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Old 15-11-2016, 07:16   #91
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Re: Battery lifetime/cost vs. state of charge

Quote:
Originally Posted by kmacdonald View Post
Maine Sail, do you think cycling batteries to 80% DOD is OK if they are charged to 100% SOC routinely?
You can always cycle to 80% DOD but your batteries still won't deliver lab like cycles nor anywhere even close. There are too many variables that we can't deliver on out in the real world, even dock side, that they can easily deliver in a lab.

-In the real world our discharge rate will often be slower & varied & dragged out thus longer times will spent with heavy sulfation on the plates.

-In the lab this time is shorter, discharge and recharge rate do not vary, and the cycles are much more controlled.

-In the real world our chargers & terminal voltage settings are often incorrect and our absorption duration's are usually grossly inadequate.

-In the real world our float voltages are often incorrect leading to plate wear that is not accounted for in lab cycle life data.

-In the real world, with flooded batteries, the orientation in the vessel is often incorrect thus exposing the plates and degrading and damaging them.

-In the lab the batteries are stationary.

-In the real world our batteries often exceed 80F which can drastically shorten cycle life..

-In the lab the batteries are cycled in a temp controlled environment that does not exceed 80F.

-In the lab the charge voltages are precise and battery temp is closely monitored and charge voltage adjusted accordingly.

-In the real world perhaps 70% of the chargers I see in use out there don't even offer temp compensation and the rest that do offer it only provide ONE temp sensor. Great for one battery but what about the rest or that one up against the engine room bulkhead? Even those that do offer temp compensation often don't allow an owner to adjust the "compensation slope" to match the battery.

-In the real world we lose dock power, for many reasons, and parasitic loads then discharge the batteries often to a voltage where the charger can't even turn back on due to safety thresholds..

-In the real world bilge pumps check valves stick or bilge switches stick on and deeply discharge batteries.

-In the real world parasitic loads kill batteries slowly.

-In the real world, even when plugged in and dock side, many chargers are not smart enough to maintain a float voltage and can be tricked into another absorption cycle from such devices as refrigeration, even when the battery is already at 100% SOC. This again leads to plate wear.

-On cruising or mooring sailed boats we have chronic PSOC use which really serves to murder batteries especially the deeper you go.

-We have stock alternator charging that usually nets far too incorrect a voltage for the batteries to get them to a respectable SOC in a reasonable time frame.

-We have no good way to test state of health of our batteries so the abuses are essentially blind to the owners. Often it is not until an owner can no longer start the motor or the lights dim when the fridge kicks on or the batteries thermally run away due to an internal failure that any issues with SOH are noticed..




Quote:
Originally Posted by kmacdonald View Post
In other words, is it the 80% DOD or the PSOC that is the worst for batteries.
Both are bad for batteries but PSOC really tends to be more of the icing on the cake. Stack them and it gets insanely bad. In my experience 80% DOD is worse in the real world than the lab graph suggests it would be.

PSOC cycle testing is not publicized by any battery maker I know of other than Firefly. Lots of companies are now marketing PSOC batteries but until they provide legitimate PSOC testing data, & the BCI has a test that all battery makers follow, it represents nothing more than a glossy BS marketing sticker.

The testing conducted by Practical Sailor is an excellent test to see how quickly PSOC use affects batteries.

#1 Cycle batteries for break-in until they attain their Ah Capacity Rating. Once rated capacity has been attained conduct a second capacity test. The two 20 hour tests are "averaged" for baseline Ah capacity.

#2 Discharge battery to 11.7V

#3 Recharge battery for exactly 1 hour at .XX of Ah capacity. For the Practical Sailor testing a .46C charge rate was used for the AGM batteries.

#4 Repeat the discharge to 11.70V / recharge for 1 hour for 30 cycles

#5 At cycle 31 re-charge to 100% SOC

#6 At cycle 32 conduct another 20 hour test

#7 If battery did not deliver full Ah capacity run a second Ah capacity test.

