Battery Advice from the USA A'vanners Club

This meter is ideal for following Jay's advice

Batteries are 100% charged when their voltage reads 12.6v or higher. When they reach 12.2v they are at 50% charge. You can check this with a digital voltmeter by opening the battery box and connecting the volt meter to the correct leads. You can also install (or have one installed) a voltmeter and switch so you can check this more easily. Stay away from battery level indicators which just use LED lights.

Three ways to determine the state of charge:

1. Hydrometer, available at auto parts stores, can be used to measure the specific gravity of the acid in batteries that have removable cell caps. Takes some work and care, but is most accurate method.

2. Amp hour meters, that measure the charge and discharge current to estimate the percentage of charge remaining. Need to be re-sync'ed to a full battery periodically to eliminate cumulative error from nonlinear battery charge/discharge characteristics. Expensive, but continuous reading. Also shows real-time voltage and Amps charge/discharge. Probably overkill unless you are a glutton for tech info (like me).

3. Accurate voltmeter, measuring battery voltage after a suitable no-load period. Quite usable if you give the battery time for the electrolyte (acid) to stabilize after a period of charge or discharge.

With a standard charge line through the trailer light plug, you will probably not overcharge your battery while driving and may not even achieve a full charge. Battery chemistry is accelerated by high temperature, so most car alternators decrease the alternator output voltage when hot to keep from overcharging the car battery. This, coupled with the resistance of the small wire back to the usually cooler trailer battery, makes it hard to top off the trailer battery. The topology of my vehicle made it possible for me to employ a large diode to raise the alternator voltage to the trailer while maintaining the proper vehicle battery voltage. But even that, coupled with large (#6) wire and a separate large charging plug allow me to only obtain about 30 Amps charge once the engine heats up. The same problems would affect your ability to charge while sitting at your site.

Alternatives:

1. Install a second battery tray under you hood and charge your battery (or a second one) there to match temperature and eliminate long wires. But...you may not have room, some work, hassle transferring battery.

2. Use inverter powered from battery in car to operate converter (or better yet a good battery charger) in trailer to help compensate for wire losses and voltage differences. But... inverter expense, converter
may not have good charge ability, converter or battery charger may not like "modified sine wave" power from inexpensive inverters, inefficient of fuel, possible dead car battery.

3. Small generator to power converter or battery charger. Expensive, noisy, may still be limited by converter charging ability.

4. Solar panel(s). Best long term power source as long as you have enough panels and sun to offset your power usage. Quiet, but expensive.

A lot depends on how frugal you are of the power you do have. A single light for a few hours a night might go the entire 2 weeks without a battery charge. Lots of furnace and light usage, and maybe a sleep apnea CPAP machine all night (like me), could reduce your battery to nothing by morning.

Lots of good information in the archives of this list. A web search for "boondocking" will yield a trove.

-Jay