Charging the House Batteries off the Alternator

[Biscuitsjam]

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?

[Johnny Mullet]

I been using the isolator method and it has been working great for me.

[ol trunt]

Like JM, I've had good luck with an isolator in my older motor homes. However, on my present bus with it's multiplex wiring system, the alternator simply shuts down when connected to an isolator. I ended up installing a Sterling Power battery to battery charger and it works fine. A nice thing about the unit is that it is a true 4 stage charger and is programable for all sorts of batteries.
Jack

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[Biscuitsjam]

Originally Posted by ol trunt

Like JM, I've had good luck with an isolator in my older motor homes. However, on my present bus with it's multiplex wiring system, the alternator simply shuts down when connected to an isolator. I ended up installing a Sterling Power battery to battery charger and it works fine. A nice thing about the unit is that it is a true 4 stage charger and is programable for all sorts of batteries.
Jack

That looks like a really neat solution. The battery-to-battery chargers automatically shut down when the voltage drops too low, so they don't drain your engine batteries. Also, as 3-stage chargers, they'll charge your house batteries faster and not over-charge them.

The Sterling Power units are expensive at $300-450, depending on size. Off-brand are $100-200, but it sounds like there is a decrease in reliability. Importantly, the off-brand units also seem to generally have incomplete or inaccurate instructions (misleading pictures on dip switch positions, etc.).

Have you noticed any major downsides from using the Sterling Power unit? Is your alternator powerful enough to effectively charge your engine batteries and the Sterling Power unit simultaneously?

[ol trunt]

I've had the Strerling BB1230 unit for a year now and even with my small 110 amp alternator I've had no problems (but then I have only two golf cart batteries to keep charged). I hooked up my dealer grade scan tool and watched the alternator output in real time with the BB1230, AC, lights and engine on and and all was within specs. There are several options as to how to hook up the BB--automatic, ignition feed with low voltage/regen and ignition feed alone. I chose ignition feed and added a switch which allows me to manually turn the system on and off. I always give the alternator a chance to recharge the start batteries before I ask it to charge the house bank-- just to hedge my bet. The Sterling user's manual is nearly excessively complete and requires several re-reads to make clear sense. Part of the reason for this is that the manual is written with a British lilt which makes for interesting sentence structure. At least it wasn't first written in Chinese, translated in India and then printed in Mexico like so many instruction sheets seem to be today.
Jack