Starting vs. Deep Cycle Battery

[Dan Hayes]

My batteries need replacing, and I'm considering using a Deep Cycle battery for the house battery, and one of those combination deep cycle/starting batteries for the engine battery. I charge off the engine, and use a battery combiner.

Thought I'd throw this out to the electrical gurus and see if there are pros and cons that I won't find anywhere else.

Any thoughts, you guys?

 

[wurzner]

Your question will be very difficult to answer based on your posting. To figure it out, you need to really do an analysis of what your electrial draw requirements are going to be and then set up a system that hopefuly never draws your batteries down below 50 at absolute maximum. Nigel Calders book has an excellent description on this very topic and in short, no battery is going to last too long if you repeatably drain it down below 50 percent. Do you have radar, a fridge, inverter,... are all things to take into account, as well as what are your charging capabilities and habits. Do you go out for extended cruising? These are all factors. On my older 32, I replaced all my running lights with LED's since they draw about .1 amps per hour opposed to 1.2 amps per hour. If you do a lot of night time sailing, that is makes a huge difference since running lights alone with an anchor light could easily consume ~30 amp hours in one night. Assuming a regular type 27 or 31 battery has approximatly 100 AMP capacity, you've already used up 30 percent. Throw in some reading lights, spreader lights etc and you're getting close to the 50 draw on the capacity. If you have a good alternator with a multi stage smart regulator, you can replace most of that with typical motor running related to getting in and out of port, etc. Keep in mind a 120 amp alternator is not going to be 100 percent efficient, expecially if it is hot which is typically is in the engine compartment.

In short, make a spreadsheet and calculate your usage and that will help you more than anyone on this list. Then, figure out ways to minimize your draw by using LED's opposed to incadescent lights. Then factor in some of the things I've mentioned above and your good to go. On the other hand, abuse the heck out of your batteries and expect to replace them every year or two as an option; that may be a practical way to go as well. Guy put up some really useful information on batteries a while ago, so you can do a search on that. The batteries on the boat I bought are from 2000 and still work well, but have exceeded what I would consider to be their expected our useful life. I had a new deep cycle go out after a year and a half on my 32 due to drawing it down and up. Also, consider 2 x 6 volts for the deep cycle bank.

Good luck and I hope this helps. Look up Guy's posting, it was really helpful.

regards
shaun

 

[Rob Hessenius]

Dan- Without getting all technical about usage and charging issues. My belief about "dual purpose" batteries are that they are truly for a boat that carries only one battery aboard. Of all the combinations of ways and functions of batteries, the worst thing that you can do is use a starting battery for anything else but to start the engine. Deep cycles are perfectly fine to start your engine, so I would suggest you put another deep cycle vs. dual. That is my opinion that might not be everyones elses. Rob

 

[rgoff]

I've always just used 2 "marine" type batteries on my E27 with no problems. I also usually leave the battery switch in the "both" position. I'm no battery expert, but I doubt you'd have problems using a deep cycle as a starting battery. It's not like you start your boat engine as frequently as your car.

 

[Dan Hayes]

Thanks for the info. I'll look up Guy's earlier posting, his info is usually very informative.

I have two group 27 batteries, currently both deep cycle batteries, with a stock 60 amp alternator - which is absolutely not a "smart" one... I generally set the switch to "both" when running the engine, then to "house" when discharging the batteries at anchor, etc. - which may change now that I know what my battery combiner can do.

I use the batteries lightly - never intentionally running them down past 50%, as I know what that can do to them. They've been running down lately, which could be from a combination of factors. So, I'm starting with replacing the batteries, as they are over two years old, and I have no idea what the P.O. did as far as taking care of them. I am of the "buy em' at Costco" school, take good care of them, but don't cry if you have to replace them at three years. My last deep cycle battery on my O'Day 222 lasted five years, and was still in amazing condition.

I was thinking that the combination starting/marine battery would have more cranking amps, and therefore start the engine better. I would then use it for starting only, rarely (like when I forget to move the switch) for other duties.

I guess my concern has to do with the alternator charging them unevenly, which may be alleviated by the battery combiner, right?

While I am at it, I am also considering the purchase of a battery isolator, so that I can hook up a dual battery charger, without worrying about electrolysis. That way I can use the house battery "guilt free" - (well, under 50%) knowing it will be charged next time I go down to the boat.

There are a lot of reasons the batteries could be running down, but I'm starting with the batteries. Any further comments from the Oracles of "E"?

 

[rssailor]

Answers

Dan,
If you can fit two group 31 deep cycle batteries in the same spot, that would be great. First of all, how do you know when your battery reaches 50%? Do you have a link meter or equivilent? Next, stay away from isolators. These are nothing more that diodes that allow electricity to flow one way, but with heat as a by product and roughly a .7 volt drop accross them.
Your battery combiner will allow you to charge both batteries as soon as the battery hooked up to whatever sensing the combiner uses to close at around 13 volts or so. This will put the batteries hooked to the combiner together for charging no mater if the charging comes from the alternator or the shorepower battery charger. The charging happens iregaurdless of wether the switch is in 1 or 2. Leaving the switch in both with a battery combiner is a waste of the battery combiner. Hope this info helps. Ryan

 

[NateHanson]

You should not charge or discharge two batteries of different types together in the same system. If you have a deep cycle and a dual purpose type battery, each needs to have a separate charger, and they should never be switched to "both" as they discharge and accept charge at different rates because of the thickness of the plates.

 

[rssailor]

Battery types

Nate,
That is a falsehood that some people think is true. A dual purpose and starting battery can be charged using a combiner as long as the battery banks are simular size. Such as a group 27 and a 24 starting battery. Now if you put a group 27 starting battery together with a bank of four golf cart batteries, then you got problems. What I think you are thinking of Nate is the very real rule that one should stick to a single type of battery chemistry on board your boat (all gels, AGM or wet cell batteries). Ryan

 

[NateHanson]

Ah yes, perhaps your right. Now that you mention it that makes more sense. Thanks for correcting me - I didn't check my facts.

 

[stbdtack]

I have 4 golfcart batteries and a group 24 starting battery. I use a combiner to bring the starter battery online as soon as I have a charging source (shorepower charger or alternator)
This setup works fine and I leave my 3 step charger on as long as the boat is plugged in. The combiner has a switch that will force it to close to use the house batteries for starting if the engine battery has a problem. I actually used that feature once when I had the old start battery die. Worked like a charm...