Dead battery

Aug 28, 2015
190
Oday 28 St Joseph, MI
I purchased my O'Day 28 3 years ago and don't know how old the batteries are. One will not hold a charge and the other passes the load test. I'm planning to replace both in the spring. The dead one died on a day I made liberal use of my new Raymarine EV100 autopilot. I also had my IPhone and a small wireless speaker plugged in. Does anyone know how long I should be able to run that equipment and still start the engine?
 
Sep 25, 2008
7,075
Alden 50 Sarasota, Florida
Batteries come in different sizes (capacities).

Look up the current draw of your autopilot, subtract that from the 50% rated capacity of your battery and you'll have a max. run time ( discounting the fact your old battery wont meet it's as-new capacity rating and the other stuff which has negligible draw). Whether or not the remaining capacity can start the engine is a function of what engine and therefore the size (draw of the) starter.

In other words, we need more info.
 
Jul 7, 2004
8,402
Hunter 30T Cheney, KS
You may want to install a battery select switch so you can isolate the start battery from 'house' power. May also involve adding a third battery if the calculations Don recommends show that.
 
Aug 28, 2015
190
Oday 28 St Joseph, MI
I do have a battery selector switch and usually switch batteries for alternate trips. One battery usually lasts all day with autopilot use restricted to raising/lowering sails or getting a drink or sandwich. The dead battery day was the first time I had used the AP liberally. It's hard to determine current draw for my electronics. I would think the system would draw less than one amp except for the wheel drive unit which is hard to determine because it's activity varies with sea state, number of tacks etc. I think it draws 7A when actually turning the wheel. Engine always starts with one or two revolutions so a wild guess would be 75A for 2 seconds. I've thought of replacing one battery with a couple of golf cart batteries but don't know where I'd put them as they wouldn't fit in the group 24 box in the engine compartment.
 
Aug 1, 2011
3,972
Catalina 270 255 Wabamun. Welcome to the marina
Batteries don't last forever. If you have no idea what the condition is, then replace them and start from scratch. That's where you're likely going to wind up anyway.
As to load, there's no way that you're going to calculate how long a questionable battery is going to power your electronics, and the larger issue is potential damage to onboard electronics if you are in a low voltage scenario.
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
your question is not how long can a battery go and still start the motor in this case. any reasonably sized battery (100ish AH) should be able to drive the AP and "noise" gear for a full day at least. Depending on the sea state (higher sea states = more AP current draw) and sail balance (a balance boat takes less power than an unbalanced one) your AP should last from at least a day (high ss and unbalance sails) to a week or more.
You should have an idea of how long you PLAN on going before you recharge the batteries. Many think that you should do recharging by SOC (state of charge) but that really obscures the fact that we are all creatures of habit. You have a general electrical usage pattern that can be determined. Once you have that pattern down you will know;
a) what size bank you need to support your recharge PLAN
b) how long you can go beyond the PLAN till you kill the batteries
these are important things to know BTW
I agree with the resident expert "Nigel" on only having one battery bank (lots of advantages and only one disadvantage which can be mitigated by knowing what you have and what it is capable of). The wiring is easier, cheaper and you stress the batteries less BUT!!!!! you HAVE TO PAY ATTENTION to what is going on on the boat electrically. Many are not willing to do this however and rely on lots of fancy and expensive gear (all you really need is a volt and amp meter) to "take care of the batteries".
Throw out the old batteries. You don't want to have a brand new battery get killed because it is connected to a shorted on that was fine a minute ago (but really old and I knew in the back of my mind that it was a bad idea to connect them together....) Develop an understanding of how many Ah (amp-hours) you use between recharges. Size the new battery bank to fit that sailing style.
I have a spread sheet that does a lot of the math and lets you play with the charging method/bank size / loads on a typical outing etc.
 
