Marine Battery Charger Buying Guide and Charging Tips

If you have a marine battery, having a charger is a must. Without charger it is impossible to recharge the battery. When it comes to your marine battery charger, you cannot simply go and grab a charger off of the shelf because marine battery chargers are just as complicated as the marine battery itself. If you need to purchase a charger for AGM or any sealed marine battery, you must be very careful about overcharging – the charger decides the life of the battery. Also, you should know the correct charging method. Here are some basic guidelines to help you to choose the correct battery and a guide to charging correctly

Battery Type

The most important thing that you must consider when buying a marine battery charger is the battery type: flooded battery, AGM battery, or gel cell battery. For different batteries, there are different types of chargers. For flooded batteries, standard lead-acid chargers can be used, but for AGM batteries, mostly smart AGM chargers or microprocessor-controlled chargers are recommended. For some of the gel cell batteries, special gel cell battery chargers are required. Once you’ve noted the battery type, the charger must be chosen.

Battery Size

The size of the battery is one important factor to be considered when purchasing a marine battery charger. For our purposes, size doesn’t mean the physical size, but rather the capacity of the battery. For example, a marine battery with 50Ah capacity will need a 10-amp charger and will take up to 6 hours for the battery to be charged. A larger marine battery with 100Ah capacity will require 11 to 12 hours to get fully recharged. For faster recharging of larger batteries, you may want a charger with more power. Depending on the size of the marine battery and the time you can allot for charging the battery, the size of the charger should be decided.


Some need a charger to keep their marine battery charged during the off-season, when the marine battery is being stored and not used. In this case, you’ll need a low current rate, so it’s better to get a low current charger. Many require a charger for quick recharging of their battery. For this, you need to go for a quick and very powerful charger with a high current rate. The purpose of the charger should be kept in mind while purchasing a battery charger.


Many marine enthusiasts and professionals often carry more than one battery on their boat – whether they carry both cranking and deep cycle, or sometimes multiple batteries of same type. This requires more power. For this, it would be messy to have different chargers for each battery, so there are many battery chargers available which can be used for multiple battery types. If you need to charge multiple batteries at the same time, go for a multiple bank charger.

Charger Type

Basically there are two types of battery chargers:

  • Automatic chargers automatically sense the voltage and internal current. They will switch off when the battery is fully charged.
  • Manual chargers need to be set manually. The voltage and current required has to be adjusted, and they don’t shut down until they’re switched off manually.

Charging Deep Cycle Batteries

You should always strictly follow the charging manual while charging a deep cycle battery. For the first few charges, the battery should be cycled lightly, so that the plates are enhanced and formed completely. With a deep cycle marine battery, the battery should also be allowed some cooling time in order to improve the battery performance. Make sure not to quick charge the deep cycle battery, as this can shorten the battery life. Charging the battery with an incompatible charger can lead to sulfating or corrosion. The deep cycle batteries need to be equalized occasionally to help the cells of the battery keep balance.

Charging AGM and Gel Cell Batteries

AGM and gelled batteries are prone to the harmful effects of overcharging. Overcharging causes the electrolytes in the battery to dry, as the oxygen and hydrogen are driven out of the pressure-release valves and they are not able to recombine to form water. It’s recommended that you only charge your AGM and gelled batteries with the recommended smart chargers. AGM and gelled batteries must be charged in a well-ventilated place, as most of these batteries are likely to release harmful gases. If the battery has frozen due to extreme temperatures, the battery should not be charged until a normal temperature has returned. Chargers with constant voltage, current, and temperature should be used.

Some Safe Charging Tips

  • Carefully follow the charging instructions that come in the charging manual guide of the battery.
  • With flooded batteries, the battery should only be charged if the hydrometer reading is below 1.225 of specific gravity, and the load test is less than 9.6 volts (or 12.4 volts open circuit). The battery should not be left on the charger for more than 48 hours.
  • The charger should be unplugged before connecting or disconnecting to the battery.
  • The battery should be charged in a well-ventilated place.
  • The battery should be kept away from flammable or igniting substances, especially when charging.
  • If the battery begins to overheat, remove the charger and consult the manufacturer.
  • If the hydrometer and voltage readings haven’t changed after about two hours, remove the charger and consult the manufacturer.
  • Frozen batteries should never be charged. Allow the battery to warm up to 60° F before charging.


Buying a marine battery charger is not an easy task, as it’s not decided just by the outer looks of the charger or the features it offers. The charger should match the needs of the particular battery owned by the user and it must be compatible with it.