Did I buy right battery size?

Ron20324 · Aug 18, 2014

My B323 came with two 27 batteries. If you only have one, maybe at the minimum move up to a 31. I now have 3 31s/

CapitalistSailor · Aug 18, 2014

John_F said:
I just replaced my marine master (deka) deep cycle battery with a Group 27 deep cycle from Sears.

Although I run a battery charger, after just a few minutes with the charger off, the voltage is about 12.8 to 12.6.

If I have my running lights, VHF, instruments (no GPS) and stereo running for 5 or 6 hours as I sail at night, will this battery be large enough and have enough power to get me where I need to go?

Did I buy a battery that is too small?

John F,

12.8 volts is the normal voltage for a fully charged (after the charger has been off for a little while), a little less voltage in cold weather (gel cells and AGM have a little different voltage). Whether the battery lasts long enough depends on how power hungry your electrical equipment is. If you look at how many amps (current) your equipment draws, and compare it to how many amp-hours of battery capacity you have, you can get a feel for how fast the battery will be drawn down (amps times hours is amp-hours). Usually you won't want to go below half discharge routinely (12.4 volts when not pulling hard on the battery) or the battery life will be decreased.

Your stereo is the wild card. Some stereos are quite power hungry, especially automotive ones. Lights can use a surprising amount sometimes, especially older ones. A VHF radio shouldn't use too much if not transmitting, but an older one might.

I suggest giving it a try at the dock or during a day sail with the lights and VHF on for a few hours and see what happens. If you don't have a built-in voltmeter check the battery occasionally with a portable voltmeter. If you want to be a little hard-core you can also check the density of the battery fluid in each cell with a hygrometer (available an an auto parts store) and measure the state of charge that way (please be careful of your eyes around battery acid).

If you have too, you can always run the engine for a little while during the night and recharge the battery. You don't need to recharge the whole way, just to 85% or so. The last 10% will take a very long time unless you have a smart charger.

Sears deep-cycle batteries are pretty tough, so you can experiment. They are old school: the most pounds of lead in one package gets the most capacity.

Stu Jackson · Aug 18, 2014

You might want to Google: Boating Energy Budget.

Try this:

The All-Important Energy Budget:

http://c34.org/bbs/index.php/topic,3976.0.html

Anonymous 121765 · Aug 19, 2014

John,

I have one question for you, Do I assume by your post that, you are only running one battery & not two?

It would be prudent to have a two battery system on a 31 ft. boat running instruments, lights etc. One battery for "house" use & one dedicated battery for starting.

When cranking up the iron jib at the end of the day, simply switch to both batteries, start the engine & you are now automatically charging both batteries. By the time you get to the dock, both batteries should be topped off.

For a proper system, a battery selector switch installed would be the way to go.
As a plus, you can also add an isolator to the mix & not have to worry about a day's full of drain drawing from the dedicated battery.

Just my opinion pal.

CR

Ralph Johnstone · Aug 19, 2014

Without dissecting you electrical needs, I'd say a 31' Bene needs at least two house batteries if it's anything like our H-310.

And as captnron has said, you MUST have a separate dedicated starting battery which is kept in a holy shrine and NEVER used for anything but starting the engine.

Stu Jackson · Aug 19, 2014

John, you might want to buy a book about boat electrical systems. Or you could spend some time and read these:

Electrical Systems 101 http://c34.org/bbs/index.php/topic,5977.0.html

Benny17441 · Aug 19, 2014

A group 27 has approximately a capacity of 100ah and should be adequate to run for 5 to 6 hours the loads you have described and still have power to start the engine. What you do not have in a single battery is redundancy and backup. I would suggest you get an automotive type battery pack that would allow you to start the engine in case of an unexpected battery failure or accidental occurrence. I do not think you need a bigger battery just a backup. A two battery setup gives you a lot of alternatives.

kentobin · Aug 19, 2014

Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis.
http://www.solar-electric.com/deep-cycle-battery-faq.html#Cycles vs Life

Worksheet to determine your electrical usage:
http://newcontent.westmarine.com/content/documents/pdfs/WestAdvisor/Elecbugt.pdf

Scott T-Bird · Aug 19, 2014

On your boat, I don't know why you don't have at least 2 batteries. If your electrical use is light, then you will be fine with 2 grp 27. If you just bought your battery, go to the store tonight (or tomorrow) and buy another. They should be put into service at the same time to insure maximum life.

Doesn't a Bene 31 come with a battery switch? I'd guess that you should probably have redundancy. To me, this means you should have 2 batteries on your house bank and at least one battery on your axiliary bank.

You have many options, so read up to see what suits you best, but, I think on a boat your size, you should not rely on just one battery, unless your use is so light and you can do without a battery in an emergency. It seems a very odd question for an owner of a Bene 31. I'd guess that at least 2 batteries is the norm.

