Battery - Best Practice

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Hengus

Well-known member
Joined
Jan 6, 2015
Messages
85
Location
Uk - Worcestershire
I am considering buying an I3. I note that BMW offers an 8 year battery warranty but does it also offer advice on how best to manage battery life? I am asking the question because we have PV solar which, on a good day, could provide a chunk of the battery re-charge. My question is 'will it harm the battery if I plug it in for a slow top up charge when the sun is out, or does the battery prefer of a longer charge from a low battery state'? You can blame Apple for the question as it recommends that its batteries are fully charged from a low charge state once per month with top ups as required in between.
 
Hi there.

No there's no clear guidance on battery care apart from not leaving it at low charge or 100% for extended periods. If travelling for a few weeks, leave at 70% charge thereabouts.

I have read that the number of deep discharges affects the battery capacity. Consequently, it's better to top up when at 40-50%. Also, if you don't need a full charge, just charge to 80% instead. If charging to 100% drive off ASAP to deplete. 100% charges are required occasionally to balance the batteries... Also, limit use of rapid chargers to once or twice a week max as rapid charges can affect bettery life.

The idea is to keep charge in the 50-80% range most of the time and limit deep discharges. I am trying to follow the guidance as I bought mine.

I hope this helps.
 
All those suggestions are good except they are very, very impractical. Keep in mind that you never really deeply discharge your battery and also you never really charge it to 100%. Fully charged Lithium cell has 4.2V, so if your battery was truly fully charged you'r voltage would be ~405V. Instead it's around 393-396V since BMW is limiting charge level to about 4.1V per cell (all other EV car makers do the same). Similarly They do not allow you to go to the lower end of battery capacity. Hence the difference between total battery capacity and useable capacity. All that makes them to be very reliable and long lasting.

Also remember about discharging cycle. Discharging cycle means one full cycle from full to empty. And again with Lithium cells (contrary to some arly designs) it no longer matters if you discharge it all at once from 100-0 or do two discharges 100-50 with recharging in between. Battery degradation is a process that depends mostly on the fact of discharging, not the state of charge at which the process happens. I do not know for how many cycles is i3 battery rated, but my guess would be around 2500-3000 cycles. Which means that if you can do 65miles on one cycle you're good for about 180k miles. That's your life time range.

Having said all that (and before I put my flame suit on) batteries do degrade if sit full and empty for extended period of time. But nobody really defines what extended means. I do believe it to be 'week or more'. If my car was to be parked for a week or more I would take care about leaving it in about 80% SOC. Otherwise I do not change my daily routine:
a) I have Time Of Day rate which means cheap electricity from 8pm to 7am. I do not start charging before 8pm no matter what state of charge the car is unless I have to make another trip that evening. So sometimes that means sitting for an hour or two at 0%. Doesn't bother me at all.
b) I have two EVs and they need to be charged sequentially so one of them obviously will be at 100% well before it's being driven. Again that means maybe 4-5h of sitting at 100%. Again doesn't bother me at all
c) we usually use only one car during weekends. For that I tend to charge the other one partially (10-15% maybe) so it doesn't sit empty whole weekend, and then charge it fully on Sunday night.

P.S. I own a Leaf for 14 months now and I was following my approach to keeping it charged and I have not noticed any degradation in capacity over that year (and 11k miles) done. I own i3 for a month now, so not much experience yet, but I doubt it will be any different.

P.P.S. In MY2014 (or 15?) Nissan took away the option to charge battery to 80% only. They have discovered that it makes it too complicated for users and doesn't extend battery life at all.
 
I agree with Tomasz, these batteries are more robust then many people are aware and while some special care will improve their life I think owners are over reacting a little too much on the side of caution. I believe that leaving a battery fully depleted and letting it sit in extreme cold without recharging is the worst situation and a quick partial charge in that instance is probably advisable. However all the hype of worrying about topping it off or letting it sit fully charged for short periods of time is a wast of effort. Yes if I do not intend to use the car for several days and it is at say 35% SOC, I will let it sit that way until the morning before I want to use the car before charging to full. However if I get home with say 10% SOC and do not intend to use the car for a week let say, I will partially charge up to around 50 to 60% and then fully charge again before my departure unless it is going to be a short trip say 4 or 5 miles then I wont bother. It's really hard to say but common sense is your best guide. The batteries like to be about half charged when idle for longer periods of time, they do not like to be on empty at all for very long and full for longer periods of time.

In short, use your car just being careful to use some common sense when and if you know your schedule adjust your SOC accordingly, if you can't don't obsess over it.

