RV Repair: electric configuration, 8d battery, stereo microphone

Question
Thanks Bill,
just to be clear about your advice: i understand that i should use the
genarator when i am using large loads like the AC or any heating elements.
but in order to run lights, stereo and fan without the genarator though, i need
to find out the watts per device/bulb, multiply that number by the number of
hours used during the day, multiply that by 10 (compensation for the
inverter) and make sure my battery has enough Ah to cover it?  Example, how
much Ah are left after lighting 5 bulbs for 4 hours?  

ie. a 15 watt bulb (.14 amps @ 105 V) used for four hours = .14 * 4 * 10 =
5.6 battery amps times five bulbs = 28 battery amps so after four hours, my
8D 250Ah battery will loose 28 Ah, thus leaving approximately 222 Ah?  

also, is a $400 battery charger typical for this size/type battery or can i use a
smaller/less expensive charger? would you even recommend the 8D battery
or is there a better battery answer?

thanks and very much appreciated,
scott
-------------------------
Followup To
Question -
hi. i have an unconventional problem that i am hoping your expertise
can
help me figure out. my "RV" is not typical as i am rebuilding a '46 Ford
firetruck into my own recreational vehicle for performing/entertaining
and
vending purposes. i have basically built a small housing unit on the
back of
the truck. electrically, i am starting from scratch. what i would like to
be able
to do is run lights, stereo, microphone, fan from a battery/inverter
when i am
unable to plug into AC. [i'm thinking about an 8D battery/250 Ah, will
this be
enough for music and fans during the day and low watt lights
(flourescents,
LEDs) at night? this is a $400 battery and is recommended with a $400
charger, is this too much for my needs?] if i run the battery down, i
would
need to switch to the generator. should i make this switch manually? I
have a Honda 3000W generator for running
an air conditioner and other high watt appliances (cofee pot, smoke
machine,
etc.) used only when needed. depending on who i ask, i have gotten
many
different suggestions. i, of course, am looking for the simplest way to
achieve
and maintain a system. am i missing other pieces to this puzzle?
please
and thank you for any suggestions/advice/help?

desperatley seeking an electrical plan,
scott
Answer -
Hi Scott:
I'll do what I can.
First we should talk about inverters, they are a funderfull thing but have a
major difficulty. An inverter, at least in a motor home, makes 105 volts AC
out of 12 volts DC. What most people forget is that the current draw on the
battery is roughly ten times what the AC load of the device is. Say you are
running a TV, 300 watts at 115 volts AC. That is about 3 amps of draw in the
AC circuit. Supplying that out of a 12 volt DC battery makes the load on the
battery 30 amps, thats a lot. Even a D8 with 250 amp-hours of storage will
experience enough of a voltage drop in a few hours to force the inverter to
disconnect. A lead acid battery has almost a linear voltage to amp-hour
storage ratio. That is to say that once half of the stored energy is used up the
voltage drops to 6 volts.Inverters usually cut of at less than 10 vots of input.
Most large motor homes have battery banks that have 600-800 amp-hours
of storage and even those run down. Imagine running a refrigerator coil on
AC through an inverter. The AC inverter load is 30 amps but what is comming
out of the batteries is 300 amps, a D8 under this load lasts about 20 minutes.
What can you do? Either increase storage or decrease the load. There is only
so much available and when its gone you have to put it back. I would make
the Honda a stricktly manual system and start it when you need it. A totally
seperate battery, isolated from the rest, will let you start it when needed,
unless you have a pull start unit. A solar charger helps a little depending on
where you are. You can hook plates together to get you 5-10 amps of
charging power. I would limit the inverter use as much as possible and use
the generator, thats what it is for. Good luck.
Bill

Answer
Hi Scott:
Yup, you've got it!
The $400 charger they quoted you is probably more of a power supply than a charger. The main difference is that a charger reduces it's output as the battery gets full and a power supply does not. It looks at demand and then supplies whatever load is required. I would install two good RV or Marine batteries in parallel for your battery bank and then get a regular charger for about $50.00. Your genset should have a 12 volt output and you could use that or the charger to charge the batteries when the generator is running. It does not make sense to run the generator to charge the batteries because you are running the inverter, unless you just like the thought of having an inverter. The RV batteries will keep the output voltage of the batteries higher for longer than the D8 but when they do start to decay they go in a hurry, like falling off a cliff!.
Bill