Battery lifetime/cost vs. state of charge

[kmacdonald]

If the life cycle chart above is wrong at 80% DOD cycling then it will be wrong at 50% DOD cycling and the reasoning will still hold true. I would like to see a PSOC life cycle chart from the manufacturer. The life cycle doesn't fall off a cliff at 50% DOD and there is no reason to take it as the "reasonable" discharge max. Where the balance between weight and cycle life falls will be a personal decision. For me, I chose lithium primarily for the weight savings but if I did go with LA I would cycle to 80% DOD.

[kmacdonald]

From Trojan Battery:

Chemical reactions internal to the battery are driven by voltage and temperature. The higher
the battery temperature, the faster chemical reactions will occur. While higher temperatures
can provide improved discharge performance the increased rate of chemical reactions will
result in a corresponding loss of battery life. As a rule of thumb, for every 10°C increase in
temperature the reaction rate doubles. Thus, a month of operation at 35°C is equivalent in
battery life to two months at 25°C. Heat is an enemy of all lead acid batteries, FLA, AGM and
gel alike and even small increases in temperature will have a major influence on battery life.

Deep-cycle batteries used in off-grid and unstable grid applications are heavily cycled at
partial state of charge (PSOC). Operating at PSOC on a regular basis can quickly diminish the
overall life of a battery, which results in frequent and costly battery replacements.
To address the impact of PSOC on deep-cycle batteries in renewable energy (RE), inverter
backup and telecom applications, Trojan Battery has now included Smart Carbon™ as a
standard feature in its Industrial and Premium flooded battery lines.

[Pizzazz]

Dear Main Sail,

Thank you for sharing your experience on the matter. Very good and valid points. I agree that the published cycles are unlikely to be reached in the real world. My point is different though. The question here is, if discharging to 20% only reduces battery life by half, then it is better to have a lower capacity battery bank and cycle it more frequently. It is cheaper and more lightweight both are clear benefits for sailors. If the published "ideal" curves say 1200 cycles for 50% DOD vs. 600 cycles for 20% DOD and we only get 800 cycles vs. 400 cycles in real life, my point is still valid that there is no need of carrying a large capacity battery bank and the 50% rule is not very useful.

Your data seems to confirm this. If cruisers who charge at 12.2V (60% SOC) get 5 years and those who charge at 11.8V (30% SOC) get 2.5 years then it is better to replace the battery bank twice as often. By the way, this matches the curve above nearly exactly.

Another take on your data, I think it is totally acceptable to lose 1% capacity on every cycle if not charging to 100% during offshore passages. Once the passage is complete, the batteries can be equalized and restored to most of their original capacity. How many 30-day offshore passages away from marinas does the average sailor make in 2 years time? Not that many.

I understand that charging to 100% and discharging to 50% (or 60%) is better for the battery. However, onc you delve into the actual numbers it does not make financial sense for occasional cruisers and has negatives in terms of weight and cost (solar, generator, regulators).

SV Pizzazz

[StuM]

Based on posting histories, I certainly know which of the recent previous posters I would choose to trust on this matter.

[kmacdonald]

If you look at the cycle life vs DOD charts it shows that the cycle life drops fastest between 100% and 50% DOD and flattens out somewhat after that. This would suggest that deeper discharging would be cost effective.

[StuM]

Originally Posted by Pizzazz

Your data seems to confirm this. If cruisers who charge at 12.2V (60% SOC) get 5 years and those who charge at 11.8V (30% SOC) get 2.5 years then it is better to replace the battery bank twice as often. By the way, this matches the curve above nearly exactly.

ISTM that you are reading what you want to see in those figures. Let's look at them in more detail:

-86% of responders who deem it time to recharge at a low of 12.2V reported more than 5 years service life with 61% of the 86% responding 6 or more years.

86% got over 5 years:
14% got 5 years or less
34% got 5-6 years.
52% got 6 or more years.
Overall average? Probably at least 6 years, possibly considerably higher.

-94% of responders who deem it time to recharge at a low of 11.8V reported 3 years or less service life with 73% of the 94% responding 2 years or less.

94% got 3 years or less:
6% got over 3 years
25% got 2-3 years.
69% got 2 years or lessOverall average? Almost certainly less than 2 years.

That's got to mean replacing batteries on average at least 3 times as often.

Another take on your data, I think it is totally acceptable to lose 1% capacity on every cycle if not charging to 100% during offshore passages. Once the passage is complete, the batteries can be equalized and restored to most of their original capacity.

Depends on your circumstances - how long and how often you spend time on passage or on the hook.

And frequently, the battery will NOT be restored to its original capacity by any equalization regime - it's a permanent loss.

