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Summary of charging, split charging and battery monitoring.

The "one way valve" route simply is never going to be ideal. As shown above.

But the only real problem with the switch route is that the operator may operate it incorrectly.

Let's take another look at the switch route then to see what we can do about it.

A closer examination shows that we only actually need one switch, not two. We can connect the charger directly to one battery, and have a switch to connect the other battery when we are charging.

There is an advantage in this system. That of further chargers. If another charger is installed, no further switches are required. The same single battery paralleling switch will allow any further chargers to charge both the batteries. Nothing more needs to be added.

All of this is obviously much simpler for the operator. But there are still 3 problems....

In order for the switch to be in a convenient position for the operator this will often necessitate very long cable runs from the batteries to the switch. Another cause of losses in a heavy current circuit.

If the operator leaves the switch on when the charger is not charging, then both batteries will be flattened leaving no way to start the engine.

If the operator leaves the switch off when the charger is operating, then the other battery will not get charged.

Other than these operator problems, the single switch solution works. It has no other problems. If we could get round these problems it would be as close to ideal as we could hope for.

Well, it so happens, that there is a way round this problem. We can use a relay. A relay is a switch, but instead of being switched by the operator, it is switched by an electric current. A small electric current provided at the coil terminals of the relay, forces the switch on. If we could find the right current, to switch it on at the right time, then we would remove the possibility of operator error.

And it also happens that many alternators have a terminal on the back marked either as "D+" or "ind" that provides power when the alternator is charging. So we could use this output to switch the relay on thus charging both batteries.

Now this is starting to sound more like what we need. i.e. the ideal split charge system. In fact, it is the main system that has been used on many vehicles and boats for many years.

But it still has it's problems.....

A. The split charge system will only operate form the engine driven alternator. If we add a wind turbine, or solar panel, or single output AC powered charger, the other battery still does not get charged.

B. Many modern alternators either do not have this terminal present or have the terminal, but the signal is electronically generated (for feeding into an engine management unit) and is insufficient to energise the relay.

C. If this system is installed, and a heavy load is put on the system, a load that exceeds the capability of the alternator then the relay will remain connected and both batteries will be discharged.

These three problems thus prevent the system from becoming our ideal split charge system.

The non-existent relay based "problem"

There is an often quoted third problem. That of the relay and cabling becoming damaged due to excess current through them. This isn't a problem with the type of split charge system. This is a problem with the installer who used the wrong relay and wrong cable size. See here and here for explanations of why this is the fault of the installer as opposed to the system itself.

What we really need is the relay based split charge system, but with some device to energise the relay (i.e. switch the switch on) when we are charging the battery from any charger and that will also operate from modern alternators that do not have the capability to energise the relay.