"Scotty, I need more power!"
With all the electronics now in use when we observe, some folks are finding an almost insatiable need for electric power in the field. My power requirements are fairly modest, and I've discovered that buying a power pack from a telescope supplier is not very cost-effective.
Six years ago, I purchased a 12VDC, 17AH rechargeable battery pack from Kendrick Astro Instruments. There weren't quite so many of them on the market then, and this seemed the best way to go. It came with a single power outlet, charger, and case and cost about $120. Kendrick now sells a comparable (18AH) unit for $148. Recently I've encountered some erratic drift in the drive of my Losmandy G11 mount. I thought a possible cause could be the power supply, but every time I hooked up the charger, the pilot light glowed green, indicating a full charge. That simple pilot light on the charger was an annoyance; it was either red (meaning the battery needed charging) or green. There was nothing on the battery itself to indicate the state of the charge. I had recently noted that the slew rate indicator on the mount's control panel was flashing. The G11 manual showed the mount was telling me there was insufficient power. The battery charger light still glowed green. Time for a new battery.
Orion sells a 12VDC, 17AH power pack for about $90; it's almost identical to the comparably priced Celestron "Power Tank". But places like Walmart sell perfectly usable power packs for a lot less. I ended up paying $59.95 for a 19AH ("Vector" brand) power pack at the Farm & Fleet store in Sycamore. It has most of the features of the higher-priced ones ... and a battery charge level indicator. The Orion unit has two 12V outlet ports, plus 3V, 6V, and 9V male pin outputs. The Vector unit only has a single 12V outlet, but that's all I need. Vector does market a splitter if an additional outlet is needed.
There are less expensive power packs with a lower AH (ampere hour) rating, but that will limit how long they can power your scope, especially if you're running other accessories. The unit I purchased seems to work just fine, so why spend more money than necessary?
When you're in the midst of an observing session, you don't want your battery pack to tell you "I canna give ye any more power, Cap'n!" (With apologies to Star Trek.)