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Old 10-11-2016, 13:15   #1
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Battery lifetime/cost vs. state of charge

It is often said on this forum that it is advisable to keep wet cell batteries above 50% state of charge to ensure maximum life. I have been analyzing my battery setup and came to the conclusion that it is cheaper to abuse the batteries than to follow the advice above.

The logic is that if we discharge the batteries to 20%, they will last half as long (650 cycles vs. 1200 cycles) but will cost half as much for the same capacity (from 80% to 20% SOC vs. from 80% to 50%; I think we all agree that charging above 80% is costly, inefficient and not practical at sea).

So why do people feel so religious about the 50% SOC cut-off? This logic also defies the case for lithium batteries. I can see many benefits to "abusing' the batteries down to 20%: you carry half the weight, bulk charging from 20% to 80% is nearly 100% efficient, i.e. the batteries take in all the current from the alternator and so on. The downside is that the batteries need to be replaced every two years vs. four years (heavy), probably more maintenance (checking the water level, etc.) and slightly higher risk that one of the cells will short circuit in the most inopportune moment (we have the starter battery for that). Any thoughts?

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Old 10-11-2016, 13:40   #2
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Re: Battery lifetime/cost vs. state of charge

From where do you get the idea charging above 80% is "costly, inefficient not practical"?
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Old 10-11-2016, 13:43   #3
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Re: Battery lifetime/cost vs. state of charge

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From where do you get the idea charging above 80% is "costly, inefficient not practical"?

He is assuming I believe charging using Diesel.
He has a point but I don't believe the only cutting life in half part
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Old 10-11-2016, 13:59   #4
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Re: Battery lifetime/cost vs. state of charge

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From where do you get the idea charging above 80% is "costly, inefficient not practical"?
This is based on using the main engine to charge the batteries. Running the engine to produce only 15-25 amps for absorption charging is inefficient (you might as well wait a bit longer and then do bulk at 80 amps in one third of the time - hence my post). Solar only makes sense if you have above 300W which is difficult (and ugly) to place on a sailboat. Plus, I usually have 5 amp draw anyway (fridge, pilot), so solar is very slow. For me solar is just a safety feature in case the engine dies. I have not experimented with wind but the not pretty factor is high. Thus, I do believe that absorption charging at sea is hard to do and involves compromises.
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Old 10-11-2016, 14:45   #5
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Re: Battery lifetime/cost vs. state of charge

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

Probably depends. If we run the genset while underway, at least for a while, we can also be charging at bulk/absoprtion rates.

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Old 10-11-2016, 15:07   #6
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Re: Battery lifetime/cost vs. state of charge

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Solar only makes sense if you have above 300W which is difficult (and ugly) to place on a sailboat.
First, not sure where the 300 watt minimum comes in, but I have 260 watts of flexible solar panels sewn into the fabric of my bimini. You can't see the panels from the water and hardly notice the necessary patches when in the cockpit. Hardly an ugly alternative (unless you hate biminis in general). The solar panels won't get me to float stage while underway with the autopilot humming along, but once anchored with some sun, I'm topped off.

But on your more general point, I don't think there is much to argue. In fact, I suspect most battery banks are treated just as how you describe. When folks say to get back to 100%, they certainly recognize the fact that using your main engine to do so is terribly inefficient. That's why they follow it up with the suggestion to install solar or top off via shore power or generator once a week.

My quick calculations for fun...

1,200 total cycles, 300 cycles per year, full recharge every week, so 208 full recharge cycles. IF my new bank costs $900 (6 T-105s) and IF the abused bank gets 600 total cycles, my breakeven cost per full recharge cycle is $4.30. That is, if I can fully recharge for $4.30 or less every 4th cycle, I am better off doing so, otherwise, I should abuse the bank and replace every 600 cycles.

A lot of silly assumptions in there, but recharging based only on the main engine probably won't be cost efficient. Will take maybe 1.5 gallons of diesel ($5?), not to mention the additional wear.

My solar set up cost $800 in total and should last 12 years, so, say, $1 a full recharge cycle and I'd probably get fully recharged much more often than once a week. Suggests I should take care of the bank, even if I have to use the engine/generator from time to time on cloudy days.
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Old 10-11-2016, 15:34   #7
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Re: Battery lifetime/cost vs. state of charge

20-80% SOC is not "abusing" lithium batteries. But it is abusing FLA.

