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Join Date: Apr 2006
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Re: Battery Sizing - am I crazy?
"I know Deep Cycle batteries are not supposed to be starting batteries"
A common mistake. Deep cycle batteries are supposed to be deep cycle batteries. Starting (SLI) batteries are designed to be CHEAPER batteries with a MORE LIMITED PURPOSE and one special trick, high impulse discharges.
When you build a battery with thick plates, that physically limits how much impulse power those plates can put out, probably due to internal resistanace and heating. When you build lots of thin plates, you can put out lots of impulse power--as an SLI battery does. The problem with thin plates is that traditionally that compromises their physical strength, so they tend to fall apart and break down faster IF they are allowed to weaken (by discharging) in any significant way.
New result, a typical SLI battery can be destroyed by less than 6 deep discharges.
But if all you are doing is SLI (Starter, lights, ignition) service, an SLI battery may weigh and cost half of what a deep cycle battery does.
If your starting loads are small, as they are for most recreational marine engines, the average deep cycle battery can easily handle then without problems. Hell, I've started a Volvo MD7 with a 17AH "alarm and exit" type battery and neither one protested about it.
But if your engine is a 600hp Cummin Diesel, then sure, you may be better off paying closer attention to battery types and ratings.
Any deep cycle battery WILL have discharge ratings, and if you can't find them a quick call to the maker will tell you just how many amps that battery can safely provide in a surge for a starter motor. To put it in perspective, even a Group24 car battery will put out some 3000 Amps in a dead short, but the starter motor it is paired with usually draws one kilowatt or less (~100Amp) for all of 2-3 seconds.
So does the OP need deep cycle batteries? YES. Remember that SLI batteries have thin plates that easily take physical damage from discharge. They are designed, according to the folks I spoke to, for a MAXIMUM discharge of not more than 10%. At that point they start taking damage no matter how soon you recharge them. Ascompared to deep cycle batteries, that are designed for a30-50% typical discharge cycle.
Take an SLI battery, run the VHF all day, come home late and use the running lights, stay out for dinner and use the cabin lights, gee, doesn't the stereo sound great? and yes, you can and will damage an SLI battery with just day sailing, if you use the electronics. If you're not using much power, either one will certainly last the season, or two, maybe three. But the deep cycle won't cost much if anything more, and if will give you that extra margin for the weekend trips and longer days.