ETX TECHNICAL TIPS Last updated: 4 February 2004

Cold Weather Battery Performance

Date: 2/4/04, 12:11
From: Don Sutherland (don_sutherland@yahoo.ca)
Often, it is  very cold where I am. The telescope and accessories need
to sit outside for an hour or two to reach thermal equilibrium, so the
batteries get just as cold as the rest of the telescope does.

There are two options; install batteries in the telescope that work in
the cold or use an external battery pack that is kept indoors until the
observing session starts.

You can keep a battery pack in the warmth of your pocket, with the cable
extending out from under your jacket. However, the chance of
accidentally yanking the plug out and losing your alignment is very
good.

Radio Shack has a 8 AA cell holder (270-387) that fits very nicely
between the 884 tripod base and the tilt-plate. Strips of self-adhesive
foam can be used to make a slot for the holder to sit in.

Radio Shack has a plug that fits the ETX power jack (5.5 mm o.d. 2.5 mm
i.d, 274-1573A), but the solder tabs on this plug are quite flimsy so it
is better to use Radio Shack's "N" type Adaptaplug (273-1717) and
insulate the connections with heat shrink tubing. This is a right-angled
adapter and it is more resistant to damage and to being pulled out by
accident, than the straight type of plug.

Radio Shack also has a nice heavy duty 9-volt battery connector
(270-324) which fits their battery holders. The connector can be popped
open to allow the leads to be replaced with longer and thicker wires.

I considered using NiMH, alkaline and rechargeable alkaline AA cells to
run my ETX. I did not consider NiCad's because the capacity of a AA
NiCad cell is less than half that of a NiMH AA cell.

I tested  three cell types for cold weather performance by putting them
in my freezer at -1°F for 2 hours. The batteries were discharged into a
30 ohm resistor, which is equivalent to the current draw of an ETX with
both axes slewing at maximum speed.

The following table shows how the different cells performed.





Note that OCV is the open circuit voltage of the battery at the
beginning of the test. I was surprised how poorly the rechargeable
alkaline cells performed. The cells had been run through about 10
charge-discharge cycles prior to the test, as were the NiMH cells.

The clear winner was the NiMH cell. Even though it starts at a lower
voltage than the alkaline cells, its internal resistance stays low and
it delivers a much more constant voltage than the alkaline cells do. I
used Eveready Rechargeable NiMH AA cells, which are available at Walmart
in four-packs at a reasonable price. The manufacturer also claims they
can be recharged 1000 times. In a pinch though, regular alkaline cells
will do the job.

I hope this helps.

Don Sutherland

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