Battery capacity needed?
 

I'm looking at setting up my extra dual bander and my 10m rig in an old
metal desk for base station use, and one of the local stores is closing out
their old brand of 7AH 12V gel cell electric fence batteries. Given that
each rig has a maximum draw around 8-10A at full power transmit, and few mA
otherwise, that I won't be transmitting simultaneously on both, and that I
will likely only have 500mA-1A of continuous charging current available,
how many batteries should I plan to stuff in there? I do have the ability
to charge at higher current temporarily if they become depleted during a
power outage or something, but it's not convenient for daily use. (that
charger has other uses that are generally more important)

hi joe,

http://www.eham.net/forums/EmergencyCommunications

lots of info on this topic and many others related to emergency comm.



"Joe Bramblett, KD5NRH" wrote:

I'm looking at setting up my extra dual bander and my 10m rig in an old
metal desk for base station use, and one of the local stores is closing out
their old brand of 7AH 12V gel cell electric fence batteries. Given that
each rig has a maximum draw around 8-10A at full power transmit, and few mA
otherwise, that I won't be transmitting simultaneously on both, and that I
will likely only have 500mA-1A of continuous charging current available,
how many batteries should I plan to stuff in there? I do have the ability
to charge at higher current temporarily if they become depleted during a
power outage or something, but it's not convenient for daily use. (that
charger has other uses that are generally more important)

Joe Bramblett, KD5NRH wrote:

I'm looking at setting up my extra dual bander and my 10m rig in an old
metal desk for base station use, and one of the local stores is closing out
their old brand of 7AH 12V gel cell electric fence batteries. Given that
each rig has a maximum draw around 8-10A at full power transmit, and few mA
otherwise, that I won't be transmitting simultaneously on both, and that I
will likely only have 500mA-1A of continuous charging current available,
how many batteries should I plan to stuff in there? I do have the ability
to charge at higher current temporarily if they become depleted during a
power outage or something, but it's not convenient for daily use. (that
charger has other uses that are generally more important)



You could do a bunch of research on this. That's advisable.
Otherwise here's a general estimate: 4 or 5 batteries. In very rough
terms: while drawing transmit power, that's 2 - 2.5 amps per battery.
Figuring 5 batteries, say you transmit 20% of the time, and using the
radio for 1 hour. That's 12 minutes at 10 amps, so you use 2 AH
hours of capacity. Assume a charge rate of ~ 350 ma with the radio
off, and a receive draw of 350 ma - so during receive there is no
battery drain or charge. You need to put 2 AH back into the battery,
and charge efficiency can be esitmated at say 80% - so the total
charge must be about 2.4 AH. You'll need to charge for a bit over 4
hours for every hour of use.

Your 500 ma - 1 amp charger could support twice the number of cells
at 750 ma, cutting your charge time to a bit over 2 hours. With ten
batteries, you could run for 3 - 4 "transmit hours" at a 20% duty cycle
or about 20 hours total. That's figuring discharging the batteries 50 %.
You DON"T want to discharge them completely. In fact, you should
plan on including a low voltage shutdown circuit to protect the batteries.

Note how I'm getting you into bigger and bigger dollars. Say each
battery costs $10.00 I've run your cost from $40 or $50 to $100,
and that doesn't include cables, connectors, relays, whatever. That's
one reason it is more advisable to do some research. Another is to
get some real numbers on your batteries in terms of discharge rate,
charge time and so forth. How they are charged and discharged
has a direct bearing on their longevity. They are easy to ruin with
incorrect charging rate or too deep a discharge.