Behavior of marine batteries

Fredct

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Sep 21, 2020
86
Oday 28 Westbrook
Newish sailor. ODay 28 with 2X 27M batteries. No solar.

Background: We are the habit of turning on engine, motoring for 10 minutes or so and then sailing with selector to #2 (drain is limited autopilot, VHF and Navigation). At the end of the day, we start engine from #2, dock, plug in and all is well. On a recent adventure, we anchored overnight, running lights and water pump sparingly (all on #2). Two false anchor alarms forced us to restart the engine at night. In the morning, battery #2 read 10 volts and could not start the engine. We were fine with #1.

Question #1: Does starting the engine consume a lot of energy? What is a rule of thumb time to keep engine one to '"top off" batteries? In other words, should the starter battery be left on alternator for more than 10 minutes? In other other words, did starting the engine twice without sufficient running time effectively drain the battery?

Question #2: How would one know when to replace a battery? Marine batteries can withstand deep discharges but how does one decide to replace? I am guessing it is a function of the stored energy but how to measure? I change a battery once but it was because it would not charge. I never attempted to measure the actual stored energy.

All help and comments appreciated.


 
Feb 6, 1998
11,454
Canadian Sailcraft 36T Casco Bay, ME
#1 10 minutes of alternator charging will do almost nothing to "charge" a depleted battery. Without knowing the alternator & its amperage it is impossible to say how much energy you can expect to put back in 10 minutes, but it is low.

#2 10.0V is grossly over-discharged and will lead to very, very short battery life.

#3 Your average under-load voltage should not dip below 12.2V. This is the point at which you should start recharging.

#4 If you want to know amperage or energy into and out of the bank you'd need a battery monitor such as a Balmar SG200 or a Victron BMV series. An accurate digital volt meter, sensing directly at the battery terminals, can also be a good "bottom voltage" guide (12.2V).

#5 A single G-27 battery is typically insufficient for a 28' boat unless you are very miserly with consumption. Once away from the dock it will be very rare to see any more than 80-85% SoC from recharging by alternator alone. This means your usable capacity is just 30-35% of that bank not the 50% most assume they have. If your battery is say 85Ah this means you only really have 25-30Ah of usable capacity and this is only if you motor for a number of hours to get back to 80-85% SoC from 50%..

#6 Starting an engine uses high-current but for a very short duration. It typically consumes less than 0.2Ah.

#7 Routinely taking your house bank below 12.2V will ruin it in short order.

#8 Trying to start your engine, at below 50% SoC (12.2V), can mean difficult starting if your house bank consists of only one G-27 battery.
 
Sep 25, 2008
6,353
Alden 50 Sarasota, Florida
Your batteries, when new, had approximately 80 amp hr capacity. Likely less now. For it to get down to 10V after light duty overnight, probably significantly less as 10V would indicate dead meaning you drew down whatever capacity the battery actually has now by using only 10 - 20 amp hrs. running "lights and water pump".

Moral of that story - your battery needs to be replaced. Assuming they are both the same age/use, that applies equally to the other battery as well.

Also, running batteries on the alternator for "10 minutes" wont add anything of any significance to recharge them even if you have a high current, high rate alternator and ext regulator which I presume you don't.

Look up "load testing batteries" - you will find how to verify the actual remaining battery capacity you have.
 
Jan 11, 2014
7,976
Sabre 362 113 Fair Haven, NY
Question #1: Does starting the engine consume a lot of energy? What is a rule of thumb time to keep engine one to '"top off" batteries? In other words, should the starter battery be left on alternator for more than 10 minutes? In other other words, did starting the engine twice without sufficient running time effectively drain the battery?
Starting the motor consumes a lot of energy for a very short amount of time. The starter will draw about 200 amps for maybe 5 to 10 seconds. When this is converted to amp hours, the number is very low.

Question #2: How would one know when to replace a battery? Marine batteries can withstand deep discharges but how does one decide to replace? I am guessing it is a function of the stored energy but how to measure? I change a battery once but it was because it would not charge. I never attempted to measure the actual stored energy.
Age is a good indicator and ability to hold a charge. Five to six years is probably about the norm for reasonably well cared for battery. An older battery may have a "normal" voltage reading, but the depth of charge can be very low. With 9 AAA batteries you can get 13 volts, but no one would suggest that there is the same amount of stored energy as in a 12v battery.

I suggest you read the article below and the other articles on the MarineHowTo website about batteries and charging.