BTW, I don't know why you need to have a dedicated start battery. I think most sailors don't ... they start the engine with the house bank, there is no reason not to. It's personal preference ... that said, I have a dedicated start battery. I selected a start battery rather than having a redundant house bank. I have 2 grp 27's deep cycle on the house bank and a start batt on the auxiliary bank for my light use.

You should also make sure you have an Automatic Charging Relay (look up Blue Seas ACR) to keep your banks charged. Sorry to add to your complexity, but you asked for it with your large boat!

Ralph Johnstone · Aug 19, 2014

Good Question ......................

Scott T-Bird said:
BTW, I don't know why you need to have a dedicated start battery. I think most sailors don't ... they start the engine with the house bank, there is no reason not to. It's personal preference ...

..................... for the weekly quiz.

Phil ? ? ? ? ? ?

Anonymous 121765 · Aug 20, 2014

Actually,

Most sailboats 27 ft. & up have a 2 bank system. When it comes to power, redundancy in batteries is the wiser path.

When it comes to boats, you can never carry too much, only too little.
I only use my engine battery for starting. Why worry if you have enough starting power after draining your only HOUSE/START battery for long periods of time. I promise you, those with only one battery all have the same thought right before turning the key, "Will it start"?

Lake boaters don't have to worry as much, but if you sail offshore, having reserve power is a must.

And I assure you, most boaters that had their only battery run too low when attempting to power up, have gone the 2 bank way.

CR

Scott T-Bird · Aug 20, 2014

I'm assuming that most of us have 2 battery banks but what does that mean exactly? For me that means 2 12V deep cycle in parallel for house bank and 1 start batt for the aux/start bank. But that's not redundancy. I assume that most cruising boats have identical sets of deep cycle batts (that is true redundancy) for both banks. Starting the engine just happens to be with whichever bank is in use.
My next question is ... Just how necessary is a redundant bank if you are monitoring the health of your batteries carefully. For those with redundant banks, do you use both banks equally or do you save one bank just for emergency? If so is it really necessary to have a redundant bank (a full set of deep cycle batts) for an emergency that virtually never happens?

Anonymous 121765 · Aug 20, 2014

Two Batteries by definition is, redundancy. It is not only having to relying on one battery.

I dedicate my house battery only for the house. The additional isolated battery is for starting & as my safety backup if my first battery is drained. I don't have to monitor these, as having a selector switch & isolator, I have not found a need. I keep my batteries topped of at the dock, so when I go out, I'm confidant of knowing I have full & backup power.

I almost always start my engine with the switch set at both batteries. Engine running, I'm now charging both. If doing an overnighter or weekend cruise, I motor to the anchorage & make sure my engine runs for an hour (total) before shutting down. This includes channel time to hook set. This charges my house battery for the nighttime drain. A half hour before picking the hook, I start the engine to recharge my house back up from last nite's drain. My longest continuous cruise so far has been about two months. This schedule that I use has never failed me or weakened my reserve.

From my years of boating down here with both power & sail, I find one thing that is always common. When you lose power here, it's usually at a critical time in what can bad weather, ugly seas, approaching shoals & fading light. These are circumstances I don't want to find myself in.

CR

mitiempo · Aug 20, 2014

A redundant house bank makes no sense. A large bank is not only more efficient than 2 smaller ones but the batteries will last longer, given comparable loads.

A start/emergency battery makes sense and should be installed on any boat with an electric start engine.

Scott T-Bird · Aug 20, 2014

Well you are redundant if you have 2 identical batts - 1 for each bank. But my question is about those who put 2 batts on one bank. Do they have true redundancy for the 2nd bank meaning they have 4 identical batts in total? I use 2 banks also but they aren't redundant. I have a house bank and a start bank. The purposes are different. I can start on my house bank if I need to so I have some redundancy but I'm not going to use the start battery for a house bank purpose so I don't have redundancy in that regard.
I'd bet that Stu & MS use either bank equally so they would have true redundancy

mitiempo · Aug 20, 2014

Scott T-Bird said:
I'd bet that Stu & MS use either bank equally so they would have true redundancy

You would be wrong about that. One large house bank used for everything including starting and an isolated emergency battery charged by an ACR or similar.

Scott T-Bird · Aug 20, 2014

That's why I ask the question because that isn't the impression that I have had based on their many discussions about battery banks.
Charging with ACR is understood. I have one as well so both banks are charged as needed

Ron20324 · Aug 20, 2014

Scott T-Bird said:
...I'm not going to use the start battery for a house bank purpose so I don't have redundancy in that regard....

Wrong. You WOULD use the start battery as a house bank if that's the ONLY battery source you had working. Redundancy is in use, not necessarily in capacity.

Scott T-Bird · Aug 20, 2014

It seems like we're splitting hairs over the def of redundancy. That not an issue for me.
So is that the answer for most? The second bank is just a small emergency battery? I never had that impression. I was always under the impression for those cruisers out there that the second bank was more or less identical to the first bank. I'm surprised!