# How often to charge...



## Ziggythewiz (May 16, 2010)

I don't think we have enough data to know what's best yet, but from the little I've seen it seems Whs over lifetime are maximized with a DOD somewhere in the middle. Does it max at 30% DOD or 70%? Who knows? If I were in your shoes I'd charge to 90 or so every 2-3 days.

If you only had to charge every few days then 8-10 years from now you'd need to be charging every other day...then another theoretical decade later you'd be charging every day...then eventually you'd let us know when the shelf life becomes a greater issue than the cycle life.


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## lowcrawler (Jun 27, 2011)

Ziggythewiz said:


> ...then eventually you'd let us know when the shelf life becomes a greater issue than the cycle life.


haha.. I'll be sure to report back in 30 years. 

I'm leaning towards doing a charge (to 90% SoC) every time I drop to 60% or less.

I'm also charging when I know I'm going to charge wihtout filling up completely. Like, right now, for instance... the car is at ~70%SoC. I know it'll take about 2 hours to charge to 90% and I need to drive somewhere and back that will use about 30% SoC. I'm leaving in about an hour and 45 minutes. So I'm charging it now until I leave...and then will immediately use that energy, so the pack won't sit at a higher charge rate for very long.


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## dougingraham (Jul 26, 2011)

I bought 60 cells intending to use 54 of them. I capacity tested at C/3 all the cells and kept the worst 6 out. Of those I decided to do a test with three of them. I stored one fully charged, one back at the 3.30 volts level and one that I discharged to 2.0 volts. The discharged cell bounced back up to 2.7v and was still there 6 months later. The cell stored at 2.7v resting lost about 1/2 percent of its capacity in 6 months. The other two cells I couldnt tell any difference from the just out of the box capacity. I suspect this is one of those cases where it hardly matters.


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## Zak650 (Sep 20, 2008)

In one of EVTV shows Jack presented some internal data from one of the battery companies and basically to put in a one sentence form:

The shallower the discharge before recharging the more charge cycles in a rather exponential curve.

Or strung out to two sentences:

Conversely the deeper the discharge the fewer charge cycles in the same sort of curve.


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## Ziggythewiz (May 16, 2010)

Here's a "study" that suggests you're best off charging as often as possible. I've seen others that indicate the peak is between 40 and 60% DOD, but can't find that one at the moment.

http://www.golfskatecaddy.com.au/gsc-lifepo4.rhtml

This one indicates exponentially better life with frequent charging, while the other one (that I'd like to believe more) indicated only slightly better lifetime throughput at the higher DOD, so you're probably better off plugging in every chance.

When I've ran other calendar life scenarios higher DOD (70% vs 35%) won out because while lower DOD had far better life time, it wasn't the 2x+ better that would justify the higher up front cost. In your case the money's already spent, so that bit is irrelevant.


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## bruceme (Dec 10, 2008)

lowcrawler said:


> So I use only about 10% of my cars range traveling from home to work. I can plug in at both locations.
> 
> I'm using 48 100ah CALB SE cells and MiniBMS.


Almost exacactky my setup



lowcrawler said:


> ...snip...
> Anyway, my question is this -- is it better to constantly 'top off' the pack at all times (ie: use 10% going to work, recharge, car sits for 7 hours 'full', use 10% getting home, recharge, car sits for 12 hours while 'full'.... lather, rinse, repeat the next day..)


I can only tell you my results. I have been driving my EV 30mi/day, typical discharge is 50% for two years. I top off when I can, but so far after ~800 partial cycles, I only perceive minor degradation. 

So the motto is.. charge'm if you got'm. The rest is a pedagogical argument.


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## swoozle (Nov 13, 2011)

Ziggythewiz said:


> Here's a "study" that suggests you're best off charging as often as possible. I've seen others that indicate the peak is between 40 and 60% DOD, but can't find that one at the moment.


The data is shown in cycles, which is interesting but marginally relevant. A shallow DOD gives more cycles, great, but they are shallow cycles. I get less work out of every cycle.

If I'm driving 30 miles a day, what's the gain by charging when I get to my destination and when I get back vs once a day? I'm burning through cycles twice as fast.

I bet the 40-60% DOD studies you refer to take that into account. That sounds more reasonable and fits in better with the "don't store your batteries fully charged" guidance.


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## Ziggythewiz (May 16, 2010)

swoozle said:


> If I'm driving 30 miles a day, what's the gain by charging when I get to my destination and when I get back vs once a day? I'm burning through cycles twice as fast.