#8 Any battery that can be equalized or has special deep discharge recovery charging can be charged using this criteria.

#9 Another 20 hour capacity test is conducted.

#10 Compile 30 cycle PSOC Ah capacity loss and the rate of capacity walk down during the 30 cycles.


Quote:
Originally Posted by kmacdonald View Post
I have always assumed it was the PSOC that is worse for them and that is not the use profile of boats plugged in to the dock. In Maine most boats a kept on moorings and therefore are not plugged in to run a charger so you may experience more of the worse case scenario.
PSOC is worse than just DOD (if the DOD energy is replenished after each cycle), to a point, but both are bad. When PSOC and 80% DOD are stacked on top of one another it gets really bad.

In Maine PSOC use is extremely wide spread but we add solar & occasionally wind to take the batteries from 80-85% (alt / gen) to 100% SOC in as few days as is possible for the array sizing we can fit. Obviously a 100A to 150A etc. inverter/charger would get them back to 100% faster, and result in less damages from PSOC, but we do the best we can on actual cruising boats (I count mooring sailed boats in the cruising class due to similarities in battery abuse).

If you discharge to 80% DOD for a weekend here or there then charge to 100% and EQ regularly you can still get a quasi respectable duration from the batteries provided they were quality batteries to start with.
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Old 15-11-2016, 07:51   #92
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Re: Battery lifetime/cost vs. state of charge

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Originally Posted by kmacdonald View Post
I agree 100%. And how long does the average person keep a boat anyway?
Gee, I dunno.

I had our C25 for 13 years and this one for 18.
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Old 15-11-2016, 07:59   #93
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Re: Battery lifetime/cost vs. state of charge

IS IT BETTER TO HAVE ONE OR TWO BATTERY BANKS FOR HOUSE USE?
(By Nigel Calder - I DIDNíT write this!!!)

The popular arrangement of having two house banks alternated in use needs scrutiny before I go any further.

LIFE CYCLES: As we have seen, the life expectancy of a battery in cycling service is directly related to the depth to which it is discharged at each cycle - the greater the depth of discharge, the shorter the batteryís life.

This relationship between depth of discharge and battery life is NOT linear. As the depth of discharge increases, a batteryís life expectancy is disproportionately shortened. A given battery may cycle through 10% of its capacity 2,000 times, 50% of its capacity 300 times and 100% of its capacity around 100 times.

Letís say, for arguments sake, that a boat has two 200-ah battery banks, alternated from day to day, with a daily load of 80 Ah. Each bank will be discharged by 40% (80 Ah of one of the two 200 Ah banks) of its capacity before being recharged. The batteries will fail after 380 cycles, which is 760 days (since each is used every other day). If the two banks had been wired in parallel, to make a single 400 Ah battery bank, this bank would have been discharged by 20% of capacity every day, with a life expectancy of 800 days, a 5% increase in life expectancy using exactly the same batteries!

But now letís double the capacity of the batteries, so that the boat has either two 400 Ah banks, or a single 800 Ah bank, but with the same 80 Ah daily load. The two separate banks will be cycling through 20% of capacity every other day, resulting in a total life expectancy of 1,600 days. Doubling the size of the battery banks in relation to the load has produced a 210% increase in life expectancy. The single 800 Ah bank will be cycling through 10% of capacity every day, resulting in a life expectancy of 2,000 days - a 25% increase in life expectancy over the two (400 Ah) banks, and a 250% increase in life expectancy over the single 400 Ah battery bank!

There are two immediate conclusions to be drawn from these figures:

1. For a given total battery capacity, wiring the (house) batteries into a single high capacity bank, rather than having them divided into two alternating banks, will result in a longer overall life expectancy for the batteries.

2. All other things being equal, any increase in the overall capacity of a battery bank will produce a disproportionate increase in its life expectancy (through reducing the depth of discharge at each cycle).

