Keeping two batteries happy (separated)

truepath wrote:
even more concerning was one comment I read that said if the two batteries are not roughly equal in charge, then when they are linked there is a massive movement of electrical power from the charged one to the other one (think hot wires, blown breakers, etc.)

pianotuna is correct. This is a myth. Think of it this way - how much charge current do you get into a battery with a charger set to 12.5 volts? it's almost none. You can't charge another battery from a battery that has only a few tenths of a volt greater voltage. It just doesn't happen. I've used a solenoid/relay for 40 years and it's still going strong. You need a constant duty solenoid/relay and one with silver contacts will last longer.

2. Smart solenoid. This is a switch, but it has some intelligence that will block charge to the auxiliary battery until to main battery is at full charge and then I believe it meters the charge to the other battery. There are various options with these devices that will allow flow in both directions between the two batteries and allow use of both to assist with engine starting. One brand I have seen is Smart Power, but I'm sure there are others. These devices are more expensive than the basic solenoid and don't seem to have the same number of significant comments about failure, although there are some. They also seem to be more challenging to install.

This is a good option also. Although I've gone with a manually operated configuration, automatic isn't bad.

3. Diode based isolator. These devices keep the two batteries separate through the use of diodes that allow a charge from the alternator to be delivered to each battery, but won't allow a charge to return from one battery to the other. These devices seem to usually function without much trouble, but mine did fail. Their downside seems to be the small voltage drop they cause in the charge from the alternator. They also don't allow the charging of the starting battery from the converter (either shore power or generator power).

There seem to be two basic options for the diode based design. In one, you just live with the voltage drop difference. This is unacceptable. In the other, you compensate for it. There are various ways to compensate - one is used if your alternator has a voltage sense lead. Modern alternators often don't have that lead, but older ones do. You can buy diode based isolators with a separate terminal that you connect the alternators voltage sense lead to. Then the alternator outputs a voltage just high enough to compensate for the voltage drop of the isolator. The other option is to use an external voltage regulator for the alternator and set it to compensate for the diode drop. I don't like either option much.

I would appreciate comments, insights, clarification or recommendations.

Another option, not in your list, is to buy a high power MOSFET isolator. They have much less voltage drop than diode based designs, but they are more expensive. Hunt around for a recent thread on this. It has links to some MOSFET based isolators.

Here it is: http://forums.trailerlife.com/Index.cfm/fuseaction/thread/tid/27324318/srt/pa/pging/1/page/1.cfm