Here's a reprint of some battery information as provided by I-Boats.com in their latest newsletter:
Question: I went through 2 marine batteries before someone told me there is a difference between a starting battery and deep cycle battery. What is the difference?
Answer
You’ll need one battery to start the engine, and one to operate the electronics, or in other words, one
starting battery and one
deep cycle battery. Here’s why:
Starting Batteries vs. Deep Cycle
Batteries are constructed of a series of positive and negative plates. In starting batteries the plates are thinner and there are more of them. This allows the batteries to produce high, brief amp bursts and crank your engine starter.
You will also want a deep cycle battery to operate you boat’s electronics. Deep cycles store up energy to gradually power electronic devices. The deep cycle has thicker plates internally, and it can be used for starting in a pinch but they do not like to repeatedly dump the big amps in one quick chunk that a starting battery (with thinner plates) is designed to do.
Battery Types
Wet or flooded batteries cost less and you have the advantage of being able to ‘equalize’ the cells, should you choose to do so, at a higher voltage than is possible with gel or AGM batteries. However, they require routine maintenance (filling with distilled water), are not sealed and can leak, and do not hold up well under strong vibration.
Gel batteries, although slightly more expensive, are maintenance free, non-spillable, and often vibration resistant. When charging one of these, though, care must be taken not to over charge.
The third type of marine battery,
AGM (Absorbed Glass Mat), is the most durable against vibration and shock since the plates are surrounded by dense glass mat. AGMs are also quick to recharge and have a long battery life. Similar to gel batteries in many features, AGM batteries also should not be over charged.
Suggestion: Keep both batteries the same type - wet/wet, AGM/AGM or gel/gel. If you use a wet battery for starting, then use wet batteries for the electronics. This makes charging and maintaining a lot easier since they both use the same charging profile. Regardless of your battery type, consider using a
3–stage charger that utilizes a bulk/acceptance/float-charging scheme to maximize your battery’s life.
Common Battery Measurements
There are 4 measurements of battery function that should be considered when choosing a marine battery:
CCA and MCA – Cold Cranking Amperage is the amount of current the battery can deliver for 30 seconds at 0 degrees Fahrenheit without dropping below 7.2 volts. Marine Cranking Amperage is very similar to CCA. The only difference is that it is measured at 32 degreed Fahrenheit. So, if a battery has an MCA rating of 800, it will deliver 800 amperes for 30 seconds before falling to 7.2 volts.
Reserve Minutes – This is how long a load of 25 amps can be sustained at 80 degrees Fahrenheit before dropping to 10.5 volts. So, a battery with 120-minute reserve can power a 25 amp load for 2 hours.
Amp Hours (Ah) – Ah describes how much energy can be delivered at a constant rate for 20 hours before dropping to 10.5 volts.
Cycle Life – This measures how many complete discharges a battery provides until it no longer holds half its rated capacity.