FOR BATTERY LONGEVITY, A SINGLE LARGE (HOUSE) BANK, THE LARGER THE BETTER, IS PREFERABLE TO DIVIDED (HOUSE) BANKS.
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Old 15-11-2016, 09:18   #94
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Re: Battery lifetime/cost vs. state of charge

^^ I have always used them as a single bank (3 x group 27) for 2 reasons.

One is the DOD argument.

The second is that I have always sailed cats, and even moderate sized cats have outboards that can be pull started. I avoid it, and in 25 years of sailing this type of boat I have only done it once. Even then, you only have to pull one engine, since in a few minutes you will have enough juice to start the second. And the amps to kick over a 9.9 outboard are pretty low. Heck, a few hours of solar would probably do it, but that was before I added panels, and it was easy anyway.

If I had to switch back and forth I would probably screw up anyway. This is simpler. Just remember to check the charge before going to bed.
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Old 15-11-2016, 09:27   #95
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Re: Battery lifetime/cost vs. state of charge

All of this conversation has been about cycles. But surely, there are other aging factors in play. Put another way, a golf cart battery or group 27 both have a typical maximum life, even if maintained as a typical dock queen and barely cycled at all. Many of the readers on this forum do cruise, but are not likely to anchor out more than 20 nights a year. The rest will be either at home or marina hopping. Thus, they will never reach more than a few hundred cycles before the batteries are retired, and many cases less than 100 cycles.

I wonder how that changes the logic? I'm pretty sure the damage contributes to aging more than the cycle count alone suggests, but I do wonder how much.

I buy cheap group 27 batteries and get 5 years from them. They are in the cockpit and changing them is quite simple. The bank is only 100 AH, but I also save weight, which I value at about $20/pound. A big bank of golf dart batteries doesn't make sense for the way I sail (fast). I probably spend about 30 nights a year at anchor and some more marina hopping.
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Old 15-11-2016, 09:43   #96
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Re: Battery lifetime/cost vs. state of charge

A few more thoughts. The lighter the boat the more attractive deep cycling becomes because the battery weight will be a proportionally larger.

From the cycle life vs depth of discharge curves it is apparent that the greatest gains in battery life are obtained by cycling batteries to less than 50% DOD. Above 50% DOD the degradation tapers off. The strongest argument is to cycles batteries to less than 50% DOD. If you are presently cycling to 50% DOD there is less to loose by cycling to 80% DOD.

My earlier post on lifetime power delivered showed that there is no difference between 20 and 80% DOD scenarios. That assumed 100% recharge between cycles. If we ran the numbers for 80% SOC recharges the 80% DOD would be most cost effective in terms of lifetime energy delivered.
Quote:
Originally Posted by kmacdonald View Post
From Trojan Battery:
20% DOD 4000 cycles
50% DOD 1600 cycles
80% DOD 1000 cycles
If you cycle to 50% DOD instead of 20% DOD you loose 2400 cycles of battery life to gain 30% capacity.
If you cycle to 80% DOD instead of 50% DOD you loose 600 cycles of battery life to gain 30% capacity.
From an energy delivered standpoint it looks the same for 20, 50, and 80% DOD:
If x is the total energy stored in the battery then at 20% DOD the battery will deliver .2(4000)x=800x over its life
At 50% DOD: .5(1600)x=800x over its life
At 80% DOD: .8(1000)x=800x over its life

This tells me to cycle deeper and save some weight.

Tanglewood said the same thing as I just showed above.

The above assumes you are recharging to 100% SOC.
All the "real word" caveats apply to any DOD scenario and are not limited to 80% DOD scenarios.
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Old 15-11-2016, 13:53   #97
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Re: Battery lifetime/cost vs. state of charge

Quote:
Originally Posted by Stu Jackson View Post
IS IT BETTER TO HAVE ONE OR TWO BATTERY BANKS FOR HOUSE USE?
(By Nigel Calder - I DIDNíT write this!!!)

The popular arrangement of having two house banks alternated in use needs scrutiny before I go any further.

LIFE CYCLES: As we have seen, the life expectancy of a battery in cycling service is directly related to the depth to which it is discharged at each cycle - the greater the depth of discharge, the shorter the batteryís life.

