J Shrum wrote:
(top-posting fixed)
"J Shrum" wrote in message
...

Hey guys... I've spent lots of time looking over google archives for the
answer to this, and God knows this has been covered many times, but I've
not quite found my answer.

I have the following QRP power supply/charger
http://www.mfjenterprises.com/produc...rodid=MFJ-4114 .
Basically it comes standard to charge a 12 nicads, but I took out the
nicad holders and put in a 12v 5AH gel cell. I modified it to charge
directly from the 13.8v power supply instead of the 18v nicad charger
circuit. I charged the battery to about 13.8v (w/ no load) by floating. If
I measure the battery while having my radio on receive, which only draws
maybe 80mA's or so, I see the battery dropping by .10's of a V in no time.
The battery is brand new.
I've noticed that the battery only draws about 30mA's while charging at
any voltage. I even added a switch so I can switch extra diodes between
the regulator ground so I can charge it at 14.X volts.

So my question... If it measures 13.8-14v after charging, it should be
fully charged, right? Is there something going on since it is only drawing
10's of milliamps while charging? I've read that they charge at several
hundred milli's. I'd like to use my QRP rig remotely, and at the rate its
dropping, I just don't think it will serve the purpose.

Thanks for all the help.

Jim, AB9LM





Me again.
Thanks for the info. I just decided to run it down to see how long it
would go. Like I said before, after fresh charge, and the battery
showing 14v, after applying the slightest load, the voltage started
dropping... it dropped down to 12.6 in no time, but.... its been
staying there. I've had the radio on for a couple hours, and its not
moved from 12.6v. I even keyed a bunch, and that draws about an amp.
Seems its holding steady at that voltage under load.

Thanks again guys, your always more than helpful.

Jim, AB9LM

I was going to suggest that you do this. If you have the patience keep
it up until the battery voltage really starts doing a nosedive. Radio
Shack sells (or used to sell) a DVM with a serial output and a little
scope program that was nice for collecting this sort of data.

"Rechargeable Batteries Application Handbook" by the "Technical
Marketing Staff at Gates Energy Products" indicates that a SLA battery
isn't really discharged until it's down around 1.8V per cell. I'm not
sure that I'd take it that low, but 12.6 - 12.8 is what you expect from
a six-cell lead-acid battery that's not being charged, and it will hold
that voltage for 70-80% of the time that it's discharging.

It's a good book, by the way. It's no more technical than the ARRL
Handbook, and it has slews of information about NiCd and lead-acid
batteries. It's a bit light (and a bit out of date) on NiMh batteries
and it came out before Lithium Polymer batteries were on the market, but
other than that it's a fine resource.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com