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BMS stands for Battery Management System; its job is to monitor and manage the state of the battery.
Whenever you're charging, the BMS will oversee this, monitoring things like temperature, charge speed, etc and managing these. If you've set the charge limit to 80%, the BMS will be what manages this and calls a halt to charging when 80% is reached, even if the vehicle is still connected to a charger.
 

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vauxhall corsa e elite nav
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Discussion Starter · #11 ·
The reason i asked is i said this on another forum.

The reason it takes longer to charge to 100% is for the last 20% the BMS (BATTERY MANAGEMENT SYSTEM) will slow the charging rate down, 1 to let the battery cool, 2 to balance the battery cells so they are all charged to the same level, an EV should be charged to 100% at least once a month just for balancing. No need to charge 100% every time, because I only charge once a week I charge to 100% but if I needed to charge every day I would only charge to 80%.
I have only used a public charger once and charged at 90 kWh for 20 mins just to make sure everything worked.
The Corsa EV will charge at a rate up to 100 kWh and it's the car that has the charger built-in, not the charge point that's just a power supply.
 

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GOLF GTE PHEV
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Doesn’t the charger control the charging?
Which charger - external or internal?
The BMS controls (indirectly) an external DC charger's current output.

I believe the BMS is in control of the onboard AC-DC charger and will command it to stop.
 

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vauxhall corsa e elite nav
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Discussion Starter · #13 ·
and this is a reply that i got

Sorry to disagree. It is not the BMS that limits the charge rate. It is the nature of charging in that it is CCCV.

Constant Current Constant Voltage.

A battery can be charged at up to a specific current maximum. It can also only accept a charge voltage up to it's maximum charge voltage.

This explanation is not just aimed at you, but for anyone interested in what I mean by CCCV.

Lets use an imaginary battery so that the maths is easy.
The battery is flat at 2V and full at 4V and it is a 1AH battery.
The charge voltage is directly related to the current. So if you increase the charge voltage you increase the charge current proportionately.
Let's say the internal resistance of battery is 1Ω and the max charge rate of the battery is 1C So the maximum charge rate would be 1AMP.

If the battery is flat and showing 1V, we cannot charge at the full 4V maximum charge voltage because 4v-2v = 2v (difference) and 2v / 1Ω = 2Amp which is double the maximum charge current.
So you set the charge voltage at it's maximum which is 1Ω x 1A = 1V above battery voltage.

At the start when the battery is empty at 2V the charge voltage would start at 3V. As the battery charges it would increase to stay 1V ahead of the current battery voltage as it charges.
The charge voltage would stay 1V ahead of the battery voltage until it reaches 4V at which point it couldn't get any higher.
That is the constant current part of the charge cycle. The fast bit and usually around 80%.

We then move to the constant voltage part where the charge voltage is capped. The charge voltage cannot go any higher than 4V so as the batteries voltage continues to rise the difference between them falls and the charge current falls proportionally.

Here is a quick rough and ready table. The 1st two show constant current and the rest show the slow fall in charge current when it enters constant voltage mode.


1628685422380.png



The BMS will kick in when one cell reaches the maximum voltage. It will apply a resistance between the cathode and anode of that cell to drain it a little while the rest catch up. rinse and repeat.
 

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GOLF GTE PHEV
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The reason i asked is i said this on another forum.

The reason it takes longer to charge to 100% is for the last 20% the BMS (BATTERY MANAGEMENT SYSTEM) will slow the charging rate down, 1 to let the battery cool, 2 to balance the battery cells so they are all charged to the same level, an EV should be charged to 100% at least once a month just for balancing. No need to charge 100% every time, because I only charge once a week I charge to 100% but if I needed to charge every day I would only charge to 80%.
I have only used a public charger once and charged at 90 kWh for 20 mins just to make sure everything worked.
The Corsa EV will charge at a rate up to 100 kWh and it's the car that has the charger built-in, not the charge point that's just a power supply.
Wrong.
On a DC charger, the 400V DC goes direct to the traction battery whilst being monitored by the BMS. It is this that will command the charger to alter the charging current at stop at 80% or 100%.

The onboard charger is only used when connected to an AC source.

You need not have any charging regime as the BMS will take care of the battery - that is what it's for.
 

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GOLF GTE PHEV
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and this is a reply that i got


Sorry to disagree. It is not the BMS that limits the charge rate. It is the nature of charging in that it is CCCV.

Constant Current Constant Voltage.

A battery can be charged at up to a specific current maximum. It can also only accept a charge voltage up to it's maximum charge voltage.

This explanation is not just aimed at you, but for anyone interested in what I mean by CCCV.

Lets use an imaginary battery so that the maths is easy.
The battery is flat at 2V and full at 4V and it is a 1AH battery.
The charge voltage is directly related to the current. So if you increase the charge voltage you increase the charge current proportionately.
Let's say the internal resistance of battery is 1Ω and the max charge rate of the battery is 1C So the maximum charge rate would be 1AMP.

