Std vs. ext charge vs. charging cycles vs. battery life

Toyota Rav4 EV Forum

Help Support Toyota Rav4 EV Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Dsinned

Well-known member
Joined
Nov 6, 2012
Messages
1,213
Location
San Jose, CA
I generally do extended charges about once every couple months, or every 8th charge. So, I only do extended charges less than 15% of the time. Furthermore, I usually only have to recharge once per week by sticking to a regime of only doing standard charges. I bought the car last November and so far have only driven it ~4200 miles, or roughly the equivalent of about 60 standard charges (conservatively assuming ~70 miles driven per charge). I actually get an average range per standard charge of ~100 miles, and I almost never recharge with much less than 30 miles range remaining on the GoM.

Given this information, and assuming ~125 miles range per charge on extended charges, I'm wondering if I only do extended charges, and I only recharge once every two weeks (not always, but mostly), would I be better off in terms of battery degradation? Ideally, which is the best alternative for the long term life of the battery?

What I am getting at here, is there is perhaps one traditional school of thought that says, cutting down on total number of recharge cycles over time, has the biggest advantage in extending battery life.

All comments and/or opinions welcome!
 
I would not do an extended charge unless you would use enough energy to get it back down within the standard charge window within 24 hours. With your driving pattern, you may take a couple days to use the energy "above the bars". Keeping the battery between 20% and 80% (3 bars to 16 bars) will probably maximize the lifetime of the battery. In fact, I only do an extended charge if I plan to travel more than 80 miles the next day. I've only done it three times in 4 months. All my other charges are standard charge when down to 13 bars or less.
 
When you're asking about the long term health of the battery, it's REAL simple:

1. Remain at 50% SOC (that's about 10 fuel bars segments)
2. Keep the battery cool

Anything you do outside of that is "worse". The absolute worst is high SOC% and high battery temperature. So, based on that, I recommend charging every day that you drive the car to keep the battery at about 10 fuel bar segments and keep the car stored in a cool place.

Just to be clear, again for longest battery life, I would never do an extended 100% charge unless you needed it. There is no reason to do so otherwise, unlike old NiCad batteries with memory.

Also, you may consider a garage air conditioner (or hot water heat pump) to cool your garage in those blistering hot San Jose summers.
 
Thanks!

I do not dispute what either of you say, but it is simply not practical to maintain as close to a 50% SoC from day to day, and my RAV4 EV seldom is parked out of direct Sun light. It is usually parked in my driveway most of the day, so my wife has a car to run errands nearby. I don't trust her trying to park it in the garage. Nor, is it possible to keep it plugged in all time when not in use. For one reason, I have a Chevy Volt that requires more frequent recharging, typically a couple times per week. Both cars are sharing the same Leviton L2 EVSE. Of course, I still have the OEM (120V) charger, but it would just be too much to expect my wife to actually use it during the day. However, it is certainly not an issue at all to continue doing mostly standard recharges, except for those rare occasions when I anticipate driving over 100 miles in one day.

Basically, what I'm asking, what is more detrimental to the long term health of the battery . . . consistently doing standard charges weekly or extended charges bi-monthly, all other factors being the same?

I have read that all batteries - Lithium ion included - have a finite number of discharge/recharge cycles. I have also read that "partial" discharge/recharge cycles are supposedly not nearly as tough on a battery as FULL discharge/recharge cycles. Of course, by design we are never truly allowed to do the latter. However, an extended charge is a "wider" overall cycle in terms of the total SoC delta variation, but still in actuality a "partial" discharge/recharge cycle. So, the question is, assuming the battery spends a majority of its life hovering around 50% SoC, what is worse, recharging the battery to an upper limit of 80% SoC or closer to 100%, where the amount of discharge below 50% is approximately the same on average, but never all the way to fully depleted?
 
Bottom line, don't do extended charges unless you need the extra range. If possible, time the extended charge so it completes just before you're ready to drive.
 
Dsinned,

My cell testing is up to 35k simulated miles and I can only see ~1% difference in capacity from standard charge vs extended charge. That surprises me but that's what the data show.
I say do whatever is easier for you (go with extended charge and charge less often?).
 
Dsinned said:
Basically, what I'm asking is what is more detrimental to the long term health of the battery . . . consistently doing standard charges weekly or extended charges bi-monthly, all other factors being the same?

