Battery Banter


(published November 2012)

Chuck Hawley, West Marine’s  Vice President of Product Development, recently bought an Alerion Express 38 Yawl and is learning how to be a cruiser all over again. Since Chuck is not only a gear expert for our industry, but also in the middle of doing upgrades for his own boat, BWS asked for his opinion on the wide range of battery choices available for cruising boats today.

What are the best battery choices for cruisers in particular?
When it comes to larger sailboats, we’re talking about deep cycle batteries, not starting batteries. Starting batteries are a non-issue, because it’s rare that cruising sailboats have engines that justify specialty batteries for starting. There are really three categories of batteries for sailboats: wet cell, gel cell and absorbed glass mat (AGM) batteries. Wet cell batteries have to be maintained and topped up with distilled water regularly, while gel cells and AGMs are sealed with pressure relief valves and require no maintenance.

Are battery types really all that different?
If you tour the East Penn factory, you’ll see that most high quality sealed batteries are nearly identical until the end, when they’re filled with electrolytes. They’re more similar than dissimilar. When pressed, the manufacturers came up with different numbers to indicate cycle life. Gel batteries are closer to 500 cycle life, while dual-purpose AGMs are around 300 cycle life and 6 volt cells are around 1000 cycle life. 6v cells have a different ratio of plate to separator to gel, so they have traditionally had twice the cycle life of other batteries.

How do most batteries lose capacity?
From chronic undercharging or chronic overcharging. Batteries are remarkably tolerant of normal cycling down to some level of discharge. The oft-quoted advice is to not discharge your batteries below 50% of capacity, but I think of this as a general guide and not a hard and fast rule. I am not aware of batteries failing prematurely because you go down to 70% depth of discharge on occasion. However, if you discharge a flooded lead-acid battery to 70% depth of discharge and leave it in that dischargted state, you get a harder crystalline lead sulfate deposit that is difficult to force into solution and the battery will lose capacity because its specific gravity drops over time.

How do the three battery types differ in charging rates and voltage?
First and foremost, any reference to battery charging has to also reference the temperature of the batteries. For brevity, it’s common to use a single voltage, but has to be accompanied by a footnote describing the temperature. Because flooded batteries can gas to the atmosphere, they can tolerate charging at higher voltages. Sealed valve-regulated batteries should not be charged at higher voltages because it creates internal pressure from the gassing, which is forced past the valves, thereby reducing their capacity and life. This is why battery articles and experts are so insistent on accurate voltage regulation when charging sealed batteries. If pressed for a single maximum voltage for the various chemistries of batteries, I’d say 14.1V for gels, 14.3V for AGMs and 14.6V for flooded batteries. Of course, this is the maximum voltage; long term “float” voltages are up to a volt less. Since there are many providers of batteries, the right answer is to consult with the battery supplier for their recommendation.

If you add a smart regulator, should you also add a temperature sensor for the smart regulator?
Yes, or set the regulator at a conservative voltage. Gels and AGMs don’t benefit as much as wet cells from a high voltage cycle. You want to have a uniform surface on the plates from top to bottom. This is not a problem with sealed batteries. But with wet cells, you need to condition or equalize the batteries with a higher voltage that reduces sulfation.

You mentioned overcharge—what are the dangers?
With flooded lead-acid batteries, the dangers are minor other than the production of hydrogen gas, which becomes an explosion issue, and the consumption of electrolyte. A certain amount of gassing is going to occur; it’s a good thing, because it churns the electrolyte. When you equalize batteries, you’re doing a controlled overcharge.

With gel batteries, you don’t want to exceed the internal pressure so the pressure caps bleed off pressure. In normal use, they should never open. If the cap opens, it will release a burp of hydrogen and oxygen, which depletes the electrolyte. Pressure should be in the range of 2.5 psi; correct pressure encourages the gaseous hydrogen and oxygen to recombine and re-moisturize the battery over time. It’s like cooking pasta with a cover on the pot; the moisture condenses on the lid and drains back into the pot. We want to create gasses inside the battery without letting them escape.

