# Batteries last longer if not fully charged



## EVComponents (Apr 20, 2009)

There are 4 different settings for a Tesla that go with each voltage level.

*6,831 cells total*

*Storage mode (3.8 V)* = approx 95 miles of range (50% of standard). This is if you are planning to be away for a few weeks, leave the Tesla plugged in. This allows the pack to self discharge slowly until 50%. Then it will pull enough energy from the outlet to maintain 50% charge. Storage mode expends just enough energy to keep the battery pack at the best temperature for long term battery health. Self discharge is about 0.5% per day.

*Standard mode (4.1 V)* = approx 190 miles of range (normal default charging mode each day). Tesla goes back to this mode by default under a few different conditions. You have to specifically go into the user screens to leave this mode. 

*Range mode (4.15 V)* = approx 244 miles of range (only used if you know you are planning a longer drive). In this mode, Tesla limits the max power for acceleration. Before entering this mode, the user screen warns that it is not good for the long term health of the battery pack.

*Performance mode (4.15 V)* This mode allows the components to operate at a warmer temp and achieves slightly better acceleration and longer performance before overheat warnings. Before entering this mode, the user screen warns that it is not good for the long term health of the battery pack.


----------



## EVComponents (Apr 20, 2009)

CroDriver said:


> OK, Li-ion and LiFePO4 are not the same chemistry but maybe this approach could be better for LiFePO4 too.


I believe that ThunderSky ships their LiFePO4 cells with a 50% charge.


----------



## JRP3 (Mar 7, 2008)

I think it's wise to avoid fully charging, but it's more important to avoid over discharging, and with the limited pack size of most DIY EV's you're probably better off charging higher than 50% so as to not take the cells too low. If you have enough pack size to safely operate in the middle range most of the time then it makes sense.


----------



## MN Driver (Sep 29, 2009)

To coincide with the information that you are discussing, the Battery University site discusses storage voltage and temperature as well as charge constant current voltages too. This information is specific to Lithium Cobalt batteries however, which have different nominal voltages and this information is fairly out of date as far as cycle life is concerned IMO.
http://www.batteryuniversity.com/parttwo-34.htm

I've never come across any information with tests that show what the ideal constant current voltage is for LiFePO4 cells. I know that most manufacturers indicate somewhere between 3.6 volts(CALB, A123), 3.65 volts(Lifebatt specs, Headway and a few other manufacturers who I can't seem to find the spec sheets to verify with), 3.85 volts(HiPower) and 4.25 volts for Thunder Sky. I'm not sure why Thunder Sky specs their cells so high but based on peoples experiences here, there isn't really anything as far as capacity when you reach that high.

So we have manufacturers that throw numbers out but we don't have any idea as there isn't extended cycle life testing that tells us how far we should go, at least that I've come across. I'd venture a guess that since the lowest recommendation to get the most we can out of cells for both usable capacity and cycle life is 3.6 volts, it is likely close if it isn't our number already.

I still think that ending state of charge at the end of discharge is more important though as pretty much every manufacturer shows that its ideal to not run them down all the way every time, with the exception of high-end cells like A123 and LifeBatt(I think). ...but they should both be considered.


----------



## EVComponents (Apr 20, 2009)

JRP3 said:


> I think it's wise to avoid fully charging, but it's more important to avoid over discharging, and with the limited pack size of most DIY EV's you're probably better off charging higher than 50% so as to not take the cells too low. If you have enough pack size to safely operate in the middle range most of the time then it makes sense.


That reminds me of an interview I saw where Elon Musk was explaining why the GM Volt battery pack and the Tesls battery pack were not comparable in terms of estimated life and number of miles before replacement.

With the GM Volt battery pack (16 kwh, 8 usable kwh) only providing 30-40 miles of range in pure EV mode, it is likely going to have a deep discharge cycle every day. Perhaps even two times per day. So over the course of time, that battery pack is experiencing much more wear from many more deep cycles.

The pure EV battery packs will be much larger. Nissan Leaf has 24 kwh. Tesla has 53 kwh. So average daily driving of 50 miles or less will be about a 50% discharge for Nissan and a 25% discharge for Tesla. The daily abuse on those battery packs is much more gentle.


----------



## CroDriver (Jan 8, 2009)

EVComponents said:


> With the GM Volt battery pack (16 kwh, 8 usable kwh) only providing 30-40 miles of range in pure EV mode, it is likely going to have a deep discharge cycle every day. Perhaps even two times per day. So over the course of time, that battery pack is experiencing much more wear from many more deep cycles.


