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  #1  
Old 07-10-2012, 04:37 PM
lowcrawler lowcrawler is offline
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Default How often to charge...

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. I have my elcon charger set to charge as if I'm only using 44 cells. When I went to 45 cells, the MiniBMS would shunt for a long while at the end of the charge, I did this for about 1500 miles until almost every single shunting light came on within about 30 seconds of each other.... Then I changed the elcon to charge as if I only had 44 cells.
Now, shunting' lights to 'flick' on for about a minute before the elcon shuts off. (ie: I'm charging to 3.5V) Given I don't need 'max range' I think this softer charge is probably good for my pack over trying to 'fully charge' all cells to 3.6V.

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..)

Or, given CALB shipps the cells at less-than-full (which makes me think it's better to not have them sit 'full') would it be better for me not to charge at home, letting the batteries sit at 10% less than 'full' for the evening/night rather than being full at that time?

Make sense?

In other words... What's better... To be at 90% full most of the time and with two short short drops to 80% SoC and then immeditaly recharge back to 90%.... or to sit at 80% SoC most of the time with one drop to 70% per day and then recharge back to 90%...
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  #2  
Old 07-10-2012, 04:47 PM
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Default Re: How often to charge...

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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Old 07-10-2012, 04:53 PM
lowcrawler lowcrawler is offline
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Default Re: How often to charge...

Quote:
Originally Posted by Ziggythewiz View Post
...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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Old 07-10-2012, 05:05 PM
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Default Re: How often to charge...

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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Old 07-10-2012, 09:20 PM
Zak650 Zak650 is offline
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Default Re: How often to charge...

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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Old 07-10-2012, 11:53 PM
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Default Re: How often to charge...

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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Last edited by Ziggythewiz; 07-11-2012 at 12:05 AM.
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Old 07-12-2012, 10:19 AM
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Default Re: How often to charge...

Quote:
Originally Posted by lowcrawler View Post
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

Quote:
Originally Posted by lowcrawler View Post
...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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Old 07-12-2012, 12:55 PM
swoozle swoozle is offline
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Default Re: How often to charge...

Quote:
Originally Posted by Ziggythewiz View Post
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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Old 07-12-2012, 01:30 PM
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Default Re: How often to charge...

Quote:
Originally Posted by swoozle View Post
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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Last edited by Ziggythewiz; 07-12-2012 at 01:49 PM.
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  #10  
Old 07-16-2012, 08:05 AM
Jesse67 Jesse67 is offline
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Default Re: How often to charge...

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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