Battery lifetime/cost vs. state of charge

[ranger58sb]

Originally Posted by Camille1

My worst experience was with Odyssey whose batteries I bought to be sure of reliable power crossing the pond. What a waste of money! They lasted no more than eighteen months and Odyssey told me, in the most courteous way to 'get lost'.
Maybe Golf cart batteries are the way to go for us non techies. Or an I wrong?

Probably depends on a given situation. In your case, that might also mean it depends on what caused your earlier batteries to fail. Do you know the cause?

-Chris

[transmitterdan]

Originally Posted by mitiempo

Not really. With a given load doubling the battery bank in size increases the AH by more than double, thanks to Peukert.

That is correct. Plus, the purchase cost per rated AH is typically less for larger batteries.

[tanglewood]

Originally Posted by mitiempo

Not really. With a given load doubling the battery bank in size increases the AH by more than double, thanks to Peukert.

True, but by how much for normal loads? Peukert really comes into play with large loads, and I expect the typical cruising boat has loads well under the 20hr rate.

Regardless, I agree with the OP that running with fewer batteries and correspondingly deeper discharge is a viable strategy that is systematically overlooked because of this somewhat arbitrary 50% DOD "rule". There is lots more range to work with, trading off the battery bank size against DOD, yielding lower capital cost, but proportionally more frequency replacement intervals. And the relationship is more linear than not, as long as you stay away from the 20% extremes of DOD.

If you don't like replacing batteries, then by all means spend more up-front for a larger bank that will last longer. Just remember that a longer period of ownership gives you more opportunity to wreck your batteries via partial state of charge (PSOC) abuse per MaineSail's warnings and experience.

Or you can spend less on batteries, knowing you will have to replace them sooner. And because you will own them for a shorter time, you have less opportunity to wreck them with PSOC abuse.

[kmacdonald]

Originally Posted by tanglewood

True, but by how much for normal loads? Peukert really comes into play with large loads, and I expect the typical cruising boat has loads well under the 20hr rate.

Regardless, I agree with the OP that running with fewer batteries and correspondingly deeper discharge is a viable strategy that is systematically overlooked because of this somewhat arbitrary 50% DOD "rule". There is lots more range to work with, trading off the battery bank size against DOD, yielding lower capital cost, but proportionally more frequency replacement intervals. And the relationship is more linear than not, as long as you stay away from the 20% extremes of DOD.

If you don't like replacing batteries, then by all means spend more up-front for a larger bank that will last longer. Just remember that a longer period of ownership gives you more opportunity to wreck your batteries via partial state of charge (PSOC) abuse per MaineSail's warnings and experience.

Or you can spend less on batteries, knowing you will have to replace them sooner. And because you will own them for a shorter time, you have less opportunity to wreck them with PSOC abuse.

I agree 100%. And how long does the average person keep a boat anyway?

[mitiempo]

Originally Posted by tanglewood

True, but by how much for normal loads? Peukert really comes into play with large loads, and I expect the typical cruising boat has loads well under the 20hr rate.

Puekert's rule, if you want to call it that, is a disadvantage with larger loads but an advantage with loads smaller than the 20 hour rate of discharge.

A 100 AH battery is rated at a 5 amp load. If the load is 10 amps the battery is effectively much smaller than 100 AH. But for loads under 5 amps the battery is effectively larger than 100 AH.

A 100 AH battery with an 8 amp load is effectively 88 AH.

A 300 AH bank with the same load is effectively a 355 AH bank.

See Mainesail's post #36 for a more complete explanation.

[s/v Beth]

I think you guys are ignoring a big part of purchasing new batteries, and that is finding a dealer who will be honest with you, getting the best deal possible and transporting the batteries down to the dock onto the boat and then cramming them into that cubby hole that is set aside for batteries.
My batteries, as I sit here tonight are at 13.4-14 volts. They are equalized every month by my battery charger.
I hate replacing batteries. Hate it! Hate it! For me it is much more cost effective to buy three times the amp-hours I need and never drop my voltage below about 12.5-12.4. I now have close to 7 years on these Crown 106 clones. On passage, as said above, I get down to 12.4 but put them back up every two days by Honda generator or solar or both.
Hey, when I cruise through Kali, could you install my batteries? and maybe I will be out of that &*^%^& hole for another 10 years!

[transmitterdan]

There is lots of confusion in this thread. Generally batteries are rated at 0.2C which means a 100AH battery is rated at a 20A draw. So it takes 5 hours to pull 100AH from the battery. If your load is less than 0.2C then the available AH will be more. Some manufacturers "cheat" and specify the AH at a lower amperage but they should still give the 0.2C capacity. Good manufacturers provide the capacity at a range of current values so you can see what the AH capacity is for your specific situation. But with LA it's always the case that less current draw means more AH capacity.

The 50% rule is not "arbitrary". It is based on well reasoned logic and real world testing. I admit it does not take into account the cost of recharging to 100%. It assumes that battery replacement is the dominant cost. It also assumes that cruising makes it harder to replace batteries and thus more expensive. So the 50% rule along with frequent charging to 100% gives many of us 10 year battery life. Especially if we buy quality batteries and maintain them properly.

