We installed the Engel MT45 fridge this weekend. That's not the "Oh, Crap!" moment.

This fridge is 1.43 cu ft; 1/10^th of the volume of the original, complex built-in fridges with their 12V water pump, 110V compressor and engine-driven compressor. "Chilly Willy", the little Engel, just plugs in to a 12V outlet I added. No water pump. No cold plates.

Yes, it ties up some floor space. And it blocks two drawers. It's easy to put stuff in those drawers that we don't access very often. For example, the bottom drawer is where we store things like spare hose and rigging wire.

Lacking a built-in 6 cu ft freezer, we have to be judicious about buying frozen food. The reduction in complexity is ample recompense for giving up ice cream. Also, since we don't have a 8 cu ft fridge, we can only chill a few beers at a time. Since I can only drink a few at a time, this doesn't look like a big problem.

Other Simplification

Another big simplification was replacing the old (energy-robbing) battery isolation diodes with a modern voltage-sensing relay (VSR). The idea is that the relay parallels the batteries during charging; otherwise it leaves them isolated.

The definition of "charging" is simple: the primary battery is over 13.7V. In our case, the primary battery is the engine starting battery. The alternator, the shore power charger and the free solar panel will all charge the starting battery. When it's topped up, then the relay will engage and charge the house battery bank, too.

Installation is not difficult. First, disconnect the old diodes and selector switch. Second, connect the various wires to the new BEP 716 switch/relay combination.

There are four gigantic #1 AWG wires. They're stiff and heavy; about ½" in diameter. Two wires are starting battery to the starter motor. Two wires are for the house battery to the house electrical panel.

There are two little volt-meter sense wires. These has to be extended from the old selector switch to the new switch.

There's the "alternator field" wire. It uses battery voltage (via an oil pressure switch) to energize the field in the alternator so that it starts working after the engine has warmed up.

Finally, there are the various sources for energy. These use moderately large and stiff #8 wire. It's difficult to wedge these into the BEP switch housing along with the giant battery wires. Also, it's the kind of thing that we'll be expanding. Who wants to take that giant cluster of wires off the engine room wall? So I added a ¼" stud (with a red cap) to which various energy sources can be added.

Oh, Crap

Everything went smoothly. There was only one emergency run to West Marine. Somehow, I had mis-measured the original diode connectors. I thought they were ³⁄₈" when they were actually ¼". So I needed a ¼" power-input stud. That's not the "Oh, Crap!" moment. That's just ordinary mis-measurement.

The "Oh, Crap" moment came when it was time to head back to Norfolk.

When I turned off the charger, preparatory to disconnecting shore power, I saw that there was a 2A load on the batteries.

2A! That's the fridge. That's the water pump. That's one of the last few incandescent bulbs we haven't replaced with LEDs. That's a crazy level of battery drain.

I checked to be sure everything was off. Yet, we're still bleeding current at a prodigious rate.

What's wrong? What did I do wrong?

Sadly, it was time to race back to Norfolk to go to work. I can't leave the boat like this. After only four days, the batteries will endure damage from a too-deep discharge.

What Changed

One significant change is the removal of the diodes. The other change is moving the alternator field exciter line from engine battery to house battery. Everything else is just moving wires around from old selector switches to new selector switches.

The diodes isolated the batteries from each other. They also isolated the various charging devices from each other.

The batteries are now kept isolated by the BEP Voltage Sensitive Relay. The engine battery isn't over 13.7V; the blue LED on the relay isn't illuminated. Unless the relay and the new volt meter are both defective, the batteries are isolated.

There's a remote possibility that the yellow emergency parallel switch is defective. However, it seems to behave as expected. When in parallel, the charger jumps into overtime to charge both batteries. When in isolation, the charger settles down to trickle-charging the engine battery. It doesn't seem to "leak".

What's left? The two charging devices.

• The 2A is current flowing through the shore power charger.

• The 2A is current flowing through the alternator.

The shore power charger seems an unlikely culprit. It's large, over-built, and pretty reliable. It has no moving parts to wear out.

The alternator and its built-in regulator are the more likely culprits. The alternator has moving parts which can wear out. Perhaps a diode in the regulator is shorted. Perhaps one of the innumerable coils has a short.

Also, the alternator's field exciter line takes battery current through the alternator to ground. This is controlled by an oil pressure switch. Perhaps the switch is stuck on.

I'm unclear on why only 2A are bleeding away. If it's shorted, shouldn't I see full power? Shouldn't I be blowing the fuses on the batteries themselves? I'm hoping that this is due to resistance in the regulator circuit. Perhaps a shorted diode still offers some resistance?

The alternator appears to be original equipment: circa 1981. Before the "Oh, Crap!" moment, it was low priority.

It appears to have changed from "seems to be working" to "needs to be replaced ASAP".

I don't want to drop $$$ on a new ZRD or Balmar 60-100-SR-IG without some confirmation. Maybe I really did miswire the BEP. Maybe the shore power charger is dead.

It all seemed so simple when I started.