Battery Cycles and Depth of Discharge Graphs

[bdosborn]

I just found a wealth of information regarding batteries, it's heavily slanted towards Trojan but it's still great information:

Trojan Batteries for Renewable Applications

And some cool graphs!

The one below really puts depth of discharge versus number of cycles into perspective. You can see why the battery manufacturers would much prefer you cycle them down only 20% DOD. The number of cycles goes down from 3000 to around 1200 if you cycle down to 50% instead of 20% DOD. This also shows how a solar panel can really extend your battery life by minimizing your DOD each day.



Here's an interesting graph for the next time someone asks about how long they can run a big inverter load. I didn't realize how much battery capacity varies inversely with the size of the load. That's Peukert's law showing itself.



And finally, here's why I moved my batteries inside the trailer where they wouldn't see big temperature swings, both cold and hot. The ideal battery temperature is 80F, anything lower reduces capacity and anything above reduces battery life.



Batteries sure are fragile for the 60 pound lump of lead that they are .

Bruce

[astrotrailer]

I have the solar on the roof so I never discharge more than one nights use on the batteries. This keeps the discharge low and the life of the battery long. The solar also keeps the batteries at optimal charge levels 12 months a year so they don't sulfate the plates. I just add a bit of distilled water to my pair of Trojan T-105 batteries every few months as needed. I prefer flooded batteries because you can add distilled water back to the battery unlike gel or AGM.

I work at a university in networking. I see a lot of larger AGM UPS batteries fail do to complete discharges under several hour long power outages. If we have enough long outages we end up replacing batteries after 3 years. The UPS batteries that have generator backup last much longer because they never have to discharge more than about a minute before the generator takes over the load.

[gdpipkorn]

Struggling with this very issue right now. Thank you for this valuable info.

[Shadow Catcher]

One thing not addressed is batteries that for all intents are not cycled. Only a few times has our 150AH been cycled even to 80% DOD and only once has it been down to 64% SOC.

[afreegreek]

total

[MtnDon]

total life of most deep cycle batteries is optimal at 50% DOD but still better at 80% DOD than 20%...

say a battery is 100 amp hours..

20% DOD @ 3000 cycles (3000 x 20 AH = 60,000 AH)
50% DOD @ 1500 cycles (1500 x 50 AH = 75,000 AH)
80% DOD @ 800 cycles (800 x 80 AH = 64,000 AH)

no sense in doubling the size of a battery bank to avoid a DOD greater than 20% when you can go to 80% DOD with a half as big, half as heavy, half as expensive battery bank that will give more hours of use over it's life....

I totally disagree with those statements. In the realm of off grid, PV powered lead acid storage battery based systems, longer battery life is enjoyed, with less total $$ spent for batteries, when the battery bank is sized to keep the DOD in the 25 to 50% maximum range on a day in day out basis. That is assuming that most days the battery bank gets recharged fully.



With a trailer sometimes a trade off of shorter lifespan for fewer batteries / less weight could be considered advantageous I suppose. But it is a trade off.

[afreegreek]

you

[MtnDon]

Maybe I am misinterpreting what you wrote.

I have read your post several times. When I read the last paragraph I understand you to mean that a deeper discharge is better ... "go to 80% DOD ..... .... will give more hours of use over it's life...."

If that is what you mean that flys in the face of what is generally accepted as truth. The depth of discharge chart in the OP above clearly indicates a lesser depth of discharge results in more cycles. As does the Trojan chart on this page.

You seem to be equating the theoretical amp-hour total as the criteria that make it better to discharge more deeply. I've never seen anyone calculate in that manner.

