Volts, Amps and Other Nerdy Stuff

Friday I had the six "house" batteries replaced on Division Belle.  I've been testing batteries and looking at alternatives for weeks, and my bride has made it abundantly clear that most people have no interest in such things. So you are welcome to skip this post if you'd like.

Heavy Lifting

The batteries that came with the boat in October of 2018 were not marked with the date installed, and were depleted to a fraction of their original capacity. The new batteries are a different brand (Lifeline) but are the same size as the old. They are "8D" batteries that are quite large and weigh about 160 lbs. each. The installation was a 10-hour job for an electrician and a very strong young man who both work for Performance Power Systems nearby in Savannah. They did a great job that was enormously difficult given the location of these batteries in fiberglass boxes in the four-foot tall engine room.

Engine Room battery boxes

So what are the house batteries and why are they important? Virtually all of the lighting and electronics on the boat as well as the refrigerator, water and other pumps, and vent fans are powered by these batteries. The batteries are nearly always being charged by a charger powered either by shore power or our generator, or by a large alternator attached to the main engine when we are underway. Since they are nearly always fully-charged, the capacity of the house battery bank is only important if we are not running the engine, connected to shore power, or running the generator.

Anchored out for example, we run our generator when we need hot water or when we need heat or air conditioning, but we do not wish to run it any more than necessary. It consumes fuel, it needs to be serviced and have its oil changed after set periods of use, and most importantly, it is noisy. So for example if we are anchored out in beautiful weather for days at a time, we would run the generator periodically to charge batteries and produce hot water, but there is nothing nicer than shutting it off for a quiet dinner on the aft deck or on the bridge.

Each of these batteries is nominally 12 volts and is rated at 255 amp hours. Each pair of batteries is connected "in series" creating 24 volts rated at 255 amp hours. And then the three pairs are connected "in parallel" to create a bank producing 24 volts rated at 765 amp hours. A bank rated at 765 amp hours means it has a capacity to produce 765 amps over a 20-hour period, or 38.25 amps per hour. 

Amp hour ratings are important for comparing batteries but running batteries all the way down shortens their life considerably. In fact, most battery makers and experts recommend never discharging this type of battery by more than 50% of capacity. In my case that would mean I could use 38.25 amps for about 10 hours or, more likely, around 25 amps per hour for 15 hours. While sophisticated gauges are available to monitor this precisely, I don't have one. In my case a decent estimate of 50% depth of discharge can be made by monitoring the battery voltage and never allowing it to drop below about 24.2 volts while under a load.

When we are asleep, our boat will draw about 8 amps just from things that are always on. When we are up with lights on, the refrigerator opening and closing, the water pump running on occasion, etc. the average amp draw might approach 25 amps. So by my estimates, shutting off the generator to have dinner we should be able to easily leave it off until some time the next day. This can be tried out and monitored to get a good idea of our usage.

The new batteries and the ones they replaced are called Absorbed Glass Mat (AGM) batteries. These are maintenance free and have tremendous capacity without worrying about filling water as required with conventional "flooded" batteries. 

Many boats being produced today and many older boats are converting to Lithium Ion batteries, and I looked at a conversion carefully. Lithium batteries have tremendous advantages such as fast charging and being able to use nearly 100% of their rated capacity. Thus, lower amp hour capacities are needed allowing for smaller, lighter batteries. In our case a conversion to Lithium would have had benefits but it would have required more expensive batteries, relocating the batteries to a cooler spot out of the engine room, a new charger, a battery monitoring system, and all of the rewiring to accomplish such a change. For us, it came down to how we use the boat and the fact that almost all of the time our batteries are fully charged and we do not need to go for long periods without recharging.

Installing the new batteries did give me an opportunity to figure out the wiring on the boat and look at our charging systems. I have figured out the mysteries and I now think I understand how the system works.

So all I need to do now is use the boat. It will be nice to anchor out in mild temperatures and just see how these batteries perform. I hope it is soon. In Georgia now it is cold and it has been raining, seemingly forever.