Here are some differences between deep cycle, marine and starter batteries:
- Thick lead plates. This is one of the reasons they are expected to last longer than the other types of batteries IF USED PROPERLY. Battery plates slowly corrode. Thick plates take longer to corrode away.
- Not designed for outputting significant current. You might be able to start an engine off of one, but it would not be good for the battery.
- Capable of outputting a modest current (amps) for a long time (running lights, appliances, etc.)
- Medium thickness lead plates.
- Moderate current output. Is designed to have the current output to turn over a smaller engine (i.e. boat motor).
- Capable of outputting a modest current for a while (still ok for running lights, appliances, etc.)
- Thin lead plates.
- Are designed to output LOTS of current very quickly. For an 8D, tossing out 400 amps to turn over a motor is expected.
- They don't do as well when outputting low current. Of course they'll work, but for their size they won't give you the length of low-draw use as a deep cycle or marine battery of similar size would.
For house batteries you will want to look closely at the Amp Hours over 20 Hours (Ah/20h). This is a good measure for low-draw, but long term use. I don't feel like explaining Ah/20h, but here's a link with a quick bit of info: https://ca.answers.yahoo.com/questio...5083822AARlga4
Now, if we want the most run-time out of our batteries we have options. Some just make more sense than others. Let's say we have an averaged load of 10 amps for a 20 hour span for running lights, pumps, whatever (this just makes the math easier). To keep the batteries above 50% we will need a battery bank with a total of 400Ah/20h (10amps * 20 hours / 50%).
Now lets look at some products:
A nice looking 8D battery. Measures in at 9.8"x11"x20.6". 170Ah/20h rating. So to get above 400Ahs, we would need 3 of these (170Ah/20h * 3 = 510Ah/20h)
Also a nice looking deep cycle battery. It has a smaller Ah/20h rating; only 155Ah/20h. To get above 400Ahs with these batteries we would also need 3 (155Ah/20h * 3 = 455Ah/20h). A little less capacity, but... it only measures in at 7"x11.375"x13.125"!! A single 8D battery is twice the size as one of these!
Marine batteries fall somewhere in the middle. They are jack of all trades, but masters of none. Overall, you'll find that marine batteries have higher Ah/20h ratings than starter batteries, but lower cranking amps. To get the most out of your battery bank you want to get the battery best suited to the job. Starting an engine? Get a starter battery. Running lights and electronics? Get a deep cycle. Marine batteries will work, but they aren't the right fit for most bus set-ups I've seen around here.