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

[Chuck Vaughn <aa6g@aa6g.org>]

Patrick W. Greenlee wrote:
> It will not supply 10 amps for 10 hours (think of it as a kind of
> reciprosity failure).  Take this into account  when sizing your battery.  I
> am guessing that you were thinking about 3 amps for about 12  or less hours
> and that 40 AH would be fine.  3 amps for 20 hours would be 60 amp hours.
> Given the non-linearity you would want something between 40 and 60 before
> derating for low temps.
> 
Here's something else to consider in regards to battery capacity. As you
use the battery the terminal voltage decreases. Typically on a gel cell
the voltage starts at about 12.5 volts. The manufacturer's amp hour
rating
is when the terminal voltage decreases to 10.5 volts. I don't believe
the ST-4 will run on this voltage. I think the minimum is around 11V.
So besides cold, the amp hour rating needs to be decreased if the lowest
usable voltage is 11V. I prefer 11.5V. Also the AH capacity is
calculated
at 1 amp for so many hours. As Patrick pointed out, >1amp current draw
means a lower AH rating but <1 amp current draw means a higher AH
rating.

And if all this wasn't enough the AH rating decreases over time. The
rating when the battery is new is greater than what it is at 1 year,
2 years, etc. I have graphs on this if you're interested.

If you have a battery and want to find out what the AH rating is to 11V
then buy a 12 ohm/20 watt wirewound resistor, fully charge the battery
and
connect the resistor across the battery and time it until the battery
goes down to 11 volts. Then you'll know for sure. This is how I found
out
that my flea market batteries were not too good.

Despite what appeared on the list, charging a gel cell from a car is
not an acceptable method unless you have a way to regulate the voltage
and limit the current. The info I gave on my previous message is
directly from the manufacturer's data sheets. Of course the
manufacturers
sell chargers.

So Jerry, are sick of batteries yet? :-)

Chuck Vaughn  <aa6g@aa6g.org>