If the battery is flat and showing 1V, we cannot charge at the full 4V maximum charge voltage because 4v-2v = 2v (difference) and 2v / 1Ω = 2Amp which is double the maximum charge current.
So you set the charge voltage at it's maximum which is 1Ω x 1A = 1V above battery voltage.

At the start when the battery is empty at 2V the charge voltage would start at 3V. As the battery charges it would increase to stay 1V ahead of the current battery voltage as it charges.
The charge voltage would stay 1V ahead of the battery voltage until it reaches 4V at which point it couldn't get any higher.
That is the constant current part of the charge cycle. The fast bit and usually around 80%.

We then move to the constant voltage part where the charge voltage is capped. The charge voltage cannot go any higher than 4V so as the batteries voltage continues to rise the difference between them falls and the charge current falls proportionally.

Here is a quick rough and ready table. The 1st two show constant current and the rest show the slow fall in charge current when it enters constant voltage mode.


1628685422380.png



The BMS will kick in when one cell reaches the maximum voltage. It will apply a resistance between the cathode and anode of that cell to drain it a little while the rest catch up. rinse and repeat.
Sorry that's rubbish.
 

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The reason i asked is i said this on another forum.

The reason it takes longer to charge to 100% is for the last 20% the BMS (BATTERY MANAGEMENT SYSTEM) will slow the charging rate down, 1 to let the battery cool, 2 to balance the battery cells so they are all charged to the same level, an EV should be charged to 100% at least once a month just for balancing. No need to charge 100% every time, because I only charge once a week I charge to 100% but if I needed to charge every day I would only charge to 80%.
I have only used a public charger once and charged at 90 kWh for 20 mins just to make sure everything worked.
The Corsa EV will charge at a rate up to 100 kWh and it's the car that has the charger built-in, not the charge point that's just a power supply.
I think you are confused. Probably it's a rubbish owners manual.
 

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The BMS DOES control DC charging, just as it controls AC charging. The only difference is that when AC charging everything is within the car, so the BMS just communicates to the OBC directly, whereas with DC charging the BMS communicates with the charger.

Simplistically, what happens is that as the highest voltage cell/cell group approaches shunt activation voltage (where charging current to that cell/cell group starts to be shunted past that cell/cell group) it signals to either the charger (for DC charging) or the OBC (for AC charging) to reduce the current. It does this because the shunts only have a limited power dissipation, so cannot operate at very high charge current without generating a fair bit of heat.

An out of balance pack will throttle back the charge current more quickly than a well-balanced pack. This is one good reason why an AC charge is a good idea every now and again, as the lower charge current allows the pack to balance towards the end of charging (that very slow stage over the last couple of percent), and a well-balanced pack can then take a rapid charge for longer before throttling back next time a rapid is used.

This issue can be resolved by using active balancing, something I've been doing for a while with my motorcycle battery pack. Not sure if many EVs yet do this, but it does have the advantage that the pack will try and balance all the time. The system I used has "flying capacitors", that transfer small amounts of charge from the highest voltage cells/cell groups to the lowest voltage ones. Over time this brings every cell/cell group to the same voltage, and tends to maintain that all the time. It's also less wasteful, as there's not as much charge power wasted as there is with a cell/cell/group shunt system.
 

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Nissan Leaf Accenta 30kWh, 3.3kW, 2017
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Sorry that's rubbish.
It is and it isn't. When I am charging cell(s) using a power supply, I will set the desired voltage and the current limit. It will then do the whole CC to CV thing that was mentioned. Note, if doing more than one cell in series to keep an that all cells stay balanced throughout a charge in case one has a lower capacity and you end up with one being overvoltage! This is one reason why it isn't recommended to charge batteries just using a power supply without a feedback from... a BMS.

As has been mentioned above, the BMS will be able to monitor all cell voltages and set the limit (either in voltage or power, not sure of the details of the various protocols) to the appropriate charger based on the maximum cell voltage, or cell temperature. It therefore controls the charging process, based on the whole CC/CV principle, along with any temp issues that might also occur. It can also decide to be programmed to limit the SOC to 80%.

Balancing is sort of different. The amount of current you can "shunt" or "bleed" from the highest cell will be minimal; less than 1A and even then, probbaly not full time depending on how beefy they made the CMU's. I guess the balancing strategy will vary battery to battery, but generally you want to do it at a high SOC and when not under load, which is why I guess at the end of charging is the most likely time to do it.
 

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EV batteries certainly do charge CCCV. You can see it just watching the voltage and currents on LEAFSpy or whatever.

Its an intrinsic characteristic of Lithium Ion batteries.

Not sure why everyone seems to think the car is "balancing" at the end of the charge as its tapering, its clearly not, its just holding a constant voltage while the cells saturate.

Balancing happens at a tiny current. 10-20ma perhaps, and it happens over a much longer time than one charging session.
 
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