AGM batteries, overcharging on long drives

loving retirement wrote:
I have 4 deep cycle lead/acid batteries in my 2002 Chevy 190 RT and have had zero problems in the last 4+ years and I bet I will get another 3 or 4 years from them. Using a 3 stage charger with almost no loss of water, no boil over or gassing. I don't believe that this overcharging became an issue until the isolator was replaced by the battery separator and they started using AGMs. The loss of that 0.7 volt drop across the diode in the isolator resulted in an additional 0.7 volts being delivered to the house batteries when they use a separator and lead/acid can handle the additional volt better than AGMs.

You are exactly correct. If you have an isolator, there is really no issue with overcharging while driving, but you do lose the ability to charge the starting battery on shore power, unless you put in a bypass.

I have 4 deep cycle lead/acid batteries in my 2002 Chevy 190 RT and have had zero problems in the last 4+ years and I bet I will get another 3 or 4 years from them. Using a 3 stage charger with almost no loss of water, no boil over or gassing. I don't believe that this overcharging became an issue until the isolator was replaced by the battery separator and they started using AGMs. The loss of that 0.7 volt drop across the diode in the isolator resulted in an additional 0.7 volts being delivered to the house batteries when they use a separator and lead/acid can handle the additional volt better than AGMs.

The two Bs I've owned (P-W and Great West) had battery disconnect switches but I have rarely turned them off. My current B gives a voltage number read out and I don't recall it ever over 14 when sometimes checking while driving.

I don't know what my chassis battery is as it is buried under the driver floor and I have never checked it. I've been trusting the periodic service to deal with it if needed and so far so good. I believe Mercedes Benz is putting in an AGM battery now. Not sure about 2010. My auxiliary batteries are lead-acid.

IMO this can be a problem, especially on the Chevy based vans. Our 07 C190P Roadtrek on the Chevy would run down the road at 14.5-14.5 volts hot. This is too high for AGM or wet cells. I have found two solutions, and now have both in place. We have cut our water use in the batteries in half.

You can, at least for Chevies, change the regulator in the alternator. I think they rate at 14.3 or so, but run at about 13.9 hot.

You can add a disconnect switch for the separator, so you can manually shut it off. It is also handy so you can shut the separator off when on shore power, if you want. We do that, as we have a disconnect on the starting battery that we use when stored in the garage, so the separator is not needed to be on.

If you take a look at the recommended float voltages for batteries, both AGMs and wet cells are in the low to mid 13 volts range, so most of the vehicles do run well above recommended voltage.

Sometimes the charging the vehicle voltage regulator may be set at a higher voltage for conventional lead/acid batteries. This cause to lead the possible over charging.

Thank you all for your information and comments. I will be monitoring the Voltage level as part of my system and will be able to then determine if there is a potential issue before it happens.
thanks again

I have AGMs in my van and in my boat. Modern day regulators built into altenators do a pretty good job of sensing actual demand and limiting output to match demand, included replenishing batteries. Don't think you have to worry.

This is a legitimate concern and part of the answer is the battery manufactures own literature i.e. when I bought Lifeline 150 AH AGM battery I checked the specifications put out by Concord/Lifeline. An AGM starting battery is a different animal than a deep cycle AGM, as much as conventional starting battery is to a true deep cycle.
One alternative is to use a three stage regulator i.e. Balmar http://www.balmar.net/regulators.html

14+ Volts is just fine. I suspect the Amps are very low.

I wouldn't worry, I am 100% AGM and my youngest battery is in its forth year.

HTH;
John

I've also wondered about possible overcharging of AGM batteries while driving. Since the 2 original equipment AGM batteries in our Sprinter seem to be fine after 7 years and 90,000 miles so apparently the problem is not too significant.