EV battery management

ElefantEd

Our first Leaf (Gen 2, 24kwh) had an option to limit the charge to 80%. But our current Leaf (30kwh) doesn't offer this. I suspect because there is already a built in limit so the battery never goes to an actual 100% even though the on board display says that it is.

Petriix

ElefantEd said:

Our first Leaf (Gen 2, 24kwh) had an option to limit the charge to 80%. But our current Leaf (30kwh) doesn't offer this. I suspect because there is already a built in limit so the battery never goes to an actual 100% even though the on board display says that it is.

You're getting into semantics now. '100%' simply means charged to the maximum usable capacity allowed by the BMS, likewise '0%' means discharged to the lowest level the BMS will allow. 

These 100% and 0% are obviously lower (and higher respectively) than the theoretical extremes of charge and discharge the cells are capable of. However, as this portion of the battery is not made available, there is no point in considering it. Were the battery to be truly fully charged or discharged then it would quickly lose capacity.

But, as regularly repeated, battery degradation is not binary or linear. It's a curve which is greater when closer to the extreme ends of the voltage range, when the charge/discharge current is higher and when the temperature is higher.

Because the Leaf in particular has no active thermal battery management, it is especially susceptible to the effects of excess heat. The 'rapidgate' issue is a result of the BMS trying desperately to protect the battery from serious harm by severely limiting the charging speed.

(Nearly) all EVs ramp the rapid charging current down as the battery nears full. This is simply to protect the cells. If rapid charging at higher states of charge wasn't damaging then this would not be a ubiquitous feature.

mgfvvc

Oneye12 said:

I've posted this on other forums - but the message is the same.

In verycold / winter / inclement weather, the EV battery will not give the range as suggested. This is due to 'calculatable losses' such as power steering, wipers and in particular heater. An ice (internal combustion engine) has very similar losses but no-one does calculations on these.

The heating losses are the major component in winter range reduction for most EVs. An ICE engine generates so much waste heat in use that heating has negligible impact on range.

[Deleted User]

Grumpy_chap said:

It might be impossible to answer if people just have not reached this point but I'll ask anyway.

When a battery degrades on an EV, is it a steady rate of degradation or does the capacity of the battery suddenly fall off a cliff?

My comparison is a mobile phone.  For a long time, the battery is just fine, then after a while you notice that the battery is not lasting as long as it once did.  Once that point is reached, the rate of decline seems to be very rapid and it is not long until the shortened battery life is an inconvenience / annoyance of sufficient magnitude as to impact the usability and enjoyment of the phone.

Does a degrading EV battery also go to that curve of swift decline as to impact the use of the vehicle?

That's because phones have small batteries and when they reach a certain point they can't provide enough power anymore, so their performance drops off a cliff.

In a car the battery can provide a LOT more performance than the car needs in most cases, so this isn't going to happen. Also car batteries are made up of many, many cells, thousands of them in some cases. Your phone has just one or two. So a bad cell in a car isn't the end of the world.

As EVs become more common we will see more battery refurbishment services offered, where they replace weak modules in old packs. You can get it done already for many models but it's going to become more common.

[Deleted User]

Petriix said:

ElefantEd said:

Our first Leaf (Gen 2, 24kwh) had an option to limit the charge to 80%. But our current Leaf (30kwh) doesn't offer this. I suspect because there is already a built in limit so the battery never goes to an actual 100% even though the on board display says that it is.

You're getting into semantics now. '100%' simply means charged to the maximum usable capacity allowed by the BMS, likewise '0%' means discharged to the lowest level the BMS will allow. 

These 100% and 0% are obviously lower (and higher respectively) than the theoretical extremes of charge and discharge the cells are capable of. However, as this portion of the battery is not made available, there is no point in considering it. Were the battery to be truly fully charged or discharged then it would quickly lose capacity.

