Battery and 12 VDC power talk #1
03 January 2012 | Bimini, Bahamas
Benno
For some weeks I've been harboring in my mind to write up something about batteries and charging. This theme is quite complex, it forced me to turn it into a four part small series of write-ups. Here is the first part:
On a boat you have a lot of equipment which runs off your onboard 12 Volt battery system and some equipment which run on 120 Volt, like a laptop computer, TV, power or kitchen tools such as perhaps a toaster, mixer or even microwave oven. To get 120 Volts to run the equipment you may have a portable gasoline driven Honda generator on board, or even a built-in 120 VAC diesel generator. When no generator is on board, another way to get 120 Volt is to have an inverter. An inverter is a unit which converts direct current into alternating current, in plain English it changes 12 Volt DC into conventional 120 Volt AC and allows you to use your favorite 120 Volt device when no AC outlet otherwise is available.
To your information, 120 Volt is called AC and 12 Volt is called DC. AC stands for alternating current and DC stands for direct current. The AC and DC explanation is beyond the scope of this write-up. You may read up about this topic in electrical engineering books. Never touch a live wire of 120 VAC, it could kill. On the other hand, you could touch a 12 VDC wire and not feel a thing. Power voltage up to 40 Volts is in most cases not life threatening. But if you have to work in your boat on a live 12 VDC system, it is common sense to remove all jewelry like rings, bracelets and wristwatch from your hands or wrists just to be safe and not to shorten out any blank wires or terminal screws accidentally. This is really very important, when you work with live battery cable. These heavy gauge cables could hold a lot of current (amps) and when shorted out by jewelry, it could melt the jewelry gold or silver metal and burn the wrist, finger or hand severely. So please remember this precaution!
The 12 VDC power is generally stored in one or more batteries and this is called a 12 VDC battery bank. This battery bank provides you with 12 VDC power to run your equipment like: lights, radios, depth sounder, radar, GPS and all the 12 Volt things you have on board, even the inverter, which makes the 120 Volt AC. This kind of 12 VDC battery bank is called normally a house bank. The other kind of 12 VDC bank you probably have on board is for starting your diesel engine or the diesel generator and consists of one or two batteries, is called a starter bank. Talking about batteries, the first electro chemical battery was invented 1800 by the Italian physicist Alessandra Volta, hence since the name Volt.
The Starter Bank
The starter batteries are engineered differently to batteries you would use for a house bank. Starter batteries have thinner and more plates, they can deliver a power burst to your engine starter, but do not like to be deep discharged from a night long of watching movies on your TV, while fridge + freezer all is hooked up to your 12 VDC system, it cuts down their lifespan. This is the job of a house bank. The starter bank provides quick enough power to the engine starter to start the diesel engine. The starter on the diesel is a very powerful electric 12 VDC motor which turns the engine's flywheel and this fires up the engine. This starter motor draws for a very short time an incredible amount of power out of the starter battery bank. Once the diesel is started and running, then the engine's alternator which really is a small generator that generates 12 VDC while running, will pump quickly enough 12 VDC power back into the starter battery bank to fill it back up with sufficient power to run your instruments and other things. It also provides enough standby power to start the diesel engine the next time you have to use it. On "Diesel Duck" the starter battery bank consists of two DEKA Group 27 AGM 12 VDC batteries which are installed in the engine room and they are charged by the original 14 VDC 70 amp PERKINS diesel engine mounted alternator, which is made by Lucas in the U.K.
This alternator is an of the mill alternator with a standard built-in regulator. This alternator was built to EC standards and the charging voltage does not exceed 14 VDC and this is within the compliant spec in charging GEL batteries, where the charging voltage should not exceed 14.1 VDC.
A battery starter bank's location is best close to the diesel engine to avoid long battery cable runs. Taking in consideration that the diesel engine generates high temperatures in the engine room or compartment, it is advisable to mount the starter battery very low toward the fiberglass hull or hull plating of a metal boat to benefit from the water cooled hull's skin temperature. If the engine compartment is very small, the starter battery might be mounted next to the engine outside of the compartment or engine box. This will keep the battery temperature down. During a charging process the battery generates its own temperature additionally to the ambient temperature.
As for the battery, please shy away from using a flooded wet cell as a starting battery in a boat. This type of battery will produce a gas when being charged. This gas is hydrogen gas and explosive. A flooded wet cell battery being confined to a not well aired engine room or compartment could present a danger of explosion in contrast to an automobile, where the engine battery is well aired underneath the hood. It makes more sense to use a GEL or AGM battery for a starting bank, they are virtually maintenance free, don't gas and will not require to be topped with water (eliminating to check the water level periodically and adding water when necessary).
On our Diesel Duck, the starter bank is mounted just above the keel, close to the aluminum hull plates underneath the diesel generator's mounting platform. Originally in 1999 we started out with two DEKA made Group 27 GEL batteries and they lasted up to April of this year. That is 12 years! From these 12 years, the batteries were stored after a short 120 nm trial run of the under construction Diesel Duck in Aug. 1999 for 6 years and since the launching of our boat in 2005, these batteries were in normal service. In April of this year these batteries showed signs of not holding a charge anymore and the time had come to replace them. We exchanged them with two DEKA made AGM batteries in Colon, Panama in April of this year!
In the next write-up, I will talk about the different battery types and battery maintenance.