1/2/BOTH Battery Switch Considerations
 
Oct 26, 2008
5,047
Catalina 320 Barnegat, NJ
If your engine usage is typical 2 occurrences per outing for 10 minutes each engine start, your alternator essentially isn't re-charging your batteries. You say that you "plug in" back at the dock, so I assume that you have a charger on board that utilizes AC power. It begs several questions ... how long do you charge with your charger (assuming you have one)? Overnight or just for a little while? Do you leave the boat on the dock with the charger running? How is the charge directed to the batteries? Do you have echo charger, ACR or some method to charge both batteries?

Your battery usage and use of the selector switch is very curious. Why do you switch from 1 to 2? What are you trying to accomplish by doing that? You would be much better served if you ran your 2 batteries in parallel, which would provide you with a larger battery bank. But then you have no auxiliary battery. I think most sailors combine 2 x 12v batts to make a larger bank and then get a smaller 12v batt for auxiliary. Starting the engine is not taxing at all on a healthy battery bank, so I would re-think your use of the switch. There is no need to start your engine with one battery and run your boat's other electrical needs with another battery.

If the batteries are not very old, it sounds like you may be chronically under-charging at least one battery. That is a good way to kill it quickly.
 

Fredct

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Sep 21, 2020
86
Oday 28 Westbrook
Clarification. We have 2 batteries (1 and 2). One 5 years old, other 3 years old. We have AC charger which we leave on for days on end. We save #1 for emergency when on water and run everything else off of #2 (autopilot, VHF, lights, water pump). i would call us miserly for energy but I am not sure how much the autopilot at idle consumes.

I will definitely replace the one battery based on responses.

Thanks for comments.
 
Oct 22, 2014
16,409
CAL 35 Cruiser moored EVERETT WA
:plus: to all of the above.
MaineSail lays it all out.
Time to become more knowledgable about your actual power consumption and resupply.
 
Jan 1, 2006
6,166
Slickcraft 26 Greenport, NY
Instead of holding one battery in abeyance to start the motor if battery #2 can't, consider buying one of the li battery charging packs. Then combine the two new batteries into one bank and have more usable power and an easier charging regimen. Most of those packs will charge devices such as a cell phone and provide a light which can be a flasher.
 
Mar 2, 2019
210
Oday 25 Milwaukee
Ok ,here is my take . MaineSail aas usual is spot on. Take the time to learn from his website . If all else fails and your engine doesn't start ,do what we do and get that teenager in the water and have him push start the boat ...Be ready to dump the clutch !
 
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Fredct

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Sep 21, 2020
86
Oday 28 Westbrook
@shemandr thanks for tip. my boat is old. Not sure of ROI + I have a charger/inverter that may need to be modified... Would definitely do on newer vessel.
 
Nov 13, 2013
646
Catalina 34 Tacoma
#6 Starting an engine uses high-current but for a very short duration. It typically consumes less than 0.2Ah.
I hear people say this all the time without any condideration for the energy draw from glow plugs. What is the energy draw using glow plugs for 30 seconds?
 
May 17, 2004
3,541
Beneteau Oceanis 37 LE Havre de Grace
I hear people say this all the time without any condideration for the energy draw from glow plugs. What is the energy draw using glow plugs for 30 seconds?
Google seems to say glow plugs draw 10-20 amps. If we assume 15 amps and 2 cylinders for 30 seconds that’s 30 amps for 0.0083 hours, which is 0.25 amp hours. Still pretty negligible.

Also, 30 seconds seems a long time to hold glow plugs. My Universal M12 needed about 10 seconds when cold, and on my Yanmar 3YM30 I haven’t had to touch them yet. In repeated start/shutdown situations like after a daysail the engine should still be pretty warm, so minimal glow plugs needed.
 

BarryL

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May 21, 2004
839
Jeanneau Sun Odyssey 409 Mt. Sinai, NY
Hey,

Lots of good advice and information above. The basic point is that your battery is old and worn out. The simplest thing to do would be to just buy a new deep cycle battery and replace the worn out one. You don't have to change anything, just understand that in another 4-5 years your battery will need to be replaced again. Since you can buy a group 27 deep cycle battery for under $100, there is nothing wrong with that plan.

One simple way to monitor the battery strength would be to just check the voltage before you start the engine to return to your home port. Over the years you will see the voltage become lower and lower as the battery loses the ability to store energy. When you start to see voltages under 12 volts (at the battery) then it's probably time to replace the battery.

Barry
 
Feb 6, 1998
11,454
Canadian Sailcraft 36T Casco Bay, ME
I hear people say this all the time without any condideration for the energy draw from glow plugs. What is the energy draw using glow plugs for 30 seconds?
I have included that in the calculation.. 30 seconds is a long glow and will lead to short glow plug life. As glow plugs die folks tend to hold them longer and longer until they are all dead. We replace a lot of glow plugs, in the fall...The typical cranking duration we measure is 0.65 seconds to about 1.5 seconds as defined by loaded starter motor to unloaded.
 