Except that this 'data', if you trust it, indicates much more than double the life.

300,000 cycles at 5% DOD = 15,000 cycles at 100% DOD
100,000 cycles at 10% DOD (OP's scenario) = 10,000 cycles at 100% DOD
20,000 cycles at 20% DOD (If OP charged daily) = 4000 cycles at 100% DOD
5,000 cycles at 50% = 2500 cycles at 100% DOD

So according to this data, charging as often as possible could get you not only more than 2x the cycles, but more than 2x the total lifetime throughput.

Since this certainly isn't concrete evidence, let's call it a gamble. The other study I recall indicated ~5% better lifetime throughput for the peak vs a shallow DOD. So I'd gamble by trying to win 2x my value instead of playing it safe to get 1.05x my value.

Now, if I were planning a pack in this scenario instead of trying for 2x the value, I'd buy half the cells and come out the same with deeper discharges, but OP already has the cells, so irrelevant. 


Just a theory here, but what might be going on (and would be much more plausible), is that the cycle life shown is not till the cell reaches 80% spec capacity as done in typical studies, but until the cell can no longer discharge the required % of original capacity; ie after 100,000 cycles to 10% DOD a 100AH cell has less than 10AH capacity.


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## Jesse67 (May 12, 2009)

I works out better for the cells to do a shallow discharge because it's a non linear relationship. Discharging to 50% has a larger impact on your cell's health than discharging to 25% twice. The same thing goes for any two different DOD charge scenarios you compare. Discharging to 80% is worse for your cells than discharging to 20% four times, etc. 

The scale on the graph is also logarithmic and the added points for 70, 60 and 50% discharge are incorrect, they correspond with 4000, just under 3000 and 2000 cycles, not 5000, 4000 and 3000 cycles, but this is just me being picky, it doesn't change anything!

With such a short commute and charging only to 3.5V your cells should last a long time, unless you drag race to work and back everyday....


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## lowcrawler (Jun 27, 2011)

Jesse67 said:


> With such a short commute and charging only to 3.5V your cells should last a long time, unless you drag race to work and back everyday....


Haha... yeah.

Well, my new property (which I just bought) is only 3 miles to work... and it's almost all 65mph freeway (litterally, my workplace is the second building off the exit and my new house is like the 7th house off the exit)... so that's like a drag race. 

(actually, that's one reason why my pack is so oversized -- so I can run on the freeway without being at too high of a 'C' rate)


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## Jesse67 (May 12, 2009)

Well any environmentally minded person like myself (tree hugger) should be asking you why you don't bike to work with a 3 mile commute.... that said I understand the joys of biking in traffic through industrial areas and fully realize its not for everyone. 

It would be interesting if there was some more hard data on this, pay more for a large pack to have lower C rates and theoretically longer cell life, or pay less initially for a smaller pack which is worked a little harder but may not last as long? With less weight though the actual current draw and thus Ah draw would be less and the cells wouldn't have to work as hard as you might think. I would say you want your cells to be worn out from use just before they become no longer useful due to age, but you want this to be far enough in the future so your replacement pack is only a fraction of the initial price with 3 x the performance...whenever this will be I have no idea.

The mini truck is using 2 1/2 year old (calander life) 100Ah TS cells, 24 of them, and our current limit is at 450A which gives reasonable acceleration but it's also about as much as I can draw without the cell voltages sagging quite low, normal draw is around 100A. I would say these are being worked pretty hard, the range is only 50 km by design to keep the cell cost low. They only have maybe 1000km on them so we'll see how they hold up. My daily commute uses between 25-30Ah (or about an extra 500 calories when I bike...) so I have the same choices you have. I can technically drive to work for three days on one charge but I'll likely charge every day. I personnally tend to do better with frequent snacks as well, I get much less grumpy.


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## lowcrawler (Jun 27, 2011)

Jesse67 said:


> Well any environmentally minded person like myself (tree hugger) should be asking you why you don't bike to work with a 3 mile commute.... that said I understand the joys of biking in traffic through industrial areas and fully realize its not for everyone.


Yeah, it's 3 miles on the freeway... nearly 6 if I bike. Still reasonable distance-wise, but only when it's between 35-60 or so degrees. Hotter and I arrive sweaty, cooler and it's too cold. ... and only when there is no threat of rain all day ... and only when it's not snowing or icy. ... and only when I don't have a meeting where my appearance matters... etc.