But, as regularly repeated, battery degradation is not binary or linear. It's a curve which is greater when closer to the extreme ends of the voltage range, when the charge/discharge current is higher and when the temperature is higher.

Because the Leaf in particular has no active thermal battery management, it is especially susceptible to the effects of excess heat. The 'rapidgate' issue is a result of the BMS trying desperately to protect the battery from serious harm by severely limiting the charging speed.

(Nearly) all EVs ramp the rapid charging current down as the battery nears full. This is simply to protect the cells. If rapid charging at higher states of charge wasn't damaging then this would not be a ubiquitous feature.

It's a bit more complex than that.

In the Leaf specifically 100% means that the battery is at maximum allowed state of charge AND that it has been balanced. That's why it is recommended to charge most EVs to 100% periodically, say once a month. It allows the cells to be balanced.

If you look on some of the EV forums people have done detailed studies of how different cars do it. It's easy to see, the power draw from the charger drops off, the car usually lets the cells cool for a while, and then it starts balancing.

Petriix

[Deleted User] said:

Petriix said:

ElefantEd said:

Our first Leaf (Gen 2, 24kwh) had an option to limit the charge to 80%. But our current Leaf (30kwh) doesn't offer this. I suspect because there is already a built in limit so the battery never goes to an actual 100% even though the on board display says that it is.

You're getting into semantics now. '100%' simply means charged to the maximum usable capacity allowed by the BMS, likewise '0%' means discharged to the lowest level the BMS will allow. 

These 100% and 0% are obviously lower (and higher respectively) than the theoretical extremes of charge and discharge the cells are capable of. However, as this portion of the battery is not made available, there is no point in considering it. Were the battery to be truly fully charged or discharged then it would quickly lose capacity.

But, as regularly repeated, battery degradation is not binary or linear. It's a curve which is greater when closer to the extreme ends of the voltage range, when the charge/discharge current is higher and when the temperature is higher.

Because the Leaf in particular has no active thermal battery management, it is especially susceptible to the effects of excess heat. The 'rapidgate' issue is a result of the BMS trying desperately to protect the battery from serious harm by severely limiting the charging speed.

(Nearly) all EVs ramp the rapid charging current down as the battery nears full. This is simply to protect the cells. If rapid charging at higher states of charge wasn't damaging then this would not be a ubiquitous feature.

It's a bit more complex than that.

In the Leaf specifically 100% means that the battery is at maximum allowed state of charge AND that it has been balanced. That's why it is recommended to charge most EVs to 100% periodically, say once a month. It allows the cells to be balanced.

If you look on some of the EV forums people have done detailed studies of how different cars do it. It's easy to see, the power draw from the charger drops off, the car usually lets the cells cool for a while, and then it starts balancing.

Rather amusingly, my MG5 says "100%, Finished" when it's balancing before reverting to "Connected, uncharged" when it's actually finished. 

Herzlos

Oneye12 said:

In verycold / winter / inclement weather, the EV battery will not give the range as suggested. This is due to 'calculatable losses' such as power steering, wipers and in particular heater. An ice (internal combustion engine) has very similar losses but no-one does calculations on these.

I think everyone acknowledges that combustion engine economy is vastly impacted in winter, but given the massive range of a tank of fuel it's not much of a concern beyond having to visit a petrol station a bit earlier.

Though having to go outside and supervise an ICE car as it warms up is a complete nuisance; the pre-warming is the main reason I'm going to electric next time.

BOWFER

Herzlos said:

Though having to go outside and supervise an ICE car as it warms up is a complete nuisance; the pre-warming is the main reason I'm going to electric next time.

Agreed.
My BMWs used to have a 'secret procedure' a driver could do, to get the engine running and then take the keys out and lock it.
That gave you unsupervised warm up, of a sort, in that the car was locked and no keys
But god knows what the insurance would have said if someone smashed a window, jumped in and drove off.
In that scenario I'd have had to resort to saying the thief had hot-wired it (and I'd have both sets of keys to assist proof)