Charging above 80% is a not "costly, inefficient, not practical" if you use solar, wind or motor a lot. Only if you rely on a genset or running an engine at anchor.

Apart from the reduced life, you will very quickly get reduced capacity and charging efficiency as the 80% charged batteries start to sulphate.

I also doubt your 2 versus 4 year figure. Properly looked after you should get a lot more than 4 years out of a good set of LA batteries.
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Old 10-11-2016, 15:36   #8
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Re: Battery lifetime/cost vs. state of charge

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Originally Posted by Pizzazz View Post
It is often said on this forum that it is advisable to keep wet cell batteries above 50% state of charge to ensure maximum life. I have been analyzing my battery setup and came to the conclusion that it is cheaper to abuse the batteries than to follow the advice above.

The logic is that if we discharge the batteries to 20%, they will last half as long (650 cycles vs. 1200 cycles) but will cost half as much for the same capacity (from 80% to 20% SOC vs. from 80% to 50%; I think we all agree that charging above 80% is costly, inefficient and not practical at sea).

So why do people feel so religious about the 50% SOC cut-off? This logic also defies the case for lithium batteries. I can see many benefits to "abusing' the batteries down to 20%: you carry half the weight, bulk charging from 20% to 80% is nearly 100% efficient, i.e. the batteries take in all the current from the alternator and so on. The downside is that the batteries need to be replaced every two years vs. four years (heavy), probably more maintenance (checking the water level, etc.) and slightly higher risk that one of the cells will short circuit in the most inopportune moment (we have the starter battery for that). Any thoughts?

Regards,
SV Pizzazz
If one could operate lead acid batteries between 20% and 80% charge your concept would be right. The ownership cost would be less than running from 50% to 80%.

But the fallacy in that argument is that you can actually stop charging at 80% with no effect on lifetime. Unfortunately this assumption is not correct. If you only charge lead acid batteries to 80% they will quickly develop sulfates and the capacity will diminish fairly quickly. It is common that batteries only charged to 80% of capacity die in less than 1 year. This is especially so for AGM batteries. Flooded batteries (and some sealed types) can be "equalized" to restore some of the lost capacity but that process takes many hours of charge time under well controlled conditions.
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Old 10-11-2016, 16:01   #9
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Re: Battery lifetime/cost vs. state of charge

There is no fallacy. The charging above 80% is a separate issue (not dealt with in this post) also affecting battery life but it is common to both setups. You charge to 100% when you have access to shore power or when extensive motoring is called for.
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Old 10-11-2016, 16:40   #10
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Re: Battery lifetime/cost vs. state of charge

I don't know where you got the 650 vs 1200 cycle idea. All of the information I've ever seen is that the curve is nonlinear the other way--more like 400 cycles vs 1200 cycles.
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Old 10-11-2016, 16:51   #11
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Re: Battery lifetime/cost vs. state of charge

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Originally Posted by Pizzazz View Post
There is no fallacy. The charging above 80% is a separate issue (not dealt with in this post) also affecting battery life but it is common to both setups. You charge to 100% when you have access to shore power or when extensive motoring is called for.
You clearly have your mind made up. So just do it and please report back when you have to replace your batteries.
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Old 10-11-2016, 16:54   #12
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Re: Battery lifetime/cost vs. state of charge

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There is no fallacy. The charging above 80% is a separate issue (not dealt with in this post) also affecting battery life but it is common to both setups. You charge to 100% when you have access to shore power or when extensive motoring is called for.
If you run 20%-80% a lot I suspect you will get something like 6 months life. Try it and let us know how it works out.
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Old 10-11-2016, 17:30   #13
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Re: Battery lifetime/cost vs. state of charge

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I don't know where you got the 650 vs 1200 cycle idea. All of the information I've ever seen is that the curve is nonlinear the other way--more like 400 cycles vs 1200 cycles.
I am going by this chart from a manufacturer. You are right that for small AGM batteries, the curve says even less 300 cycles at 80% discharge. However, for wet cell golf card batteries, it says 675. I did not mean to be steadfast, I just wanted to separate a popular belief from the facts. My batteries are four years old, may be due for a replacement so I will definitely experiment with the idea.
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Old 10-11-2016, 18:14   #14
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Re: Battery lifetime/cost vs. state of charge

It really makes sense to use more than 30% capacity of lead acid batteries in a boat installation. The weight savings is all important.

