What do batteries want?
Battery power is a baffling topic to most boaters, but is also critical for starting engines and running lights and accessories. If you don’t happen to have an abundance of “smarts” when it comes to managing your charging needs, it helps to have a system with the “smarts” built-in. Fortunately, modern “smart” charging options are available to make proper care of batteries almost idiot-proof. We have onboard battery chargers, portable battery chargers and combination inverter/battery chargers for any size or style of boat. To find out more about chargers that are combined with a power inverter, see our West Advisor, Do-it-Yourself: Selecting an Inverter or Inverter/Charger.
Marine deep-cycle batteries last the longest and charge the fastest if they are charged in distinct phases, which we refer to as the “Ideal Charge Curve,” the charging schedule recommended by virtually all makers of marine batteries. In the description that follows, remember that recommending precise voltages for batteries is subject to at least two caveats: 1) Gel batteries charge at different (and lower) voltages than flooded-type and AGM batteries. 2) The voltages stated are temperature-dependent.
This makes it difficult to recommend precise charging voltages, since they vary according to the temperature of the battery. Most of us generally operate our boats in temperatures between 50°F and 90°F, and the values used in our West Advisors reflect that. Higher temperatures require lower voltages, and lower temperatures require higher voltages. Note: In the following section, we define the capacity of the battery bank (in amp-hours) as C.
What size charger do I need?
Deciding how big your charger needs to be is determined by the size and type of your batteries, and whether your boat has a continuous or intermittent source of AC power to run your charger. Boats that spend most of their week at a dock, constantly hooked up to shore power, require smaller chargers. You need enough capacity to run the continuous loads on your battery system, like DC refrigeration (frequently the biggest user of battery power) and lights, plus enough power to float-charge your batteries. A good rule is to have enough amperage to equal the sum of the DC loads plus 10 percent of the amp-hour capacity of the batteries.
If you’re cruising or anchored out, and aren’t plugged in except intermittently, you will want enough capacity to recharge in the time you have available, if possible. You need enough juice to replace the power consumed by all the DC loads, as above, plus an average charge rate equaling the amp hours required divided by the hours available. Maximum amperage that batteries can accept during the Bulk Phase of the charging cycle vary depending on battery chemistry: flooded batteries can accept a charge rate of up to 25 percent of C; gel batteries have a higher acceptance rate of as much as 30 percent; AGM batteries accept the highest charging amps, as much as 40 percent of C.
How long does it take to charge?
If you operate your house bank between a 50 percent and 85 percent state of charge, as many experts recommend, and charge once daily, you should be able to return the 35 percent of battery capacity by operating a properly-sized alternator for slightly over an hour. More deeply discharged batteries, or smaller alternators, will require more time. Surprisingly, using a massive charger or monster alternator to pour on the current can be counterproductive, and can shorten your battery life. Excessive current makes batteries heat up and gas excessively (when the electrolyte dissociates into hydrogen and oxygen). Soon the voltage limit is reached, causing a downshift from the Bulk to Acceptance Phase. Adding more battery capacity is a better solution, and may even decrease the needed recharging time. Bigger battery banks can accept more charging amps, so they replenish more quickly and you don’t need to run your engine as long.
Max Charge MC-614-H has seven charging programs for your externally-regulated alternator.
Multi-stage smart charging while under way
When it comes to efficient battery charging, a multi-stage smart shore power charger is only part of the story. You need a voltage regulator with intelligence, plus a high-output alternator, to maximize the efficiency of your charging system. In automotive applications, the alternator’s function is to keep a starting battery happy while supplying power to electrical/electronic systems needed to operate the car (headlights, CD player, vanity mirror lights, etc.). In this environment, a simple, internally-regulated alternator is usually more than sufficient to get the job done.
Onboard a boat the alternator and regulator must support a much greater battery capacity to fulfill engine starting and house battery loads. The alternator and regulator must be able to charge effectively at lower rpms and live in an inhospitable environment. In addition, the alternator and regulator may sit for long periods between uses (surprisingly, one of the toughest aspects of alternator life). At the same time, this system must meet the expectations of boaters who want to have their batteries charged with the least amount of engine running time.
Intelligent voltage regulation meets the needs of newer battery types, and the added load that modern electronics and inverter technology demand. Intelligent regulation matches the output of the alternator to the specific needs of your batteries (AGM, gel, Optima, deep-cycle flooded, etc.) so batteries charge faster based on the needs of their unique constructions, and so the charging system can most effectively utilize the increased voltage high-output alternators provide. Max Charge and ARS-5 regulators are equipped with engineered programs for each battery type—just tell the regulator what battery type you are using and the regulator handles the rest.
In addition, many smart voltage regulators are equipped with the ability to monitor both battery and alternator temperature, and respond by increasing or decreasing voltage levels to maximize both safety and performance. In the event of a condition that poses danger to the system or the vessel, the regulator has the ability to discontinue charging completely.
Our 15 Amp Automatic Battery Charger with Engine Start Timer delivers three-stage charging for a flooded starting or deep-cycle battery. It even monitors how well your alternator is pumping out power.
Tips for battery longevity
Shallow discharges lead to a longer battery life.
80 percent discharge is the maximum safe discharge
Don't leave batteries deeply discharged for any length of time
Charge batteries after each period of use
Don't mix old batteries with new ones
How to rapidly kill a battery
Undercharging: consistently failing to fully recharge batteries leaves them with lead sulfate that hardens on their plates—they become sulfated—and gradually lose their ability to perform. Increased resistance when charging causes falsely elevated voltage readings, essentially fooling the battery charger, leading to further undercharging, in a downward spiral. Beyond a certain point, a sulfated battery cannot be returned to a healthy state, and you need a replacement. Keep your batteries charged, and equalize your wet cell batteries every six to eight weeks in temperate climates, and more frequently in the tropics.
Overcharging: especially fatal to Gel and AGM batteries, consistent overcharging (NOT equalization) boils the electrolyte out of the cells, and can even lead to thermal runaway, with the battery becoming hotter and hotter. One of our writers experienced thermal runaway on his liveaboard Catalina 30, caused by a ferroresonant “dumb” charger, with nearly catastrophic results.
Excessive deep discharge: don’t completely discharge a deep cycle battery if it can be avoided. The deeper the discharge the less life you will get from the battery. The ideal method is to charge and discharge the batteries through the middle range (50 percent to 85 percent) of their capacity and, if they are flooded batteries, to equalize them periodically. Leaving the battery in a fully-discharged state, for example during winter storage, causes it to become sulfated.