How to determine a battery's REAL capacity

Sep 30, 2013
3,538
1988 Catalina 22 North Florida
Trying to determine what size bank I need ...

If a battery is rated at, say, 100 Ah reserve capacity, does that mean 100 Ah in total? In other words, does it mean the battery actually has only 50 Ah you can actually USE, before reaching the critical 50% level? Or does it mean there are 100 Ah available for daily use?

Sorry for the rookie question. I'm reading Don Casey's "Sailboat Electrics Simplified", but have not yet come to this answer. Plus I'm a slow learner. :)
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
100Ah is total capacity. Useable is 50Ah. But really about 30 to 40 Ah because getting the last 10-15% of the capacity back into the battery takes a long time. Won't happen unless you motor for hours or plug into shore power with a good charger.
 
Sep 30, 2013
3,538
1988 Catalina 22 North Florida
Thanks jssailem. Actually, I guess I'm going at it backwards by trying to determine how much battery bank I can HAVE. (It's a very small boat.) What I can have, is going to determine what my energy budget can be. Is this idea fatally flawed somehow?

My Albin Vega was designed to hold two group 24's down in the keel. I have found that I can build more shelving in there and fit one more. G24's are rated at about 75Ah, so if I understand correctly, this would provide somewhere around 225 Ah, which means in reality I would only have about 112 Ah I can actually USE between charging (?). I'd like to have more, but this means moving the bank to a higher location, and probably further from the center line.
 
Oct 22, 2014
20,989
CAL 35 Cruiser #21 moored EVERETT WA
Stu, One of these days I'm going make a favorits list of your great documents so that they are at my fingertips. I currently do a search. And they always come up.:) But if an enterprising computer moderator somehow messes with the search system I will be SOL... (sorry old landlubber)...
 
Oct 22, 2014
20,989
CAL 35 Cruiser #21 moored EVERETT WA
Gene, No space is always a consideration. MaineSail will telll you that there is a "greater bang for the buck" using 6 volt golf cart batteries. It is tru in several ways. One they are smaller in physical size yet supply more amp hours. And two they give you more charging cycles (longer life). Problem is you need 2 to get 12 volts. If one dies all are dead. So now you need 4 to have a back up 12 volts. You do get 225 amp hours from two 6 volt batteries.
I found an alternate rout. I had room for 2 Group27 sized batteries. Then I learned that the Group 31 batteries are the same size (fit the Goup 27 battery box). I checked out Dyno Batteries here on the west coast and found they make a group 31 thick plate battery for aerial lifts - 30TMX
(http://www.dynobattery.com/products/sweeper-and-aerial-lifts/30tmx.php) 135 amp hours. Maybe not perfect but better than the group 24's I was using.
 
May 24, 2004
7,129
CC 30 South Florida
Do not think of 50% as the goal to discharge a battery to be able to use maximum a/h. Discharging to 40% will be more beneficial and if you need additional a/h just increase the size of your bank. I try to size the bank to my average usage for a discharge range of 25% to 40%. Too large a bank in relation to usage may not allow a bank to exercise properly. We need to reach a level that triggers bulk rate charging for about an hour to reduce sulfating.
 
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Sep 25, 2008
7,075
Alden 50 Sarasota, Florida
Anybody know of a reference written by a battery manufacturer or other informed source which verifies the hypothesis that exceeding 50% capacity shortens battery life?

And if so, how much useful life is sacrificed by going to 30%, 20%, etc...?

Like others, I've likely exceeded the 'magic' 50% figure probably numerous times on battery banks which have far exceed d my expectation for longevity.
 
Nov 26, 2012
2,315
Catalina 250 Bodega Bay CA
Most people do not know the relationship to % and DC readings. Such as the perception that 50% is basically 12.2 volts! Chief
 
Feb 26, 2004
22,759
Catalina 34 224 Maple Bay, BC, Canada
Anybody know of a reference written by a battery manufacturer or other informed source which verifies the hypothesis that exceeding 50% capacity shortens battery life?

And if so, how much useful life is sacrificed by going to 30%, 20%, etc...?
Gee, Don, I would think Maine Sail is a pretty reputable source.

http://www.pbase.com/mainecruising/deep_cycle_battery

Calder also discusses this in detail in his Boatowners Manual.
 
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Sep 25, 2008
7,075
Alden 50 Sarasota, Florida
Gee, Don, I would think Maine Sail is a pretty reputable source.

http://www.pbase.com/mainecruising/deep_cycle_battery

Calder also discusses this in detail in his Boatowners Manual.
Stu

I've read Calder's manual and looked at the link but neither answer the question as both omit meaningful info, that being actual useful lifetime vs. capacity draw. They speak only in terms of # of cycles.

More specifically, if one uses more than 50% capacity on a regular basis, the actual time between recharges during which battery drawdown occurs is obviously extended meaning I am using the battery more per recharge interval.

