1988 Mondial 3.2 Cab I have used an AGM battery for some time, and always note the battery when put on an AGM compatible charger takes more charge top up time than I would expect. I have always wondered if the charging voltage of the alternator is sufficient to deal with the higher charge rate required of the AGM chemistry. After fully charging my battery, and waiting a day later before testing with a volt meter, resting the voltage measures 12.74 volts. The voltage when started and running without load at idle is steady at 13.82 volts, and with lights and heater fan on it is 13.69 volts. This seems consistent with the Bosch alternator putting out about 14 volts from what I have been able to find, with a small voltage drop when measuring at the battery posts rather than at the alternator directly. This would appear to not be sufficient to fully charge an AGM battery from what I read, and as I am experiencing. Perhaps we should all be using flooded lead acid, which seems to be what our alternators are optimized for. AGM's clearly work ok, but one is probably getting much less than the rated capacity of the battery, all at a higher cost, and probably with reduced AGM lifespan with it being chronically undercharged. I am surprised this does not seem to have been a topic of much historical discussion, everyone seems fine with AGM's. Or maybe my original alternator is not putting out its full voltage - I am not really certain what the Bosch spec for the unit actually is? Thoughts?
Interesting thread. I had not thought much about AGM vs lead acid but you may be on to something. I know my alternator puts out pretty much the exact same voltage you are seeing. So maybe a switch to a regular old lead acid battery may not be a bad idea. I wonder if the draw of charging the AGM causes an additional drain on the system as well that makes our already iffy electrics a little less happy. Would be interesting to hear others thoughts. Sent from my iPhone using FerrariChat
My understanding is the AGM battery with a stock alternator requires additional charging like a trickle charger to maintain the battery. If not, the battery will only get to an 80% charge which over time shortens the life of the battery. I use lead-acid batteries and I get at least 7 years on a battery and I dont even trickle charge them off-season through the winter in unheated storage. On my old motorcycles (Harley's) with AGM batteries to prevent acid spills and paint damage I get 2 years max on a battery, I dont trickle charge them because Im not a fan of middle-of-the-night infernos in my garage. The lead-acid sport bike batteries or dirt bikes I get 5-7 years.
The best service life I ever have with these cars is with conventional flooded lead acid batteries. Save your money and buy a cheap battery.
If AGM didn’t exist when the original alternator was designed, seems logical that you’re going to have issues. I believe this is a well known problem.
I get around 14.0 volts on my 328. Engine idling, no load. Drops to around 13.6v with lights and fans on. Not much increase when engine RPM is raised. This was measured at the cigarette lighter socket. I had an AGM battery on an old LR Defender.. It was used daily but only lasted five years. The Bosch sealed lead acid battery I have on the 328 is 12 yrs old and still shows full capacity on a load test. I've come to the conclusion that AGM batteries are only useful on modern cars where charging rates are higher, but even then I would question their cost v service life against a good quality sealed lead acid battery.
On lead acid batteries whether serviceable, sealed, AGM, gelled, etc. Bulk charging is fast. It is that last 20% that can take a long time to charge and that is perfectly normal. For best lead acid battery life, it is best to keep the battery topped up with battery maintainer CTEK, etc. Modern cars really need topping up because of all the microprocessors that never really turn off when the car is parked.
Best reason I buy AGM is that they are sealed and do not cause rust on my battery box or supported ferrous zone. In addition AGM do have deeper Depth Of Charge and CCA is superior to Lead Acid battery. . Lead Acid battery charges at 14.2V and AGM charges at 14.7V, not that huge of a difference. I drove 1000 miles to Las Vegas Concours on AGM and had no problems over 5 days. At home, it is always on a CTEK SnowFlake MODE (yes real name) It did cost $225 but i am very happy with AGM
