The three-way Dual Circuit Plus is simpler than the four-position switch, but you still have to remember to switch to “combine” so that both batteries are charged when the engine is running, and back to “on” when the engine’s off, which means there is still room for confusion. I wanted more of a set-and-forget system, so I decided to add a battery combiner. Also called automatic charging relays (ACR) or voltage sensitive relays (VSR), these devices connect two or more battery banks together when charging and isolate them when charging stops.
Don’t confuse ACRs with battery isolators. These contain silicon diodes that let current flow only one way; the current can flow from the alternator to the batteries, but not from one battery to another or back to the alternator. This means you can’t accidentally run both batteries down, which is good, but there is a sizable voltage drop across the diodes—up to 0.7 volt—that can leave your batteries undercharged.
You can overcome this by installing a “smart” regulator that senses the undercharging and compensates for it, but this would be a whole other project, so I decided to follow the path of least resistance (pardon the pun). An ACR contains a relay that closes when one battery reaches its peak charge, allowing charging current to also flow to the second battery. When charging stops the relay opens again, isolating the batteries. There is no voltage drop.
There are several makes of ACR on the market, some more expensive than others, and some with more functions than others. I chose a Blue Sea 7610-SI Series unit (Photo 1), which supports alternators up to 120 amps and can cope with temporary immersion, always a handy attribute for marine electrics. The combination of the new switch and the ACR meant that I would merely have to turn the switch to the “on” position, and both banks would be charged according to their needs. When the engine was switched off, I could leave the switch in the “on” position in the knowledge that I was using current only from the house bank. It would no longer be possible to flatten the starting bank accidentally.
It was difficult to choose a battery monitor. There are many on the market, from the simple analog type with its moving needle to the sophisticated microprocessor-controlled units from companies like Link and Xantrex, which have a multitude of functions and can cost hundreds of dollars. With my simple system I couldn’t see the need for anything too complex; all I wanted to know was the state of charge of each bank and how much current was flowing into or out of the batteries.
In the end I decided on a Microlog DMM-1 (Photo 3), a Canadian-made unit that measures battery voltage on both banks and lets you know in amps how much current is going into or out of them. You can switch through four modes to display (1) the voltage of the first battery; (2) the voltage of the second battery; (3) charging current in amps; (4) total current drain in amps. It has alarms for both low and high voltage (handy to know if you’re cooking your batteries) and looked like a simple, cost-effective solution.