Mobile Home Power Backup

You may think that you will have power all the time if you have a dual battery setup where the
auxiliary battery is charged along with the vehicle battery and where you have some solar
modules for when you stay put in one place for extended periods. But this may not be the case.
Even when your power consumption is within an acceptable limit which had been accounted for
in the design of the system, you may still unexpectedly find yourself short on power for any of a range of reasons:

1. The vehicle alternator is voltage limited such that the auxiliary battery never reaches full
charge, particularly when the auxilliary battery is being cycled on a daily basis. This is
not generally a problem for the vehicle battery as the only discharge is probably when you
start the engine, after which the battery recharges when you are driving. The vehicle
battery is thus generally close to full charge most of the time. Solar modules connected
to your auxiliary battery bank via an appropriate regulator should allow the auxiliary
battery to achieve full charge on a regular basis, as long as incoming power is allowed
to exceed outgoing power by at least 15% (to allow for battery inefficiency).

2. It is very difficult to design an appropriate solar power system for a home with no fixed
address. It is difficult enough when there is a fixed address, because even though you can
get a fairly accurate average solar radiation value for each month of the year for that
location, there is no guarantee that you won't have two months of predominantly overcast
weather.

3. The standard practice of designing a solar system based on a given load profile is an
inexact science at best. How can anyone say that he/she will use a particular light for 3
hours per night and watch TV for 2 hours per day. Anyone filling out the load profile
form in order to have an appropriate solar power system designed, will use educated
guesses at best or be totally misleading to avoid embarrassment at worst. Even though a
person may volunteer that he/she watches TV for 2 hours per day, he/she may neglect to
divulge that the TV is often left on as background noise or as a 'baby sitter'. The system
design can never be more accurate than the information it was based on.

4. According to Murphy's Law, something will invariably go wrong, and usually when you least expect it. There can be any number of faults that could cause the battery to go flat.
For all of the above reasons, it is good to have some kind of backup. So what is suitable for a backup to get the charge back into your battery bank? Keeping the engine idling or going for a long drive is not an acceptable option if you don't really need to go anywhere. Using a vehicle
engine just to charge a battery and nothing else is not good fuel economy. This is where you need a small efficient motor (petrol, diesel or LPG) driving either a 240V AC or 12/24V DC generator.

The standard transformer style battery charger is quite inefficient (50% to 60% efficiency is quite common) and the generator to provide the power to such a battery charger may need to be able to produce four times as much power as the battery charger puts into the battery (some of this to account for poor Power Factor).

There are two better options:
1. Use a DC generator which is dedicated to be a battery charger and does not produce 240V AC at all.

2. Use a switch mode battery charger that uses solid state electronics instead of a transformer plugged into a 240V AC power source (eg your 240V generator). A switch
mode battery charger can be in excess of 90% efficient and will work successfully from a considerably smaller generator.

If you need more information, contact the friendly staff at Rainbow Power Company info@rpc.com.au