The determination of the proper size of your battery bank is a subject very important to understand.
Depending on your needs, it may be necessary to upgrade or add to it.
For those boondocking i.e. dry camping without a full hookup, you may need a battery bank of four to six 12 volt deep cycle batteries allowing extended stay.
Battery sizing must be well thought. A bank of one or two batteries is normally insufficient unless you want to recharge them constantly or you are dry camping only one night.
Because house batteries are bulky and heavy, the battery bank should be located over an underneath structure strong enough to support it. Reinforcing or adding support under the RV battery bank may be required.
The extra weight should also be well distributed without creating an overload for the recreational vehicle.
You need sufficient space in a ventilated storage compartment for wet cell type batteries since they release hydrogen when they charge. Sealed AGM batteries do not outgas but ventilation is always beneficial.
This is a subject requiring a lot of information and I will try to summarize it as well as possible.
Step 1 - BE AWARE OF YOUR DAILY ACTIVITIES
In order to know how many 12 volt batteries (twice as many 6 volt) must be part of your battery bank while boondocking, you need to have a fairly good idea of your daily demand in electricity.
This is an exercise that you probably never had to do at home. You were just flipping switches and paying the bill at the end of the month. If you are in a campground with full hookup, it is pretty much the same, you just have to pay the bill.
Now, boondock RVing is different and this is why I love it so much. It saves on energy and can be called green RVing.
To determine your daily consumption in energy, start thinking of a specific day!
After getting up, you brush your teeth and have a shower. Right there, in addition to having to be careful to conserve your water, your water pump is running, which means consumption of energy. Then the hair dryer which is very power hungry for such a small item.
Later, you have breakfast., taking milk from the fridge. Even if the refrigerator runs on propane, it still has a dead load (even when left on propane) which means it still uses electricity; not much but it accumulates over a 24-hour period.
After cereals and toast (the toaster being another power hungry apparatus), you have coffee. Dishes need cleaning. You turn the computer on to check your e-mails. More electricity... Getting the idea.
Electricity use (as water) accumulates quickly. Energy conservation is important.
In fact, everything you do entails a demand in electricity. Even the items in your living quarters plugged in but not even turned on, use some power (dead or phantom load).
It gets complicated, does it not? Well... not really after you get the twist of it. It is just fun and makes you aware of things.
Step 2 - FIND OUT THE AMPERAGE OF ALL APPLIANCES USED
Make a list of all electrical devices in your RV and write down its amperage.
The info is available in the owner's manual or on the item itself (the typical U/L tag). You can also visit the manufacturer's website. Another way to measure very precisely the power usage of individual appliances and equipment is by using a meter like the kill a watt meter which can count consumption.
Note that you always work with the ampere (A) or amp: the unit of measure for the flow rate of electric current, more precisely DC amps since the batteries are DC amp rated.
It is also important to remember that when you add electrical devices to the existing circuits, you always need to check if the circuit has the capacity to accept this extra load.
Your RV has both DC and AC appliances. DC works on a 12 volt current and AC on 120 volt current. Does 1 DC amp equal 1 AC amp? No !
The ratio (DC/AC) is approximately 10:1.
10 DC amps at 12 volts converts to 1 AC amp at 120 volts. On a DC circuit, an appliance will use 10 times as many amps as it will on an AC circuit.
However to complicate things and to be more precise, since you know that an inverter is necessary to convert DC to AC current and the conversion is not 100 % efficient, there is a loss.
Assuming 10% loss as a worst case scenario, then 11.1 DC amps (12 volt) equals something like 1 AC amp. Better efficiency is attainable with new units.
For now, let's just use the ratio 10 DC amps:1 AC amp. But be aware!
There is something else worth mentioning. If you do not find the amperage (amp) but you have the wattage (watts) of an appliance, you can get the amperage with the following formulas.
DC amps = WATTs / 12 (for 12-volt DC current) ex. a 60 watt water pump equals 5 DC amp (60 watts/12 volts = 5 DC amps)
AC amps = WATTs divided by 120 (for 120 volt AC current) ex. a 900 watt AC microwave equals 7.5 AC amps or 75 DC amps (900 watts/120 volts = 7.5 AC amps) (7.5 AC amps X 10 = 75 DC amps)
Why do I make you do all that? It is much easier to simply take the wattage and divide it by 12 to get the number of DC amps directly. (Watts/12 = DC amps)
Are you still with me? Do not get discouraged! Everything will make sense.
