Battery management or the BMS

[vitesse]

I can't believe that we've had our classic MS70D for 5 months now and almost reached 7,000 miles in our tenure!

It appears to have a battery in very good condition and even after 7K miles (37K in total) it still charges up to an indicated 200 miles (Typical) at 90% which equates to 222 at 100% - roughly 2.2% degradation from what I have gathered from reports of range when brand new of 226-227 miles.

Our longest single non-stop drive was 186 miles from home to Darts Farm (Exeter) on the way to Falmouth. Teslafi reckoned we used 80% battery or 51.26kWh. Arithmetically, that's 100% = 64kWh.

On that trip we managed an average of 5% better than the car's predicted energy efficiency/range for the destination.

I've only seen one suggested real capacity for a new classic 70D battery and that was 68.8kWh - which suggests degradation is actually much worse. But I have no idea how Teslafi works things out.

Anyway, I digress. Battery Management.

I understand that periodically the BMS needs to be recalibrated. I've read that this should involve running the car down to significantly below 10% and then charging back up to 100% and the keep plugged in while the cells are levelled.

So I have a few questions:

1. Is that how to recalibrate?
2. How often should it be done?
3. Do you need to use a Supercharger or is home charging OK?
4. Does this make a difference to the reported Typical miles (up or down)?

Over to you!

 

[vitesse]

The info about real usable battery capacity for various packs was found here:

Tesla’s hacked Battery Management System exposes the real usable capacity of its battery packs - Electrek

 

[vitesse]

Nerd-overload or just too boring?!

 

[Roy W.]

Nerd-overload or just too boring?!

I’m my case, wary to share limited knowledge

As for the recalibration, I think that just achieves more accurate range predictions, and although your fully-charged range displays as higher, I think that’s mostly what you’ve achieved, not a real increase.

I had a discussion elsewhere on here about the need to balance the cells, and my view of needing to do it periodically seemed to be out of step with the other viewpoints.

As to whether it’s necessary, I don’t know, but I don’t see that it can do any harm. As it happens automatically when you charge to 100%, that’s all I rely on. I have a long trip at least once a month where I charge up to 100%, so job done!

Other than that my normal charging regime is up to 90%.

 

[vitesse]

Thanks Roy

I haven't done a 100% charge for several months. I also charge to 90% most of the time but I did go for a few weeks as low as 70% because there was no need to go any higher - the car is only using about 20% of range per day max for my wife's commute. We're visiting our daughter in Portsmouth next weekend so that will be a 180 mile round trip. a 90% charge should be more than enough but we might top up at Liphook. Then again I could run it down and charge to 100% after that trip (and not let it rest at 100% of course).

It's mostly idle fascination but I do like to know how these things work.

 

[WannabeOwner]

I have a long trip at least once a month where I charge up to 100%, so job done!

That's what I do too

Do you need to use a Supercharger or is home charging OK?

In terms of the 100% charge and cell-balance then best not to do it at Supercharger as it takes a long time (and doesn't benefit from high kW supply)

I've done it once at Supercharger (needs-must on that occasion), graph below along with a recent home-charge to 100%

 

[vitesse]

Thanks @WannabeOwner - I also want to have confidence that the BMS doesn't over-estimate the charge I have left when, in the hopefully rare situation, I need every last drop.

 

[WannabeOwner]

I also want to have confidence that the BMS doesn't over-estimate the charge I left when, in the hopefully rare situation, I need every last drop.

Good points. I want the battery to run out when Display Hits ZERO. Not 10 miles before that, and not keeping a just-in-case reserve of 10 miles (I need those ... ). There was a Spanish test of a variety of EVs, a bit old now, but quite a number ran out 10+ miles after hitting zero. From memory Tesla was one of the ones that ran out very close to zero. I've not tested when mine runs out but I do know it doesn't happen at 5%

I rely faithfully on the SatNav Energy : TRIP graph showing the original estimate of arrival SOC% and the revised-prediction as I drive. After one trip (fairly early on in ownership) where I limped home and arrived somewhere between 2% and 1% I don't want to repeat that ...

I tend to aim for 20% on arrival (e.g. if Supercharging I wait until destination arrival prediction is 20%) and then hammer it ... easing up if it starts nudging down to 10% (that hasn't happened, normally if I Supercharge 50 miles from home I get home with more than departure prediction ... hadn't thought about ti before, but perhaps prediction is "off" after Supercharging, and/or battery is nice-and-optimally-warm for the next few miles?)

I have had road closure and "Could you just swing by XXX and pick up YYY" on the way home ... then there is torrential rain which also kills range.

 

[vitesse]

Yes, the lowest I've arrived at a Supercharger on a long journey was 9% and my wife was NOT happy I wanted to go to the next Charger (Not really)

 

[MrBadger]

We're visiting our daughter in Portsmouth next weekend so that will be a 180 mile round trip. a 90% charge should be more than enough but we might top up at Liphook.

Nice place Liphook SC to do a 100% charge. Lots of stalls and minimal activity so you won't be inconveniencing anyone.

 

[vitesse]

Jane's t

Nice place Liphook SC to do a 100% charge. Lots of stalls and minimal activity so you won't be inconveniencing anyone.

Long walk to the facilities - hope it won't be raining!

 

[MrBadger]

If it’s raining don’t sit in the downstairs coffee bit as the roof leaks !

 

[vitesse]

I may well charge at home to 100% for our trip to Portsmouth on Sunday. Depending on what we do when we are down there and our route home we could arrive back with around 10% remaining. I'll then charge overnight to 100% again.

If I understand this - just charging to 100%, from a relatively high SoC - say, 80%, just performs the cell levelling.

Charging from a very low SoC (say, 10% or a little under) to 100% will help BMS calibration AND perform cell levelling?

Is that right?

 

[vitesse]

So I got home last night with 9% battery and attempted to charge to 100% by this morning. However, the charge completed at 97% rather than 100%. Here's the Teslafi chart:

I'm pretty sure I didn't make a mistake with the charge setting as in the app there is a small gap between the green and the charge % line. So I am rather bemused now.

 

[MrBadger]

What does TeslaFi show as charge limit?

ie 97 / 97 % or 97 / 100 % ?

 

[vitesse]

My wife has just parked up and it says:

Charger Disconnected
93 / 100 %

 

[MrBadger]

So charge limit is 100% as you thought you had set. hmmm...

anything in the minute logs?

When you normally charge with 100% limit, does it charge to 100% and this is first time this shortfall has happened?

 

[vitesse]

So charge limit is 100% as you thought you had set. hmmm...

anything in the minute logs?

When you normally charge with 100% limit, does it charge to 100% and this is first time this shortfall has happened?

I have only very rarely charged to 100% and they all completed normally.

There was a short dip in charger amp at 06:15

The Typical miles report for 97% still extrapolates to what I expect for 100% (222 miles).

 

[vitesse]

Will Fealey at TOGUK suggests it may be a consequence of vampire drain (long charging time) plus the BMS recalibrating and that it's quite normal.

 

[vitesse]

The fact that the car completed a charge set for 100% at only 97% has been bugging me so I did another 100% charge (at home 7kW) today and this time it completed at 99%. That's better but still not 100%

Is it just paranoia or should I be concerned?