Battery lifetime/cost vs. state of charge

[StuM]

Originally Posted by kmacdonald

Yes, that is what the Trojan Chart tells us, within it's limits of course. Again, I have not verified the accuracy of the Trojan Chart but I am not going to dispute it without hard evidence.

No, the Trojan chart tells us how how many cycles to expect under one set of test conditions i.e. discharged under a certain constant load to a particular DOD and then immediately recharged to 100% using an optimal charging regime. It bears no resemblance to real life conditions and tells us nothing about how many Ah in total you can put a battery under other conditions.

[kmacdonald]

Lets look at what you say Stu.
1) rate of discharge is irrelevant in this discussion. It will depend on the particular use but not on DOD. In other words it will be the same for any DOD.
2) Ambient temp will not depend on DOD.
3) The charging regime will be the same and not dependent on DOD
4) Time in PSOC may or may not vary much.

Let's try to keep our eye on the ball.

[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.

The above really sums it up.

[kmacdonald]

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.

I agree.

[kmacdonald]

Originally Posted by tanglewood

Another way to look at this, and a lot of battery people do look at it this way, is that any given battery has a fixed number of life-time Ahs of power that can be stored and retrieved. The cycle life vs DOD charts plot this. You can utilize those life-time Ahs in lots of smaller chunks (50% DOD), or fewer large chunks (80% DOD). If you take the cycle life charts and convert it into a chart plotting life-time Ahs (or kwh) against DOD, it's a remarkably flat curve, save for the extreme ends.

That is true. That's the way RE folks look at it.

[kmacdonald]

Remember, this thread is about battery cost vs DOD. It's not about what "kills" batteries or how to charge them. Focus gentlemen focus!!!!!!!!!

[mitiempo]

Originally Posted by kmacdonald

Remember, this thread is about battery cost vs DOD. It's not about what "kills" batteries or how to charge them. Focus gentlemen focus!!!!!!!!!

What kills batteries in the real world - not in a lab or on paper - is extreme DOD. As has been stated in several posts by Mainesail (#122 for example) the greater the depth of discharge the more accelerated the loss of AH.

The only manufacturer comparison that I believe to be valid is cycle testing of one battery vs another from the same company. The numbers are exaggerated by a large amount even here but the comparison model to model is accurate.

[StuM]

Originally Posted by mitiempo

What kills batteries in the real world - not in a lab or on paper - is extreme DOD. As has been stated in several posts by Mainesail (#122 for example) the greater the depth of discharge the more accelerated the loss of AH.

Exactly - which results in less total usable Ah over the battery's life time.

Unfortunately, our swinging, rafting expert is again going by what he reads on the internet rather than real world experience.

[conachair]

Originally Posted by kmacdonald

Remember, this thread is about battery cost vs DOD. It's not about what "kills" batteries or how to charge them. Focus gentlemen focus!!!!!!!!!

Focusing on the real world, how does your fridge and the rest of the kit onboard cope with the low voltages when the battery is discharged so much?

[kmacdonald]

It's amazing how such simple concepts present such a challenge to so many. Good thing I am so tolerant.

[StuM]

Originally Posted by kmacdonald

It's amazing how such simple concepts present such a challenge to so many. Good thing I am so tolerant.

I'm starting to get the feeling that km is actually an A grade stirrer with his tongue firmly in his cheek and that he doesn't actually believe anything he writes.

That's about the only explanation I can think of.

[kmacdonald]

Originally Posted by StuM

I'm starting to get the feeling that km is actually an A grade stirrer with his tongue firmly in his cheek and that he doesn't actually believe anything he writes.

That's about the only explanation I can think of.

If only the facts didn't get in way.

The simple answer to the OP's question is discharge anywhere from 20 to 80% DOD. Battery cost wise it makes little difference.

Good morning Stu.

[valhalla360]

Originally Posted by kmacdonald

Remember, this thread is about battery cost vs DOD. It's not about what "kills" batteries or how to charge them. Focus gentlemen focus!!!!!!!!!

Except killing the batteries quickly with fewer overall amp-hrs does cost more.

The charging/discharging pattern have a major impact on the overall amp-hrs a bank will produce.

Reality is bigger banks produce more lifetime amp-hrs per individual battery within reasonable ranges.

[kmacdonald]

Originally Posted by valhalla360

Except killing the batteries quickly with fewer overall amp-hrs does cost more.

The charging/discharging pattern have a major impact on the overall amp-hrs a bank will produce.

Reality is bigger banks produce more lifetime amp-hrs per individual battery within reasonable ranges.

The charging pattern does have a major impact on battery life but DOD within reasonable (20 - 80% DOD) limits does not. It's nice to see people cling to their long term beliefs despite the facts. Speaks volumes for their integrity and tenacity.
Just out of curiosity, where do you draw the line on battery bank size?

[valhalla360]

Originally Posted by kmacdonald

The charging pattern does have a major impact on battery life but DOD within reasonable (20 - 80% DOD) limits does not. It's nice to see people cling to their long term beliefs despite the facts. Speaks volumes for their integrity and tenacity.
Just out of curiosity, where do you draw the line on battery bank size?

Oddly if you look at the graph on post #13, it certainly does impact expected battery life based on DOD within the 20-80% limits (assuming 100amp-hr flooded battery)

20%DOD ~ 3300 cycles 66,000amp-hr
50%DOD ~ 1150 cycles 57,500amp-hr
80%DOD ~ 675 cycles 54,000amp-hr

That sure looks like you get fewer amp-hr out when you deeply discharge. Of course, when you account for duration at deep discharge levels, I would expect even further losses.

So I'm not clear why you are clinging so dearly to your beliefs when the data doesn't support your position and then you go on about how it shows your true integrity?

As far as your curiosity, I size the battery bank based on needs. Haven't had to mess with it on the new boat but the last boat, I swapped out 4 - 6v golf cart batteries for a single 12v. Of course, the fridge ran on propane when away from the dock, so 12v use was negligible for lights, water pump and occasionally some fans.. Never dropped below 75% with just the solar panels and you could pull start the motor easily, so no worries about having significant battery reserves.

If you get to the point where you can't fit in the battery bank at 50% DOD, you probably need to either reduce consumption or switch to a generator as you probably don't have room for the solar array needed to support it.