I believe the life cycle chart above more than the pseudo experts here. Stick with the facts and avoid the noise when making rational decisions.
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Old 10-11-2016, 18:32   #15
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Re: Battery lifetime/cost vs. state of charge

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I am going by this chart from a manufacturer. You are right that for small AGM batteries, the curve says even less 300 cycles at 80% discharge. However, for wet cell golf card batteries, it says 675. I did not mean to be steadfast, I just wanted to separate a popular belief from the facts. My batteries are four years old, may be due for a replacement so I will definitely experiment with the idea.
The problem with those numbers is that they are derived from "lab cycling" tests which are not at all realistic out in the real world. They are Walt Disney level fairy tale numbers. Those cycling numbers are essentially misleading lies unless you plan to operate your batteries in a white coat, white glove laboratory.

RV, off-grid and marine use is some of the most abusive battery use there is, with marine usually taking the solid abuse edge over the other two. Sure floor scrubbers, fork trucks & other industrial batteries are deep cycled (most to 50%-60% DOD) but they are plugged in every night and receive a 100% SOC charge before the next cycle. They are also equalized and maintained on a regular schedule and these are not your typical Group 24, 27, 31, 4D or 8D type of batteries that are not really designed from the ground up for deep cycling..

Even in industrial type of use, with daily 100% SOC recharges, the batteries rarely if ever approach the lab cycle life data. Without the daily 100% SOC recharges it would be needle in a hay-stack odds at approaching lab numbers.

When you get out into the real world of deep cycle partial state of charge cycling use (PSOC), most lead acid batteries are murdered very, very quickly by deep discharges. Regular 70-80% DOD deep discharges stacked on-top-of PSOC use is about as murderous as it gets for most lead acid batteries due to the effects of sulfation.

During PSOC use the usable capacity quickly "walks down" and each PSOC cycle requires longer charging intervals to attain the same usable capacity you had on cycle 1 or 2. PSOC walk down begins as early as just one to two cycles off a full charge.

In PSOC testing I have seen some batteries permanently lose as much as 1% of rated and tested baseline as-new Ah capacity for each PSOC cycle. This is 30% of the Ah capacity permanently gone in just 30 days of 80% DOD PSOC cycling with partial recharges to approx 80% SOC.

Until the BCI (Battery Council International) implements a PSOC rating & testing system, for all deep cycle batteries, and the industry accepts it, then the lab data means about as close to nothing as it gets, for real world marine use.

If you want to go to 80% DOD regularly then you'll want very frequent full recharges between cycles to get any sort of respectable cycle life. Just look to the AGM makers, who initially launched their batteries as capable of being regularly cycled to 80% DOD. They now strongly advise not cycling any deeper than 50% because 80% DOD was murdering their reputations.

Only one AGM maker today adamantly recommends 80% DOD as the regular discharge level and that is the Firefly Carbon Foam battery. It actually works but is not at all an inexpensive battery. Northstar TPPL AGM's and Odyssey TPPL AGM's "market for" 80% DOD cycling but they strongly advise 50% as a max DOD for cycling use..

There are very few lead acid batteries that will yield respectable cycle lives when PSOC cycled to 80% DOD. The most tolerant would be GEL's (Sonnenschein Prevailer & the East Penn GEL) and the Firefly Carbon Foam AGM.

I would not advise taking any other marine use lead acid battery to 80% DOD on a regular basis unless you can tie to a dock after each discharge or ensure daily or at least bi-daily 100% SOC recharges.

In a 2015 survey conducted by Sailboatowners.com, where there were over 1300 responses for a Marine Battery Use Survey, the take away on deep discharges was this:

-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.

-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.


Sure, for the occasional week or shorter off-shore passage, dipping to 80% DOD can actually make a bit of sense. Bulk charging is very, very efficient so you'll get more usable capacity for the short duration engine runs. The key here is that you don't want to do this all the time, only when necessary and quickly followed up with a 100% SOC recharge. As long as the off-shore deep discharges can be very quickly followed up by a 100% recharge, and an equalization, if the batteries are designed for this, you can do marginally okay dipping to 80% DOD on occasions where it may be necessary. Dipping to 80% DOD for regular PSOC use will drastically shorten cycle life.
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