May 24, 2004
7,129
CC 30 South Florida
Look up the power load rating on your new autopilot. it should be given in amps at 12V. So you have a battery that will not hold charge so ditch it. You have another battery that passed a load test which just means it can be marginally good but we don't know how much storage capacity or useful life is left in it. You have had the those batteries for 3 years and don't know how old they were so I think it is time to ditch that one too and start anew with two fresh batteries. There is a test that would tell you the capacity of that battery but it is not worth the time and effort. You would not want to pair a new battery with that one as if it dies shortly like its partner did it could kill the new one too. I would recommend a pair of group 27, deep cycle, wet cell batteries. Shop at a dealer that has a fresh stock of batteries with a manufacturing date of not more than 60 days. Batteries will discharge and tend to sulfate when they sit on the shelf for too long and that limits the storage capacity. If each battery has a rated capacity of 105 Ah (amp hours) then when combined you will have a bank of 210Ah. Batteries will last much longer if you limit the deep discharge cycle to less than 50% of capacity so your usable capacity will be 105Ah between recharges. You can divide the 105 Ah by the rating in amps of your autopilot and that will give you an approximate figure of how many hours you can anticipate for usage. Remember batteries must be recharged to 100% as soon as possible after usage to again prevent sulfating. If you want to continue just using one battery until spring then I would recommend you purchase and carry an automotive emergency battery pack.
 
Jan 7, 2011
4,723
Oday 322 East Chicago, IN
We really need more information as others have said, but I have 2 group 27's in my O'Day 322. The batteries are fairly new. When I went out last weekend for an overnight on the hook, I isolated one battery to use, and 1 in reserve for starting the engine if I ran down my house battery.

I used the AP some on my 5 hour sail. Used the stereo and Sirrius sat radio all day. I kept my iPhone plugged in to run my anchormdrag alarm all night, used the house lights (not LED) and an LED anchor light all night.

That battery started the shaft engine just fine in the morning, and ran AP and stereo on the way home.

My other tip, if you are not going to replace the batteries this season, is to get a jump start battery pack. I kept one on my boat when the batteries were weak and I was concerned about getting the engine started.

Greg
 
Jul 26, 2016
94
American Sail 18 MDR
Get "Battery Equalizer" online and add it to your batteries if you intend to keep them till they die in about 2 yrs. It extends the life of old batteries and new.
 
Aug 28, 2015
190
Oday 28 St Joseph, MI
Thank you thank you all for your input. I'm going to measure my battery boxes tomorrow to see if they can hold Grp 27 batteries. Benny17441, I didn't mean I'm running on one battery until spring. My boat is in Michigan and yesterday was pulled from the water until Spring. I'm headed south after bottom paint and putting on the winter cover. A boats electrical system is more complicated than initially meets the eye. Now I have to determine how long it will take my 55A engine alternator to recharge a 50% discharged battery.
 
Feb 26, 2004
22,759
Catalina 34 224 Maple Bay, BC, Canada
Now I have to determine how long it will take my 55A engine alternator to recharge a 50% discharged battery.
Ain't gonna happen. Why? 'Cuz alternators are good for providing some replenishment but because of battery acceptance the fuller they get the longer it takes/the harder it is to pump in more juice. The only sure way to get a bank fully charged is with either a full night on shorepower or solar. Trying to use an alternator for this is a waste of engine run time. Before solar, cruisers would run the engine to get the bank to around 89-85% and stop there.

Battery Acceptance by Stu http://c34.org/bbs/index.php/topic,4787.0.html
 
Oct 22, 2014
20,989
CAL 35 Cruiser #21 moored EVERETT WA
The "Stu" knows... cause he's been there, done that, and written about it.
Thanks Stu. Love your straight to the point insight. you must be on the computer and not on the boat. Rain?
 

jviss

.
Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
The only sure way to get a bank fully charged is with either a full night on shorepower or solar.
I disagree. A smart regulator can run in acceptance mode (14.4V or so, constant voltage) until you get to a charging current of about 5%C, say, 15A or less for a 300AH bank, and then switch to float. Soon after entering float, the current will reduce to a trickle, at which point you are at, or very close to 100% state of charge.

