Quick discharge

[fnegroni]

I would just like to query why so many people seem to believe that you need to use the Granny Charger (typ. 2kW only) in order to perform either the circa weekly or 2-week (depending on your typical mileage) charge to 100% including the balancing charge at the end of the charge?
Here is pg.173 of the manual (may have been updated now, since I downloaded this in July 2023):

Slow Charging

Note: Carrying out a full slow charge is the only way for the high voltage battery to reach the optimal equilibrium state (equalisation charge).

High voltage battery chargers are available with various power outputs. Chargers with outputs of up to 11kW are generally considered as slow chargers, larger than 11kW are considered as fast chargers and rapid chargers are available in AC or DC outputs. Generally the AC chargers are rated at 43kW and the DC chargers at 50kW plus.

Charging times are dependent on charger output.

To carry out an equalisation slow charge, it is recommended that the charger output does not exceed11kW.

Note: Chargers of up to 7kW power output are supplied via standard household single phase power. Any chargers that are rated above this, 11kW for example, will require a 3 phase power supply.

Further - in the Section on pg.295 of the manual under the title: "Precautions and Restricted Conditions for use of Battery" the last sentence in the first paragraph states:

"Every 3 months to half a year, a low-power (less than 10% or 1 grid) full charged is performed."

Apart from the fact that this is not exactly clear (I assume due to translation errors), I believe that this could be refering to charging at a 'low power' level of less than 10% of the cars maximum charge rate of 135kW i.e. < 13.5kW or '1-grid' which would be less than a single grid connection i.e. 30A or 7kW (assuming a 230V ac supply from the grid), to a full charge level of 100%.
Hence the definition earlier of low-power chargers being anything less than or equal to 11kW being suitable.
I will therefore persevere in trying to get clarification from MG on this - unless that is anyone else has already got a clearer answer already from MG on this topic?

My interpretation of that last sentence is that you should perform a full slow charge from less than 10% (state of) charge or 1 grid (of the 10 grid charge level meter on the dashboard).

 

[tsedge]

My interpretation of that last sentence is that you should perform a full slow charge from less than 10% (state of) charge or 1 grid (of the 10 grid charge level meter on the dashboard).

Yes, much more likely to be this.

The slow charger things is a bit of a misnomer anyway - on a Rapid DC charger it will drop as low as 2kW on my car at high SoC, actually lower than my home charger tries (keeps 7kW right up until 100%).

It is the fast bit of the Rapid charge curve which will do some damage over time with lots of rapid charging plus leaving the car at high states of charge (or constantly charging to 90%+).

It makes sense that every so often it needs to deplete the battery to 10% to help calibrate the lower end of the scale.

 

[Rolfe]

What I'm not sure about is whether the full charge has to be uninterrupted. I have my battery down to 36% now and was planning to take it down below 10% by Monday evening. Then as I thought I didn't need the car on Tuesday I could let it charge from Monday night through as long as it wanted and it would be done by Wednesday morning.

Curses, I forgot there was something on on Tuesday evening. So either I stop the charge on Tuesday evening to use the car, then start it again, or I beg a lift to that event (in an ICE-cold Fiesta), or I put enough in the battery overnight on Tuesday to do me on Wednesday but no more and do the charge Wednesday night to Friday morning instead.

Or I ask for a shot on my neighbour's wall box on Tuesday night.

 

[Museruk]

OK thanks, I understand why you used the granny now!

I assume that the reason for this low battery level (<10%) to 100% charge (if that's what they intended to state) is simply to reset the GOM accuracy?
Otherwise if it isn't this reason, I don't understand why we have to do it and I suspect many owners, like me, will find it is highly inconvenient as it means that the car is not available in an emergency for quite some time until it has had a reasonable charge level in it once more.
Indeed people that rely on the car to drive some distance to work and back each day would struggle to follow this procedure - even though it is only required every 6-months or so.

I've had my MG4 Trophy now nearly 7 months and I still haven't done this; yet I haven't noticed anything detrimental - although it is difficult to estimate given the low temperatures we've been experiencing of late that obviously significantly lowers the mpkWh. I'm currently getting around 2.9 to 3.2 on local journeys with outside temperatures hovering around -2 to 4 deg. C (and of course with the air cond. on)
After a 100% long range charge + equalisation (which I tend to do once a week) I get around 220 miles range on the GOM which I guess is not too bad at this time of year.

 

[TerryN]

I too don't like letting the SOC go too low. The lowest I've let it go down to is 19%. I'm always charging on my home charger and let it go to 100% plus balance charging. I wonder how important the "below 10%" advice is?

 

[Daniel Brown]

The 10% to 100% has nothing to do with balancing the battery which is something different to this

The reason for the below 10% and up to 100% is that the BMS (battery monitoring system) counts the electricity coming nto the battery when charging and the electricity going out of the battery when discharging or driving

This counting isn't 100% accurate as there are losses in heat and resistance and while the BMS does account for these it can't be done perfectly.