Step 3 - EVALUATE YOUR DAILY ENERGY CONSUMPTION
After you have your list completed and all the figures in DC amps for each item, you have to consider how much time an appliance is used in hours or fractions of one hour and calculate the consumption or demand in electricity in DC amp-hours.
a) Time conversion in hours
ex. 37 minutes equals 0.62 hour (37 minutes / 60 mins. in one hour = 0.62 hour) 2 hours 16 minutes equals 2.27 hours (136 minutes / 60 = 2.27 hours)
b) Usage in amp-hour (Ah)
ex. I use the same 7.5 AC amp microwave (you remember 7.5 AC amp = 75 DC amps) for 4 minutes.
4 minutes / 60 = 0.07 hour X 75 DC amp = 5.25 DC amp-hours of consumption
ex. I take a 5 minute shower, running the DC water pump (5 DC amp)
5 minutes / 60 = 0.08 hour X 5 DC amp = 0.40 DC amp-hour of consumption
Now, I think you are starting to get the idea!
Next, you have to calculate the total consumption of DC current per day that your battery bank will have to supply.
Example of a daily usage:
TOTAL: 40.31 (Ah)
To give you an idea, below are average daily electricity consumptions by most RVers in the spring, summer and fall seasons.
Note that it would be much higher in winter.
Step 4 - FINALLY DETERMINE THE SIZE OF YOUR BATTERY BANK
You now have a better idea of how much DC current you would consume during a typical day, This knowledge is crucial in order to determine how large the capacity of your battery bank needs to be.
In order to do that, however, we have to respect one important rule.
Never apply a load higher than 25% of the battery bank capacity.
What does this mean?
It means that if our daily use or load is above 40.31 Ah (amps-hour), in all respect to the one important rule, we need a battery bank with a capacity 4 times higher than 40.31 Ah (25 % load) which equals to 161.24 Ah in size.
Since most RVs come with one single 12 volt group 24 battery with a capacity of 70-85 Ah, most of us will have to make some changes to the battery bank.
For the above example, two group 27 12 volt batteries of 100 Ah capacity each for a total of 200 Ah (2 X 100 Ah each = 200 Ah) wired in parallel, or even better, two 6-volt batteries (2 X 6-volt = 12 volt) with capacity ratings of 220 Ah wired in series would be adequate (two 6 volt 220 Ah = one 12 volt 220 Ah).
You have to understand the principles of wiring for a better comprehension.
Keep also in mind that the temperature affects the efficiency.
Knowing that the house batteries are tested at 77 F (25 C), have a look at the following table to give you an idea:
Ambient temperature versus factor of efficiency
This means that a battery with a 220 Ah capacity in reality is equivalent to a 185 Ah capacity battery when exposed to an ambient temperature of 50F/10 C.
This factor is also important to remember when calculating the size of your battery bank. It is better to have more capacity than less, all depending on your budget of course.
When you identify exactly the amp-hour capacity of your battery bank corresponding to your needs and you go selecting your batteries, it is important to notice that an amp-hour rating of a battery can be based on a "20 hour rate" or a" 100 hour rate".
This means that the rating is based on a battery discharged from a 100 % charge to 0% (10.5 volts) over a 20 or 100 hour period to establish a rating for this particular battery.
Therefore it is important to compare batteries rated at a same discharge rate (20 or 100 hrs.)
Another reminder is not to forget the rule that it is best to keep your discharge as low as possible (not more than 25%) and to fully recharge the battery bank daily.
It is worthwhile to note that when a battery is discharged, the voltage decreases. A lot of equipment will stop functioning when it drops below 11.8 volts (normally around 50 % discharge).
I mention recharging daily because in order not to take more out of your batteries than you put in, the replenishment has to be done within a reasonable time.
The battery capacity is then only one part of the equation. The other limiting factor is the charging capacity.
Whatever the method used ( solar power for example) it has to be well planned.
A properly sized battery bank will provide to all your electrical needs with enough capacity not to be overly discharged on a daily basis.
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