Biking is a GREAT option, but it has limited use in a state like MN due to the weather extremes. (it was triple digits here yesterday with the high humidity (humidity is generally high in MN anyway)... and it's below freezing for roughly 4-5 months of the year.




> It would be interesting if there was some more hard data on this, pay more for a large pack to have lower C rates and theoretically longer cell life, or pay less initially for a smaller pack which is worked a little harder but may not last as long?


Even as-is, I'm pulling 2-4C pretty consistently. I felt 15kwh was about the smallest I could go and not be over-stressing the batteries.

Interesting idea, though. I wish we had SOME data on calendar life... I mean, as far as I know, we don't even have estimates... do we?



> less [cells] and the cells wouldn't have to work as hard as you might think.


My pack isn't a huge percentage of my entire car weight -- only about 15-20%. Removing cells is going to shrink pack size (and increase C) at a rate faster than weight is removed... no?




> I can technically drive to work for three days on one charge but I'll likely charge every day. I personnally tend to do better with frequent snacks as well, I get much less grumpy.


Yeah, I'm leaning that way too....


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## dtbaker (Jan 5, 2008)

Jesse67 said:


> It would be interesting if there was some more hard data on this, pay more for a large pack to have lower C rates and theoretically longer cell life, .... I'll likely charge every day. ... much less grumpy.


extrapolating manufacturing estimates on cycle life as affected by C rate in either charge or discharge, and %DOD..... It is logical that designing with a pack 'oversize' on range to obtain average C discharge no more than 2C with only occasional accel to 4C or 5C to avoid internal heating, and %DOD no more than 50% or 60% with daily charge will likely provide *much* longer life.

HOW much longer under 'real use', I don't think we'll know for a couple more years, until the early adopters here start posting decreasing capacity on 'normal use' packs.... but the goal I think would be to find a pack size yielding 10+ year, or 100k-ish miles which would make the pack essentially a 'lifetime' pack for a given donor.


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## scott (Feb 15, 2009)

Hi all, found this post and am interested in how things are going, or turned out. I just upgraded to a 180Ah 153volt lithium pack in Little Red, and would like to know.Thanks


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## jwiger (Oct 18, 2014)

lowcrawler said:


> ...it has limited use in a state like MN due to the weather extremes...



The wife's car (CMAX Energy) uses power from the J1772 port to turn on the heater/defroster/seat-warmer or A/C depending on the weather. Also considering the temperature happy zone of the batteries might impact your charging, charging could help prevent cold-soaking your pack. Charging them when they are warm (a hot parking lot) might cost you some cycles.

just my $0.02


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## lowcrawler (Jun 27, 2011)

Mine worked great for about 10,000 miles....

Then it got too cold to charge (MN has winters cold enough that the batteries got way below the safe charge temp). Unfortunately, with the MiniBMS system there was no way to 'turn it off' all the way and the parasitic charge of the MiniBMS took my cells way, way down...

Long story short, trying to bring them back up did a little damage to some of the cells. I got some more milage out of it, but it no longer was 'fun' after the MiniBMS issues (the alarm was always going whenever it was connected).

Lastly, the breaks in the car went out and I haven't fixed them.


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## Moltenmetal (Mar 20, 2014)

The literature I've read doesn't agree. You're looking at only half the problem.

Time at high voltage, and especially time at high voltage while hot, are both bad for the battery capacity. Two bad things happen at high voltage: thickening of the solid electrolyte interface on the anode, and electrolyte oxidation at the cathode. One eats lithium, tying it up in species that don't store energy while also making a thicker layer for the Li+ ions to diffuse through on their way to the anode. The other does more or less the same thing at the cathode. Both of those add up to degraded capacity and increased effective internal resistance under load.

Cycles AND calendar life are both ticking away at the life of each cell. Shunt charging is definitely something to avoid in my opinion, as it has minimal effect on the useful pack capacity at a risk of much longer time at high voltage.

There's a difference between high voltage during charge and high voltage during storage. Voltage during charge is a few tenths of a volt higher than the resting voltage post charge, and that few tenths of a volt mean a lot in electrochemical damage terms.

What's better: to recharge at 10% DOD to 0% DOD three times, or to recharge from 30% DOD to 0% DOD once? I'd guess the latter. What's the optimum? I'm sure the answer is "it depends".

Tony Bogs posted some excellent links on the ripple charging effect thread that are well worth a read.


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