In article ,
Doug Smith W9WI wrote:
Dave Platt wrote:
It's not a bad one-afternoon project and the components
are the sort of thing most homebrewers are likely to have in their
junk-box.

Schematic is at http://www.radagast.org/~dplatt/hamradio/charger.pdf

Interesting circuit.

I have to beg a fair amount of ignorance about battery behavior. I've
got a couple of the smaller sealed lead-acid batteries - the one on the
desk right now is 4Ah.

Judging from what (little) I know about these batteries, I presume it
would be safe to use your circuit as-is - that the maximum 200mA
charging current, while intended to protect the unregulated source,
would also be low enough to not overheat the battery.

Should be fine. According to the Power-Sonic Technical Handbook
(http://216.87.171.32/manuals/techman.pdf) charge currents should not
be allowed to exceed 0.20 * C amperes - which would be 800 mA for a
4Ah battery. I prefer to stick to 0.10 * C amperes if I can afford to
wait overnight for the battery to charge.

I mis-stated the actual current limit provided by the schematic I
posted... this schematic is a modified version of what I actually
built, and has a higher current limit which would allow it to
be used with a larger wall-wart. The current limit is set by the
value of R4 - the charger goes into current-limiting when the voltage
drop across this resistor reaches 0.7 volts and turns on Q1. The
1-ohm value in the schematic sets the limiting point at 700 mA (not a
bad value to use when charging a standard 7Ah gel cell). 3 ohms would
give you a bit more than 200 mA.

In actual use, when charging gel cells, you're likely to find that the
charging circuit only goes into current-limit if the battery is quite
deeply discharged. Once the battery charges up to a significant
fraction of its total capacity, its terminal voltage will rise up to
13.5 (or whatever float voltage you've tweaked the charger for) and
the current flow will decrease, bringing the charger out of limiting.
It will then charge at a slower (and steadily decreasing) rate until
fully replenished, and will then draw perhaps 0.01 * C amperes of
float/trickle charge current indefinitely.

In order to charge the battery more rapidly, you'd need to set the
charge voltage up to 14.4 volts or so (the "fast charge" voltage
region) and then set it back down again once the current flow had
dropped below 0.01 * C. Commercial battery chargers often use this
sort of dual-voltage charging scheme to allow for faster recharge.

Note that you should _not_ use this sort of "higher voltage for fast
charging" technique when charging starved-electrolyte (glass mat)
lead-acid batteries - they are rather intolerant of higher charge
voltages and should not be charged at above 13.8 volts. However,
using a higher charge voltage really isn't necessary with these
batteries... they'll soak up charge amazingly fast at 13.6 - 13.8
volts.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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