Ron at MTR Design sells a product that may be of interest to members of
the amateur astronomy community. His product announcement is as
follows:

"Dew heaters, cooling fans and go-to motors are wonderful, but don't
inadvertently jeopardize your battery by discharging it beyond
recommended levels. Doing so can significantly decrease battery life
and wind up being a costly mistake.
A variety of low battery alarms are available, but most have a fairly
high price point. The Low Battery Alarm XL can be pre-programmed to
sound a tone and LED lamp to warn you when your battery has reached its
discharge limit. We can offer the alarm at a reasonable and affordable
price.
For ordering information and pricing email or see the
product at www.batterycellmate.com. Click on the Low Battery Alarm
heading at the top of page.
Thank you."