From: grande@cbnewsd.cb.att.com (brad.l.grande)
Subject: Re: My battery charger exploded!!
Organization: AT&T
Distribution: na
Date: Tue, 12 Jan 1993 13:58:38 GMT

In article <1682@blue.cis.pitt.edu>, jwgst2+@pitt.edu (James W Gourgoutis) writes:
> Hi!  I hate to bring up this subject again, but I was charging the battery for
> my HBL (12 volt, 3 amp-hour) with a manual 1-amp automotive charger, and the
> charger blew out.  My apartment lights got dim for a second, and then I heard
> a popping sound, and then I smelled smoke from the charger.  What did I do 
> wrong?

Did you have positive of the charger hooked up to positve of the battery?  If
not, nasty things can happen because the foward resistance of the cell is very
low and excessive current will generate excessive heat for excessive time.

> The charger had only been hooked up for about 2 hours, and I figure
> that since the battery is a 3-amp-hour battery, it should take 3 hours to
> recharge.  Is this correct?

Charge time is dependent on the remaining charge state of the battery, how
much charge needs to be replaced, and how much current you can deliver.  A low
current charger, such as yours, will take longer to charge an automotive battery
than a higer current one.  The easiest way to determine the amount of time
needed to charge is to measure the current, when it gets very low and stays
low, the cell is charged.  If you can't measure the current, but can measure
the voltage, run the light until the voltage is around 10V or a little less
(DON"T TOTALLY DISCHARGE THE CELL!).  Charge for 1 hour, run the light until
you reach the same voltage and record the time.  Next charge for 2 hours, and
repeat.  When the time to discharge reaches a constant, your charge time is
known.  Also, this method allows you to determine how long your light will
give you service.  It is possible that a bulb at a given rated wattage may
consume more power, also an older gel-cell will have a lower charge storage,
both of which will result in shorter run times than you calculated when you
designed the system.

> 	I was able to exchange the charger for a new one, and I want to make
> sure that this doesn't happen again.  Is there some way that I can monitor
> the charge-state of my battery, so I know how long I need to charge it?

See above and below.

> The charger's package says that even if you leave it hooked up to a battery
> overnight, you cannot overcharge your battery; however, I suspect that they
> mean a full-sized auto battery, not a small one like I am using.  And, to 
> further confuse me, the guy at the auto parts store where I bought the charger
> said that it was not an overnight charger.  So I don't know what to do.

First, never trust ANY INFORMATION from anyone who receives compensation from
the automotive industry (by and away the greatest fraud ever perpetrated on
the american public).  The best way is to measure the unloaded output of the
charger.  If it is greater than 15V, you want to carefully monitor your
charging.  If it is between 14 and 15 volts, you do not want to leave it on
too long.  If it is under 14V, relax, don't worry, have a homebrew.


> 	I was thinking of getting a small voltmeter from radio shack to monitor
> the voltage of the battery during charging--will this work?
> 

A more acurate measure is the charge current.  You can measure this easily
by placing a series resistor in the path and measuring the voltage drop
across the resistor.  Send me email and I will help you with the math.

Finally, I assume that this charger has worked for you in this system before.
If so, then you may have just experienced infant mortality of the charger
or you connected the battery wrong and ended up frying the charger.  If this
was the first time, check your connections!  If they are correct, let me know
and we will see if we can figure it out.

> James Gourgoutis       				   


Brad Grande	WB0OYX
Another bike riding beer brewing kite flying amateur radio operator who
	still likes CD's!

AT&T Bell Labs
Naperville Illinausea
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