Marine Batteries and Charging

Written by Harvey Gilbert
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Marine Batteries and Charging

by Harvey Gilbert (Captharv)

I will attempt to explain which batteries that you need, why you need them, and how to properly charge them. I will speak of regular flooded electrolyte batteries. The gell and special batteries are similar, but require a slightly different charging voltage. Do not mix AGM, Gell, and flooded electrolyte batteries. The alternator/charger can not generate different voltages.

Marine batteries come in 2 general types: Starting and Deep Cycle. A starting battery has many thinner plates for much current, for a shorter time period. Their total amp-hours are much lower than the deep cycle for the same size. You will see instructions in the engine and/or battery literature about not cranking the engine for more than 15-20 seconds. During this very high current state, the battery is venting hydrogen gas, among other things, and if it gets hot, can ignite. So, it is designed to produce the kind of current to allow the engine to start relatively fast. Problem is, some engines don’t know that…….

The deep cycle battery has fewer, much thicker plates and is designed to give a small current for a long time period, like 5-10 amps to run a refrigerator, lights, TV, etc Their starting amps are low, but the amp-hours (AH) for smaller currents is much higher. Most deep cycle batteries are rated at 5 amps; 50 amp loads will cut the AH to about 70%. Fact: most “deep cycle” batteries are a compromise: they fall somewhere between a starting and a true deep cycle battery and can start an engine. The total amps is slightly less, but usually they are just plain big enough to do the job.

Another fact: batteries are designed to be discharged less than 25% of AH capacity. Running even a deep cycle completely dead harms it. They are called deep cycle, not run-dead. Why? When a battery is run down, one cell will be dead before the others. The remaining cells are passing current thru the dead one, and trying to charge it backwards. Think about it! This causes a condition called sulfation. Sulfation is not reversible, regardless of the snake oil ads. The lead combines with the sulfuric acid. After it breaks off when its charged, the missing lead can not be replaced from outside the battery.
The battery may not immediately accept a charge, but it may after a few hours. This battery cannot be trusted. It will fail, but how long is merely a guess. Replacement should be done.
Charging. Here’s where it can get a little complicated. Simply put (I hope) is the battery has an internal voltage that is proportional to it’s state of charge. While being charged, a dead battery is about 12.6 (remember, while being charged) and a fully charged about 14.4. The voltages are dependant on temperature, but the alternator knows that and automatically compensates for it. The alternator is regulated at about 14.4 volts and is obviously connected direct to the battery. The difference between the battery’s internal voltage and the charging source determines how much current flows. The battery voltage starts coming up fast, and the charging current drops off in proportion. The battery receives a tapering charge until it and the source are somewhat equal. Then the circuitry considers the battery charged. The current tapering off does not put a lot of heat into a battery. Heat is also an enemy of the battery.

The tapering causes the battery to take a lot longer than one expects for charging. An example here: Say you anchor overnight, and the combination of accessories draws 55 AH. You run an hour back to the marina and have a 65 amp alternator. The battery should be 100%, Right? Wrong. First of all, the engine electricals take about 10 amps, and lets say your electronics take 5. So, now you are down to a 50 amp. The alternator will bring the battery’s initial charge up fast, and the current tapers off to 25 amps, then 10, etc. It would take about 4 hrs to top it off. Some boaters have a bypass switch on the regulator to hot charge the battery. Bad. You will overcharge and overheat the battery, and ruin it. Simply put, the alternator can not fully charge the battery, itself.

This brings us to chargers. Don’t even think about a cheapie auto charger or a “trickle” charger. They can ruin a battery, because they are not regulated. A “trickle” charget is a device which sells batteries. Get a “smart” charger. It has 3 stages: 1. A bulk mode. As much current as possible, for a very short time, 2. An absorbsion mode, where it delivers the taper charge, like an alternator, and 3. A maintenance stage where the circuit reduces the charge voltage to about 13.6 to avoid boiling the electrolyte out. These chargers are a plug in and forget until you go out again, and the batteries are near 100%. It will also make the batteries last longer. As I said above, improper charging is the #1 killer.