If you think it is cost effective to buy small batteries and intentionally "murder" them then go ahead. No one will argue with your logic except to say that for most cruisers replacing batteries every year is not our idea of fun as a/v Beth so eloquently stated.

[tanglewood]

Originally Posted by s/v Beth

I hate replacing batteries. Hate it! Hate it! For me it is much more cost effective to buy three times the amp-hours

So you are a perfect candidate for buying a larger bank, shallower cycling, and less frequent battery replacement. Just be sure to manage them properly so you get the full life out of them, which is sounds like you do.

Just remember that nobody is saying you HAVE to do it differently. They are just saying to CAN, and that for some people it might be attractive, and that financially it is about the same cost over time.

[tanglewood]

Originally Posted by mitiempo

Puekert's rule, if you want to call it that, is a disadvantage with larger loads but an advantage with loads smaller than the 20 hour rate of discharge.

A 100 AH battery is rated at a 5 amp load. If the load is 10 amps the battery is effectively much smaller than 100 AH. But for loads under 5 amps the battery is effectively larger than 100 AH.

A 100 AH battery with an 8 amp load is effectively 88 AH.

A 300 AH bank with the same load is effectively a 355 AH bank.

See Mainesail's post #36 for a more complete explanation.

Like I said initially, I agree. And I understand Peukert's equation and it's effect. I haven't run the numbers, but would estimate that maybe it makes a 10% difference the way we typically use battery power?

[transmitterdan]

Originally Posted by tanglewood

So you are a perfect candidate for buying a larger bank, shallower cycling, and less frequent battery replacement. Just be sure to manage them properly so you get the full life out of them, which is sounds like you do.

Just remember that nobody is saying you HAVE to do it differently. They are just saying to CAN, and that for some people it might be attractive, and that financially it is about the same cost over time.

Somebody needs to show us the math on that claim. The theories so far have ignored the Peukert effect of LA batteries. Bigger batteries provide more AH dollar for dollar and recharge more efficiently than smaller batteries. I would like to see a technically accurate spreadsheet that proves the theory proposed in this thread.

[tanglewood]

Originally Posted by transmitterdan

There is lots of confusion in this thread. Generally batteries are rated at 0.2C which means a 100AH battery is rated at a 20A draw. So it takes 5 hours to pull 100AH from the battery. If your load is less than 0.2C then the available AH will be more. Some manufacturers "cheat" and specify the AH at a lower amperage but they should still give the 0.2C capacity. Good manufacturers provide the capacity at a range of current values so you can see what the AH capacity is for your specific situation. But with LA it's always the case that less current draw means more AH capacity

I think batteries are generally rated at their 20hr rate, i.e. their capacity when you drain them over the course of 20hrs. So for your 100Ah battery, that's a 5A draw, not 20A. 20A would drain the battery in 5hrs, so that's the 5hr rate. So it works out to be C/20, or 0.05C. Note that these numbers are just to illustrate since the Peukert effect will cause lower capacity at a 20A draw, as pointed out by a number of people. But for the sake of understanding what a battery's 20hr capacity rating is, those numbers should illustrate.

[StuM]

Originally Posted by transmitterdan

There is lots of confusion in this thread. Generally batteries are rated at 0.2C which means a 100AH battery is rated at a 20A draw. So it takes 5 hours to pull 100AH from the battery. If your load is less than 0.2C then the available AH will be more. .

The iconic Trojan T105 is generally reckoned to be 225Ah, and that is at the "20hr rate" or 0.05C.

At the 5 hrs rate (0.2C) , they are only rated at 185Ah.

[tanglewood]

FYI, I played around with some Peukert calculations. There is a 10% penalty in battery capacity if you cut your battery bank capacity in half, yet maintain the same discharge current. Similarly, if you were to double your battery bank capacity while maintaining the same discharge current, there is a 10% increase in capacity.

So for anyone considering running with a smaller battery bank and deeper discharges, be sure to factor this 10% loss in capacity into your plans. This puts some numbers around the Peukert side of this discussion.

[Maine Sail]

Originally Posted by transmitterdan

There is lots of confusion in this thread. Generally batteries are rated at 0.2C which means a 100AH battery is rated at a 20A draw.

This is actually not how it works. Most all marine use batteries are rated at a 20 hour discharge rate or rated Ah capacity ÷ 20 hours. While some may be rated at a 10 hour or 2 hour or 100 hour discharge rate the most common Ah capacity rating, the one most often found on the sticker/label, is the 20 hour rating for ampere hour capacity.

A 100Ah battery would be 100Ah ÷ 20 Hours = 5A load. Any load above 5A shrinks usable battery capacity and any load less than 5A can actually increase usable capacity.

Originally Posted by transmitterdan

So it takes 5 hours to pull 100AH from the battery. If your load is less than 0.2C then the available AH will be more.

