# Is there any approximation to know how many will last my battery?



## mnikhilesh (Jan 10, 2013)

FriedChip said:


> The data provided is that they can last up to 2,000 cycles discharging them *@0.3C* and to *80% of DoD *(Calb cells). I am planing discharging them lets say 80 times a day at a 4C rate but just for few seconds (17seconds) and recharging them at 0.5C till the initial SOC. Is there any way to approximate how many will they last? The cycle is about 2% of the total energy in the battery. Also what initial SOC will you fix them at? Thank you.
> 
> Note: It's for other application different to electric vehicle.


If the discharged caacity is only 2% of total capacity it will be almost micro cycles and battery should easily last 100,000 of same but your discharging current is quite high @4C, CALB cell is designed for 2C continuous and 4C will be severe for them but again only 17 seconds should not do any harm. As for calculation there is no direct way to calculate the number of cycles but a linear calculation of (80%/2%)*2000 = 80,000 is good to start with. Lithium cells generally gives better cycle count with lower DOD i.e. if it's giving 2000 cycles @ 80% DOD it will give more than (80/70)*2000 = 2285 cycles @ 70% DOD. Some company quotes 3000 cycles @ 70% DOD. But again it all depends on several factors like discharge rate and specially woking temperature. Lithium technology is now so mature that some company have started quoting temperature life instead of cycle life i.e. 2000 cycles @ 30 degree centigrade compared to 3000 cycles @ 25 degree centigrade (just for illustration). My point is in your use case life will more depend on working temperature than cycle counts.
As for initial SOC anything in the range of 3.25 volt to 3.32 volt should do the job. The main idea is here to keep cells in the flat voltage range not in the initial high voltage and not in last low voltage range. For long term storage these cells are stored at 60 % SOC so I think it will suit your need.

Regards
Nikhilesh Mishra


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## FriedChip (Apr 29, 2013)

Hi Nikhilesh Mishra, thank you for such a detailed answer. Without considering the discharge rate as you say it's not going to do any harm, i was thinking in getting out of it a bit more cycles. According to the next graphic.

My principal worry was if this was true even at different discharge rates but I assume that my clycle is not very severe as you say. Also, I should consider the calendar lyfe but I am not considering this at the moment. Again, thank you for the information given.


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## mnikhilesh (Jan 10, 2013)

FriedChip said:


> Hi Nikhilesh Mishra, thank you for such a detailed answer. Without considering the discharge rate as you say it's not going to do any harm, i was thinking in getting out of it a bit more cycles. According to the next graphic.
> 
> My principal worry was if this was true even at different discharge rates but I assume that my clycle is not very severe as you say. Also, I should consider the calendar lyfe but I am not considering this at the moment. Again, thank you for the information given.


I will recommend consulting some expert before going 4C, though it seems it should not do any damage but better be sure cause 4C seems little high to me. Also I was thinking if you are going for 4C why not consider Li-ion cell instead of Li-polymer (if you have not procured cells yet). Boston power can easily give you 4C and they quote better cycle life. (FYI i am using CALB CA40 cells in my converted vehicle.)


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## FriedChip (Apr 29, 2013)

mnikhilesh said:


> I will recommend consulting some expert before going 4C, though it seems it should not do any damage but better be sure cause 4C seems little high to me. Also I was thinking if you are going for 4C why not consider Li-ion cell instead of Li-polymer (if you have not procured cells yet). Boston power can easily give you 4C and they quote better cycle life. (FYI i am using CALB CA40 cells in my converted vehicle.)


Here says they can deal with 10C during 10 secs: http://www.i4wifi.cz/img.asp?attid=255162
I can't find were to buy the ones you recommended to me. Do you know an orientative price? Thank you!


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## mnikhilesh (Jan 10, 2013)

Which country you are from, they have reseller in several countries. The price I saw somewhere for Sonata 4400 cells are
below 5,000pcs (FOBHK): USD5.5;
​ 5,000 - 10,000pcs (FOBHK): USD5
above 10,000pcs (FOBHK): USD4.5
SPQ: 84 pcs per box.


​


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## FriedChip (Apr 29, 2013)

mnikhilesh said:


> Which country you are from, they have reseller in several countries. The price I saw somewhere for Sonata 4400 cells are
> below 5,000pcs (FOBHK): USD5.5;
> ​ 5,000 - 10,000pcs (FOBHK): USD5above 10,000pcs (FOBHK): USD4.5
> SPQ: 84 pcs per box.
> ...


Nice price. I come from Europe. Also I have seen some interesting battery modules, but I guess this is high priced. Interesting alternative, thank you for the masterclass. 

P.S. You said they quote a better cycle life but I read in the datasheet just 800 cycles? Am I missing something? Maybe it is @100% DoD?


