"Jim Wilkins" wrote in message
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"Ignoramus30995" wrote in
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[Old batteries] also need an ammeter to show if they aren't
accepting a charge despite apparently correct charger output
voltage, which might be your real problem. A solar panel charger
forces current at whatever voltage is necessary. The danger is their
open circuit voltage 20V which could damage electronics if the
battery refuses to accept charge current.


This is the accepted reason why batteries fail:
http://www.batteryrestoration.com/im...0Failure.pd f

The symptom is increasing intenal resistance that eventually won't let
the battery accept charging current at the usual float voltage.
Increasing the voltage to 16 - 17V may force enough current in to
begin slow recovery. The charger needs to limit the current which may
otherwise rise quite high at 15V or above when the battery recovers.
Pulse reconditioners automatically apply high enough voltage at
limited current, though I'd rather watch the progress with a metered
DC supply and adjust the charging current to a few percent of the
Amp-hour rating of the battery

I've made neighbors' "dead" batteries last for several more years that
way, but they needed annoyingly frequent attention. Since I don't like
trickle charger power cords lying around I check and top them off on a
schedule.

When old batteries reach that condition the trickle charger voltage
doesn't tell you enough. I bought the HF carbon pile tester to measure
how much starting current is really available. A cheaper fixed
resistance load tester is nearly as good.

-jsw