A given battery may cycle through 10% of its capacity 2,000 times, 50% of its capacity 300 times and 100% of its capacity around 100 times.
The data given above is there just to suit the argument.

Quote:
Originally Posted by kmacdonald View Post
From Trojan Battery:
20% DOD 4000 cycles
50% DOD 1600 cycles
80% DOD 1000 cycles
Quote:
Originally Posted by Stu Jackson View Post
If the two banks had been wired in parallel, to make a single 400 Ah battery bank, this bank would have been discharged by 20% of capacity every day, with a life expectancy of 800 days, a 5% increase in life expectancy using exactly the same batteries!
This is guesswork because it is unlikely that paralleled batteries will share equally the load.

To consider:

Quote:
Originally Posted by Maine Sail View Post

-In the real world our discharge rate will often be slower & varied & dragged out thus longer times will spent with heavy sulfation on the plates.

-In the lab the batteries are stationary.

-In the real world parasitic loads kill batteries slowly.

-On cruising or mooring sailed boats we have chronic PSOC use which really serves to murder batteries especially the deeper you go.

-We have no good way to test state of health of our batteries so the abuses are essentially blind to the owners. Often it is not until an owner can no longer start the motor or the lights dim when the fridge kicks on or the batteries thermally run away due to an internal failure that any issues with SOH are noticed..

In Maine PSOC use is extremely wide spread but we add solar & occasionally wind to take the batteries from 80-85% (alt / gen) to 100% SOC in as few days as is possible for the array sizing we can fit. Obviously a 100A to 150A etc. inverter/charger would get them back to 100% faster, and result in less damages from PSOC, but we do the best we can on actual cruising boats (I count mooring sailed boats in the cruising class due to similarities in battery abuse).

If you discharge to 80% DOD for a weekend here or there then charge to 100% and EQ regularly you can still get a quasi respectable duration from the batteries provided they were quality batteries to start with.

Quote:
Originally Posted by Stu Jackson View Post
FOR BATTERY LONGEVITY, A SINGLE LARGE (HOUSE) BANK, THE LARGER THE BETTER, IS PREFERABLE TO DIVIDED (HOUSE) BANKS.
Divided and multiple banks give the opportunity to fully charge, even equalize the bank resting. Are easier to monitor and maintain. Are safer and provide redundancy.
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Old 17-11-2016, 10:19   #98
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Re: Battery lifetime/cost vs. state of charge

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.....Divided and multiple banks give the opportunity to fully charge, even equalize the bank resting. Are easier to monitor and maintain. Are safer and provide redundancy.
Let me give my 4 reasons why one big bank is better than two small banks.....

In the past it was often popular to have two service banks and no starter battery. It was thought a smaller bank would charge quicker. Current thinking is that it is much more efficient to have just one large service bank and a much smaller starter battery.

1. Doubling the service bank size means the life cycle is longer as the DoD is unlikely to fall so close to 50% so often. Life cycle at 50% DoD may be 1000 charge and discharge cycles. At a DoD of only 25% the life cycle may be 2500 or more.



2. Doubling the service bank size also means the ďapparent capacityĒ is greater. Peukertís law says that the apparent Ah size of a bank changes depending on the current draw.

A bank is designed to deliver a capacity with a current discharge that will flatten the battery in 20 hours. (The 20 hour rate) 

So with a 100Ah battery, a 5A load will flatten the battery to 10.5v in 20 hours. 

When drawing currents higher than 5 amps the "actual" bank size will be much smaller, so the bank will not last as long before it needs re-charging. Conversely when using much less than 5 amps the bank size will be larger and will deliver more Ah. 

If a 100 Ah that battery has a Peukert value of 1.25, then higher or lower loads than 5 amps will change the actual capacity of the battery by the following amounts.

With a 10A load for 20 hours there are only 84Ah's in the bank.

With a 1A load for 20 hours there are 150Ah's in the bank.


