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Question on Tesla Battery Cell Charging Currents

Question on Tesla Battery Cell Charging Currents

I have read up a little on the battery packs, 6 groups of 74 cells in each moddule, and 16 modules in series etc, but does anyone know what current is being put into each 18650 battery cell when the car is plugged into a Supercharger station?

I'm just interested in how hard each lithium cell is being pushed with regard to the Amperage being put in.

Thanks

Dramsey | 27. kesäkuu 2016

Good question. I suppose you could come to a reasonable estimate by looking at the amps shown on your instrument panel when charging and dividing by the number of cells in the car.

So if I'm charging at, say, 250 amps, and divide that by 7104 cells, then that would be 40 milliamps per cell. But that doesn't seem right-- way too low.

bb0tin | 27. kesäkuu 2016

You can work it out by considering the pack voltage and cell voltages.

Dramsey | 28. kesäkuu 2016

Well, let's see: a typical LiIon cell has a potential of about 3.6 volts. I understand that the main battery pack voltage is 400 volts, which implies cells arranged in groups of about 111. That divided into 7104 total 18650 cells is 64 cell packs-- hey, at least the numbers are making sense-- but then I run out of information, since I don't know how the cells in packs are arranged, in series or in parallel, and for that matter the same thing applies to the pack arrangement in the larger battery pack.

bb0tin | 28. kesäkuu 2016

You do not need to know how the packs are arranged. You only need to know how many packs there are and how many cells in each pack. The cells will be arranged in series.
PS: By 'pack' I am not even talking about the real packs. It is the theoretical 'pack' to charge each cell at the required voltage. The real packs need to obey the same rule per cell.

Dramsey | 28. kesäkuu 2016

Ok, so each of the 64 packs outputs 400 volts from its series-connected cells. In that case we'd just divide the hypothetical 250 amps by the number of packs to get 3.91 amps or so per pack at whatever voltage (480 volts if we assume the Supercharger is cranking at its full 120-kW capacity). Since the cells are connected in series each cell is getting the full 3.91 amps. Which is pretty high, but does that sound right?

bb0tin | 28. kesäkuu 2016

Your numbers are a little high but in the ballpark. Standard 18650 batteries can be charged at about 3A without active cooling. Tesla charges at a high rate initially and then drops the current as the battery charges. They also actively cool the battery. They are charging it at the accepted rate.

stuart | 28. kesäkuu 2016

Don't overthink this. Amps is a measure of how many electrons per second are going past some point, like the business end of your charging plug. No matter how you arrange the batteries, all those electrons are going somewhere, like to be used to store chemical energy. If all batteries are treated equally (as I suspect they are), then divide the total current by the number of batteries to get the current per battery. Dramsey is more or less right the first time. I get about 35ma per cell at 250 amps total. My home wall outlet is supposed to deliver just 40 amps,so when using that plug just about 5.6 ma per cell.

All the discussion in other posts is more likely to change the number of volts per cell. I have no idea what that is. Each cell needs to store about 12 wh for a 90 Kwh battery pack. If I want to spend about 12 hours loading that energy, I need to have a rate of 1 watt. That looks to me like 5.6 ma at 178 v. Seems doable, but unlikely at that high voltage. Since you can always trade volts for amps, they could use twice the current at half the voltage, or any other similar combination since w=a*v. Someone said they use a 3A rate. In that case they are charging at about 1/3 v. That seems low to me. The higher the amps the bigger the conductor. If I had that much time to charge I'd lower the current and rase the voltage to maybe 5v. That would give you 0.2a. Now you are into battery engineering and I know nothing.

My wall plug is supposedly 208 v (YMMV) so that could be all you need to know. But if they want to use a lower voltage to make the wiring easier or to avoid voltages that would cause bad things to happen to the battery, they increase the current. Knowing the incoming amps at the charger plug is insufficient information. You need to know the battery charging voltage.

This is all ideal, no losses so adjust number accordingly.

bb0tin | 28. kesäkuu 2016

@stuart
You are totally wrong, as a simple sanity check would have shown.
A 18650 battery has about 3000mAH. At 35mA it would take 85 hours to charge. It does not take 3 days to charge.
At the correct 3A it would take 1 hour to charge, about how long it does take to charge.

bb0tin | 28. kesäkuu 2016

@stuart
The rest of your post was also rubbish, so bad that it may even be trolling.
Assuming it is not trolling, you should know what you are talking about before trying to correct someone's post.

stuart | 30. kesäkuu 2016

as wikipedia says:An ampere-hour is not a unit of energy. In a battery system, for example, accurate calculation of the energy delivered requires integration of the power delivered (product of instantaneous voltage and instantaneous current) over the discharge interval.

Voltage matters. Dramsey's post talked about 250 amps at the charging plug, so that is what I was referring to. He didn't mention voltage.

