ETX TECHNICAL TIPS Last updated: 18 March 2003

Info on Batteries and Power Supplies

Subject:	ETX-105 and battery power
Sent:	Sunday, March 16, 2003 4:08:43
From:	guenter.nill@gmx.de (Guenter Nill)
First of all, many thanks for a great website, which did not only help
me to purchase the right scope for a beginner in astronomy (an ETX-105
with UHTC; Feb. 2003), but also was indispensable when "tuning" a few
little things, that I just thought could be done in a "more elegant
fashion" than with the standard setup. Very helpful was the hint about
the polarity of the external power input of the ETX-105 (center pin:
positive; 12 V DC). As an "old pilot' of radio-controlled model gliders,

I am quite familiar with rechargeable battery packs (NiCad, NiMH) used
to power the radio control and the motors (in powered gliders). As you
can imagine, if these items fail, the result is not just an error
message in the AutoStar, but rather severe damage and hours of repair
(not to speak about the dollars involved). As, as a chemist by
profession, I also feel, that the use of standard batteries is not only
a cost factor, but also environmentally irresponsible, please let me
share some hints, that may be useful to some of the visitors of your
site.

!. Rechargeable NiCd/NiMH batteries: Even when good care is taken
(microporocessor-controlled charging to the correct final voltage), a
set of 8 batteries almost always contains one cell that is not
performing as desired (lower voltage, lower power output). This is why
experienced RC modelers always "select" their cells. You can do this
only through one of the fancy charging/control systems mentioned above,
by charging, discharging and measuring the charge that the cell actually
is able to deliver when "loaded" with the current that your device
(telescope) actually demands. When the selection procedure is performed
as above, 8 batteries (NiCd or NiMH) with nominal 1.2 V (after charging
~1.4V) deliver enough voltage and certainly current to operate the
ETX-105 flawlessly (If everything else is OK) over ~ 20 hours of
operation, as the rechargeable batteries deliver a more constant voltage
through their "life". As I live here in Germany, I cannot provide
specific recommendations as to the brands and types of batteries for
most of your visitors, but generally, amongst the RC-flyer community,
Sanyo and Panasonic are the most popular brands for high-performance
applications. The same applies to brands of computer-controlled
chargers, but looking at some of the "hands-on" comments of your
audience, I am sure, people will be able to deal with these hints. By
the way, if you do not own or do not wnat to purchase one of these fancy
chargers, a simple test can also help: Charge the batteries for 15 hours
at a current which is 10 percent of their nominal capacity (1,500mAh:
Charge for 15 hours at 150mA current), measure voltage (should be ~1.3
to 1.4V), then let them "sit" for 1 day by themselves and measure again.
If the voltage has dropped to below 1.2V, you can be sure, that this is
a "bad" battery. However, this test does not reveal any problems with
the chemistry when discharging, which means, this still might be a "low
performing" battery during operation. However, the test at least helps
to sort out the "lowest performers". The two types of tests should be
performed whenever you observe any problems with your telescope or every
three months, as a rule of thumb.

2. External power source: As the exchange and charging of the built-in
batteries is rather cumbersome, I have bought for myself a 12V (10 cells
= nominal 12V output) NiCd battery pack, which provides the higher
nominal voltage of 12 V (as an external power supply) and 1,700mAh of
capacity, and should last for about 20-30 hours with an ETX with
AutoStar. It is available in most model stores and sells for
approximately 30$. It can be recharged with a small charging device
(~40$) in about 15 hours at a current 150-200 mA. A quick recharge
(30-60 min. at 3 A current) is also possible with a "Delta Peak"
charger, which cuts off the charging current after the battery pack is
fully charged. This is required to prevent from "overcharging", which
severly limits battery life. By the way, I own battery packs, which are
5 years and older. Usually, the more you use them, the longer they
usually live (within limits, about 1,000 charging cycles can be
achieved). To me, this setup is far more convenient, as you just connect
the external battery pack after connecting Autostar, then you switch
Power to "ON" and there you go. As a last hint regarding polarity of the
external source: The center pin of the power connector (specifications
see elsewhere on Mike's site) needs to be "Positive". This can be asily
ensured, as the battery pack usually comes with installed "Tamiya"
connectors. If you build a connector cable, that has one of these Tamiya
connectors (opposite gender to the connector on the battery pack cable)
on one side and the correct plug for the ETX-105 installed at the other
end (a 1A  quick fuse in the line makes it even safer), you only have to
check polarity with a digital voltmeter once, as the connectors all are
"self-aligning", and you can never erroneously apply the wrong polarity
to the ETX power input socket (even in the darkest night).

I hope this helps and I certainly would like to add the disclaimer, that
all these modifications, additions and recommendations come without any
warranty, neither expressed or implied, that the person performing those
should have a basic understanding of electronics and that extreme care
should be taken when testing and before connecting to the telescope.

I hope that these hints provide some useful inputs to your audience.
Many thanks again for all the valuable help that you are providing and
the tremendous amount of work and dedication that shows on your website.

Best regards
Guenter Nill

Return to the top of this page.

Go back to the Tech Tips page.

Go back to the ETX Home Page.