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Path: utzoo!watmath!clyde!bonnie!saf
From: saf@bonnie.UUCP (Steve Falco)
Newsgroups: net.auto.tech
Subject: Re: Maintenance-Free Batteries
Message-ID: <694@bonnie.UUCP>
Date: Sun, 19-Jan-86 12:12:04 EST
Article-I.D.: bonnie.694
Posted: Sun Jan 19 12:12:04 1986
Date-Received: Mon, 20-Jan-86 05:43:06 EST
References: <216@hropus.UUCP> <128@linus.UUCP>
Organization: AT&T Bell Laboratories, Whippany NJ
Lines: 43

> Is there
> any harm in pumping 2 amps or so into a fully charged battery for a few
> hours? If all it does is generate heat, it could possibly help by raising
> the temperature of the battery a few degrees. Or am I boiling off or
> electrolysing some of the water?

I think this amount of excess current on a regular basis would be
harmful.  Fortunately, there is a way to check!  Globe-Union puts out a
flyer on their gel-type lead-acid batteries - it should be applicable to
conventional batteries as well.

They recommend 2 charging methods.

Method 1 - used with discharged batteries.  Limit the current
to 3 or 4 times the 20 hour discharge rate.  For example, if the battery is good
for 100 A-hr, the 20 hour rate would be about 5 amps and charging
current should be held to 15 to 20 amps.  NOTE: I don't know what the
amp-hour rating of your battery is - I used 100 just for illustration.
When using this method, you charge the battery until the voltage reaches
2.4v per cell (14.4v for a 12v car) and HOLD that voltage until the current
drops to about 1/5th of the 20 hour rate (1 amp in the above example) then STOP
or drop the voltage to between 2.25 and 2.3v per cell (13.8v).  But the
key is: hold the voltage.  Many cheap chargers won't do this because they have
no voltage regulators - the voltage can easily rise to 16v or more.
Normally, the lack of a regulator doesn't matter because the charger is
used infrequently - and only on a fairly dead battery.  This is of course
why your car has a voltage regulator - to prevent frying the battery
during continuous/long-term operation.  

Method 2 - used to maintain a charged battery.  HOLD the voltage across
the battery to between 2.25 and 2.3v per cell (13.8v).  The battery
will automatically pull the correct amount of current to maintain full
charge.  Once again, most chargers won't do this.

I mentioned above that there is a way to check whether the 2 amps is
harmful.  Simply check the voltage across the battery when you
stop charging.  If the voltage is much higher than 14v, you are
probably dumping too much into the battery.  A final note.  This is
temperature dependent - a cold battery will tolerate more voltage than a
hot one.  I can't lay my hands on a table at the moment - check a good
automotive book.

	Steve Falco