What's bad for our batteries?

[eNate]

I noticed somebody lumped 32a level 2 charging in with DCFC as possibly harmful. Is there an actual consensus on this? I just bought a 2014 Rex, and while I don't plan to obsess over battery life, I'd obviously like it to last as long as possible. I installed a 32a charger at home because I do almost 40 miles on most days and more than that on some, so I wanted to ensure I'd have full juice everyday. With the battery buffer and BMS, I figured that was a non-issue. Incorrect?

I'd say, in order...

1. Don't worry about it.
2. Select "Reduced Rate" from your charging menu if you are worried about it.

I wouldn't lump L2 and DC together, but I'd agree that heat is the enemy of batteries. There used to be some general agreement in the battery community (ha! -- this is pre-EV, talking NiCAD RC airplane days when NiMH was new tech) that a 10 hour charge rate was deemed "safe" for battery health, but that was in the days of not very intelligent fast chargers that might monitor temperatures externally as a safety and taper the charge near the top.

Our EV batteries have plenty of head room so they're never fully topped off, and have advanced charge management systems, internal temp monitoring, and cooling systems.

But in the case of your 60Ah battery, you can go from empty to full in 3 hours with that 32A L2 charger, and you can recoup your 40 miles in just over an hour and a half.

There's an argument to be made for charging at a reduced rate if you have the luxury of time. It will reduce the cooling demands of the thermal management system, which itself is an electrical draw. And it certainly reduces the instantaneous load on your neighborhood's grid. Though I doubt either of these is a huge impact. Yet still... a three hour charge is a three hour charge, and there's going to be excess heat associated with that no matter how well it's managed. Detrimental? Probably yes to a small degree. Worth worrying about long term? I don't think any i3 owners have presented a compelling case against this.

 

[jadnashuanh]

The manual discusses that you shouldn't leave the car with a low battery charge when it is likely to get cold overnight.

 

[panamamike]

Think leaving the battery at a near 0 charge for an extended duration is probably the worst thing you can do. I'm assuming it's bad not because being a user 0 is bad, but rather that the car will slowly dissipate the batter to below zero and can reach or get to actual 0 charge which is bad for lithium batteries. Leaving the car idle not connected to a charger a lose a few miles of range over night. Multiply that by a month and you could be in trouble.

 

[alohart]

Leaving the car idle not connected to a charger a lose a few miles of range over night. Multiply that by a month and you could be in trouble.

Li-ion cells have a very low self-discharge rate. Unless battery pack or cabin preconditioning is enabled, the battery pack is electrically isolated when an i3 is off and parked overnight, so there are no vampire loads on the battery pack as there are on battery packs in Tesla EV's. Therefore, an i3 battery pack won't lose enough charge overnight to be noticeable. That's different from the range estimate decreasing overnight due to lower temperatures.

I have left our 2014 BEV in storage for periods of 3 to 9 months on 4 different occasions with the charge level when put in storage ranging from ~40% to ~60%. When I was preparing to put our i3 back on the road, the battery pack's charge level had decreased ~1% per month as best I can tell. Because the charge level displayed is a calculated value, its accuracy was probably not great under these circumstances, so I calculated the charge level based on the Batt. Kapa. max usable capacity and how much energy was added when charging to full (assuming a 92% charging efficiency). There's significant error in this calculation, but it seemed to result in a reasonable charge level estimate.

So no one should be concerned about an i3's battery pack discharging significantly overnight or even over a month.

 

[panamamike]

Check out this thread regarding the long term storage.

https://www.mybmwi3.com/forum/viewtopic.php?t=4544

 

[abeln2672]

I noticed somebody lumped 32a level 2 charging in with DCFC as possibly harmful. Is there an actual consensus on this? I just bought a 2014 Rex, and while I don't plan to obsess over battery life, I'd obviously like it to last as long as possible. I installed a 32a charger at home because I do almost 40 miles on most days and more than that on some, so I wanted to ensure I'd have full juice everyday. With the battery buffer and BMS, I figured that was a non-issue. Incorrect?

I'd say, in order...

