Deep Cycle Battery Charging From Alternator

atp20 wrote:
Hello,

May I ask for advice regarding charging deep cycle batteries from the alternator? I have read older threads, and understand opinions vary.

The previous two 100Ah deep cycle AGM coach batteries each failed in well under two years of intermittent use in my 2003 Sprinter Class B.

Would non-AGM deep cycle batteries last longer? AGMs have been said to be sensitive to overcharging. Any battery recommendations? Maintenance-free would be preferred because of awkward battery access.

Do I need to look into some sort of smart charger, alternator-to-battery charger, or inverter/charger setup? I'd like to keep things simple.

Would installing a second coach battery affect the situation?

The RV is used as a daily driver and for long distance trips, boondocking mostly. When traveling, the principal load consists of two refrigerators, drawing 2.5A each when active, plus a high power blender that draws about 95A (12v) through a 2kw inverter for two minutes a day. The coach battery is located under the rear floor, about 20' from the alternator. Not sure of the connecting wire size, which runs through an isolator and 50A breaker. The 120A(?) alternator charges at 14.4 volts when the battery is full.

Thank you. Any advice would be appreciated.

Jerry S
2003 Freightliner Sprinter Class B
S Florida

Yes ... it is possible to properly, and for good long term service, keep charged RV coach batteries from the main engine's alternator. We have been doing it for 13 years with our Class C's Ford E450 stock 130 amp alternator. The Class C is now on only it's 2nd set of two 12 volt AGM deep cycle batteries. I replaced the first set of AGM coach batteries after about 8 years "just-in-case" - as they were still functioning well after the 8 years.

However, there are some things to be aware of, such as:

1. Is the wiring between the alternator and coach battery bank of large enough gauge size so as to minimize voltage drop along the wire's length while the charging is going on?

2. Is the alternator large enough capacity such that it's not getting stressed too much when the coach batteries are down a bit when starting to charge?

3. Do you have a permanently mounted voltmeter that shows alternator output voltage at all times, and a permanently mounted voltmeter that shows the voltage on the coach batteries's terminals at all times?

4. Do you have a permanently mounted ammeter that shows the current flowing from the alternator into the coach batteries at all times?

5. Is the alternator mounted under the hood such that it gets pretty much a solid and direct blast of air continuously from the radiator fan?

6. Is the alternator connected to the coach battery bank with a heavy duty interconnect solenoid - with silver coated contacts - so that the interconnect solenoid's contacts don't corrode much, or at all ... and hence not reduce the actual voltage that is on the batteries's terminals versus what voltage the alternator is holding on it's terminals?

Our small Class C motorhome's setup meets the above criteria. Our coach battery bank consists of two Group 31 12 volt deep cycle AGM batteries of 115 amp hours each - hooked together in a balanced parallel configuration - for a total capacity of 230 amp hours. When we're camped without hookups and our battery bank is down around 50%, the Ford's V10 alternator can dump up to 70 amps at engine idle into the coach battery bank (according to the ammeter). I sometimes charge this way for about an hour before firing up one of the RV's generators to finish the charging ... or use only the alternator to partially charge the batteries in noise senstive camping situations due to the idling V10 being way quieter than any generator.

When we're traveling after camping and starting out with the coach batteries down around 50%, the alternator takes about 3 1/2 hours to charge the coach batteries up enough such that the ammeter reads about 1/2 amp going into the coach batteries - which means that the batteries are fully charged.

The reason for the dual voltmeters - one for alternator output and one for coach batteries's terminal voltage - is to keep a close watch on voltage drop between the alternator and the coach batteries. If these voltage readings aren't real close to equal whenever the alternator is charging the coach batteries, it most likely means that the interconnect solenoid's contacts have degraded and hence the solenoid must be replaced ... this has happened to me twice in 13 years and ~71,000 miles of camping travel.