piperMember2018-12-02 at 17:20Post count: 3
My 1988 780 kills batteries. It is in storage from April to November in Florida with the neg. battery cable detached. This November the battery was completely dead and needed to be replaced (for free under warranty). Next day the battery was dead. I probably put five miles total on the car in that time. I read a brief mention in this forum about the radio amplifier being a constant drain on the battery. Does anyone know how to fix this problem?onakawParticipant2019-01-07 at 10:44Post count: 18
I had a similar problem, turned out to be the alternator. Wouldn’t come off line when engine was shut down.Johan VlagsmaParticipant2019-01-08 at 12:39Post count: 108
What I would first do is measure the actual drainage: switch off everything electrical in the car, then disconnect the negative lead on thebattery and connect an Ampere meter between lead and battery negative terminal. Make sure to select the highest Amp setting first and gradually switch to lower settings until you measure a current. Then pull fuses until you see the Amps go to zero. This provides the offending group and you can look further for the cause in that group.
Causes I’ve seen: alternator with a defective diode (like onakaw mentioned), trailer hitch connector with wet sand shorting the constant plus (interesting fault), light in the boot or under the hood/bonnet not switching off, Alarm system, bare wires shorting, water in lamp unit.
If the drain current is high it should not be too difficult to find
Best of luck and let us know when you find the cause. We’re always eager to learn 😉piperMember2019-02-05 at 17:22Post count: 3
Latest on battery killer. I have kept the trickle charger on pretty much all the time. With it unhooked i get an amp reading of 0.01. Amps which isn’t much but is it enough to discharge the battery? Also now my battery gauge and gas gauge are playing dead. Any suggestions?
PiperJohan VlagsmaParticipant2019-02-09 at 13:38Post count: 108
0.01 Ampere from a good 65 Ah battery will last for 6500 hours, that’s about a year. Are you trickle charging the battery with the car circuit connected? The leakage may be bigger than the trickle charger can supply and then battery voltage will drop and gauges etc won’t work but also Amps will drop because there’s no voltage left to drive it. What is now the voltage across the battery terminals?
You might want to disconnect the car from battery (neg lead is sufficient), charge the battery until it is full, confirm it has >12 Volts across the terminals, then measure leakage current from battery neg terminal to car neg terminal. If the battery by itself doesn’t reach >12 Volts when fully charged it is (partially-) dead.
Note the trickle charger needs voltage across the terminals in order to start charging. If the battery has <8 Volts you could use another good battery to force charge it some and then complete charge with the trickle charger.DaviesKeymaster2019-02-27 at 17:31Post count: 183
The battery killer on 780s, from my experience, is often the stock amplifiers. Over time they build up resistance and continue to pull power even when off. There should be one in the trunk for the rear speakers and one under the driver’s side knee brace for the front. Keep us posted on what you find.piperMember2019-03-03 at 16:56Post count: 3
First let me thank the group for very insightful input. Since last posting I have re-lugged the wires to the pos side of battery. No lost amperage now! Also I pulled out the gauge cluster, tightened all the little screws, and sprayed all connections with electronic contact spray. All gauges work!ARPParticipant2019-05-22 at 22:13Post count: 19
On both my 91, I put a fluke 375fc meter on the battery + paired it to fc3000 and my cell phone then pulled the fuses one at a time and it was the amps in the trunk, I install a bosch relay in the trunk and problem solved.
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