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Author Topic:   Why Use a Deep-Cycle Battery
pglein posted 03-31-2010 09:50 AM ET (US)   Profile for pglein   Send Email to pglein  
Something has been perplexing me for some time. It would seem to me that, if you were to check under the cover of the battery box of 90% of small powerboats, you would find, hiding in there, a Marine Deep Cycle battery. As I understand it, the benefit of a deep cycle battery over a traditional starting battery is that it can withstand being "deep cycled", or drawn down to a very low charge, without permanent damage. They provide more amp-hours (Ah) in exchange for fewer cold cranking amps (CCA). This characteristic is beneficial when used as a "house" battery to supply power to accessories when no charging source exists.

Yet, if you look at how most of us use our batteries, aren't they primarily used only for starting the engine? How often, in a small powerboat, are you running accessories without the engine (and therefore the alternator) running? In my case, almost never.

Even in my 36' fast-trawler, which carries a total of seven (yes, I said SEVEN) batteries, every single one of them is a deep cycle battery of one sort or another; even the one designated exclusively as a starting battery for the main engine (a 420hp diesel). It seems that the manufacturers buy into this though process.

I'm not trying to argue against using a Deep Cycle battery, just trying to understand why we do it. Am I missing something?

gnr posted 03-31-2010 10:09 AM ET (US)     Profile for gnr    
Electric downriggers.
VHF radio
AM/FM radio (sometimes)
Nav lights
Accessory lights
Cell phone charger

All these run at one time or another while trolling on the kicker. A full day of fishing will take the charge right out of a battery for me. The downriggers really suck the juice.

I have to charge my accessory battery before every trip.

sosmerc posted 03-31-2010 10:15 AM ET (US)     Profile for sosmerc  Send Email to sosmerc     
Mercury has always recommended conventional flooded Marine Starting Batteries for their outboards with the exception of Verado. They now require AGM batteries with a minimum of 1000 Marine Cranking Amps.
pglein posted 03-31-2010 11:56 AM ET (US)     Profile for pglein  Send Email to pglein     
I must be the only one with a kicker that has an alternator. Not just that, but one that puts out more power than my main engine.
Tohsgib posted 03-31-2010 12:09 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
Using a deep cycle to start your engine will most likely result in premature failure of the battery. They are not designed to be drawn down that hard like when you are cranking it over. There are however deep cycle cranking combo batteries, I have never owned one though.
jimh posted 03-31-2010 12:38 PM ET (US)     Profile for jimh  Send Email to jimh     
Moved to SMALL BOAT ELECTRICAL for discussion.
jimh posted 03-31-2010 12:45 PM ET (US)     Profile for jimh  Send Email to jimh     
Most outboard motor engine starting is done in moderate temperatures. Most outboard motors are not big displacement, high compression V8's. You can crank most outboard motors with less battery power than would be needed to crank over a big-displacement high-compression V8 when the temperature is minus-10-degrees-F. As a result, you can crank over most outboard motors with a battery that is not entirely engineered and manufactured for starting purposes
David Pendleton posted 03-31-2010 11:46 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Using a deep cycle to start your engine will most likely result in premature failure of the battery.

I do it all the time, and have not experienced "premature" battery failure. In fact, I haven't experienced battery failure at all.

I have two deep-cycle batteries on my boat. I normally leave the battery switch to "Both," and I have one of those "high-compression" V8's jimh is referring to.

davej14 posted 04-01-2010 08:37 AM ET (US)     Profile for davej14  Send Email to davej14     
I don't understand how a deep cycle battery could be harmed by using it for starting so long as the MCA rating of the battery is adequate for the motor. Please explain.
Tom W Clark posted 04-01-2010 09:45 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
I certainly have drained more than a few batteries dead while trolling. Not all kickers having alternators and not all trolling utilizes a kicker.

Recreational boats are often left unused for long periods of time. Batteries will discharge on their own. Not everybody understands or practices good battery management.

For those and a number of other reasons, it is not unreasonable to expect a boat battery, unlike a car battery, will be drained from time to time, hence the prevalence of deep cycle batteries.

For the vast majority of little Boston Whalers, the cranking needs of the outboard motor is not much and a deep cycle battery will suffice.

I prefer to use a Dual Purpose battery that has some qualities of both types. They have served me well over the last decade and do not seem to mind being discharged and then recharged.

jimh posted 04-01-2010 12:23 PM ET (US)     Profile for jimh  Send Email to jimh     
Tom makes a good point, which I will restate: Boat electrical systems are more prone to situations where the battery will be accidentally deeply discharged (than otherwise might occur in operating a vehicle). Most vehicles are driven daily, and most give lots of warning bells and buzzers if you walk away with something left on. Boats are often used infrequently and at longer intervals. A boat electronic instrument inadvertently left on, a boat cabin light left on, or a over-active boat sump pump are all common causes of the boat battery being deeply discharged. Deep-cycle batteries, as their name implies, can recover from deep discharges with less loss of battery life and capacity that a battery designed strictly for engine cranking. This may account for the preference to use a deep-cycle battery by some boaters.
fishgutz posted 04-05-2010 06:55 PM ET (US)     Profile for fishgutz  Send Email to fishgutz     
I'm with Tom. I use 2 dual purpose batteries. They happen to be maintenance-free. It is so hard to get inside that little door on my console to do any regular maintenance like checking the water level.
David Pendleton posted 04-05-2010 09:46 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
For me, the benefit of two large, deep-cycle batteries means I can run any of my boat's systems with little fear of draining the batteries.

I've converted my navigation lights including my all-round/anchor lite to LED to reduce the draw on my batteries.

I replaced my spreader light with a HID model to reduce it's power requirements.

My cockpit and cabin lights are LED for the same reason. This is also true for the courtesy lights in my cockpit.

I have two giant batteries; group 27, 105aH. They both (by virtue of the battery switch), also act as my my starting batteries.

When I overnight on my boat, I run the following systems continually: VHF, GPS and RADAR.

I have a GPS anchor alarm set at ~10 meters, a RADAR warning zone set at .25 NM and the VHF is on all the time scanning locally important channels.

This is in addition to my CPAP machine, which I use to sleep, and often times my laptop and an AM radio.

I've never had a battery (or starting) problem.

It's probably a bit different for you outboard guys, but I say carry as much battery as you can charge and afford.

Bella con23 posted 04-05-2010 10:46 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
I agree with the deep-cycle theory.
The battery in plain view is a group 31 Nautilus and the battery to the right is a group 24 "starting" battery.

I always start the day with the "24" then switch over to the "31" when we reach our destination including trolling or drifting. This method keeps them both charged at all times.

The only problem I had was with the charger being hardwired into the batteries was actually discharging them after several weeks of not being plugged in.

Thanks to jimh for pointing out that an unplugged charger connected to a battery can draw on the battery over a period of time. Apparently some chargers more then others.

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