When preparing your RV for boondocking, it becomes important to have your battery bank adequately sized for your needs. This usually tends to involve a littel trial and error to get an accurate number as your needs may change from day to day. Eventually you will figure out if you have enough battery capacity or not. Here are some general guidelines to help you get a ball park estimate of your power requirements.
Estimating Your Power Needs
Your battery bank should be sized to match your estimated daily power consumption. Consider a typical 24 hour period. What electrical items will you use and for how long? Determine the amount of current (in amps) that each piece of electrical equipment uses. Finally add it all up and compare it to the capacity (in Amp Hours) of your battery bank at full charge.
Tip: When comparing DC components with AC powered components it is easier to first convert both measurements to watt hours.
Note: AC powered appliances will state their current draw in AC amps (at 120 volts AC). These amps need to be converted to DC amps (at 12 volts DC). Applying Ohms Law, we know that the conversion is roughly [Amps @ 120 volts AC] X 10. In other words, a 5 amp AC appliance will draw roughly 50 amps from your 12 volt battery bank. This conversion is not necessary for DC components and fixtures such as your RV lights, vent fan, and water pump.
To get a handle on your "actual" usage, you would have to experiment and measure over a period of time. There are many variables that could affect this number and there is certainly no need to be exact. I recommend you simply add around 20% to your initial estimate and design your system to be expanded should you need more charging power in the future.
Here are some tips
- AC power ratings (in amps or watts) are usually posted on the back or bottom of each electrical component
- For AC components you can also use an inline AC power meterthat plugs into your power cord to measure the amps (or watts)
- Replace all of your incandescent light fixtures with LEDs which consume roughly 1/10th the power of incandescent bulbs (Video: Comparing LED power consumption to Incandescent bulbs)
- Install a quality battery monitor like the TriMetric Battery Monitor to get a real-time measure your voltage, current (amps), power (watts) and amp hour (battery capacity) draw
- Use a digital clamp meter to measure the DC amp draw on components like your detectors, water pump, and gas refrigerator
Understanding Amp Hours
Amp hours (Ah) are a measure of a battery's capacity. A piece of equipment that draws 1 amp of current for 1 hour will consume 1 amp hour from a battery. Larger batteries are rated in amp hours (Ah). Smaller batteries like "AA" or cell phone batteries are rated in milliamp hours (mAh) where the "milli" translates to 1/1000th of an amp.
For example: A typical 12 volt RV battery may have a 220 amp hour capacity. A typical alkaline “AA” battery on the other hand, has a capacity of around 2 to 3 amp hours (or 2000-3000 mAh).
Consider a standard incandescent light bulb that draws 1 amp at 12 volts DC. Each hour it will draw 1 amp hour of capacity from your battery. If you left the light on for 24 hours, it will draw 24 amp hours. Using this calculation (24 amp hours X 9 = 216 amp hours) we conclude that this single bulb would stay lit for about 9 days on a fully charged 220 amp hour battery bank. At that point the battery will be nearly depleated (to 10.6 volts).
Of course this is just a simple illustration of how to calculate amp hours. In reality there will be more things drawing power from your batteries than just a single light bulb, but you get the picture.
Monitoring How Much Power Are you Using
The only way you will really know how much battery capacity you have available is to monitor amp hour consumption. For this purpose, I highly recommend installing a quality battery monitoring system such as the TriMetric Battery Monitor. Simply checking your battery voltage gauge will NOT indicate how much battery capacity you have left.