If I understand this right, the upper and lower 4kWh (below 25% SOC and above 75% SOC) are considered as the limits. So the car needs to recharge when it still has 25% SOC left, and is "fully" charged when it reaches only 75% SOC. So the pack is treated very gently IMO


----------



## major (Apr 4, 2008)

CroDriver said:


> If I understand this right, the upper and lower 4kWh (below 25% SOC and above 75% SOC) are considered as the limits. So the car needs to recharge when it still has 25% SOC left, and is "fully" charged when it reaches only 75% SOC. So the pack is treated very gently IMO


Hi Cro,

Listening to horse's month, so to speak, this is not the way they are used. http://www.diyelectriccar.com/forums/showthread.php/chey-volt-some-new-pictures-38627.html From those presentations, this is what I understand. The battery is fully charged from the grid. The car runs on battery only until it reaches 50% SOC. Then the engine/generator comes on and the car runs as a series hybrid. But the battery is not charged above the 50% level by the engine. 

Regards,

major


----------



## MN Driver (Sep 29, 2009)

I read through a great deal of speculation in the comment sections of the GM Volt blogs/news releases that the split is a 'charge to 80%, discharge to ~30%', at which point it begins its perpetual 'charge sustaining' mode where it maintains that 30% until the car is shut down. The point that it is at is confidential according to the official information that GM releases but reading through what they have said, it will not charge the vehicle back up, and this is by design so it doesn't use more fuel than it needs in case you are about to plug it in. ...so basically once the engine starts, it doesn't shut off until you turn the car off. I'm sure there are some exceptions such as extremely light loads(25 mpg, extreme stop and go, or sitting at a 'stop light' type situation for a long time where it might catch up to a point where it powers down to save fuel.

The 80% to 30% state of charge should make it easy on the batteries but it also allows them the room to slowly 'extend the gap' so to speak as the car ages so that once that battery is nearing end of life, it probably won't be noticed until well over the standard 80% of total capacity is lost. It allows the car to 'hide capacity' when its new and use more of it when it goes through the cycles.

I have a feeling it probably does the same as the Tesla design does, with a charge ending at 4.1 volts, which coincides with what Major is saying with it being fully charged. I also figure that GM will want to tell people that it is fully charged too so that way they don't get confused and ask questions about 'why can't I get 60-80 miles and use all of the battery for my commute?'. ...but who knows, we get what GM tells us and what you said could very well be right, but I'm sure that there is a 5-10% headroom in there at least as it provides for longer cycle life.

Either way you cut it though, I buy cars in the used market and I see myself as always doing this. I see this as a car that people are afraid to buy used after that warranty is up and if things look good in the GM Volt forums that exist and amongst the ones that crop up, you bet I'll be buying a used one for dirt cheap. A similar thing is happening to the 1st generation Prius at the moment, although the battery isn't the only concern for that car, it has a great deal of bells and whistles that can't be fixed(steering column/eps, hv trans) and without the battery, it can't start moving without), have a high replacement cost so I can see where people are getting leery of buying a used one. The leery buyers are making them cheap to buy. I personally have my eyes on the 1st generation Honda Insight at the moment, simpler car with less to break and it can drive without the battery, also much easier to rebuild or replace too, but it will likely get complimented by the Volt as a second car when the time comes. I think that time will tell with how the Volt performs in the long term but if things pan out decently, I'd love to have one for my 30 mile round trip commute and still be able to go the 100 miles I go on some weekends and also have it be compatible as a road trip car.

...of course, that is if I don't build me one as a DIY project first, or if Tesla produces its third vehicle at a price that I can swallow, because I know I can't work in the $50k cost after $7,500 rebate when a conversion practical to me is cheaper.


----------



## EVComponents (Apr 20, 2009)

CroDriver said:


> If I understand this right, the upper and lower 4kWh (below 25% SOC and above 75% SOC) are considered as the limits. So the car needs to recharge when it still has 25% SOC left, and is "fully" charged when it reaches only 75% SOC. So the pack is treated very gently IMO


That is true. But they have to do this to allow the battery pack to survive 10 years. They are having to really baby that battery and limit the usage to only 50% of the battery. The upper and lower limits of the battery pack have to leave a wide margin with such heavy use.

Tesla does not have to do that with such a large battery pack. Because owners rarely do a deep discharge cycle, Tesla can allow more EV range from their battery pack. 

Only 2 times in the past 7 months have I taken my battery pack below 50 miles of estimated range remaining. So I have likely never been below 35% SOC on the cells. That is very gentle on the cells. Compare that to the use of a GM Volt that will likely reach the lower limits of the battery pack every day.


----------



## major (Apr 4, 2008)

MN Driver said:


> ....it also allows them the room to slowly 'extend the gap' so to speak as the car ages so that once that battery is nearing end of life, it probably won't be noticed until well over the standard 80% of total capacity is lost......


Good point, MN,

But the charts presented at that SAE event showed 16 kWhr to start and then hovering at 8 kWhr while the engine was on. Now maybe it is actually an 18 kWhr battery new to start with. They didn't get into those particulars. And from my work with other electrical energy storage devices, it is common for the actual new capacity to be higher than spec such that the rated specified capacity will hold at half way through the specified life.

A side bar: It was mentioned by Mr. Klein of CPI that the batteries could find "second life" use after they no long meet the minimum Volt vehicle requirements as stationary energy storage devices. 

Regards,

major


----------