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## mnikhilesh (Jan 10, 2013)

For cell they quote 3000 cycles @ 80% DOD. The 800 cycles you are seeing should be for 100% DOD (I remember that being 1000 but that may be with swing 5300) but keep in mind the number of cycles for individual cell and battery pack are quite different.


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## FriedChip (Apr 29, 2013)

mnikhilesh said:


> For cell they quote 3000 cycles @ 80% DOD. The 800 cycles you are seeing should be for 100% DOD (I remember that being 1000 but that may be with swing 5300) but keep in mind the number of cycles for individual cell and battery pack are quite different.


Sorry but why are they different?  I don't see that...


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## mnikhilesh (Jan 10, 2013)

If all cells are made equal and they are put in exactly equal conditions theoretically they should all reach same cycle count but in reality not all cell are exactly equal because of manufacturing inequalities (very minute but its there) also every cell see different internal resistance and temperature et. Any cell reaching the extreme (low side of voltage or high side of voltage) they wear out fast and ultimately the weakest cell ruins whole pack. A good manufacturing process, good pack of very similar cells and a good BMS will extend the life but no company gives warranty of that so they quote really low cycle count for battery pack. This is the simplest explanation, though there can be more reasons.


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## dougingraham (Jul 26, 2011)

FriedChip said:


> The data provided is that they can last up to 2,000 cycles discharging them *@0.3C* and to *80% of DoD *(Calb cells). I am planing discharging them lets say 80 times a day at a 4C rate but just for few seconds (17seconds) and recharging them at 0.5C till the initial SOC. Is there any way to approximate how many will they last? The cycle is about 2% of the total energy in the battery. Also what initial SOC will you fix them at? Thank you.
> 
> Note: It's for other application different to electric vehicle.


A 4C discharge for 17 seconds will use about 1.9% of the capacity of the battery. The recharge time will be a little more than 136 seconds at 0.5C. If you keep the state of charge away from the ends, above 20% and below 80% you are probably looking at calendar life for useful life. The capacity would need to drop below 2% of new before it would fail to perform the mission so the usual cycle rating is not meaningful to you. Even if you assume the wrost case that 40 of your cycles is equivalent to one 80% DOD discharges that would mean you would be doing the equivalent of two 80% DOD cycles per day and thus at least 1000 days of cycles before you reached 80% of original capacity. Making another worst case assumption that each 1000 days of use like this would further reduce the original capacity by an additional 20% means you can do this 4 times total for 4000 days of cycles (11 years). Since there is evidence that if you keep to about the middle 40% SOC you will get 10000 cycles before reaching 80% of original capacity I expect that your micro cycles if kept near the middle of the state of charge of the battery would have essentially no effect on the life of the cell. I would guess at least 15 to 25 years before it wouldn't do these little cycles anymore. Certainly more than 10 years. No way to tell for certain without running some long term tests.


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## Ziggythewiz (May 16, 2010)

4C is not high at all for CALB CA. Even the SEs were rated for 10C pulse and the CAs far outperform them.


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## FriedChip (Apr 29, 2013)

dougingraham said:


> A 4C discharge for 17 seconds will use about 1.9% of the capacity of the battery. The recharge time will be a little more than 136 seconds at 0.5C. If you keep the state of charge away from the ends, above 20% and below 80% you are probably looking at calendar life for useful life. The capacity would need to drop below 2% of new before it would fail to perform the mission so the usual cycle rating is not meaningful to you. Even if you assume the wrost case that 40 of your cycles is equivalent to one 80% DOD discharges that would mean you would be doing the equivalent of two 80% DOD cycles per day and thus at least 1000 days of cycles before you reached 80% of original capacity. Making another worst case assumption that each 1000 days of use like this would further reduce the original capacity by an additional 20% means you can do this 4 times total for 4000 days of cycles (11 years). Since there is evidence that if you keep to about the middle 40% SOC you will get 10000 cycles before reaching 80% of original capacity I expect that your micro cycles if kept near the middle of the state of charge of the battery would have essentially no effect on the life of the cell. I would guess at least 15 to 25 years before it wouldn't do these little cycles anymore. Certainly more than 10 years. No way to tell for certain without running some long term tests.


Thank you for your great analysis. I thought usual cycle rating was because after the capacity past the barrier of 80% of the nominal it started going down even faster (internal impedance goes up). Am I wrong?  Anyway the numbers you gave me look nice!


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## FriedChip (Apr 29, 2013)

Ziggythewiz said:


> 4C is not high at all for CALB CA. Even the SEs were rated for 10C pulse and the CAs far outperform them.


I forgot to tell mines are SE. The bad ones I'm afraid.


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## dougingraham (Jul 26, 2011)

FriedChip said:


> Thank you for your great analysis. I thought usual cycle rating was becouse after the capacity past the barrier of 80% of the nominal it started going down even faster (internal impedance goes up). Am I wrong?  Anyway the numbers you gave me look nice!


