Expert
Posts: 3853
    
Location: Vermont |
IMPORTANT NOTE ABOUT THE SUITABILITY OF
ABSORPTIVE GLASS MAT (AGM) AND
GELLED ELECTROLYTE BATTERIES
IN RV'S
That First Important Step
There is a lot of interest in high-tech batteries these days. While Valve Regulated Batteries possess many traits and characteristics that seemingly make them a desirable replacement for standard (flooded) lead acid batteries, care must be taken to insure that they are not subjected to environmental and operational abuse that would dramatically shorten their life span. Valve regulated batteries are expensive, sometimes double or even treble the cost of standard RV deep-cycle batteries.
CHARGING VOLTAGE VALUE IS CRITICAL
AGM and Gelled batteries require a charging voltage that does not exceed 14.00 volts (summer temperatures may require even lower voltages). Unfortunately almost all automotive charging systems have a permanently fixed set point voltage that exceeds 14.00 and this spells trouble. Subjecting the batteries to (commonly found) 14.6 volts for a prolonged period will eventually destroy them. Pusher diesel motorhomes normally use truck-type alternators and voltage regulators and most have internal set screws which can be fine-tuned (To lower the voltage set point). Automobiles (pickup trucks) and standard motorhome charging systems can be modified to accept an exterior adjustable voltage regulator. Your local automotive electrical rebuild shop can be a lifesaver if you elect to go that route. For your edification, 14.05 volts is my personal "upper limit" for charging valve regulated batteries.
THERMAL RUNAWAY
The name "thermal runaway" sounds ominous and when it happens to an Absorptive Glass Mat battery the results can be dramatic. A curious electro chemical reaction can take place inside an "AGM" battery if it is overheated while being charged. The phenomenon can be the result of too high a charging voltage, which will overheat any battery, too high an environmental temperature or a combination of both elements. Instead of tapering off, the charging current actually increases as the battery temperature increases. In extreme cases the electrolyte and binder material can be forcibly ejected from the battery vents. Obviously the battery will be destroyed. The point here is to pay attention to the battery's location and charging voltage limit.
MODERATE TEMPERATURE ENVIRONMENT
Both gelled electrolyte and AGM batteries insist on being located in an environment well away from high underhood engine or radiator temperatures. As a matter of fact the manufacturers of these types of batteries insist that all charging must cease altogether if the core of the battery reaches one hundred twenty degrees Fahrenheit. This point becomes critical in some pusher diesel bus applications that fit the house batteries in the path of radiator exhaust heat!
DON'T FORGET THOSE "OTHER" CHARGING SOURCES
When fitting valve regulated batteries to an RV care must be exercised to insure that the vehicle converter or battery charge finishing voltage does not exceed the 14.00 Volt limit. Solar voltaic charging systems must have a voltage regulator (commonly referred to as a "controller"). Auxiliary generators with integral charging systems must have their charging systems adjusted or defeated. And finally the owner of the Valve Regulated Battery must understand that even one unsupervised service station "quick-charge" will destroy even the best maintained gelled or AGM battery.
http://www.batteryfaq.org/ |
 Elite Veteran
Posts: 736
Location: Western WA | This is good information, thanks for sharing. The 120 degree limit on charging is a deal-breaker for me. Was a show a couple of years ago near Sacramento where the daytime high hit 124. The daytime high was over 110 every day for a week. And that's without any other heat source. Seems like the temp issue would be fairly limiting in the application of these batteries. |
CHARGING VOLTAGE VALUE IS CRITICAL
