Re: [APML]: Gell Cells vs Deep Cycle Marine Batteries for powering a scope

[Chuck Vaughn <aa6g@aa6g.org>]

Jerry Lodriguss wrote:

> I'm interested in purchasing a battery to power my scope setup for when I do
> astrophotography in the field.  All of my equipment (Drive corrector, ST-4,
> and anti-dewers) draw about 3 amps.
> 
> I was looking at maybe a 40 amp hour battery minimum, but I have a couple of
> questions on gell cells vs lead-acid deep cycle marine batteries.
> 
I can tell you about gell cells but I haven't used the deep cycle
batteries.
I've used them for years for astronomy and currently have 40 and 33 amp
hour
batteries.

> Any difference in performance in cold weather, near freezing and
> sub-freezing temperatures? Would keeping the battery in an insulated
> container help here?

As far as I know all of these batteries have lower capacity in cold
weather.
 
> Can you recharge either one of these batteries from a running car engine?

Gell cells need to be charged as follows: Apply a maximum of 2.4V per
cell
(6 cells in a 12V battery = 14.4V) and the charge current must be
limited to
battery capacity divided by 5. That's 8 amps for a 40 AH battery. You
really
need an adjustable voltage/adjustable current limiting power supply.
Charge
the battery until the current drops to about 100-300mA for a 40 AH
battery.
Disconnect the battery from the charger when charging is complete.

Alternately you can "float" charge these batteries which means leaving
the
battery across the charger. You can do this for the life of the battery
if
you want which is about 5 years. To float charge, reduce the voltage to
2.25
to 2.3 volts per cell. Float charging is often used in battery back-up
systems to instantly switch to the battery if the power supply fails. 

> Would the alternator/regulator (I know next to nothing about electricity!:-)
> ) in the car put out too much, what? "juice" that would damamge either type
> battery?

Over charging is one of the few ways you can damage a gell cell.
 
> Do either of these types NOT like to sit around unused for extended periods
> of time (couple of months)?

Gell cells should be charged at least once every six months. If the
battery sits
around too long a chemical process occurs in the battery that causes it
to be
unable to charge to full capacity. This is why it is a bad idea to buy
those
"new" batteries at the flea market. Generally they are unsold batteries
that
sat on the shelf too long and are now for sale cheap at the flea market.

You cannot over-discharge a gell cell. They are not subject to cell
reversal
like some batteries. They also do not have the "memory" effect like some
Ni-Cads.

Gell cells are more expensive than deep cycle batteries. A 40AH on runs
about 
$100. I got a lower price ($65) on my 33AH one because it was a
wheelchair battery
and they sold a lot of them.

Chuck Vaughn  <aa6@aa6g.org>