Charging AGM trailer batteries from vehicle

[mswartz]

Has anyone had successful long-term experience charging AGM batteries in a trailer with the tow vehicle charge line?

Following several weeks of testing, this weekend I am installing AGM batteries in my Bambi along with a Xantrex Prosine 2.0 inverter/charger and Link-10 choice monitor. Due to the very specific charge voltage requirements of the AGM batteries, I'm disconnecting the factory-installed tow-vehicle charge line to the Bambi's DC bus so I won't overcharge the new batteries.

After getting some experience with the new DC system on the trailer, I am installing an Analytic Systems BCD-605-12-12 DC charge controller I purchased to regulate the tow vehicle charge circuit to the AGMs.

Any experience or thoughts would be appreciated to help me down the learning curve.

[bryanl]

Here is what Arizona Wind Sun Solar says about AGM charging:
http://www.windsun.com/Batteries/Battery_FAQ.htm

"The charging voltages are the same as for any standard battery - no need for any special adjustments or problems with incompatible chargers or charge controls. And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents. The Concorde (and most AGM) batteries have no charge or discharge current limits."

While some of the better battery chargers (such as your inverter) may have AGM settings, I don't think you would have any problems leaving the tow vehicle charge line as is. But, as always, there are things to be learned.

1) if your prosine has an AGM setting, does its manual describe what it does different? (I have an older one and I think the manual said something about this - will have to see if I can find it again)

2) why do you think you need a charge controller on the tow vehicle charge line? What is is going to do for you?

[RoadKingMoe]

There are hundreds, if not thousands, of AGM batteries in off-road vehicles and boats, that use the same vehicle charging system used for flooded cell batteries, and they work just fine. Besides WindSun, Optima says use the same charging system.

Many confuse AGM with gel cells, which ARE sensitive to charging voltages and temperature. Lifeline had to overcome this misconception for the government, by grossly overcharging their batteries in extreme heat temperatures to prove there was no problem with AGMs. AGMs are NOT gel cells.

AGMs not only charge faster than flooded-cell with the same flooded-cell charging equipment, but they can tolerate even higher charging voltages for even faster charging.

[mswartz]

Thanks for your reply.

I'm using East-Penn VRLA 8A31DTM AGM batteries. I've read the comments from many manufacturer and vendor sites regarding AGMs, and they are generally consistent. Some say that AGMs require no special charging considerations. East-Penn, the manufacturer of my batteries states in their VRLA technical manual (available at: http://www.eastpenn-deka.com/products/pdfs/0139.pdf, page 9) about charging their AGMs:

"Automatic temperature sensing, voltage-regulated chargers must be used"
... and
"Charge voltage must be limited (14.4 to 14.6 volts maximum at 68°F)"
(their emphasis on must, not mine.)

Where I work we have alot of experience with them and prefer to use specialized chargers to assure availability and long life. These AGMs with Analytic System chargers have worked well for our shipboard UPS and navigation systems with many installation-years of operation.

1. The Xantrex PROSine 2.0 does have a generic AGM setting, but also allows the user to set customizable charging algorithms for nearly all charging parameters. I'm using the optional temperature sensor attached to a negative terminal on the batteries to monitor battery temperature for charge compensation.

Differences in the PROSine charge profiles for AGM vs other batteries include:
a) If the user has configured the PROSine 2.0 battery algorithm for AGMs, there will be no "overcharge" charge stage.
b) No "equalization" charge stage is available if AGM is selected.
c) Temperature compensation coefficient is different (-27mV/°C for flooded cell vs -21mV/°C for AGM) along with different absorption stage voltage limits. The correct settings depend on the battery manufacturer's recommendations.

I've tested both Concorde PVC-12100 and the East-Penn 8A31DTM with the PROSine 2.0 and their charge behavior is slightly different, but since I cannot rigorously test them I can't say if it is due to age (Concordes are about a year old with more cycles on them, East-Penns are new) or due to fabrication and chemistry differences. Both seem to be suitable batteries for my application.

2. The reason I'm installing a DC/DC charge controller from the tow vehicle is to provide for controlled 2-stage charging to the endpoint voltage of the AGMs regardless of the tow-vehicle source voltage. The BCD-605-12-12 provides a voltage regulated (within 50mV of the setpoint) charge at currents up to 42A with inputs from 10.5 to 14V from the tow vehicle. It can boost or reduce the input voltage as necessary to maintain user-selected charge voltage and should prevent overcharging.

[bryanl]

Good stuff! Here is what I get.

Battery temperature probes are good for avoiding excess charge rates and can be used to finely tune battery charging. (shouldn't be needed for anything less than C/10 <AGM. C/20 for wet cell> charging rates where the battery is in close proximity to the charger and in same environment)

AGM's don't want or need equalizing (one reason for max voltages Marshall described).

AGM's may have slightly different float voltage preferences from wet cells.

A charge controller on the tow vehicle battery line can compensate for wiring resistance and switching voltage loss and be set to more quickly charge the house batteries.

Good chargers can be tuned to battery manufacturers specs and recommendations for optimum life and capacity. (but discharge and storage characteristics need to be considered, too)

What we are talking about here is the difference between "what works OK for most RVer's" and "finely engineered and tuned for opitimal life and use."

And two good links to peruse. Thanks.

[RoadKingMoe]

Well said, Bryan!