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C4s are known to have batteries that need to be charged if the car is not driven for a couple of weeks. We have had rain for the past 2 weeks and the car has not been out. I read in some Corvette magazine that when charging the battery on these C4s it is best to leave it connected rather than use the disconnect switch. The supposed purpose it to avoid giving the ECM a surge when connecting up the charger. I am using an automatic maintanance charger.
What is the correct way to do this?
I do not use a disconnect switch for my 1985's battery, because I don't believe that it is necessary, as in the case of the older models. Battery power must ALWAYS be applied to maintain ECM and clock memory. The current draw for these two devices is negligible. I have been using a battery minder for the past seven years, and the battery is always at full charge. If you MUST use a disconnect switch, because you suspect a "gremlin" current draw somewhere, then use it as recommended, because the ECM is always energized, even WITH the switch, through the bypass lead.
Modern cars have up to about a 50 ma current drain to keep electronic memories refreshed and power the clock and alarm, so if you disconnect the battery, radio presets may have to be reset and the PCM may have to "relearn" some data, so it's probably best to leave the battery connected and charge it at least every month or before you drive it if it's been more than about a week, certainly two.
That's how I manage the batteries on my "modern" cars. Bringing the battery to full charge before you apply cranking load will get you seven to ten years of life on inexpensive lead-acid batteries. If you consistently apply cranking load when the battery is partially discharged, life may be shortened to as little as a couple of years.
If you want to know the static current drain, connect an ammeter to the negative battery post and cable. Then remove the cable and read the ammeter.
On my '88 MBZ the draw jumped between about 20 and 30 ma every second. The jump was the clock movement!
I have one battery charger connected to my 96's battery all the time it is parked since it is driven less. Plug it in to electrical power about once a week to maintain charge. Always charge it before driving after it has been sitting. My 85 usually is driven during a two week period, but if it has not been driven I connect another battery charger and charge it. I don't have any disconnect switches. Using battery chargers like this works fine for me.
Thanks for the advice. I put the disconnect switch on when I had my '86 judged two years ago in order to pick up the bonus points. I began the habit of disconnecting the battery when I put the maintenance charger on. The magazine article said to leave the battery connected when charging to avoid any possible damage to the ECM. I have had the '86 twenty years now and I am on my 4th battery and my 2nd ECM. At any rate, I plan to keep the battery connected during charging except during the winter storage months.
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