House Power

You would think house power would be simple with all those kilowatts sitting on the boat. But there are a number of different options.

    • Tap off of one of the batteries
    • Add a DC-DC voltage converter; converting the motor voltage to 12V
    • Add a dedicated house battery

Tapping off the batteries (for example 1 battery in a 48V string) will work, but will cause that battery to dis-charge un-equally. Long term this will lead to problems.

Because in most cases our boats will be a bit overweight, most people will want to try some form of voltage converter to avoid carrying even more batteries. These electronic devices take 24, 36, or 48V input and convert to 12V output. More expensive units will put out slightly higher voltage so they can be used to charge an auxiliary battery. The more expensive units can handle up to 20amps @ 12V (~ 250watts) or more.

However, I happen to favor a slightly different strategy. Thunderstruck EV (see vendors) sells a very inexpensive "Green Galaxy" converter (about $25 as of this writing) that can handle 10amps and can be put into parallel operation. I recently found another version (not yet tried) which is potted and should be fairly waterproof.

So I recommend calculating your maximum amperage draw for all appliances on your vessel. Then buy enough of these inexpensive units for what is known as "n+1" redundancy. So for example, if your peak load is 30amps, n=3 (30//10) ergo - n+1 is 4. So buy 4 of the converters for parallel operation. This way you can lose one and still have ample power. These units are still so inexpensive that even buying 4 they are about 1/2 the price of the more expensive units - and you have the redundancy to cover you in the event of a failure.

These units are non-isolated. This is not normally a problem unless you are trying to use digital volt or amp meters which require separate power from the bus being measured. The more expensive isolated units will let you do this.

[Untested idea] Also interesting would be to run a 48V positive lead closer to the load points (say, a TV) and do the conversion right there. This will minimize losses due to higher amps over what is usually overly thin wire. The negative lead from the converter connects to your negative bus (both 12V, 48V are connected) - this sounds wrong, but it works.

[Untested idea] Finally, for very high amp appliances such as windlasses you can try a technique of drawing "from the middle". The voltage across the middle of a 48V string for example is 24v and 24v windlasses are easy to find. You're only using brief bursts of pwer, so even though you would be drawing unequally from the batteries, it should not have much of an effect.