I don't think most of the industry would agree that you can never get a bank to 100% on alternator alone. And, it won't require that the engine run all night, if you have a good, properly programmed, smart regulator, a sufficiently large alternator, and batteries in good condition, i.e., not excessively sulfated.
 
Aug 1, 2011
3,972
Catalina 270 255 Wabamun. Welcome to the marina
Not unlike pumping air into a sealed container. The more you pump in, the harder it gets to pump.
Good thing we have a solar bank to take care of this crap while we're away from the mooring and WalMart down the street with the three year warranty to supply new batteries if one goes south.
 

jviss

.
Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
You have to be careful with solar, too. A solar charge controller can be tricked into entering acceptance mode, with full batteries, every day; and, the current drop-off when the sun goes down can fool a battery monitor into thinking the battery is full.

I used to buy fresh, Wal-Mart, deep discharge batts, but after reading MaineSail's tutorial on his marinehowto.com site, I will be getting something different next time.
 
Oct 22, 2014
20,989
CAL 35 Cruiser #21 moored EVERETT WA
Meriachee... Those 3 days are 23 hours long. Only way you can get enough energy stored to last the 11 months of boat storage.:laugh:
 
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Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
You really need to do a loads/production/storage analysis. Little loads (anchor lights, salon lighting, pressure water pump) add up over time. the big uses (moving heat around) clearly are design elements but ignoring the numerous smaller loads can get you into trouble too. which is why I developed the spreadsheet that I (shamelessly) offer to anybody that asks these type of questions.
Trying to determine exactly how solar/wind/alternator effect your systems SOC on an hour by hour basis so you can determine when to fire up the engine (or if you even need to fire up the engine because solar extends time between alternator recharges) is really a lot of stubby pencil work. I don't want to imply that my spreadsheet is perfect (it is not) but it sure does let you get a good grip on what you are up against.
 
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Feb 6, 1998
11,665
Canadian Sailcraft 36T Casco Bay, ME
I disagree. A smart regulator can run in acceptance mode (14.4V or so, constant voltage) until you get to a charging current of about 5%C, say, 15A or less for a 300AH bank, and then switch to float. Soon after entering float, the current will reduce to a trickle, at which point you are at, or very close to 100% state of charge.

I don't think most of the industry would agree that you can never get a bank to 100% on alternator alone. And, it won't require that the engine run all night, if you have a good, properly programmed, smart regulator, a sufficiently large alternator, and batteries in good condition, i.e., not excessively sulfated.
Even a dumb regulated alternator can get batteries to 100% SOC, but it takes lots of time, time most sailors will rarely allow for. The issue is one of time with dino juice vs. battery acceptance.

Even with some of the highest acceptance rate AGM batteries it takes approx 5.5 hours from 50% SOC to get back to 100% SOC and this is at a charge rate of .4C or 40% of Ah capacity/ 40A for a 100Ah battery or a 180A charge rate for a 450Ah bank. The majority of boaters still don't have AGM's nor do they charge at anywhere close to .4C.

It's important to understand that this 5.5 hours, measured and tested using lab grade equipment, is without dropping to float too early. With a shore side battery charger premature floatulation is not as big a problem, but if an alternator or solar controller drops to float too early it can lead to considerably slower charging.

In the real world it will take a solid 6.5 to 10 even 12+ hours, depending upon battery health & battery type, to attain 100% SOC so long as you don't drop to float too early. Drop to float too early and the time to 100% is going to be closer to 15-20+ hours depending upon the float voltage.

This is why Stu says: "Trying to use an alternator for this is a waste of engine run time."

He says this also because very few sailors will run the motor anywhere near as long as it takes unless they are doing the ditch or a very long no-wind passage.
 
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Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
This is also why having a larger battery bank is helpful from both a charge (Ah not SOC), and battery life point of view.