3. Doubling the service bank size also means it will be more efficient and accept more Ah more quickly from all charging sources during the boost phase up to 80%.



It takes a bit of very over-simplified maths to prove the point, but a 100 Ah battery that is discharged to 50% may accept 20Ah in the first hour during the boosts stage, maybe 10Ah in the second hour during the start of the less efficient absorption phase, and the remaining 20Ah in another 5 hour. Doubling the battery size to 200Ah, with the same charging source of 20 amps, will accept 10Ah into each battery in 1 hour, thatís 20Ah into the bank. In the second hours it will store another 20Ah. Thatís 40Ah replaced in two hours, as compared to 30Ah with a single bank. In the 3rd hour it may still accept 20 amps into the bank because a single battery in the start of the absorption phase could accept 10 amps. Thatís 60Ah in three hours. The key point is that for two hours it is still in the more efficient boost stage where the battery is taking all the current the charge source can give it. Note that the initial boost charging stage has captured 40Ah in two hours and 60 Ah in three hours. With the smaller bank it could only capture 20Ah in the first hour during boost and 30Ah after the second hour during the start of absorption. The third hour may add another 5 amps. Thatís 35Ah with one batteries and 60Ah with two batteries. So a bigger bank will be more efficient and accept more Ah more quickly from all charging sources.

Since a lot of the time we are only charging up to the absorption stage which is about 80-85% then this increase in stored Ah is significant.


4 If you have a larger bank - or many smaller batteries in one large bank, it is easy when they start failing to just disconnect the bad ones and run on the others as long as you can until you can replace the whole bank. This may also allow skippers to search around and find the batteries they really want - not just be forced to buy the local "rubbish" because they are desperate
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Old 17-11-2016, 10:54   #99
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Re: Battery lifetime/cost vs. state of charge

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Originally Posted by chala View Post
Divided and multiple banks give the opportunity to fully charge, even equalize the bank resting. Are easier to monitor and maintain. Are safer and provide redundancy.

Big benefit being able to equalize living on the hook.
Plus most of the time they can be just one big bank.
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Old 17-11-2016, 11:19   #100
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Re: Battery lifetime/cost vs. state of charge

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Big benefit being able to equalize living on the hook.
Plus most of the time they can be just one big bank.
Then it's simply a wiring and switch management issue that is essentially one big bank anyway.

Equalizing should take maybe four hours. Evn with a wired-in big house bank, not split, you could run your boat off the reserve bank, even the fridge, for that mount of time. Plus, if most folks equalize with a gen set or plugged in, then it's a no brainer.

So, if you want to wire your house bank into a split house bank, but use it as a big bank, could you please explain the advantages of the additional wiring and connections?

I've been into larger boats for 18 years now. I ran across the Calder post in 1998. Remains true today, but there are still some who fight it. Same guys probably have CQRs and guns.
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Old 17-11-2016, 11:26   #101
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Re: Battery lifetime/cost vs. state of charge

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Originally Posted by Stu Jackson View Post
Plus, if most folks equalize with a gen set or plugged in, then it's a no brainer.

So, if you want to wire your house bank into a split house bank, but use it as a big bank, could you please explain the advantages of the additional wiring and connections?
Not everyone uses genny or shorepower. , good windy day then equalize. If there's not much solar or wind about then at least get each set fully charged every few days.
At least with a little extra install you have the choice. More options are good when your a long way from anywhere.
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Old 17-11-2016, 11:34   #102
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Re: Battery lifetime/cost vs. state of charge

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Originally Posted by sailinglegend View Post
Let me give my 4 reasons why one big bank is better than two small banks.....

In the past it was often popular to have two service banks and no starter battery. It was thought a smaller bank would charge quicker. Current thinking is that it is much more efficient to have just one large service bank and a much smaller starter battery.

1. Doubling the service bank size means the life cycle is longer as the DoD is unlikely to fall so close to 50% so often. Life cycle at 50% DoD may be 1000 charge and discharge cycles. At a DoD of only 25% the life cycle may be 2500 or more.