Reading several studies I found that over 3 or 4 years and say about 30000 or 40000 miles the capacity loss of someone who is very careful compared to someone who is not was only about 3 to 5 percentage points different.
 
The only thing I'd heard that shortens battery life quickly is extremes of temperature. Cold as posted previously with a low battery, but also heat. They've been testing a bunch of Leafs in Phoenix, Arizona IIRC, running full discharge every day then topping them back up to full on a fast DC charger. I think they have seen battery degradation of about 30% in 18 months but as it's over 100 degrees F over there most of the year it is not surprising. And they have been put there on purpose as the high temps are known to accelerate battery degrading.

This is just personal opinion, but I'm pretty sure one thing to try and avoid is fast charging straight after a high speed run. Though that's when your most likely to need it. .. on a long trip. But if you can hang on a couple of minutes before plugging in it might give the batteries chance to cool down before you start whacking 50kW into them. Its high temps that kills lithium ion. some kind of chemical reaction takes place which makes the batteries expand and you lose some charging ability, never to be reclaimed. A battery temp gauge night be a nice aftermarket add on if it's available.

Reminds me of running a high performance turbo car where you have to let it run down before turning off the ignition. Of course the battery management system is meant to control the temps, but there's no harm in helping it.
 
This conversation reminds of people who insist on changing their ICE's oil every few thousand miles despite advances such as synthetics and monitoring sensors that have made such practices archaic. I see no issue with following BMW's advice regarding leaving an i3 unplugged about about 3/4 charge when you won't be driving it for long periods. But otherwise, drive and charge as needed.
 
New i3 owner here. My other car is a Tesla so I'm trying to learn my way around the i3 and its differences. The first issue I'm trying to understand is charge scheduling. In this thread, people talk about charging to 80%. How do you do that? I can't seem to find a way. The Tesla has a slider in the touchscreen that's allows you to set the charge limit. On the i3, it seems to shoot for 100%.

I have only had the car for one day but, so far, I like it.
 
I do not know if 100% on a Tesla is really 100%, but it is not on the i3, nor is zero really zero. The computer keeps a buffer and won't let you use it to make the battery last longer. Just charge it and use it.
 
Gavine said:
In this thread, people talk about charging to 80%. How do you do that?
Unfortunately, the i3's software isn't as capable as Tesla's. It doesn't support charging to anything but 100%. To charge to less than 100% as some of us typically do, an EVSE that supports setting a charge start time and duration or end time is required. My JuiceBox has this capability.

Most i3 drivers don't manage their battery packs; they just charge them to full with no regard how long they remain at full charge. All EV manufacturers, including BMW, know that consistently discharging a battery pack to near actual empty and charging to actual full charge would shorten its life, so their battery management software includes buffers at the low and high charge levels to prevent this harmful behavior. The ratio of the i3's usable to total capacity suggests that these buffers together are ~15% of the total capacity, but we don't know how this 15% is distributed between the low and high charge buffers. This total buffer size is considerably smaller than the Volt's whose battery pack life has been outstanding, probably because of its large low and high charge buffers. It's too early to know how well the i3's battery pack will age, but without the Volt's large buffers, I think that an i3 owner who wants to treat his battery pack well needs to do a little battery pack management.

Most i3 drivers lease rather than own, so they don't have a long view of battery pack life. A battery pack that isn't managed well will be some future owner's problem. Because of this, I fear that i3's out of warranty will require expensive premature battery pack replacements that could tarnish an i3's reputation, hurt future sales, and reduce already low resale values.
 
Hengus said:
I am asking the question because we have PV solar which, on a good day, could provide a chunk of the battery re-charge.

Hi Hengus,

This is an interesting point and I've had the same idea after my PV system was installed. Mine is a 4 kWp system mounted on a garden summerhouse and I have to see what the output during the summer is to get anywhere near something that would provide enough peak output to charge the car without taking anything from the grid. However, if you have anything closer to 10-12 kWp then you might be able to charge straight from the array during a good day with optimum conditions, using your 7kW EVSE.

This chap here owns an i3 and blogged about solar charging his i3, which has to be done using the standard 3-pin charger, as the peak output is not close to 7kW.
https://shkspr.mobi/blog/2015/11/solar-charging-electric-cars/

So your solar charging very much depends on your PV setup. Hope this helps. Let us know what your PV capacity is, I'd be interested to hear more.
 
I know on the level 1 side you can tell the car to adjust the charging rate but I do not remember if there is a setting for the level 2. IF there is, you could lower the maximum demand from the EVSE by a few clicks.

