Note that an LX200 mount is a MUCH bigger mount than an ETX and made to carry much greater loads so it will draw more power. Many consider this an observatory class set-up.
A note about capacity.
I work a lot with batteries for my electric airplanes and gliders.
Capacity, measured in Amp Hours, AH, or miliamp hours mAH tell you how much capacity the battery has. However it tells you nothing about how fast it can deliver the power. 1 AH = 1000 mAH
Any rechargeable pack that is rated for 3AM or 3000 mAH, should be able to run your ETX. That pack will be able to put out 1C at a minimum, which would be 3 amps. Your ETX 90 will likely never pull 3 amps unless you are hanging cameras and stuff off of it and even then I doubt it will pull 3 amps. The only way to really know is to put a meter on it and measure the draw, but these are my estimates. Or call Meade and ask if they publish an amp draw for the mount. I don't think they do.
The higher the capacity rating of the pack, the higher its amp delivery capability. So a 6000 mah or 6 AH pack that is rated for 1C can deliver 6 amps. You don't need that much so the pack will not be working hard at all and will last longer. Most lithium and NiCd packs can deliver more than the 1C rate.
All batteries experience a voltage drop as they discharge. That goes for AA dry cells, car batteries, jumper packs and the like. They all do this. This is the basis of a capacity meter that some packs offer. They read the voltage and know how much capacity is left.
As this is normal, the mount will be able to handle it as long as the pack isn't approaching discharge. If you never use more than 50% of the capacity you will likely never experience a significant voltage drop and everything will be fine. A Lithium pack is likely 12.4V when fully charged but will drop to about 11 for most of its useful capacity under a 1C load. The mount will handle that just fine.
If we assume that the scope is slewing 100% of the time, drawing about 2 amps then, if you run that for an hour you will use 2 amp hours of capacity or 2000 miliamp hours of capacity. So you would want at least a 4 AH or 4000 mAH pack for a 1 hour session.
But normally we slew and then we track. During tracking the amperage demand drops dramatically. It may be less than .5 amps. If you tracked for 1 hour you could use .5AH or 500 mAH.
So, how long is your typical session, or how long do you want to plan for? If you use an average of 1 AH of capacity for every hour you run the mount then you want 2-3 AH of capacity for every hour you run the scope.
This does not take into account cameras, dew heaters or other accessories, only the mount. If you put a lot of weight on the mount it will draw more.
If you are doing AP where tracking errors must be kept to an absolute minimum, then Multiply that by 4 so you never get down to more than 20% discharged which will minimize the voltage variation and lead to better tracking accuracy.
So, going back to your original question. Is a 6AH pack big enough? I would say that should be good for about 1.5-2 hours in cold weather and 2-3 hours in warm weather. Likely it will last longer than that, but I am talking about the minimum you can expect as every session will be different and you want to have plenty of extra capacity.
If you can get a 10 AH, that is even better. 15 AH, even better. More capacity will never hurt you.
Edited by aeajr, 25 November 2019 - 10:54 AM.