Charging Your Boat Batteries

Battery and charger technology alike have made some incredible advances since those days a long time ago when I worked in a gas station and filled new car batteries with fresh sulfuric acid before charging them on a linear trickle charger...

Today is a different story; batteries designed for cars are one thing -- those designed for boats are a totally different story. Each of today's boat batteries has a different purpose, and each has various advantages and disadvantages (For more information, check my article on Boat Batteries: What You Need To Know).

Trolling batteries

Making the right choice when buying a new battery will eliminate many worries when you are on the water so you can concentrate on the most important reason you bought your boat in the first place: having fun.  But what about after you get off the water?   Those brand new batteries you just bought -- whether deep cycle or cranking variety -- will obviously need to be recharged at some point. More importantly, marine batteries, no matter the type (flooded wet cell, gel, AGM, or even the newer Lithium-ion variety, respond to charging in different ways, and using the right battery charger in the proper way will help preserve the life of your boat's batteries.

Charger Types

Most chargers that were designed to charge you car batteries were not designed to charge your boat batteries... for a number of reasons. For one thing, many chargers -- especially the less-expensive ones -- don’t always completely shut down. And how often do you think that you can remember to unplug that inexpensive charger when things “look about right?” Importantly, Buying a low-cost charger with limited charging capabilities will shorten the life of the batteries rather than prolong them. So, what is the best charger to buy in order to keep those expensive marine batteries in top shape?

There are two types of chargers to consider: portable or onboard. Portables are great for many applications -- especially when your batteries are at home and/or if they are out of the boat. These chargers also tend to be a bit less expensive then onboard models. The primary disadvantage of portable chargers is that they can be somewhat inconvenient to hook up and switch from battery to battery in the confines of a boat's battery compartment. And because they are portable, they are more subject to being stolen if you need to use them in places like motel parking lots, boat stalls or other public places.

On board battery charger

Onboard chargers, although more expensive, can easily help pay for themselves when it comes to convenience.  Since your whole system is already wired, simply plug it in to a 120-volt outlet and let the charger do the work; because it's permanently installed, it certainly deters would-be thieves from helping themselves...  Perhaps most importantly, on-board chargers are generally more technologically-advanced units, providing the necessary multi-stage switching to manage and maintain a boat's batteries during charging.  I'll therefore center the discussion to onboard chargers; in specific, the desired variety is a multi-stage charger.

Multi-Stage Charging

Leaving behind most, if not all, of the chemistry and physics involved, batteries will do best on a regulated diet of amperes and volts served up in the right amounts… Current battery charging technology relies on computer chips (microprocessors) to do the job; this is done in stages and there are usually three of them: 1) bulk, 2) absorption and 3) float. The chargers engineered for the staging tasks are referred to as "smart chargers", designed to provide maximum charge benefit with minimum observation on your part.

In the bulk stage, which involves about 75-80% of the recharge, the charger delivers as much current as it is capable of safely doing -- up to charger capacity -- at a constant rate while the voltage in the battery is increasing.

Note: This ampere amount could be anywhere from 10 -25% of the battery capacity in amp hours and should not raise a wet battery temperature above 125° F, or an AGM or GEL (valve regulated) battery over 100° F.

The absorption stage is where the voltage is now constant but the amperage is tapered and regulated while the battery is “topped off”; this constitutes roughly the remaining 20-25% of the charge and the battery should now register anywhere from 14.1 volts to 14.8 volts. Depending on something called a "set point" in the charger, the current must be varied in a precise way in order to hold the voltage constant (at, for example, 14.4 volts) while the battery absorbs the remaining energy to reach a fully-charged state. (Otherwise, under- or overcharging could occur),

And finally, the float stage is where the voltage is just enough to keep the battery from losing any charge. The charging current has dropped back to almost nothing, allowing the battery voltage to drop back to around 13.5 volts - 13.8 volts. The neat thing is that, since the battery is now fully charged, this "float" mode can be used to maintain a fully charged battery indefinitely.

Smart, multi-stage chargers are designed for faster and fuller charging, which ultimately leads to longer battery life, more useable capacity from your batteries, less electrolyte loss (in the case of wet flooded batteries), and/or very little chance of over-charging and ruining sealed batteries.

The Right Charger

Opinions differ on the "right" charger. Recommendations for charger size range from 10% up to 25% of the battery's amp-hour rating. Bottom line? The higher the charger ampere rating, the quicker the recharge time. Recharge time can be approximated by using a fairly simple formula: Divide the number of amp-hours to be replaced by 90% of the rated output of the charger.

As an example: Say you have a 100 amp-hour battery that has been discharged by roughly 50 percent. You would therefore need to replace 50 amp-hours. Using a 10-amp charger, take 50 amp-hours and divide by 90% of 10 amps (or 9 amps) = an approximate 5.6 hour recharge time. A deeply discharged battery deviates a bit from this formula, requiring more time per amp-hour to be replaced. A 6-amp charger would require over 9 hours, while a 15-amp charger would take less than 4 hours to replace the 50 amp-hours of charge.
Charger used in garage for winter storage

I have a Dual Pro Recreation Series Triple Output Charger in my garage with three 6-amp banks to use at home over the winter (when time isn't an issue), to which I connect my cranking and my deep-cycle batteries, as well as an onboard charger in my boat that has two 10-amp banks just for my trolling batteries (two 12-volt batteries for the 24-volt system). 

Note: The Recreation Series charger has been replaced by the Real Pro Series.

 In terms of the right charger, the decision is yours and depends on your needs and how much you want to spend.  Higher amperages and more charging banks add more to the price.  Also keep in mind that longer charge times require lower currents in the bulk stage and are a bit more gentle on the batteries.

If you like quick charge times, I recommend the Dual Pro Professional Series 2-Bank Charger, 15-amp/Bank PS2. And, all Dual Pro Chargers - Professional, Sportsman and Eagle Performance Series are Made in the USA!

Whatever your choice, a good charger is important in maintaining healthy batteries.

See you On the Lake!

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