[StuM]

Originally Posted by kmacdonald

If you look at the cycle life vs DOD charts it shows that the cycle life drops fastest between 100% and 50% DOD and flattens out somewhat after that. This would suggest that deeper discharging would be cost effective.

Quite the opposite. It suggests that shallower discharging would be more effective. That's why I try to keep my batteries above 75% as much as possible.

You do realise that the Y axis on such graphs are logarithmic, not linear?

[kmacdonald]

Surveys are useless here. DOD is not the only factor in battery life. If we isolate DOD as the only variable and look at DOD vs cycle life charts, with all other variables being equal, it is clearly more economical to deep discharge LA batteries. There is no gray area here.

[kmacdonald]

Originally Posted by StuM

Quite the opposite. It suggests that shallower discharging would be more effective. That's why I try to keep my batteries above 75% as much as possible.

You do realise that the Y axis on such graphs are logarithmic, not linear?

Wrong. Get your facts straight. Here is the link: http://www.trojanbattery.com/pdf/dat...ata_Sheets.pdf

The chart is not logarithmic so maybe that will help you understand a little better.

[Pizzazz]

It is interesting how people can look at the same data and come up with different conclusions. I think the facts are clear that the slope of the curve is decreasing which means it is more cost effective to deep discharge and replace batteries more often. Even if it is 2 years vs. 6 years (based on Stu's interpretation of the survey), it is not a big deal to replace the batteries.

Many boaters can benefit from this analysis and save weight and space by having a smaller wet cell battery bank. Some would still prefer to carry 75% dead weight and that is also a valid choice if space, displacement and money permits. Let's go sailing now

SV Pizzazz

[conachair]

Originally Posted by Pizzazz

I think we all agree that charging above 80% is costly, inefficient and not practical at sea).

We all do not agree
Wind and sun is free.

[ErikFinn]

Originally Posted by Pizzazz

Another take on your data, I think it is totally acceptable to lose 1% capacity on every cycle if not charging to 100% during offshore passages. Once the passage is complete, the batteries can be equalized and restored to most of their original capacity.

No. It does not work like that. Equalizing will not restore bats to "most" of their original capacity after extended abuse. It will help a little bit but your original capacity will have been permanently lost and its gone forever.

But if it works for you then go for it.

Cheers

[StuM]

Originally Posted by Pizzazz

Some would still prefer to carry 75% dead weight and that is also a valid choice if space, displacement and money permits. Let's go sailing now
SV Pizzazz

To me it's not 75% dead weight. It's reserve, just like the diesel, water and food I carry on a long passage. I don't plan on running any of them down to a minimum, but they're there when I need them.

While I try to keep above 75% SOC, I am quite prepared to use another 25% intermittently and know that I've got at least another 25% still there if I lose all charging sources.

Habitually discharging to 80% DOD leaves you little or no reserve when your engine dies and you still need your radio, instruments, nav lights etc.

[transmitterdan]

The only data that should be given weight in this discussion is empirical results using the OP's proposed usage profile. Lots of people have unintentionally tried this concept and gotten short battery life. That prompted experts in the field such as Lifeline, Morgan's Cloud, Maine Sail and Nigel Calder to do detailed research. From that came the recommend 50% recharge point and frequenct 100% top ups.

If you want to follow the advice of some Internet junky relying on charts based on a completely different charge regimen to "prove" these experts are wrong then I think you can guess what will happen. But if you think the experts are wrong please do the scientific research and publish the findings. I suspect the proposed charge/discharge scheme will take no more than 2 years to prove the experts are right and the proposed scheme is a money losing proposition.

[kmacdonald]

trans, the experts in the field are the battery manufacturers with multi-million dollar R&D budgets. Trojan has published actual data. Your listed sources have given non-scientific anecdotal observations without controlling other variables in battery life. Could it be that anything beyond a binary observation is over your head?

Have you considered that the max 50% DOD advice is marketing driven? If you follow that advice you will buy twice the battery capacity. You will be cycling your batteries in the worst section of the cycle life vs DOD curve. You will be carrying twice the weight. You will be cycling the batteries in the most expensive energy delivered scenario. Your batteries will last longer leading you to think you have great batteries and when the time comes for new ones you will buy the same brand again and repeat the process. That's right, shoot yourself in the head.

Keep the batteries out of the engine compartment, charge them to 100% SOC three or four times a week, and equalize monthly. If you can do that the 80 to 90% DOD cycling will work well from an economic standpoint and weight savings standpoint. There are other factor to consider and will vary by intended use and charging sources but for the most part the 50% cycling fairy tale is BS.