Absorbed Glass Mat (AGM): Maintenance Free
An AGM Battery is a combination of the two previous types with one key difference. This difference is a lead plate separator made of woven glass fiber sheets. Adding this separator aids in providing more consistent power. To explain, each glass sheet soaks in electrolyte which in turn absorbs and suspends the fluid. This results in the electrolyte fluid always being in contact with the battery plates. Constant contact with the plates enhances both the discharge and recharge efficiency. Likewise, the container comes sealed which is spill proof and vibration resistant.
Similar to gel cell batteries, AGM batteries are sensitive to overcharging. To explain, overcharging boils off the acid, damaging the plates, and killing the battery.
In general, AGM and gel batteries need special smart chargers to prevent overcharging. Unlike a wet cell battery that can use a digital charger. Either way, I prefer safe and fast charging for my marine batteries and recommend using smart chargers.
“Smart” chargers can charge a variety of battery types. While being safe and accurate using multi-stage charges, but more about that later. To ensure safety, check for a label that states it will work with the chemical type you use. Since every smart charger is different.
One last note, do not try to charge a gel cell at the same time as the other types on one charger. Gel cells need a lower voltage at the bulk and float stage. As a result, if you mix with the other battery types, one battery (the gel cell) may become damaged beyond repair.
Step 2: Determine the number of banks needed
Simply put, the number of banks needed, is the number of batteries the charger is able to charge at the same time. So if the charger has the “2 banks” label, it can charge 2 batteries within your boat at the same time. Although, if you have one 12V starter battery and two 12V trolling motor batteries you need a three bank charger. Otherwise, if you use one battery you can use a single bank charger.
To add confusion, there are instances when a “bank” is several batteries wired together. This is to allow these batteries to act as if they were a single, larger battery. With this in mind, smart chargers will allow one output per bank.
Each charger has outputs with separate positive and negative terminal connections. Plus per-bank microprocessors. To allow a charger to handle different types, sizes, and state of charges. Remember to be careful, what you can connect to a charger depends on the battery charger size.
Step 3: Determine Battery Charger Size – Amp Hours & Voltage
We don’t mean physical size of a charger but the actual capability of the charger. This refers to its amp and voltage rating. The amp rating will answer the amount of time the charger takes to charge batteries. While a voltage rating determines if the charger matches the wiring of your batteries.
For amps the simple answer is: the higher the amp rating, the faster the battery will recharge. To calculate what is a suitable amp rating you will do some simple math.
Amp hour rating on battery(s) X 10% = charger amp rating
Example (group 31 battery) 100 amp hours X 10% = 10 amp rating
To determine your total charge time, do another quick calculation.
Total Amp Hour Capacity of the Batteries
Total Amp Output of Charger
= Total Hours to Charge Batteries
Example: 100 amp hour battery / 10 amp rated charger = 10 hours
For voltage, it is critical to make sure that your output voltage matches the voltage on the charger.
Most small/medium-sized boats use 12 volt batteries, some will use one battery but most use two or more. Most boaters will use two batteries to gain more amperage, but want the voltage to remain the same. To accomplish this they wire the batteries in parallel.
Two 12 volt batteries wired in parallel, will output 12 volts, so you need a 12 volt charger.
If you want increase the voltage but keep amps constant, you wire batteries in series. Two 12 volt batteries wired in series makes 24 volts, like when powering a trolling motor.
For two 12 volt batteries wired in series, they change to outputting 24 volts, now you will need a 24 volt charger.
Many boaters get confused, a 24 volt charger will not charge a 12 volt battery faster. Attempting this destroys the battery, the charger, and can cause a fire on your boat. To reduce this hazard 12/24, 36, and 48 volt specific chargers exist.
Step 4: Choosing A Power Source
You will either power your chargers with continuous or intermittent AC power. So you will have some things to consider which factor into what kind and the size of the charger you want.
Many boats spend most of their week at a dock, with a charger connected to shore power. For this situation you can buy smaller amp rated chargers that have constant 120 VAC power. Find one that can handle the DC system and deliver proper power to float-charge your batteries. A good rule is the following:
Have enough amperage to equal the sum of the DC loads plus 10% of the amp-hour rating of the batteries.
Or if you spend more time out on the water than on shore you need to factor in intermittent power. Keep in mind the amount of capacity you need until the next recharge. One helpful tool is a battery monitor, such as the Victron BMV-700 Monitor. Some large boaters use an inverter generator to provide power to chargers on the water. But these may overcharge Gel and AGM batteries. So I recommend adding more battery capacity to be safe. Large battery banks accept more charging amps, so they replenish faster. As a result, you spend less time worrying and more time cruising the waters.
Step 5: General Considerations
The following list contains some general things to look out for, with a basic explanation of each.
- Multi-stage Charging: A smart charger is “smart” because of how it charges and protects your batteries. To provide precision charging a smart charger will it take temperature and battery chemistry into account. It does this with built-in microprocessors that charge each battery in multiple stages. How many stages and which safety options are available will depend on the charger you choose. When using a smart charger the batteries will charge fast, be safe and will maintain them when not in use. Yes, a smart charger is more expensive than a digital model but it’s worth the extra cost. Due to it aiding in extending the life of the battery.
- Vibration resistant: A high quality 2-bank battery charger should withstand shocks from wave impact.
- Waterproof construction: It is critical that water cannot leak inside a charger. Water will destroy the microprocessor inside as well as produce shock and fire hazards. High quality manufacturers will build to a specific (IP) waterproof standard. The chart below explains each.
IP Waterproof Chart
- Mountable: If you choose an onboard 2-bank battery charger it should be easy to install. Some charger companies will include mounting hardware and others won’t. If they aren’t included, it’s important to find corrosion resistant hardware approved for use on your vessel. As well as following the manufacturer’s installation methods when mounting. Finding and installing with proper hardware will ensures a clean installation and reduce the risk of damage to the charger or your boat.