries also don't like being left in a discharged state.