Below 10% the battery voltage drops of dramatically and so the BMS knows that the battery is almost empty and above 90% the battery voltage increased dramatically so the BMS knows that the battery is amost full.

Anything in-between 10% and 90% the battery voltage is fairly level

So when your display for example says you have 50% battery and 100 miles this is purely a guess from the BMS as there is no really accurate way to measure it

So discharging the battery to below 10% and then charging to 100% allows the BMS to calculate the true capacity of your battery which In turn allows a more accurate guess to your battery state of charge

Failure to do this won't damage anything but will eventually over a long period of time lead to inaccurate state of charge being displayed to you..

 

[Museruk]

The 10% to 100% has nothing to do with balancing the battery which is something different to this

The reason for the below 10% and up to 100% is that the BMS (battery monitoring system) counts the electricity coming nto the battery when charging and the electricity going out of the battery when discharging or driving

This counting isn't 100% accurate as there are losses in heat and resistance and while the BMS does account for these it can't be done perfectly.

Below 10% the battery voltage drops of dramatically and so the BMS knows that the battery is almost empty and above 90% the battery voltage increased dramatically so the BMS knows that the battery is amost full.

Anything in-between 10% and 90% the battery voltage is fairly level

So when your display for example says you have 50% battery and 100 miles this is purely a guess from the BMS as there is no really accurate way to measure it

So discharging the battery to below 10% and then charging to 100% allows the BMS to calculate the true capacity of your battery which In turn allows a more accurate guess to your battery state of charge

Failure to do this won't damage anything but will eventually over a long period of time lead to inaccurate state of charge being displayed to you..

View attachment 23075

Thanks for this explanation Daniel - very clear and much appreciated!
I will now try to figure out how I am going to achieve this procedure with minimum disruption to use of my EV and also how to manage any range anxiety whilst discharging to below 10% on local journeys!

It would have been great if MG had explained the reasons behind this requirement within the manual, as I am sure that if everyone understood why, then it would save all of this questioning of this surprising requirement!
So I see from your graph that not only will the mileage estimate deteriorate with time (i.e. the 'GOM' guess-o-meter), but also even the estimated battery percentage capacity left will potentially become inaccurate without this recalibration every so often to identify the end points of the battery charge status.
I guess that all EV's, not only MG, will suffer from this same issue unless this procedure of <10% discharge level to 100% charge is carried out on at least a reasonable time span (in MG's case it is stated as 6 months maximum).
I can't help thinking that if this procedure were much more widely known, then it could easily put quite a few potential EV purchasers off buying an EV.
Perhaps that's why MG aren't so keen on advertising this procedure!
I wonder if some other EV manufacturer's e.g. Tesla, Hyundai, Nissan etc. also need this full discharge / recharge procedure to be done, or whether they have perhaps figured out a much more sophisticated / clever alternative that makes this procedure unnecessary?

How do phone manufacturers for example cope with this issue as they also use Lithium batteries?

 

[Rolfe]

Phone manufacturers have conned everyone into believing their batteries will only last a few years. However, most people do let their phone run pretty low and even right out from time to time and then recharge it. Newer software updates are now allowing owners to limit the charge to 85% for routine use - possibly prompted by NMC battery car owners querying why this wasn't possible.

The last time I did the <10% to 100% charge I waited until my battery was quite low but not critical - maybe 20%? - then went off on a short joy-ride which brought it to 9%. Because it was a circular drive of known length from home I knew I was absolutely fine.

This time circumstances may force me to put maybe 25% to 30% in the battery on Tuesday night so I have enough charge to do Wednesday's run, calculating it so that I get back from that at around 9%, If I have to do a handful of extra miles on Wednesday evening because I put too much in, so be it.

Any time an emergency arises I can get a lot in my battery PDQ by taking it to the 50 KW rapid charger five minutes walk away. So long as the emergency can wait for up to 55 minutes, that is.

 

[Daniel Brown]

Yes it's a pain and how many people will actually do it untill you think you have 30% battery left and then it suddenly drops to 10%

It's my belief is that this is necessary as the battery ages to account for degradation.

I've found this information for tesla

 

[SteveM]

Am I the only one confused by the manual fragment:

When using the vehicle on a daily basis, it is recommended to fully charge it at least once a week, which is conducive to the health maintenance of the high-voltage battery pack; ...

and on the next page

Where possible it is recommended that you carry out a slow charging (equalisation charging) every month...

I use the car every working day ~30miles, wait until ~20% then charge to 80% using the 7kW wall box. Then once a month (ish) I set it to go to 100%. I had not noticed the earlier paragraph before, which also includes the cryptic " (less than 10% or 1 grid) full charged is performed."