2. Doubling the service bank size also means the ďapparent capacityĒ is greater. Peukertís law says that the apparent Ah size of a bank changes depending on the current draw.

A bank is designed to deliver a capacity with a current discharge that will flatten the battery in 20 hours. (The 20 hour rate) 

So with a 100Ah battery, a 5A load will flatten the battery to 10.5v in 20 hours. 

When drawing currents higher than 5 amps the "actual" bank size will be much smaller, so the bank will not last as long before it needs re-charging. Conversely when using much less than 5 amps the bank size will be larger and will deliver more Ah. 

If a 100 Ah that battery has a Peukert value of 1.25, then higher or lower loads than 5 amps will change the actual capacity of the battery by the following amounts.

With a 10A load for 20 hours there are only 84Ah's in the bank.

With a 1A load for 20 hours there are 150Ah's in the bank.


3. Doubling the service bank size also means it will be more efficient and accept more Ah more quickly from all charging sources during the boost phase up to 80%.



It takes a bit of very over-simplified maths to prove the point, but a 100 Ah battery that is discharged to 50% may accept 20Ah in the first hour during the boosts stage, maybe 10Ah in the second hour during the start of the less efficient absorption phase, and the remaining 20Ah in another 5 hour. Doubling the battery size to 200Ah, with the same charging source of 20 amps, will accept 10Ah into each battery in 1 hour, thatís 20Ah into the bank. In the second hours it will store another 20Ah. Thatís 40Ah replaced in two hours, as compared to 30Ah with a single bank. In the 3rd hour it may still accept 20 amps into the bank because a single battery in the start of the absorption phase could accept 10 amps. Thatís 60Ah in three hours. The key point is that for two hours it is still in the more efficient boost stage where the battery is taking all the current the charge source can give it. Note that the initial boost charging stage has captured 40Ah in two hours and 60 Ah in three hours. With the smaller bank it could only capture 20Ah in the first hour during boost and 30Ah after the second hour during the start of absorption. The third hour may add another 5 amps. Thatís 35Ah with one batteries and 60Ah with two batteries. So a bigger bank will be more efficient and accept more Ah more quickly from all charging sources.

Since a lot of the time we are only charging up to the absorption stage which is about 80-85% then this increase in stored Ah is significant.


4 If you have a larger bank - or many smaller batteries in one large bank, it is easy when they start failing to just disconnect the bad ones and run on the others as long as you can until you can replace the whole bank. This may also allow skippers to search around and find the batteries they really want - not just be forced to buy the local "rubbish" because they are desperate
An excellent summary.
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Old 17-11-2016, 17:59   #103
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Re: Battery lifetime/cost vs. state of charge

This has been an interesting and informative thread. Lots of varied opinions, as usual. So I might as well join the fun. I've had varied battery systems in different boats, from group 27 and 31, 4D and 8D, all "deep cycle" None gave the performance of US Battery golf cart bats, bank of 4 so 450ah.

I switched to golfs after working for a summer (I was jobless in 2000) with a marine electrician who mainly designed and installed battery systems on boats and some RV's. He explained to me the difference between rated capacity and practical capacity. As he explained it, practical capacity of a normal deep cycle battery is 50% of rated capacity, while a golf cart is 80%, you could take it down to the last 20+ % without damage as long as you did not leave it there for long before charging. So a bank of three 8d's at about 600 ah only had a practical capacity of about 300ah, a 450ah bank of 4 golfs had a practical capacity of 360 amps, while weighing 100 lbs less.

I abused the hell out of those golfs, took them well below 40% scores of times, 20% lots too, even down to 0 a couple of times when I had a faulty dock connection, and they always came back and performed. This was not extended cruising though, after a week out, back in the slip and shore power. Charger was nothing special, an old Mariner 50 amp charger, no multistage or equalization or such. They were still going strong when I sold the boat 9 years later. Golfs do use water, you have to keep an eye on that, so you have to be able to get to them. Still they were way easier to install than 8d's, where you had to wait for high tide to get them up and down the gangway, not to mention getting them into the boat. I was a young man then, can't imagine doing so now.