I decided to look at the specs. As I read it, the battery cell delivers 2250 mAh at 3.6v. That sounds like 8 wh to me, so I don't quite understand how around 7100 of them gets to be more than 56.8 kwh, but maybe I'm missing something there. If you have an input of 250 amps at 208VAC that would be a rate of around 52kw. I suspect I have to toss in a square root of 3 somewhere by the time it gets to be DC. That class was a long time ago. Sounds like you can charge any size Tesla battery in less than 2 hours with that power if charging was linear, but it isn't. A full charge would take somewhat longer.

My battery technology info is very old, but IIRC you have to charge it at a higher voltage than the output of 3.6V. The design engineers get to choose whatever current and voltage they please that is consistent with the total power (watts) available. The tradeoffs are things like high voltage requiring better insulation and possible damaging the battery, and high currents that require thicker wires. The battery specs vary among brands, but most seem to recommend a charging voltage around 4.2, a current that can start as high as 4000mA and drops to as low as 50mA as charge increases with a total charge time of 1-2.5 hours. If you have less power available (as with my 208V 40A wall plug) that ends up closer to 8-12 hours.

Can someone other than bb0tin check my work? I'm hoping to not have to get a job in engineering ever again, but dc electricity should still be in my skill set.

bb0tin | 30. kesäkuu 2016

I won't check your work, although "Can someone other than bb0tin check my work" does not actually exclude me from doing so.
Itching itching. Don't scratch.

h.r.elenbaas | 30. kesäkuu 2016

A post on TMC covering a 85Wh teardown will give some if not all the you one need. The 85Wh(400V DC) battery has 16 modules (14 for the 60Wh pack) witch each 6x74 custom Panasonic 18650 cells. One could read this as an array of 96 (16x6) bij 74 cells with give you the 7104 number of cells. 96 cells in series wil give you the 400V DC (open) voltage, the current is delivered through 74 Cells in parallel...
The voltage of 400V will decrease when current is drawn, when charged current wil flow back at a higher voltage.
So when charged at a 120KW Supercharger a maximum initial charge current of 300A (120kW / 400V) is distributed over the 74 cells in parallel yielding a max. charge current around 4 amps.
I'm pretty sure the for Tesla customized Panasonic cells will be fine with that.

h.r.elenbaas | 30. kesäkuu 2016

witch --- with

English is only my second language

stuart | 30. kesäkuu 2016

elenbaas: Your English seems fine to me. Your probably understand this better than I do, but batteries have an internal resistance which leads to some power being wasted to heat (hence the battery cooling system). How does this change charging when you take into account the series connection? All the numbers I know about are for individual cell charging. Is there any difference at all?

h.r.elenbaas | 30. kesäkuu 2016

@stuart
My take on the Tesla’d Panasonic cells are that they are rated somewhere around 3.6V / 3500mAh for the 90kWh pack, so your ‘square root of 3’ is almost accounted for…
For the most part charging a series of parallel cells or a single cell is the same. Because of minor differences between cells even for even cells from the same production-run these differences between cells in a module has to be levelled out to maintain a uniform SOC: state of charge, this to maximize the capacity of the battery-pack.
Therefore any of the 16 modules in series has it’s own controlling electronics which monitors the charging process, any of the 6 74-cell rows are subject to ‘balancing’. This balancing scheme is an addition to the charging-scheme of cells in a multi-cell battery-pack. Tesla uses ‘passive’ balancing by using FET-controlled power resistors in the pack’s controller, these will ‘bleed’ current from the charging current from any of the 6 74 cells rows. This balancing is typically performed at significant lower currents for a non-balanced battery-pack to avoid overheating the bleeding-resistors.
My knowledge is derived from the earlier mentioned TMC (teslamotorsclub.com/tmc/threads/pics-info-inside-the-battery-pack.34934/) and previous century electronics engineering.

dansplans | 30. kesäkuu 2016

@h.r.elenbaas - your numbers and breakdown is close to spot on. Nominal voltage for 18650 calls is 3.7v, with a published capacity of 3100mAh per cell used in Tesla packs. Newer packs may well have slightly higher capacities.

4Amp charge rates are in line with best of class cells, and also conform to Mr Wormwoods' suggestion elsewhere that Tesla's cells may very well be designed for 3.5Amp currents.

