OT: battery power and generation

[Joseph Sinclair]

Nathan England wrote:
> All,
> 
> I live in a travel trailer full time and move from place to place. I always 
> hook up to power at the camp ground / trailer park and have never needed to 
> use my batteries unless the power goes out, which in northern AZ is quite 
> often. 
> 
> Anyway, I want to start using my batteries more and not paying APS for 
> electric, so I am looking into solar generation or a gas generator to 
> supplement. But I am completely ignorant of the way power works or what 
> amp/hours means and all that.

Definitions:
Amp/Hour : Power delivered at one ampere for one hour.  If your trailer has a typical load of 30 amperes, then a 90 amp/hour battery will power it for roughly three hours before running out.
Inverter : A device that transforms DC power (usually from a battery) to AC power.
Volt-Ampere (VA) : A simple measure of AC power.  AC power is not calculated as Volts*Amps the way DC is, it's a more complex equation due to RMS equations, multi-phase power, etc...  VA is a simpler proxy for the single-phase power equation.
Battery : A chemical generator.  Batteries do not store energy, they store chemicals that can generate electricity.  Rechargeable batteries use a reaction that may be reversed in-place by application of a reverse-biased electric field.
Solar Photovoltaic (PV): the use of semiconducting materials to generate electricity directly from light.  Most current commercial materials are 8-12% efficiency, and may generate around 1500 watts under full sun, which works out to around 5 kwh/day(about 20% of typical household use) year-round average (for a 3X5 meter roof-mounted unit costing around $15,000 installed).

> 
> I have two 12volt deep cycle batteries, what do I need to charge them and how 
> long could it take to charge them?

To charge your batteries and use them for primary power as well you would need a power management unit (available with most solar and generator systems, at an extra cost).  The time to charge depends on:
1) how much power can the batteries produce fully charged
2) how efficient is the recharge process (typically around 30%)
3) how much power do you have to dedicate to charging the batteries

Keep in mind that even deep-cycle batteries have a limited number of charge/discharge cycles, usually around 500-1000, depending on chemistry, so you have to factor in replacing them every 2-3 years if used for primary power.

> 
> I know I could ask this on an RV forum, but being a tech junkie, and you all 
> as well, I'm assuming someone will know some tricks so I could possibly build 
> something myself... Possibly creating an interface to my computer to 
> monitor/control any of this.

It's fine to write a software interface to a purchased power management unit (most have well-defined control interfaces), but don't even think about building your own power management unit, since the unit has to handle household power circuits, and a small mistake could easily cause fire or electrocution.

> 
> nathan