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I just posted this in the general EV charging forum, but as it's specific to the i3 I wanted to put it here too. I've seen advice both here on SpeakEV and elsewhere to keep electric car batteries at around 50-80% charge instead of 100% during the coronavirus lockdown. This is to maintain the long-term health of the car battery.

I'm not sure how to reconcile this advice with what I read in the manual of my BMW i3, which on page 203 reads:

"Do not allow the vehicle to sit idle for extended periods with a low charging state.

Before storing the vehicle for an extended period, check the battery charge indicator to ensure that the high-voltage battery is fully charged. For longer idle times, park the vehicle with a charging plug plugged into a suitable power source. Regularly check charging state. Otherwise, the high-voltage battery can be damaged when there is excessive discharge.

For idle phases that last several weeks, park the vehicle with a fully charged battery if possible."

This suggests that the i3 wants to be in a FULL (presumably 100%) state of charge when it's not in regular use. I've written to BMW asking for clarification, I'll post here if I hear back from them.
 

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i3 "full" isn't really 100% full, it's closer to 94%.

The problem with storing battery packs with low states of charge is that all cells do not retain charge evenly. As a multi-cell battery depletes, it's possible for the individual cells that run down first to be negatively charged by the other cells in series that are still producing power.

Of course, the i3 has protection against this, too, as there is about 10% reserve (more or less, I don't believe that's the exact figure which I believe varies across the 60Ah, 94Ah, and 120Ah batteries).

But as long as you're not going anywhere, there's no reason you can't leave the car at a mid value, i.e. 50%, and then a month from now plug it in to your slow charger for a couple of hours and bring it up to 60%. The reason I suggest this is because I believe system will not maintain the 12v battery unless the car is plugged in OR driving. And I gather you're trying to avoid leaving it plugged full time, or for that matter trying to avoid reaching 100%.
 

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The disconnect in that thinking though is that the i3 only has a 6% margin at the top. In other words, an owner of a car with no buffer is told to leave 20 to 50% at the top, while the advice to i3 owners is "6% is OK."

Now personally, I'm Ok with 6%, and I'm Ok with 50%, and see no problem with playing it cautious.
 

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Since most of us have to make the "essential journey" to buy food occasionally its makes sense to use an electric car - ideal for short journeys...and also keeps it ticking over. We normally keep the i3 at 100% in case we want to use it, but at the moment are letting it run down as we aren't allowed to use it. When it gets to 40 or 50% I'll charge it up to 100% again. I think not using any car (electric or ice) is bad for the car so we need to find a way to cycle them somehow at least every three of four weeks. We're lucky and have some land. I took the Rangie for a spin round the fields for 10 mins the other day to get everything up to temps and made sure all of the moving bits move and the stationary bits didn't.
 

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The disconnect in that thinking though is that the i3 only has a 6% margin at the top. In other words, an owner of a car with no buffer is told to leave 20 to 50% at the top, while the advice to i3 owners is "6% is OK."

Now personally, I'm Ok with 6%, and I'm Ok with 50%, and see no problem with playing it cautious.
Who says 50%, and on what basis?
 

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Who says 50%, and on what basis?

AFAIC it's a commonly passed around myth. Maybe there is or was some truth behind behind it with older technology NiCad, I don't know. But while I don't subscribe to it, I also don't think it hurts to do so.
 

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Maintaining the big battery is easy. But what about the 12v battery in the i3.
Does it mean running the car or charging it up while the big battery is charging?
Can one use a trickle charger on the 12v battery to keep it up?

It is the 12v battery that bothers me as it could brick the car.

Would it be better to change up the main battery using the granny charger in order to lengthen the time the 12v battery is being charged?

I have seen no clear advice from BMW.
 

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Maintaining the big battery is easy. But what about the 12v battery in the i3.
Does it mean running the car or charging it up while the big battery is charging?
Can one use a trickle charger on the 12v battery to keep it up?

It is the 12v battery that bothers me as it could brick the car.

Would it be better to change up the main battery using the granny charger in order to lengthen the time the 12v battery is being charged?