The tests that I have seen, indicate a range of charge voltages and amperages, depending on the state of charge. Charge voltage typical runs between 4.95-5.05v per individual cell, with amperage starting high and decreasing to a trickle as the cells approach maximum. Your 400v cell packs would then receive a charge voltage around 480v.

dansplans | 30. kesäkuu 2016

*cells

bb0tin | 30. kesäkuu 2016

@dansplans
If you are going to attempt to insult referencing a character then you should at least pick one that is apt.
I am glad that you realise that what I posted is correct.
Now you need to also realise that all the corrections I have done to your posts are also correct.

dansplans | 30. kesäkuu 2016

" Never argue with a moron. Anyone who overhears the argument will not be able to determine which of you is the moron."

bb0tin | 30. kesäkuu 2016

@dansplans
You have posted on many threads with patently absurd and incorrect statements. The reason you do not argue is that you cannot defend your ignorant and incorrect opinions.
If you do not want your posts to be factually challenged then either stop making them, or post correct statements rather than incorrect statements.

teslamonkey | 18. heinäkuu 2016

I parked my 2013 Model S P85 to go shopping and it had 140 miles on the battery. When I returned a half hour later the battery had gone to zero and I was unable to do anything. I had to have my $100K+ care towed to a Service center.

I was told that the HV Battery showed a cell imbalance and it had to be sent to Fremont for a refurbishing.

Is this a known or common defect? Also what exactly does this mean? My car showed no signs there was a problem before it went to zero.

Nexxus | 19. heinäkuu 2016

@dansplans,

You'll have to excuse bb0tin, as he probably suffers from Asperger Syndrome.

dansplans | 19. heinäkuu 2016

Aspergers.....now that is funny! +5

bb0tin | 19. heinäkuu 2016

@Go_Peddle_4_me
What are you asking dansplans to excuse exactly?
I guess it must be correcting his idiotic and incorrect posts, of which there are many.

@dansplans @Go_Peddle_4_me
Well done insulting people with Aspergers.
No, I do not have Aspergers, but I do know someone who does.
You should be ashamed of yourselves.
What other section of society would you two like to insult next?

Nexxus | 19. heinäkuu 2016

You should go get tested to make sure. It describes you to a tee.

Nexxus | 19. heinäkuu 2016

And I have nothing to be ashamed about. I didn't insult anybody else in the world, or a class of people at all. I described your mentality and acuity. I'm not the one calling people names and belittling their posts for their opinions. I leave that up to you.

bb0tin | 19. heinäkuu 2016

@Go_Peddle_4_me
To claim that someone has Aspergers, because of behaviour which you do not like, is an insult to people with Aspergers.

Yes, I belittlle ignorant and incorrect posts, particularly when it is an 'answer' to a question from another poster. If someone answers a question, does not know what they are talking about, has not checked that their answer is correct, and it is indeed incorrect, then I will have a go. Nobody is holding a gun to these posters heads saying they must post something. The can keep quiet like the vast majority of members on the forum. They can check that what they are about to post is correct. But instead they repeatedly post their ignorant and incorrect opinions as if they know what they are talking about.

Then there are posters such as yourself. You pop onto threads for no other purpose than to make a personal insult. You have done this on multiple threads.You provide nothing of value to the forum with these posts.

Nexxus | 20. heinäkuu 2016

@bb0tin,

BS, you are a troll and have the mentality of a 14 year old know-it-all. And if you look at my posts in the past I have offered my opinions and information, without being a bombastic cretin like yourself. I only pop onto threads to make a personal insult to your comments. I don't believe I've ever had to take someone to task, other than you. You are an idiot and the sooner you leave this forum, the better it will be.

bb0tin | 20. heinäkuu 2016

@Go_Peddle_4_me
You just illustrated my point.

Nexxus | 20. heinäkuu 2016

Point intended!!

Red Sage ca us | 22. heinäkuu 2016

Go_Peddle_4_me: Superb.

bb0tin | 22. heinäkuu 2016

@Red Sage ca us
You complain that I factually critique your erroneous posts and how unfair you seem to think that is.
But here you are supporting simeone who uses Aspergers in an attempted personal insult, follows it up with '14 year know-it-all', 'bombastic cretin' and 'idiot'.
Perhaps I should add gratuitous personal insults when I falsify your posts, and then you will be happy.

milesbb | 23. heinäkuu 2016

the supercharger max rate is 90 kW at 400 volts DC
90,000/400 = 225 amps DC

The 3.6 volt 18650 batteries are arranged in series so that they can produce the 400 volts.
400/3.6 = 111 cells in series

Each string is in parallel with other series strings to get to the 7104 cell
7104/111 = 64 parallel strings of 111 cells each.

The 225 amps DC gets equally divided between the 64 parallel strings.
225/64= 3.51 amps DC through each string and through each cell.

milesbb | 23. heinäkuu 2016

or (90kW/7104)/3.6 = 3.51 amps

bb0tin | 23. heinäkuu 2016

@milesbb
The supercharger max rate is about 120kW

milesbb | 26. heinäkuu 2016

@bb0tin
thanks for the correction. I was thinking the Super Charger pair had a 125 kW rating with 90 kW going to the first car. I now see that the Super Charger pair has a 135 kW rating with 120 kW going to the first car,

(120kW/7104)/3.6 = 4.69 amps