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Fredct

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Sep 21, 2020
86
Oday 28 Westbrook
Thanks for all the good information. I have bought a new battery (Interstate) and will replace this weekend. I will also carry a portable 1000A LiFePO battery pack that can be used to start the engine but mainly to charge USB devices. After calculating my actual energy consumption, I concluded that the battery #2 was in fact dead. Will check the other battery at the same time.
 
Oct 26, 2008
5,047
Catalina 320 Barnegat, NJ
If you are going to combine 2 batteries to make a significantly improved battery bank, then get another new battery. You will weaken your new battery by combining it with an old battery. Use good sense and get 2 new batteries to double your primary bank, use the old battery, if it has any life, as the auxiliary. I wouldn't bother with a battery pack, it's a waste. Install a USB outlet or two instead

Also, you say you are energy miserly, but you have an inverter? Why? It's a waste. If you don't have a modern charger, get one. You own a boat ... you don't get to cry poverty! :biggrin: Become a little more sophisticated about your energy needs. You have a small cruiser in a cruising environment. Treat your boat like a cruiser.
 

Fredct

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Sep 21, 2020
86
Oday 28 Westbrook
@Scott T-Bird I am not following your reasoning. We are nor miserly nor cheap, just careful and prudent. The boat has 2 batteries and a convential rotary selector switch. Based on information provided above by Mainesail, I have decided to change the one that seems faulty (and in any event is 5 years old). Should I change both including the new one that is 2 years old? There is no room for more batteries on the vessel. The volume occupied by two 27M is all that is available.

As to the emergency power bank, why not have an independent plan B and a way to charge portable devices? Charging devices on the extant batteries only increases the drain (albeit by a little).

The vessel came with an inverter. We never use it on battery power. Switch to 120 outlets is always off.

The charger circuit has been upgraded in last 5 years but I am away from the vessel so can't do a verification on brand or anything.

My bottom line: I had a faulty battery (possibly going back to last year) but only discovered problem when I was away from shore and unable to recharge from shore power. Solution: replace the battery (seems logical to me) and have an independent way of jump starting motor in case of emergency.

I appreciate all the comments and advice. There is many years of expertise to help us.
 
Jan 11, 2014
7,976
Sabre 362 113 Fair Haven, NY
@Scott T-Bird I am not following your reasoning. We are nor miserly nor cheap, just careful and prudent. The boat has 2 batteries and a convential rotary selector switch. Based on information provided above by Mainesail, I have decided to change the one that seems faulty (and in any event is 5 years old). Should I change both including the new one that is 2 years old? There is no room for more batteries on the vessel. The volume occupied by two 27M is all that is available.

As to the emergency power bank, why not have an independent plan B and a way to charge portable devices? Charging devices on the extant batteries only increases the drain (albeit by a little).

The vessel came with an inverter. We never use it on battery power. Switch to 120 outlets is always off.

The charger circuit has been upgraded in last 5 years but I am away from the vessel so can't do a verification on brand or anything.

My bottom line: I had a faulty battery (possibly going back to last year) but only discovered problem when I was away from shore and unable to recharge from shore power. Solution: replace the battery (seems logical to me) and have an independent way of jump starting motor in case of emergency.

I appreciate all the comments and advice. There is many years of expertise to help us.
The point @Scott T-Bird is making involves the size of the battery bank and the condition of the batteries in the bank. Many cruisers place 2 batteries in parallel to make one big battery bank. There are advantages to do this, especially if you can have a separate start/reserve battery.

If you combine both of your 90 ah Group 27 batteries into one bank you will effectively have one battery with 180 ah. If you should do this, then both batteries need to be of the same type and age and if possible from the same production run. On your boat, you have them separated so it is less of an issue.

Why combine the batteries? The simple and short explanation is there is less stress on the batteries. Depth of discharge and charging cycles determine a battery's life span. When the batteries are paralleled each battery is discharged less than it would be if run solo. For example, if 45 ah are drawn from a Grp 27 battery, the battery has been discharged to 50% of capacity. However, if that same load is drawn from 2 Group 27 batteries in parallel, then each battery is only drawn down to 25% of capacity.

There are ways to make your DC system more efficient that are not terribly expensive and of course there are expensive ways too. The route you go is dependent on how you sail and cruise and the depth of your bank account. There is more information on this on MaineSail's site, MarineHowto.com.