Note: As in the last two installments, this information is designed to help newbie RVers better understand the electrical needs of their RV. As with anything electrical, there are exceptions to every rule of thumb, but the basic information shared below is applicable in most RVing situations. Results will vary depending on temperature, battery condition, large loads, and loads over long periods of time.
In our previous article, we looked at how to calculate your RV’s 120-volt shore power needs by calculating watts and watt-hours. In this post, we will look at how to determine the electrical demands of your 12-volt battery system by learning about amps and amp-hours.
As we shared in the last couple of installments, the electrical needs of 120-volt appliances are typically expressed in “watts” on the back of the appliance. When they aren’t expressed as watts, we looked at how to multiply voltage times amperage to determine watts, then by including run times to determine watt-hours.
Norcold 12-volt needs – Photo from iRV2
While watts and watt-hours can also be applied to the 12-volt system in RVs, it seldom is. Rarely are the electrical requirements of the 12-volt appliances (water pump, furnace motor, lights, etc) expressed in any form other than amps.
This is definitely more convenient than watts as deep-cycle RV batteries (aka “House Battery”) are rated in amp-hours rather than watt-hours. Note that amp-hours are commonly abbreviated as “A.H.”, “Ah” or similar.
Just as we added up the 120-volt shore power needs in watt-hours, we will add up the needs of 12-volt (battery-powered) appliances in amp-hours.
Examples:
- An incandescent 12-volt ceiling light bulb has a draw of 1.5 amps. If you leave the light on (burning) for one hour it will consume 1.5 amp hours of battery power. If you burn it for 2 hours it will consume 3 amp hours of battery power, etc.
- The blower motor and related 12-volt components of your propane furnace draw 7 amps while running. If the furnace runs a half-hour, it will consume 3.5 amp hours from your battery bank. 7 amps X .5 hours = 3.5 amp hours
- An LPG leak detector that draws .2 amps and operates 24 hours will consume 4.8 amp hours from your batteries.
Water pump requirements – Photo from iRV2
Now, if you are connected to 120-volt shore power, none of the above matters much, as your converter charger will replenish the RV batteries as loads (amps) are drawn. Where it becomes important is when you are dry camping (no shore power) and the reserve power (amp-hours) in your RV batteries is all you have to supply your electrical needs.
A typical deep-cycle RV battery will be rated around 80 amp-hours, which in theory would supply one amp for 80 hours. However, in reality, if you discharge a lead-acid battery (what you likely have in your RV) more than 50% of the rated capacity you will greatly shorten the life span of the battery. Therefore, when calculating the reserve amp-hours of your RV’s batteries, the usable power will be 50% of the rated capacity.
Now, it is just a matter of adding up your amp hours for every 12-volt item in your RV (like we did for watt-hours in the last installment) and divide that number into the amp-hours available in your RV’s battery bank to determine approximately how long the battery will last.
Now that you know how long your batteries are likely to last, you can plan accordingly for your next dry camping adventure in RVing!
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Dave,
When you have two batteries say 1- 80ah and 1-70ah would that mean a total of 150 ah and half of that is usable (75ah) before being hard on the batteries? Or does it effect the first/last battery differently than the other one?
Thanks for all the help you’ve offered along the way!!!
Good information for beginners.
Hersh,
For starters if the batteries are wired together in parallel ALWAYS use like batteries. Now to answer you question – yes you would have 150 total amp hours capacity, but only 75 amp hours usable (50% of rated capacity). If the batteries are alike and wired together parallel correctly they would draw down evenly under load. If you must use dislike batteries you should keep them isolated and draw them down separately.
Dave, Oops! Just backwards.
To determine useful time divide battery Ah capacity (actually 1/2 battery capacity) by Ah usage. If usage is 20 Ah per day and battery capacity is 80 Ah you can go two days without significant battery damage. That is, 40 Ah capacity (1/2 rated battery capacity) divided by 20 Ah per day.
Thanks Dave!
Wilyoung, Thanks for the comment – I could have worded it a bit better. “divide that number into the amp hours available” AH capacity / amp hour load = hours
Is battery disconnect switch necessary. If so, what do you recommend.
A disconnect switch is not necessary, but is convenient for keeping your batteries from parasitic loads (gas detector, clock radio, etc) while in storage.
Thanks for your reply. Any recommendations on disconnect switch.
Amp hr ratings on batteries are close to true when brand new. After use it lowers and not equally for each battery. Batteries need to be well maintained and kept full of distilled water. As far as batteries I have found with experience of of about 20 yrs of using power inverters that the only battery I would buy is Trojan batteries . After using many brands and capacity I for the last 8 years would buy the highest rating amp hr they sold. They are expensive but worth the money. I found a site where I could buy them at Hodkins in Tucson,AZ and pick them up at any Trojan dealer will call and pre-paid. These batteries would perform more than 2 times better. Even over the well over states Interstate batteries.
As far as amp hr or amp ratings of devices what is posted on device is maximum and not operational. Operational is usually a lot lower than the rating. Sometimes motors like AC units have may even be higher than ratings when older. That is why putting a real high rated and well maintained battery pack will serve you well.
Someone told me that two 6 volt batteries in series is better than two 12 volt batteries in parallel. The Ah becomes higher. Is this true? I need to replace my two batteries and wonder if two 6 volts is the way to go. A little more expensive. Thanks.
I’m going to the trip soon but I scare that my RV batteries not as good as befor. The information is very useful for me. Thanks so much!
I had no idea that deep-cycle batteries are measured in amp-hours. I look forward to getting an RV before Christmas so I can go on a big road trip with my family. I’ll look into getting a deep-cycle battery that is amp-hours instead of watt-hours.
So if I have 1 80ah battery and I am using 20 amps a day it would last 2 days?
How many watts solar suit case would I need to keep the battery charged ?
Dave, good article that covers the basics.
Our trailer came with two new RV deep cycle batteries and I soon learned that I never got the expected life from them (or I should say half life as I didn’t want to drain them beyond 50%). I eventually found that the WFCO converter in the trailer did not fully charge them to 100%. I installed a Renogy DC-DC charger and they now charge to 100%. Deep cycle and AGM batteries need higher voltage than a wet cell (Maintenance Free) battery used in most vehicles.
I’m looking at a 12v fridge that has a rating of 340w/24 hours. I’m confused on how to get an amp hour rating. Help! 🙂