Some EVSE's have a user adjustable pilot signal to limit its output. If you don't have one, you might consider that as a priority. It may not be convenient to use...I've not really looked into it. If the goal is to minimize any grid usage, that should work. But, considering that it won't be taking all day to recharge the i3, and you'll likely have an excess available from your panels for the rest, your actual outlay for purchasing power may still end up with a credit. The car won't be pulling its maximum the entire time...it quickly ramps up, but gradually tapers down towards the end.
 
jadnashuanh said:
I know on the level 1 side you can tell the car to adjust the charging rate but I do not remember if there is a setting for the level 2.

The '14 that I'm driving for the weekend has settings for L1 and L2 but they use terms like "Maximum" and "Low". No idea what they mean in terms of current other than the 120V charging cable listing 12A draw which is standard derating for a 15A circuit. The owner's manual says the adjustment is for not overloading circuits, but in that case it would be nice to accommodate something stronger than a 15A circuit.

Also, per the manual the charging cable can be used for 120V to 240V but I assume that means there is a different cable for 240V.

It is certainly not as sophisticated as the Tesla system which lets you specify the actual current, which can be fairly high even without an EVSE.
 
The adjustable charging levels are only useful if the circuit you are tapping into must support other items at the same time. Ideally, it is a dedicated circuit, then, maximum would always be the norm.

FWIW, the EVSE sends out a signal announcing how much current it is designed for, and the car then will decide how much, up to that maximum or its internal capacity, that it wishes to use. So, say you had an 80A capable EVSE, plugging in an i3 to it would not overload either the EVSE or the i3 as the i3 would only draw up to it's maximum capacity. Sort of like plugging in a 100W lamp into a plug capable of 1800W (typical 15A circuit) except the receptacle is smarter than a typical wall outlet.

BMW chose 12A for the USA supplied level 1 EVSE to support the most common receptacle in US homes - a 15A device. Code says you can't draw more than 80% of the circuit capacity constantly, so that means 12A on that 15A circuit. Should you have a 20A/120vac circuit, theoretically, you could get 33% more, or 16A, but then, you'd not be able to plug it into a 15A receptacle. Finding a 20A receptacle in a typical home is just not going to happen unless it was installed for a specific purpose. This would severely limit the usefulness of the EVSE. A 20A/120vac receptacle has one slot T-shaped to accept either a 20A plug or a 15A one. A 20A device must come with the plug in that with one blade rotated 90-degrees so it can only be plugged into a 20A receptacle.
 
jadnashuanh said:
I do not know if 100% on a Tesla is really 100%, but it is not on the i3, nor is zero really zero. The computer keeps a buffer and won't let you use it to make the battery last longer. Just charge it and use it.

I'm quite sure 100% on a Tesla is 100% because regen is disabled when fully charged. On the other hand, the i3 still has regen with a full charge which confirms the fact that a full charge is not 100% on the i3.
 
Gavine said:
jadnashuanh said:
I do not know if 100% on a Tesla is really 100%, but it is not on the i3, nor is zero really zero. The computer keeps a buffer and won't let you use it to make the battery last longer. Just charge it and use it.

I'm quite sure 100% on a Tesla is 100% because regen is disabled when fully charged. On the other hand, the i3 still has regen with a full charge which confirms the fact that a full charge is not 100% on the i3.

Nope, wrong.

When the i3 is fully charged, it feels like it is using regen when you back off on the accelerator. The reality is that the electronics is operating the brakes instead of the regen. You can tell that by touching the brakes at that time, and you can tell that they are already engaged.
 
I33t said:
When the i3 is fully charged, it feels like it is using regen when you back off on the accelerator. The reality is that the electronics is operating the brakes instead of the regen. You can tell that by touching the brakes at that time, and you can tell that they are already engaged.

My suspicion also but in my case I can hear the brakes being applied on a SoC of 100%.
 
So then the i3 is charged to 100% then when fully charged. Is that what you're saying? Otherwise, the regen wouldn't be disabled. It seems the consensus is that it's not 100%.
 
Gavine said:
So then the i3 is charged to 100% then when fully charged. Is that what you're saying? Otherwise, the regen wouldn't be disabled. It seems the consensus is that it's not 100%.

Sort of. There is a difference between having the battery charged to absolute capacity and charging the i3 to 100% according to the display. There is a battery protection reserve there. The battery is 22kWh capacity and the max usable capacity is 18.8 kWh. There is a reserve at both ends to protect the life of the battery.

When the i3 is charged to the displayed 100% (or close), regen is disabled and the brakes are applied instead of regen. This gives the driver the same basic experience when lifting off the throttle, and it is one of the smarts built into the i3.
 
The i3 is capable of battery cell balancing, but I'm not sure how often it exercises that function. It tends to become more of an issue as the battery pack ages.
 
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