MG ZS EV Mk1 usable battery capacity

[xafalcon]

Hi there. I drove my Mk1 (2021, charge flap lifts out and up) MG ZS EV down to 4% remaining battery (as reported by the charger when I plugged in). I recharged it to 93%, but the charger only pumped out 35kWh

The maths say

44.5kWh x 93% x 4% = 1.65kWh left in battery (93% SoH)
35kWh x 95% = 33.25kWh added to battery (95% charge efficiency)
(33.25 + 1.65) / 93% = 37.5kWh in full battery

37.5 / (44.5kWh x 93%) = 90.6% of degradation-corrected capacity
(44.5kWh x 93%) - 37.5 = 3.9kWh missing energy

So is there some hidden capacity somewhere? Can I drive it below 0% capacity?

TIA for any insights, with such a small battery I want to know if there is any range "hiding below zero". But I don't want to get stranded on the roadside trying to find out.......

 

[N2STY]

Hi there. I drove my Mk1 (2021, charge flap lifts out and up) MG ZS EV down to 4% remaining battery (as reported by the charger when I plugged in). I recharged it to 93%, but the charger only pumped out 35kWh

The maths say

44.5kWh x 93% x 4% = 1.65kWh left in battery (93% SoH)
35kWh x 95% = 33.25kWh added to battery (95% charge efficiency)
(33.25 + 1.65) / 93% = 37.5kWh in full battery

37.5 / (44.5kWh x 93%) = 90.6% of degradation-corrected capacity
(44.5kWh x 93%) - 37.5 = 3.9kWh missing energy

So is there some hidden capacity somewhere? Can I drive it below 0% capacity?

TIA for any insights, with such a small battery I want to know if there is any range "hiding below zero". But I don't want to get stranded on the roadside trying to find out.......

There is a little buffer at the top and the bottom of the battery it's definitely not advised to run the battery down that low

 

[xafalcon]

Does MG rate their batteries like Nissan = theoretical capacity based on electrochemical max and min cell voltages? And then program the bms with more conservative min and max cell voltages, giving rise to a lower "usable" battery capacity?

If MG rate their batteries in actual usable capacity (like all other EV manufacturers do), it can't have spare capacity at the top, because charging stops at 100%

Any idea of the range or available energy (kWh) below zero percentage charge?

Or maybe the cell voltage where the car shuts down? (NMC will run down to 3.2V without damage)

TIA

PS. I'm not worried about slightly reduced battery life.

 

[N2STY]

Off the top of my head there is about 3or4 % on top of battery ie when car says 100% diagnostics on the battery says 97% and at the bottom of the battery 2or3% when battery says 0% in car diagnostics says 2% running the battery that low is a bit more then reduced battery life it's speeds up the degradation of the battery I wouldn't recommend going below 5% the end of the day the battery has a seven year warranty
Also remember when battery percentage is very low car has limited power so it's not particularly nice to drive

 

[Zoomy]

Post BMS gate the overall pack voltage dropped such that the usable is nearer 40kwh than 44.5.

 

[xafalcon]

Post BMS gate the overall pack voltage dropped such that the usable is nearer 40kwh than 44.5.

Thanks. I've searched BMS gate, but only got 3 other threads. One of those suggested fully charged voltage dropped from 455V to 449V in January 2021 software update. Clearly this would reduce stored capacity, unless the sw update only affected displayed voltage rather than actual voltage

Is there another thread which does a deep dive into this sw update?

TIA

 

[Zoomy]

Approx 108 cells in the overall pack. Top end values were brought down from 4.22V to about 4.16-4.17V increasing the margin on the narrow top buffer and now using more of the lower buffer to offset the difference. Overall this decreases storage but increases battery life.

 

[xafalcon]

Ok, so my ZS EV goes at least 8km from when the orange turtle light comes on and motor power is limited. This light first came on half way up a hill at 90kph. Battery voltage was 367 volts, and the power limiting stopped any further drop in voltage. Assuming 108 cells in series, then average cell voltage was 3.4 volts. This is below the knee in the discharge curve, but still well within the safe operating voltage range for NMC cells

When I pugged into the fast charger, it reported battery level as "unknown". After 0.9kWh had gone into the battery it reported battery level as 1%. Recharging was normal after that

So there is at least 1kWh below zero.......

 

[Zoomy]

Yes on OVMS I got it to minus 1%

 

[Peter WA]

Yes, there is hidden capacity. But no, you can't use it and drive below zero, the car won't allow you to completely drain (and likely damage) the battery.

The car does not charge to 100% actual SOC. For the Mk1, in the 2021 BMS version, the display shows 100% when the actual SOC is 94%.
And it does not drain to 0%. If the display heads towards 0%, the actual SOC is still 2.5%.

That explains most of the difference you observed. What little is left could be that the 95% efficiency figure is not quite right, or that the charger didn't accurately measure.

93% SOH, with what mileage?

 

[xafalcon]

93% SoH was at 56,000km

So you are saying that MG have taken a leaf out of Nissan's playbook, and their battery capacity is not actually usable capacity. If correct, this is rather poor form. I had hoped those days were over

Chargers must be accurate in their delivery, just like petrol pumps. They are both dispensing energy for a cost. In NZ, the weights and measures act requires product delivery must be accurate, or to the customer's benefit

 

[Peter WA]

or to the customer's benefit

Yes, that's what suspect happened here.

What if it supplied 37 or 38 kWh and only charged you for 35kWh? What if they do that to compensate for the 95% charging efficiency, to make sure you actually end up with 35kWh to use when you pay for 35kWh?

 

[xafalcon]

In general, companies don't give away anything for free. eg. Multiply 2kWh by 1000 chargers then multiply by 10 cars each day then multiply by 365 days each year. That's 7.3 GWh being given away.Not really a sustainable business model.

And definately not an energy suppliers responsibility to compensate for charging losses. Their point of sale is the connection to the car