That additional time isn't considered in the diagrams in Calder's or the Penn data.

The more time batteries are being used isn't necessarily proportional to life as the diagrams note only cycles, not life measured in practical terms such as hours.

What am I missing?
 
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Nov 26, 2012
2,315
Catalina 250 Bodega Bay CA
capta: even the charts differ: thats why I said "50% is basically 12.2v". Chief
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
The more time batteries are being used isn't necessarily proportional to life as the diagrams note only cycles, not life measured in practical terms such as hours.
Useful hours is dependent upon the current draw on the batteries. A 1 amp draw for 10 hours is equal to a 10 amp draw for 1 hour, however, when charging the total 10Ah draw counts the same towards the recharging cycles.

The number of recharging cycles is not the number of times the battery is recharged, it is the number times the battery is recharge from a specified state of discharge to a full charge.

MaineSail no doubt has more specific information on recharging cycles, so let me give you a hypothetical example. If one recharge cycle is returning a battery from a SOC of 0% to 100%, recharging a battery from a 90% SOC to 100% SOC would count as 1/10 of a cycle. A recharge from 50% SOC to 90% SOC would count as 4/10s of a cycle. The cycles are additive. These two recharges would count as .5 cycles.
 
Sep 25, 2008
7,075
Alden 50 Sarasota, Florida
Useful hours is dependent upon the current draw on the batteries. A 1 amp draw for 10 hours is equal to a 10 amp draw for 1 hour, however, when charging the total 10Ah draw counts the same towards the recharging cycles.

The number of recharging cycles is not the number of times the battery is recharged, it is the number times the battery is recharge from a specified state of discharge to a full charge.

MaineSail no doubt has more specific information on recharging cycles, so let me give you a hypothetical example. If one recharge cycle is returning a battery from a SOC of 0% to 100%, recharging a battery from a 90% SOC to 100% SOC would count as 1/10 of a cycle. A recharge from 50% SOC to 90% SOC would count as 4/10s of a cycle. The cycles are additive. These two recharges would count as .5 cycles.
Appreciate your response, however, I'm asking about useful life rather than recharge cycling and by useful life, I hoped to imply life at a typical draw, not one amp as your example cites.

I'm not surprised at the lack of info forthcoming as I suspect there may be none.
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
Appreciate your response, however, I'm asking about useful life rather than recharge cycling and by useful life, I hoped to imply life at a typical draw, not one amp as your example cites.

I'm not surprised at the lack of info forthcoming as I suspect there may be none.
Don, every boater is going to have a different typical draw. If you know what your typical draw is and you know the number of cycles for a battery it becomes easy to calculate. There are just too many variables to get reliable data from a large group of boaters. Even looking at 1 boat, the draw will vary from day to day based on how the boat is used and how the batteries are recharged.

If I understand your question correctly you are asking for information that would simply be either impossible to obtain or if presented would be not be very precise and thus unreliable.

If you looking to get the best bang for the buck, you could calculate the cost per rated cycle.
 
Nov 26, 2012
2,315
Catalina 250 Bodega Bay CA
It seems both of you are right. Battery life depends on a miriad of things but I suspect who made it to be the most major consideration. I have no problem with that, and then, its our use and care of the batteries. Through the years my battery banks life expectancy has been about 5 years. I use Walmart wet cells ($79 ea.) and that cost per year is fine with me. I do draw down my bank to as low as 11.8v on occasion but not a high amperage load at that voltage. My 2a draw TV might be on with 11.8v load reading. Just my observations, Chief
 
Sep 25, 2008
7,075
Alden 50 Sarasota, Florida
Don, every boater is going to have a different typical draw. If you know what your typical draw is and you know the number of cycles for a battery it becomes easy to calculate. There are just too many variables to get reliable data from a large group of boaters. Even looking at 1 boat, the draw will vary from day to day based on how the boat is used and how the batteries are recharged.

If I understand your question correctly you are asking for information that would simply be either impossible to obtain or if presented would be not be very precise and thus unreliable.

If you looking to get the best bang for the buck, you could calculate the cost per rated cycle.
The phrase "typical draw" was a bad choice of words I used to imply an amount greater than the one amp-hr someone used as an example.

I don't want to belabor the point that there seems to be no correlation between 50% capacity draw-down cycles and useful battery life without a consideration of actual time under load which none of usual references consider. The generalization about never going below 50% capacity is likely an over-simplification given the lack of real information on how to get the most out of a battery bank.

In my experience which spans the period in which we used and abused the cheapest batteries to trying to 'baby' (temperature compensated charging and careful recharge management routine) expensive Rolls batteries, they all lasted the typical 5-8 years, the lower end being here in Florida heat and 12 month season vs. New England's 5 month season.