Charging the House Batteries off the Alternator
I would like to be able to charge my house batteries off the bus engine while driving long distances. I've read extensively through the archives. Generally, I see several solutions:
1. Isolator method
Connect the alternator to an isolator, and then attach the house batteries to one side and the engine batteries to the other. Both sets of batteries will get charged, but no current will flow between the battery banks. There are two big downsides. First, the isolator will cause between a 0.5 and 0.9 voltage drop, reducing the charge to both sets of batteries. Second, the alternator current is not ideal for house batteries (too low for bulk charge but too high for maintenance charge).
2. Solenoid method
Connect the house batteries and engine batteries together and have the alternator charge both. This avoids the voltage drop of the isolator. By correctly wiring a solenoid, you can have the batteries automatically connect when the ignition is on and disconnect when it turns off. With a little additional wiring, you can easily use your house batteries to jump-start your bus if needed. However, there is significant danger of shortening the life of both your engine batteries and house batteries by connecting them together.
3. 2-way switch method
Connect the alternator to a 2-way switch, then manually change whether the current is going to the engine batteries or house batteries. One set of batteries will get all the charge and the other will get neither until you change it back. I see a lot of possibilities for problems with this if you forget to change, etc.
4. Second alternator method
It is possible to install a second alternator on an engine. The original alternator continues to charge the engine batteries while the new alternator charges the house batteries. An added bonus is that you can potentially generate a whole lot of extra electricity for running air conditioning while driving, etc. However, this requires a certain amount of technical know-how and a lot of effort to locate (or fabricate) the right parts (alternator, brackets, possibly new pulleys, belts, etc.).
5. Inverter and Smart Charger method
Run an inverter off of your engine batteries to a smart battery charger hooked up to your house batteries. You'll lose some energy to heat in transforming direct current to alternating current and back again, but you can get the right voltage to quickly and safely charge your house batteries. The major downside is that you can completely drain your house batteries if you forget to turn off the inverter.
I considered using an inverter and smart charger, but adding either an isolator or a solenoid so that it would turn off when the alternator isn't running and avoid draining the engine batteries. I'm curious what folks here have done and how effective it is. Thoughts?