Many cruisers have opted for solar panels and wind generators. How well do they work with typical cruising batteries?
The battery doesn’t care. It’s looking to be fully charged through a multi-step algorithm so it reaches its greatest capacity and is then allowed to float. This means being maintained at a voltage slightly above resting voltage, which is 12.7 for most batteries.

A solar panel has an advantage over most other chargers because it delivers pure direct current (DC). Batteries like DC because there’s no surge or ripple effect in the current that you get from a wind turbine or alternator.

When I was racing to Hawaii in 1980, I had an electrical system consisting of a single flooded deep-cycle battery and a 50W solar panel. During the race, I managed to stumble into the Pacific High. The solar panel was positioned flat to the sun. At one point, I went down below and flicked on the analog volt meter and it went WHAM to 16v. I could hear the battery gassing, so I turned the panel away from the sun and thought I had fried the battery. In truth, the battery had never worked better. I had inadvertently equalized it. That’s good for non-valve-regulated batteries; however, you can only take valve-regulated batteries up their maximum recommended voltage, but not over.

What would you suggest for the owner of a 38-foot yawl going to Mexico or the Caribbean for the winter?
If I were going on an extended cruise, first I would replace my current batteries with a set of new, fresh batteries from a quality manufacturer that cruisers respect. That’s one reason we’ve gone with East Penn for our branded batteries at West Marine. Personally, I’d get gel batteries with sufficient capacity so my daily energy use didn’t control my life. I’d use three group 31 batteries; they have more capacity than group 27, but are of a size that is manageable. I’d have a starting battery—group 24—that is adequate for my engine on its own charging circuit. I’d install a quality aftermarket alternator and a separate smart regulator to manage the charging cycle.

I’d have a couple of solar panels, but not mount them on my dodger, where they inevitably suffer from some form of shading. I’d put them on the pushpit, where they could be angled perpendicular to the sun’s rays.

Finally, I’d put an amp hour meter on my house bank, like the Blue Sea VSM422 monitor, which retails for $360. It monitors your starting battery for low and high voltage, it monitors your house bank, and it monitors your bilge pump cycling. It’s one tool that is a really good fit for a simple cruising boat.

Could you comment on building a house bank using 6v instead of 12v batteries?
6 volt batteries have some inherent benefits, as evidenced by their double cycle life. You get a robust single 12v bank in batteries that you can lift. If you put two 6v batteries in parallel, you get a very good 12v, 200 amp hour bank. Much easier to handle than an 8D when it comes time to replace them.

Are there issues with 6v batteries aging at different rates?
Putting two batteries in series, like a 12v bank consisting of two 6v batteries, is not a problem. There can be issues if you create a larger bank by putting multiple 12v batteries in parallel. While I have not experienced problems with paralleling battery banks, I know of other cruisers who have. The issue is if one battery has a shorted cell, the other battery can discharge its energy into that single cell and cause the battery to fail spectacularly. This is a strong argument for being able to take any battery in the system offline with a battery disconnect switch, as well as fusing batteries independently. I believe that most issues with batteries in parallel occur if you combine batteries of different ages; it’s vital that the batteries be identical in type and age. This is another reason to have a battery monitor like the Blue Sea Systems VSM 422—you can set both high and low voltage alarms, which can alert you to battery problems before they become big problems.

Any words in closing?
Many years ago, I was talking to one of West Marine’s vendors, who commented on how complicated boaters tend to make their electrical systems. They create systems that are “optimized” with lots of options for charging and lots of components to maximize their battery capacity and speed of charging. The vendor commented that the time and money spent optimizing could be better spent buying a new set of sealed batteries of the correct capacity, and having a single engine-driven alternator with modern multistep regulation. I think there’s a lot of truth to this. I’d also add that the first place to optimize a vessel’s electrical system is in the wires and switches that connect the batteries to the engine, alternator and starter. Anything more than a very small voltage drop in this circuit will adversely impact all other circuits on the boat. I realize this sounds like blocking and tackling, but large wire sizes, short wire runs and careful terminal installation are paramount for effective electrical systems.