Amp Hours and Battery Charging
Q: I really appreciated your latest articles and advice on RV batteries. As a matter of fact, I appreciate all of your advice. I usually trailer camp where there is piped water and either pit or flush toilets but no power. I go about four days before needing a dumpsite for the gray/black water. I may go a week or more without a significant recharging from my tow vehicle, so I need to do some battery power management and recharging. I also have an 80-watt solar panel.
If I have, for example, a 200-amp-hour-rated battery that is fully charged at 12.6 volts, how many amp hours have been used when the battery is fully discharged at 11.0 volts? Also if I have one of those small gasoline generators with an internal 12-volt DC 8-amp battery charger, would I have to run this generator for 12.5 hours to recharge 100-amp hours to the battery? And the last question, if I have a 200-amp alternator in my tow vehicle, what is the average driving time at say 60 mph to recharge 100-amp hours to the trailer battery?
Dona Kazmar, Via Internet
A: A battery is not like a water tank, you can’t plan on running it empty; that’s a recipe for ruining it. Deep-cycle batteries should not be discharged below 50 percent, so you should think of a 200-amp-hour battery (A-h) as having a 100-A-h capacity. Also, batteries lose capacity over time and use, so realistically it’s more like 80 A-h. Additionally, batteries are not 100-percent efficient; if you put 100 A-h in you don’t get a full 100 A-h out.
The built-in chargers on generators are single stage and don’t do a very good job of charging. Therefore, you should use a separate charger with multiple stages that will recharge the battery to a higher level and sooner. Batteries should not be charged at an ampere rate higher than 20 percent of their A-h capacity per hour. For example, a 200-A-h battery should not be charged at more than 40 amps, and if it gets hot or starts to expel electrolyte, you must reduce that rate. It’s impossible to say how quickly your alternator will charge the battery, because there are many variables, including other power demands of the tow vehicle and trailer, lights, fuel pump, ignition, air conditioning, etc. Battery condition and voltage drop in the wiring will also greatly affect charging rate. Your best bet is to take voltage measurements during rest stops when the engine is off to determine charging progress.
– Ken Freund