1. Don't worry about it.
2. Select "Reduced Rate" from your charging menu if you are worried about it.

I wouldn't lump L2 and DC together, but I'd agree that heat is the enemy of batteries. There used to be some general agreement in the battery community (ha! -- this is pre-EV, talking NiCAD RC airplane days when NiMH was new tech) that a 10 hour charge rate was deemed "safe" for battery health, but that was in the days of not very intelligent fast chargers that might monitor temperatures externally as a safety and taper the charge near the top.

Our EV batteries have plenty of head room so they're never fully topped off, and have advanced charge management systems, internal temp monitoring, and cooling systems.

But in the case of your 60Ah battery, you can go from empty to full in 3 hours with that 32A L2 charger, and you can recoup your 40 miles in just over an hour and a half.

There's an argument to be made for charging at a reduced rate if you have the luxury of time. It will reduce the cooling demands of the thermal management system, which itself is an electrical draw. And it certainly reduces the instantaneous load on your neighborhood's grid. Though I doubt either of these is a huge impact. Yet still... a three hour charge is a three hour charge, and there's going to be excess heat associated with that no matter how well it's managed. Detrimental? Probably yes to a small degree. Worth worrying about long term? I don't think any i3 owners have presented a compelling case against this.

Appreciate the thorough response. I'll definitely consider the reduced charging rate on days I can afford it (which is most, tbh). Maybe I'll assign it a button on the presets so I can quickly switch. Thanks!

 

[GoElectric]

The manual suggests: Before storing the vehicle for an extended time, ensure the battery is fully charged. Do not let the vehicle sit idle with a charge state below 80%.

 

[alohart]

The manual suggests: Before storing the vehicle for an extended time, ensure the battery is fully charged. Do not let the vehicle sit idle with a charge state below 80%.

That suggestion is for storage up to 3 months. For longer storage, consulting a BMW service department is recommended. I was told to disconnect the 12 V battery to prevent it from discharging totally which would likely ruin it.

Storing with the battery pack full or at >80% makes no sense because the cell degradation rate increases with the amount of time a cell remains at a high charge level, and because the battery pack discharges so slowly that it would not completely discharge for years at which point damage would occur.

 

[Spintrian]

The worst you can do to your battery is discharging it to 100%. It is necessary that we have ideas or guides of the do's and dont's and what we must have and don't have on our gadgets. In order for us to know what would be the problem. Just like on my engine, I make sure I have my Actron scanner https://bestobd2scanners.com/actron/ by my side.

 

[jadnashuanh]

The chemistry used in the Samsung batteries of our i3 have a fairly high rated number of discharge cycles compared to some of the others out there. If it's cold out, you go a shorter distance for one cycle. So, if you look at it that way, preconditioning the battery means on any given trip, you'll use less percentage of the charge, meaning fewer discharge cycles, which, should improve the life of the thing. Let's say you get 100 miles on a full charge when the weather is good and go 10K miles in a year, that's 100 discharge cycles. Now, let's say you get only 80 miles when it's cold (just round numbers, not what you may expect in real life)...over that same 10K miles, instead of 100 discharge cycles, you end up with 125 cycles. If you precondition the batteries, it might drop to 111 cycles and improve the range to 90 miles, and so on. So, preconditioning saves discharge/recharge cycles, and should prolong the battery life. Throw in when it's cold, cabin heating takes more range away than cooling, that makes for more discharge cycles, and makes cold weather worse. So, selecting preconditioning the cabin at the same time as the batteries should help more than just the range, but the battery life as well.

Personally, I don't worry about plugging the car in when I get home. I like always having a full charge when I want to go somewhere, and I do, if I have the time, precondition. I've not noticed any significant range lost in five years, but the thing doesn't have a huge amount of miles on it.

 

[alohart]

Fortunately, the i3's battery management system prevents discharging to 0% by limiting the discharge to ~9% which is displayed as 0%, or charging to 100% by limiting the charge to ~95% which is displayed as 100%. To reduce the battery cells' degradation rate, it's best not to allow the charge level to remain at a very low charge level or a very high charge level.