The further you stray from the "50% SOC cool battery model", the more detrimental it is.

I would do the former if that is your only two choices.
 
Kohler Controller said:
Dsinned,

My cell testing is up to 35k simulated miles and I can only see ~1% difference in capacity from standard charge vs extended charge. That surprises me but that's what the data show.
I say do whatever is easier for you (go with extended charge and charge less often?).

What voltages are you using for top and bottom of discharge? What is the overall degradation so far?
 
TonyWilliams said:
Kohler Controller said:
Dsinned,

My cell testing is up to 35k simulated miles and I can only see ~1% difference in capacity from standard charge vs extended charge. That surprises me but that's what the data show.
I say do whatever is easier for you (go with extended charge and charge less often?).

What voltages are you using for top and bottom of discharge? What is the overall degradation so far?

For standard charge, I charge at C/5 to 4V with no taper on current, simply shut off. For extended charge, I charge to 4.15V and then taper to 0.2A.
As for lower end, I stop when it is down to 15% SOC (equivalent to roughly 7kWh still left in the pack).
After 35k simulated miles, I am down to 94% of original capacity for cells being charged using Standard Charge, and 93% on those cells using Extended charge.

It takes about 2 weeks for me to go through 5k miles. The 40k mile data point will be taken early next week.
I plan on sharing all of the results/graphs when I reach 100k miles.
 
For someone who mainly has short drives during the week (< 10 miles/day) and only slightly longer on the weekend (~ 20 miles/day) would it be better to charge more frequently but not all the way (40% SOC to 60% SOC) or less frequently but all the way (40% to 80% SOC)? I've never needed an extended charge and want to preserve the battery as well as possible but since I drive so little I'm not sure if numerous charge/discharge cycles are worse or leaving it with a higher SOC more often due to limited driving. It rarely gets above 80 degrees here so heat hopefully shouldn't be too bad.
 
OUTSTANDING!!! All of us here appreciate your efforts! Thank you!

Your preliminary results are very interesting. So far, surprisingly enough, not much difference in cell degradation while comparing standard vs. extended charge testing. Of course, this does indeed seem quite counter intuitive.

Will you be simulating a lot more mileage? 100k would probably be a good milestone for measuring projected degradation.

I think most people could live with up to -15% degradation in that many miles of real world service, but any more than that would have a noticeable impact on range. In order to get about the same range as when the battery pack was new, perhaps more owners may be inclined to start doing extended charges more often.

By then, the average 2012 RAV4 EV would have been owned for about 7 years; and some perhaps closer to 10 years. At that point any "change in the rate" of degradation would be of even more interest to most owners.

Therefore, if you can continue your testing to a simulated 150k or even longer, you would be doing us all a tremendous service.

Thanks again!!
 
Is it better for the life of the Rav4 battery to charge fewer times during the week by using extended charges or a greater number of "normal" charges? My commute is such that If I extend charge my Rav4 I only need to recharge every other day, however, if I normal charge I have to charge every day. So the question is which is optimal for extending the life of the battery- normal charging every day or extending charging every other day?

Thanks
 
I posed this very same question here a while back. The consensus was to stick with doing standard charges even if more often. I wonder . . .

According to the following article, Lithium batteries repeatedly discharged by as much as 70% of total SoC, will result in an approximate cycle life of 3000, whereas 80% of total SoC cycles will result in a cycle life of 2000. This leads me to consider that extended charging, done less frequently, (in your case half as often as standard charging), may very well extend the life of the battery, or at least not degrade it any faster. A depth of discharge that uses up 80% SoC is roughly equivalent to the full usable battery capacity of our cars (41.8kWh). By the same token, 70% would correlate well with using only 33.44 to 35kWh of usable capacity (depending upon your persuasion). Another advantage of less frequent charging is your net use of electricity from the wall will be reduced by doing extended charging.

http://www.homepower.com/articles/v...-batteries-electric-vehicles/page/0/3?v=print

In my case, my Rav4 EV is only driven about 50 to 70 miles per week, so to play it safe, I think it prudent to stick with standard charging only as the norm most of the time.

Nevertheless, according to a fellow member here ("Kohler"), I believe his preliminary Li-ion testing of cells like used in our car's battery, in simulation of 100,000 miles driven by a RAV4 EV, has yet to indicate that extended charging will degrade battery life any more significantly than standard charging. Very interesting!
 
Back
Top