For the size of the battery: starting batteries are rated in CCA (cold cranking amps) or MCA (marine Cranking amps) Bigger is better, and at least 1 CCA for each engine cubic inch. Again, bigger is better. For the house battery(s) calculate the amp hours by taking the current of the device times how long it’s run at anchor. Like a 4 amp refrigerator, times 20 hrs = 80 AH. When you come up with the total, double it. That’s the size to get, or even 2 in parallel if over 115AH (size 27). Again, do not discharge the battery below 25% and this also allows for the battery aging. What I have in my cruiser is a 230 AH battery bank for house battery and another separate starting battery. An overnight with the fridge, and intermittent TV, radio, lights, etc draws about 110 AH. I let the alternator do its thing, and have an automatic shore power charger, which runs while the boat is on the trailer in my yard. Takes it about a day and a half to be back to 100%.

The simplest installation would be a sport boat, without a cabin and accessories like a refrigerator that would draw a lot of current when the engine is not running. A stereo would draw about 3-5 amps, unless you have a godzillion watt amplifier (please don’t anchor near me). The simple installation would involve 2 starting batteries, with a 1-2-all-off switch. When running and starting, “all” when the engine is off, either 1 or 2. This way, if the stereo runs one dead the other can start the boat. For the gozillion watt amps, get 2 batteries, one starting and 1 deep cycle, and a battery isolator. The stereo unit and amplifiers to the deep cycle, the engine to the starting. The isolator will charge both batteries as needed.

Next we would have a small cruiser with 1 engine, a starting battery, and a house battery (system) as above. Now here’s a way to make it boater proof: an isolator. What it does is to divide the alternators output to the batteries. Remember what I said above about the battery’s internal voltage vs state of charge? If one battery is down and the other is charged, the one that is down has a lower internal voltage, and current will flow thru it. The one that is up will not take current. This holds true for good batteries.

The isolator allows you to have a dedicated starting battery hooked to the engine only, and the house battery for all else. This way, if the house battery is run dead, the starting battery does its thing. This is as automatic as it gets.
Being that the alternator can’t replace the charge during the trip home; a “smart” charger is a necessity. When you get back to the marina, or your home on the trailer, simply connect the boat shore power up, and forget about it. The batteries will be 100% the next time out. Another advantage of the charger is the batteries will have a longer life. The worst thing for a battery is to store one discharged. I have been getting 5 years out of my batteries using this method. Almost boater proof.

With twin engines/alternators, recommend a 2 alternator, 3-battery isolator; a separate house battery (bank), and a smart charger. Not only do you have the above advantages, but also it will charge all batteries if one of the engine/alternators quits working. If there is a genset, let it have it’s own battery, and get a 4 bank charger.

Battery maintenance. Note: battery electrolyte (sulfuric acid) is some bad stuff. Please wear safety glasses when working on or near the battery. If you do get splashed, rinse with a lot of water. 1. Clean the top of the battery. Use a baking soda/water mix to neutralize the acid. Keep the terminals clean and tight. 3. Maintain the electrolyte level. NOTE: Use only distilled water to fill to mark, and only top it off when it is charged. Discharged batteries have a lower level because of the lower specific gravity.

If the covers are removable, check the specific gravity of the cells every few months. The exact reading will depend on state of charge and temperature, but should be around 1.280 at 70 deg F. However, the reading difference between cells is important. A large variation, more than .020, shows that cell is going bad. Replace it.

A relatively new device is called a combiner. It works like this: You have a starting battery (master) and a house bank (slave). After starting the engine, the master is connected to the engine alternator. When it reaches full charge, a microprocessor senses this and an electronic relay connects the slave battery to the alternator.

A very simple system is a Master Battery Selector Switch (MBSS). It manually selects bat 1, bat 2, all, or off. Simple. You start on the starting battery (1) and then switch to all for running. Then switch to the hose bank for anchoring. It requires actually throwing a switch, however, cheap and simple.
Remember this: the more “automatic” hardware you have, the greater possibility of something failing.

The batteries are an important part of boating. Have you ever tried to rope-start a 5.7L engine?


Posted with the permission of the Author: The author retains all copyright privileges. No reproduction without express written authorization of the author

Harvey Gilbert (Captharv) posts regularly on this topic on the US Bayliner Owners Club (BOC) site.

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