I have seen no clear advice from BMW.
Plugging in to the granny charger will "unbrick" it. Yes leaving unattended for a long time. it is the tiny 12v battery you should be concerned about. As suggested earlier, plugging it in for a short spell, will ensure the 12v battery is charged. The official advice is to leave the car plugged in if it is being unused for a prolonged period. That will manage everything.
 

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The correct SOC for idle storage is 42% for all cars, all battery types. It says so in the HHGTTG.

lol! :D :D :D

And for the non-Douglas Adams disciples, here is some "clear advice from BMW":

(...and with respect to the question of the 12v, leaving it plugged in will see that that is tended to.)
129431
 

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To be slightly controversial - the manual says what it says as that's the easiest advice to follow, and that which will minimise the number of call-outs / complaints that BMW get within the warranty period about the car failing to start. If you want to keep the car for a long time, you need to be worried about long-term degradation of battery capacity. Storing the battery full (or nearly full, there is a top buffer) for a long period of time will result in faster capacity loss. Lots of papers about this and the difference is significant. Higher temperatures also make the effect worse. Optimum is 50%, 20-80% is considered acceptable, being lower (<20%) is worse than being higher (>80%).
 

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Lots of papers about this
Do you have a source that includes 94Ah i3?

LEAF 24 uses a different battery chemistry, has a smaller top safety margin and the built-in charger can overcharge the pack. LEAF 30 is similar.
 

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Do you have a source that includes 94Ah i3?

LEAF 24 uses a different battery chemistry, has a smaller top safety margin and the built-in charger can overcharge the pack. LEAF 30 is similar.
Having checked - no. General lithium ion chemistries. They all seem to agree but I'm quite happy to admit that I have no specific knowledge on the BMW pack and whether it has some other chemistry that makes it behave differently.
 

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I store all my LiPo's for my RC cars at 50% - 60% depending how well I did in the race and charge fully to 100% just before their next use.

With the Zoe I just charge 100% and let it run down until 20 - 30% before charging back up to 100% seeing as it has a top and bottom buffer to protect the battery. Currently she has gone from being charged fully every 2 to 3 days to I haven't charged since lockdown began and she currently has 43% left so I might only be charging every fortnight from now on :)
 

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Having checked - no. General lithium ion chemistries. They all seem to agree but I'm quite happy to admit that I have no specific knowledge on the BMW pack and whether it has some other chemistry that makes it behave differently.
It's not just chemistry, but rather what built in buffers there are in the BMS.

If in doubt follow the manual.
 

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It will not hurt to store the lithium batteries however they should be at 75-85% SOC.

If you would like to keep using it without driving, you can pre-condition the car once a day until it needs charging again without moving it.
 

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To be slightly controversial - the manual says what it says as that's the easiest advice to follow, and that which will minimise the number of call-outs / complaints that BMW get within the warranty period about the car failing to start. If you want to keep the car for a long time, you need to be worried about long-term degradation of battery capacity. Storing the battery full (or nearly full, there is a top buffer) for a long period of time will result in faster capacity loss. Lots of papers about this and the difference is significant. Higher temperatures also make the effect worse. Optimum is 50%, 20-80% is considered acceptable, being lower (<20%) is worse than being higher (>80%).
Pure Genius😁
 

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Thanks Guys, it sounds like the best thing to do is granny charge or set the car to slow charge thus making the charge time as long as possible, to top up the 12v battery while connected and the big battery up to about 50%..Then unplug the charging and then to keep the ability to recharge the 12v battery use preheating about once a week to run the big battery down a bit, then charge the car up to 60% on the next occasion to give time for the 12v battery to charge, following week 70% etc ending up with the worst case as late as long as possible of charging up fully so the battery is once again balanced, but then having to do a long preheat to get the SOC to leave capacity for the next charge. This going to be fun!

Very clear low charge damage in the big battery is much worse and expensive to fix than replacing a damaged 12v battery.

So the i3 is not designed for self-isolation or big sleep like the LEAF, it wants to stay connected!

I know how it feels. :)
 
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