If you start with the figure that is not accurate for Ah capacity discharge rate then everything else will also be incorrect.

A 100Ah battery with a Peukert of 1.27 and a discharge rate of 20A becomes an approximately 69Ah battery for usable capacity, not 100Ah. It should deliver its rating, when new and broken in, of 100Ah's with a 5A load at 77F. Any changes in temp, Peukert, or discharge rate will change the usable Ah capacity figures.

Originally Posted by transmitterdan

Some manufacturers "cheat" and specify the AH at a lower amperage but they should still give the 0.2C capacity.

No Ah capacity is; Rated Ah Capacity ÷ 20

The only tests done at a fixed discharge rate, regardless of Ah capacity, are reserve capacity/reserve minute tests. These tests are usually 25A or 75A tests and the minutes of run timed are stated at that discharge rate such as; Capacity Minutes 447 @25A.

A 100Ah battery has a 5A discharge rate at 77F and a 225Ah battery gets the same 20 hour rating of capacity ÷ 20 or 225Ah ÷ 20 = 11.25A discharge rate. A 450Ah bank is still 450Ah ÷ 20 = 22.5A discharge rate. There is no cheating. Either the battery can run for 20 hours at 77F and at the 20 hour discharge rate, or it can't. A .2C discharge rate (20% of Ah capacity) will simply not deliver the Ah rating unless you had an LA battery with a 1.0 Peukert and this simply does not exist. Even in Lithium we have Peukert effect.

Originally Posted by transmitterdan

Good manufacturers provide the capacity at a range of current values so you can see what the AH capacity is for your specific situation. But with LA it's always the case that less current draw means more AH capacity.

Absolutely, and now we can see from the chart below that the discharge rating is not a fixed discharge of .2C or 20% of Ah capacity.


This is a 375Ah battery and 375Ah ÷ 20 Hours = 18.75A discharge rate (this is a 375Ah rated battery). Most boaters with a bank like this are drawing it down at closer to the 100 hour rating, on average, which gives you close to 486Ah's of capacity.

We can easily see the effect of Peukert on this battery. If we were to draw this battery at .2C (75A) it becomes an approx 215Ah battery as opposed to a 375Ah Ah battery at the 20 hour discharge rate, or a 486Ah battery at the lower 100Ah discharge rate.

Originally Posted by transmitterdan

The 50% rule is not "arbitrary". It is based on well reasoned logic and real world testing. I admit it does not take into account the cost of recharging to 100%. It assumes that battery replacement is the dominant cost. It also assumes that cruising makes it harder to replace batteries and thus more expensive. So the 50% rule along with frequent charging to 100% gives many of us 10 year battery life. Especially if we buy quality batteries and maintain them properly.

If you think it is cost effective to buy small batteries and intentionally "murder" them then go ahead. No one will argue with your logic except to say that for most cruisers replacing batteries every year is not our idea of fun as a/v Beth so eloquently stated.

The 50% rule is also not hard and fast and most manufacturers want to see the shallowest cycling design possible.

In a lab the cycles are discharged to cut-off voltage, then immediately charged up to 100% SOC, then down to discharge cut-off voltage then immediately back to 100%, repeat, repeat, repeat.. This is all done in a controlled temp water bath under "ideal" conditions with no breaks. In other words, not at all real world. Pauses between cycles, irregular charging, incomplete charging, low voltage charging, in adequate absorption cycles etc. all cut into the lab cycles and drastically reduce that number once outside the lab.

In the real wold no one ever cycles batteries like they do in the lab so the lab data is pretty useless for us to draw any conclusions from, except perhaps to compare batteries within a brand, for lab cycling durability.

For example if the T-105 (GC2) can deliver 1200 cycles in the lab (to 50%) and the SCS-225 (Group 31) can only deliver 600 cycles (to 50%) which battery do would you expect to be the most durable at cycling? Lab data is a guide point only and then only within a specific brand because different manufacturers often test to different BCI criteria depending upon expected use..

One extremely popular Group 27 "deep cycle" battery can only muster 200 lab cycles to 80% DOD. Out in the real world I have seen them murdered in less than one season and about 20 cycles to 80% DOD. This due to real world use and PSOC abuses.

When boaters cycle to 80% DOD out in the real world, with most LA batteries, they tend to fall off a cliff significantly faster than they do at 50% or 40% DOD. One only has to look to the early AGM marketing to see how much of a change they saw when they started advising 50% as max DOD vs. 80%.

I don't see the lab derived curve to look the same in the non-lab PSOC real world environment. From what I see, in testing of used real PSOC use batteries, it does not match. If an owner expects and accepts replacing batteries every 12 months to 2 years then I have no problem with that. Heck I get some nervous types that want to replace batteries when they are still testing at 90% of rated capacity. As a result I have a lot of premium "loaner" batteries kicking around my shop. Some owners just feel good about replacing batteries before they are really due and some owners push them a bit too far and get into trouble..

[transmitterdan]

Maine,

Thanks for correcting my understanding of the industry AH rating system.