dhf@linus.UUCP (David H. Friedman) (01/17/86)
To make a long story short, I just replaced the battery in an '81 Chevette. This was still the original battery, a Delco sealed unit, after 4 years and some months. We had a few cold mornings up here in the Boston suburbs when the thing just barely cranked the engine and died after about 20 seconds of trying. I tried connecting a 6-amp charger overnight; this helped some, but one morning at 0 degrees F. finally did it in. The replacement is an Exide, recommended by the local Chevy dealership. I'm still connecting the charger every night when the temperature is likely to go below freezing, just to be sure it starts next morning with a full charge. There are two things I'd like to know: 1) The ammeter on the charger reads about 4 amps when I turn it on, decreasing to 2 amps when the battery is presumably fully charged - this is to be expected according to the instructions for the charger. 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? 2) I replaced the battery clips on the charger cable with a plug to connect through the cigarette-lighter jack. This way the unit can be connected without raising the hood of the car. What limits the amount of current that can safely be pumped in through the lighter jack? So far I haven't blown any fuses. (When I went to shop for a charger, I initially asked for one that could be connected this way. All I got from the clerks at the auto-parts store was funny looks. I later saw one at a discount store, but it was only a half-amp trickle charger, much less than I thought I might need.) Thanks for any help from out there, and no snide comments about the Chevette, please - it does what it was meant to do, all the more so with a new battery!
saf@bonnie.UUCP (Steve Falco) (01/19/86)
> 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