Until something better comes along, golfs are the only bats I'll install. Powertron in Costa Mesa, CA has US Battery golfs for $99, T105's for $120.
that's my 2 cents!
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Old 18-11-2016, 09:47   #104
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Re: Battery lifetime/cost vs. state of charge

I've read thru the thread and I'm still hung up on the chart. It doesn't state it but I can guarantee it assumes a perfect 100% charge after each use. No over charging, no under charging, so at best it gives you a sketchy idea of what happens to cycles.


The other question I have is how many people really only take their batteries up to 80% full? With most boats being outfitted with solar arrays and with systems generally sized to allow for at least 2-3 cloudy days, I'm betting most battery banks are hitting 100% full (or close) on a daily basis. Suddenly the idea that you are getting double the power out of a battery bank of a given size, drops to about 60% increase. When you account for the lost number of cycles, it's not so great.


As others have said there is a hassle and a risk factor in having the battery bank fail frequently. If you aren't vigilant, you may find out when you have a fridge full of food go bad or you can't get the motor started. There are ways around this but they require additional cost and maintenance.


This kind of reminds me of hull speed. If your boat has a hull speed of 7.5kts, you can probably put a bigger engine in and get 8.0kts but it quickly becomes a silly question. Particularly for monohulls which aren't really impacted by an extra 100-200lbs. Even most cruising cats won't care much. It's not until you get into the high end racing boats where it comes into play.
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Old 18-11-2016, 12:09   #105
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Re: Battery lifetime/cost vs. state of charge

Let me attempt to summarize all the facts and opinions presented so far in an effort to answer my original question regarding the optimum battery bank. It would be good to remind ourselves that we use batteries to hold electricity between charging. Thus the key figure of merit for any battery system is capacity vs. cost/weight/volume/use.

- Based on manufacturers' data, going to deep discharge vs. halfway discharge reduces the battery life proportionately other things being equal. This is based on the expectation that we are going to get the same amp-hours over the lifetime of the batteries irrespective of depth of discharge. For more information see the DOD curves.

- There are other factors that could further shorten battery life, the two most important are high temperature and incomplete charging. Most people expect the combination of two or more factors to exponentially decrease battery life but we have not been able to verify this with hard data.

- It is important to take Peukert's law in consideration when sizing a battery since high current draw could significantly reduce available battery capacity.

- Charging wet cell batteries above 80% is inefficient due to the rising internal resistance of the battery. This means that if charging the battery from an alternator, we need to run the engine twice as long to get from 80% to 100% vs. from 60% to 80% assuming a three stage temperature compensated charger. With a simple dumb charger it is much longer. Thus many cruisers just give up on charging the batteries above 80% unless they have excess solar, wind or generator (all options more expensive than just bulk charging and replacing batteries). This is a separate topic though.

- Having a larger than necessary battery bank has negatives including weight, volume and upfront replacement cost (i.e. if one battery goes down you have to replace the whole pack) while providing higher margins of safety.

Based on the above, some cruisers decide to play it safe sizing their house battery bank at 400AH for a typical 120A overnight use which allows them to stay above the 50% DOD recommendation plus adds a sizeable reserve and hope to get 5+ years out of the batteries. If they want to use more than 30% of the installed capacity they invest in the three way charger or renewables and accept the inefficiencies or extra generation capacity when away from shore power.

Others, myself included, choose to be aggressive on the weight savings and install only 200AH, discharge to 20% of capacity, only do bulk charging at sea and are prepared to replace the batteries every two years (assuming a limited amount of nights away from shore power, i.e. not full time cruising). The net cost difference between the two approaches is small (Calder's article mentioned 5%, other people think it could be more) and difficult to pin point as other real-life factors also affect battery life. Both approaches are valid.

Now, let's get away from shore power.

SV Pizzazz
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