If you can find any of the few long term studies what appears to happen is the decay in capacity tends to level off as it approaches the mid 65% range of original capacity. From memory it was around 10 to 15 thousand cycles to get down to the low 60% range. As far as I know there has not been a study done of the change in "internal impedance" as these cells age. If there is a change (and there is no reason to think there would be) I would expect it to show as an increase in sag under the 4C load. It would be another aspect that would need to be taken into account. How much voltage drop can your application handle?

What appears to be the weakness with all of the lithium types cells with carbon anodes is the carbon molecules break near the ends of the state of charge when the lithium ions intercalate. If you stay away from the ends you don't break the carbon up nearly as fast. Your application would never need to go near the ends.


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## Ziggythewiz (May 16, 2010)

FriedChip said:


> I forgot to tell mines are SE. The bad ones I'm afraid.


I wouldn't consider the SEs bad at all, I just haven't seen them go up to 22C like a CA. I regularly use mine to 4 or 5C.


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## FriedChip (Apr 29, 2013)

dougingraham said:


> If you can find any of the few long term studies what appears to happen is the decay in capacity tends to level off as it approaches the mid 65% range of original capacity. From memory it was around 10 to 15 thousand cycles to get down to the low 60% range. As far as I know there has not been a study done of the change in "internal impedance" as these cells age. If there is a change (and there is no reason to think there would be) I would expect it to show as an increase in sag under the 4C load. It would be another aspect that would need to be taken into account. How much voltage drop can your application handle?
> 
> What appears to be the weakness with all of the lithium types cells with carbon anodes is the carbon molecules break near the ends of the state of charge when the lithium ions intercalate. If you stay away from the ends you don't break the carbon up nearly as fast. Your application would never need to go near the ends.


Thank you! How many would you say will be the voltage drop if I initialize them at 60% of SOC? Also do you recommend me any BMS for this application? I am using just 4 cells but 160 Amps current peak. Would you intialize them or at least balance them every certain cycles?


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## jeremyjs (Sep 22, 2010)

FriedChip said:


> Thank you! How many would you say will be the voltage drop if I initialize them at 60% of SOC? Also do you recommend me any BMS for this application? I am using just 4 cells but 160 Amps current peak. Would you intialize them or at least balance them every certain cycles?


A lot of people advocate bottom balancing and there are real world examples of it working very well long term, but honestly with only a 2% discharge/recharge cycle they're probably plenty balanced from the factory to accomplish this so long as you never deeply discharge or fully charge them.....


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## dougingraham (Jul 26, 2011)

FriedChip said:


> Thank you! How many would you say will be the voltage drop if I initialize them at 60% of SOC? Also do you recommend me any BMS for this application? I am using just 4 cells but 160 Amps current peak. Would you intialize them or at least balance them every certain cycles?


The sag depends on a lot of things. They will sag a lot more at low temps than on a hot summer day.

With only 4 cells and your micro cycles I would not bother with a BMS and if you balance them so they have the same resting voltage to about 0.001 volts that should be good enough. You will probably want to pick some arbitrary voltage say 3.4 volts per cell (13.6v for the pack) as your charge cutoff voltage. You will get some drift in the SOC with temperature but it should stay in the upper half. I think you said you had the old CALB SE cells. I think they would sag 25% with a 4C load so less than that. Maybe down to 9.9 volts worst case? You won't really know until you try it.

If you want a DIY BMS for this just get 5 inexpensive DVM's and watch the voltage of each cell and the pack voltage.


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## PStechPaul (May 1, 2012)

For a 17 second discharge at 12V and 160 amps the energy is about 32000 W-seconds. You might consider ultracapacitors, where energy is:

0.5 * C * V^2

or

C = 2 * E / V^2 = 64000 / 144 = 453 F.

You can get a bank of seven 2600 F, 2.5 V ultracapacitors, which is 371 F, for about $200, including a balancing circuit:
http://www.ebay.com/itm/Ultracapaci...oster-Car-Ultra-Super-Capacitor-/300900579759

If you do not need the voltage to remain constant during the discharge, a capacitor bank like this might be all you need. You could put the capacitor bank across the battery pack and use a current limiter to recharge the capacitors from the pack at 0.3C or 1C to maximize cycle life, and maintain battery charge with a separate charger set at optimum levels.

Ultracapacitors have a cycle life of 10,000 to 100,000 or more, and pretty much infinite shelf life, so they may be well worth considering.

Another deal is 12 capacitors for $200:
http://www.ebay.com/itm/Maxwell-260...citor-power-storage-used-Qty-12-/370819154254

They would give you up to 30 volts so you could make a regulator to get the 12V you need and you can get 

0.5 * (2600/12) * (30^2 - 12^2) = 81,900 Watt-seconds

That's more than twice what you need. A 160A 12V voltage regulator may be a challenge, but it's well within the range of an EV motor controller.

I'm curious, what are you doing? I might guess some sort of welding?


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