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Choosing and Mounting a Battery Charger

Modern BCs can charge your batteries, maintain them without overcharging and condition them.

By Jack and Alex Wilken Most boats spend a fair amount

of time at the dock, so charging and maintaining your batteries is the job of a shore powered battery charger (BC). We want to talk about some guidelines for which charger to choose, and how to install it. A battery charger is not something we want to be fussing with all the time. We put them in and we want them to do their job without intervention from us. Today’s BCs are designed to do just that. The simpler models need only be installed, while the more sophisticated models will need an initial programing. Even this programing is only a matter answering some questions like the type of batteries, their size, etc. Modern BCs charge your batteries, maintain them without overcharging and destroying them, and in some cases, conditions them. This was not the case in the past. Type of Battery Charger

The most basic choice is between

the older ferroresonant type and the newer electronic chargers. We will focus on the electronic chargers and really would not recommend anything else. What to look for depends on what type of batteries you have. Most BCs have a switch or setting for flooded and gel batteries but not all have a setting for AGMs–Absorbed Glass Mats. (Figure 1). You can use the

flooded battery setting in this case, however you may not get the most from your more expensive AGMs. Be careful. It is very common for the term “Gel Cell” to be used when referring to sealed, maintenance free batteries. Sealed or maintenance free batteries are flooded, but they are sealed, and incorporate some way to recapture the gases created during charging. They also tend to be more sensitive to over charging because there is no way to replace the electrolyte. When choosing your BC, you have 3 or 4 phase models available. The 4 phase BC is especially suited for flooded batteries, 3 phase being suitable for the others. Size of Battery Charger

The size of the charger depends, as do most things on a boat, on the use.

Amperes to Watts volts X amperes = watts

12v X 2 amps = 24 watts Watts to Amperes

watts divided by volts = amperes 25 watts / 12v =2.08 amps

Figure 2: Use these formulas to calculate amperage draw.

Figure 1: This charger has multiple choices of battery types.


How much electricity does your boat use at the dock? Lights, refrigerator, etc., all can be calculated by looking up or measuring their amperage draw and multiplying it by the number of hours of intended use. For example, a 25 watt light on a 12 volt system draws a little more than 2 amps per hour (Figure 2). The total consumption is then added to 10% of the capacity of your battery banks. If you want to be at the dock for short periods as opposed to the reverse, you will need to size your BC to be able to match the maximum charge rate. The

maximum amperage that you can put into a battery depends on its chemical type- flooded, gel, or AGM. The max charging rates for AGM batteries is 40% of the total capacity of the batteries to be charged. For gel it’s 30% and 25% for flooded. If you are using your BC at anchor off the generator, it will need to have an output equal to the max charge your type of batteries will accept. This will keep your generator running time down.

Where to Mount Your Battery Charger

You need to mount your BC securely

to a bulkhead or structural member in a place where it will not be in contact with corrosive gas. Mounting it directly over the batteries is not allowed under ABYC (American Power and Yacht Council) guidelines. It is necessary that any access panels or hinged plates must be able to be opened. ABYC also says it must be mounted at least 2’ above normal accumulation of bilge water. The space in which the BC is located must have an ambient temperature of less than 122° F and be well ventilated. You will also want to think about noise from cooling fans if you are near a sleeping space. Remember, batteries are not to be mounted or at least vented into a living space. Of course, if the BC is to be put in a space requiring ignition protection, it must be protected. Connecting the Charger

The best situation is to purchase

a BC that has separate outputs equal to the number of banks that you have (Figure 3). Using isolating diodes in place of separate outputs will reduce the voltage arriving to the batteries

Figure 3: Battery charger with three separate outputs.

Figure 4:.

Isolating diodes will not charge batteries completely.

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