Newbie with an electrical question

[beav395]

Hi all,

I recently bought a used tent trailer but it doesn't have any batteries on it. I am in the middle of deciding what type of battery or batteries I want to get. To figure this out I am trying to calculate what my usage will be.

I want to know if the following is correct. Do I have the basics right?

If amps X volts = watts, then a 15 watt dome light would require 1.25 amps (15 watts / 12 volts = 1.25 amps)

If I have 2 dome lights, then that would equal 2.5 amps.

The only other 12 volt item I have is a water pump rated at 2.5 amps.

Add the 2.5 amps for the pump and the 2.5 amps for the lights and I have a total of 5 amps.

Now If I have a battery that is rated at 105 amp hours and I only cycle it down to 50% I will get 55 amp hours from that battery (in a perfect world).

Therefore 55 amp hours / 5 amps = 11 hours of operations of my 12 volt items from my battery. Assuming I turned everything on the battery would last about 11 hours.

So, I'm thinking. If I was boondocking for 3 days and used my lights for 3 hours a day, that would be 9 hours x 2.5 amps for a total of 22.5 amp hours used by my lights. If I used my water pump for 30 minutes a day that would be 1.5 hours x 2.5 amps for a total of 3.75 amp hours used by the water pump. Therefore under these conditions I would have used 22.5 + 3.75 = 26.25 amp hours of my 55 amp hours available from my battery.

Under perfect conditions (which I'm sure I won't have) is this basicaly correct?

Thanks,

Brian

[AZsix]

Reading that made my head spin! :yikes:

I'm pretty new myself so I'm no help but I wanted to welcome you to the forum.
I'm sure someone on here will have an answer for you.

 Personally, I just know if the lights don't come on, my batteries dead.

[wavery]

Hi all,

I recently bought a used tent trailer but it doesn't have any batteries on it. I am in the middle of deciding what type of battery or batteries I want to get. To figure this out I am trying to calculate what my usage will be.

I want to know if the following is correct. Do I have the basics right?

If amps X volts = watts, then a 15 watt dome light would require 1.25 amps (15 watts / 12 volts = 1.25 amps)

If I have 2 dome lights, then that would equal 2.5 amps.

The only other 12 volt item I have is a water pump rated at 2.5 amps.

Add the 2.5 amps for the pump and the 2.5 amps for the lights and I have a total of 5 amps.

Now If I have a battery that is rated at 105 amp hours and I only cycle it down to 50% I will get 55 amp hours from that battery (in a perfect world).

Therefore 55 amp hours / 5 amps = 11 hours of operations of my 12 volt items from my battery. Assuming I turned everything on the battery would last about 11 hours.

So, I'm thinking. If I was boondocking for 3 days and used my lights for 3 hours a day, that would be 9 hours x 2.5 amps for a total of 22.5 amp hours used by my lights. If I used my water pump for 30 minutes a day that would be 1.5 hours x 2.5 amps for a total of 3.75 amp hours used by the water pump. Therefore under these conditions I would have used 22.5 + 3.75 = 26.25 amp hours of my 55 amp hours available from my battery.

Under perfect conditions (which I'm sure I won't have) is this basicaly correct?

Thanks,

Brian

You are over-complicating this. Just get the biggest 12v deep cycle battery and battery box that you can fit on the tongue of your trailer, without having problems.

I would recommend a 29 or 31 series deep cycle battery. The cost difference isn't that great and you may be sorry if you get one too small. You won't be sorry if you never run out of power. That's the bottom line.

Now, if you were installing an inverter or solar panels or some other more suffisticated system, it would be worth doing the math. In your case, it is just, "Bigger is better" (To a point).

[beav395]

Thanks for confirming what I was already thinking.  After looking at the math it seem that 2 6 volts batts would be overkill and 1 good 12 volt would be plenty to cover me for quite a while given my rather small reguirements.

Safe Travels,

Brian