[aprssig] GelCell Charger won't work off 13.8V

Jim Lux jimlux at earthlink.net
Wed May 4 14:06:30 EDT 2005

Those LM317 based chargers will NOT work off 13.8V, if for no other reason 
than the output voltage won't go high enough to charge. Not only is there a 
diode voltage drop in series, but the LM317 requires a certain minimum 
voltage across the regulator to work.  As the schematic at 
http://www.projectx.com/kits/gelcell/gcc01v1.pdf says, you need 18 to 24 V 
to make it work.

If you figure you need 14.4V to charge, you need to supply the charger with 
around 18V to get a couple volts drop across the regulator plus the 0.7V 
across the diode.

THere's also the problem that these chargers aren't very well optimized for 
rapid charging. In order to avoid cooking your battery, you'd need to set 
the output voltage (at no load) to low enough for the trickle regime, so 
that will limit the charging rate dramatically.

Real battery chargers do something like:

Current limited for first part of charge letting the voltage rise as the 
battery charges until it reaches some trigger voltage (14.4V?), then, 
dropping back to a constant voltage charge at 13.x volts to maintain the 
charge.  Typically, they'll periodically (tens of minutes intervals) bump 
up to the higher voltage for a second or two, to see if they need to switch 
back to the high current charge mode.

To do all this from an unregulated battery source (i.e. automotive 
electrical system) really means using some sort of switching boost/buck 

Later on in the digest, Jason wrote:
 >Cool, thanks very much. That's exactly the item I need. There isn't any
difference to the charger between a Gel Cell and a Sealed Lead Acid, is

 >>> YES there is a difference.  There are several forms of battery out 
there, ranging from traditional lead acid, to maintenance free lead acid 
(which can still spill), to Valve Regulated Lead Acid, to Absorbed Glass 
Mat.  They all have different required charging profiles, if you want to 
get optimum life. They also have very different current output 
designs.  Car/Marine batteries are designed for high current surges lasting 
a short time, and low loads the rest of the time. Classic "deep cycle" 
batteries are designed for moderate loads that would discharge the battery 
in, say, 4-8 hours. UPS batteries are designed for high loads that would 
discharge the battery in 30minutes to an hour. Standby power batteries (as 
for alarms and the like) are designed for very long discharge cycles (20 hrs)

If the batteries are cheap, you may not care if they die in 20 cycles 
instead of 1000.  For instance, a consumer UPS battery isn't really 
expected to be discharged more than a few times in the life of the UPS. 
Furthermore, you don't need fast recharge in a UPS, so a simple constant 
voltage float charger is cheap and works well. At least, in consumer 
UPSes.. commercial UPSes do care about recharge time and battery life, and 
cost a lot more, too.

Also, since we're talking about batteries, I bought some Yuasa MP2.3-12 12V
2.3Ah SLA batteries for tracker use. Their rated "Stand By Use" charging
stats are 13.5-13.8V and "no limit" on current. (You may see where I'm
going with this.)  Is there anything wrong with hooking it up to an unused
port on my RigRunner? (Fused, of course, but for what amperage?)

 >>> Is your RigRunner regulated to 13.5-13.8V? Have you looked at the 
datasheet for those batteries? Is that really a NP2.3-12? 

Perhaps reading over the application manual

might be a useful thing to do? (Page 11 (page 13 in the pdf) is where all 
the info on how to charge is).  They show charging with constant voltage 
charging limited to 0.1CA  (that would be 230 mA for your battery) and 
.25CA (575 mA)  (the note on the 2.5V/cell (15V) at 0.25CA says 
"recommended for overnight charge only")

Page 16 of the manual (page 18 in the pdf) shows the "recommended method 
for charging" (as described above)

Another complication in the whole charging thing, important if you're 
putting your batteries in a car or outdoors in an equipment cabinet (as 
opposed to the benign environment faced by a UPS sitting under your desk), 
is that charging voltage should be regulated according to temperature.  As 
temperature goes up, the charging voltage should go down.

Finally, we come to your current limiting question. Take a look at page 
19.  "It is recommended that charging current be normally limited to 
0.25CA."  and it goes on to say that in standby use, no current limiting is 
required, because the battery is designed to take an overcharge.

Are you going to be using these batteries in a "standby" scenario (float 
charge, almost never discharge)?  If so, the life is several years, 
depending on the temperature (20-22C is design temp).  If you're cycling 
them, then you'll get between 200 to 1000 cycles (lower number for 100% 
discharge, higher number for 30% discharge) before the capacity decreases 
to 80% of new.

Temperature is the killer for batteries.  10C rise (18F) will halve the 
life.  Think twice about how long you expect that battery to last in the 
40C trunk of the car.  If you discharge it 50% every day, you might get 
only 3 months service out of it.

 >>>I'm perplexed by the "no limit" current. I assume that means that the
battery can't be damaged by too much current when charging at that level.
(The Cyclic Use stats are 14.4-15.0V @ .58A max.)

Precisely.. The 0.58A is the .25CA value, and is for the 2.4-2.5V/cell 
charging regime.

More than you ever wanted to know about batteries...

Jim, W6RMK

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