I am counting cycles not the amp or watt - hours of power withdrawn from the batteries.
~~~~~~~~~~~~~

From a Trojan document. "Do not discharge your battery more than 80%. This safety factor will eliminate the chance
of over-discharging and damaging your battery"

This article states, "When using lead-acid batteries, one of the most common fatal mistakes is to discharge one's battery bank too deeply, causing permanent damage that impacts performance and reduces the life of the batteries"

And this site states "Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. "

Trojan has a battery bank sizing calculator here. It uses DOD figures of 10% to 50%. It goes no higher than that.

That all indicates that it is better to cycle batteries somewhere between a DOD of 20 to 50% and to do your best to never discharge as deep as 80%. If that is what you mean I somehow have trouble understanding your writing and I apologize for that. However, if you mean we would all be better off to regularly discharge our lead acid batteries to 80%, I believe you are incorrect.

[afreegreek]

Maybe I am misinterpreting what you wrote.

I have read your post several times. When I read the last paragraph I understand you to mean that a deeper discharge is better ... "go to 80% DOD ..... .... will give more hours of use over it's life...."

If that is what you mean that flys in the face of what is generally accepted as truth. The depth of discharge chart in the OP above clearly indicates a lesser depth of discharge results in more cycles. As does the Trojan chart on this page.

You seem to be equating the theoretical amp-hour total as the criteria that make it better to discharge more deeply. I've never seen anyone calculate in that manner.

I am counting cycles not the amp or watt - hours of power withdrawn from the batteries.
~~~~~~~~~~~~~

From a Trojan document. "Do not discharge your battery more than 80%. This safety factor will eliminate the chance
of over-discharging and damaging your battery"

This article states, "When using lead-acid batteries, one of the most common fatal mistakes is to discharge one's battery bank too deeply, causing permanent damage that impacts performance and reduces the life of the batteries"

And this site states "Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. "

Trojan has a battery bank sizing calculator here. It uses DOD figures of 10% to 50%. It goes no higher than that.

That all indicates that it is better to cycle batteries somewhere between a DOD of 20 to 50% and to do your best to never discharge as deep as 80%. If that is what you mean I somehow have trouble understanding your writing and I apologize for that. However, if you mean we would all be better off to regularly discharge our lead acid batteries to 80%, I believe you are incorrect.

#1

[afreegreek]

here

[MtnDon]

NAWS is a very informative site. It would be a very good place for any beginner to start looking into alternative energy and energy storage. The quote from NAWS seems to reinforce the commonly held view that it is best to operate a battery bank at lesser rates of depth of discharge, not greater rates.

I understand your method of calculating amp-hours or goo or whatever. You do not need to re-run your math. Your math is, IMO, a theory; a theory that runs against the commonly held belief that prevails in the alternative energy world when specifically working with lead acid batteries. Theory needs to be proven by experimentation. I trust you are documenting the discharge and charge cycles of your battery bank. You might be onto something.

I guess we could say I have my own experiment running. Twelve GC-2 batteries that have been in service since July 2009. And still behaving very well. Daily discharge runs in the neighborhood of 20% maximum. I won't bore you with any more details about that, other than to state that my experience goes along with what would be expected when following the present day conventional wisdom regarding the care and use of a flooded deep cycle battery.

Happy first day of a new year.

[tony.latham]

It's -11º outside this morning and my teardrop battery is reading 12.4V. My 20 watt panel that's hooked to it (through the Morningstar controller) isn't in the sun for more than three hours and of course it's flat on the tongue box. In a bit, I'm going to pull out my folding 60w panel and hook it up too.

Is that 12.4V because of the bitter cold or the fact that it's not fully charged???



Next winter I'll park her with a bit more of a southern heading.

Tony

[MtnDon]

Dang, that is cold. Reminds me of back home. Brrr!! The cold weather will reduce the available amp-hours. At -11 F the capacity loss will be about half. However the voltage reading will be as accurate as the voltmeter is. My trailer batteries are at 20 F right now, according to the temperature probe taped to one of them, and the volts are showing 12.6. It has been sitting for a couple weeks not plugged in and no solar, no loads. FWIW, at 20 F I'd expect the amp-hour capacity to be down almost 30%.

[tony.latham]

Thanks Don!

I've got the 60 watter plugged in and facing the sun. I'll be curious to read the voltage tonight.

T

[afreegreek]

the