I share with you my research on the gross capacity of the MG4 SE 51 kWh LFP. The current SOH is only an estimate, as the car continues to charge at full capacity. Here are the details regarding the usable and gross capacity.
MG advertises 50.80 kWh of usable energy, but in reality, it is much more. According to the datasheets, the usable capacity is 156 Ah for a weight of 398.7 kg and 104 cells in series. If we calculate with a nominal voltage of 3.2 V per cell, this gives 51.92 kWh "usable" (I emphasize this term).
However, the CATL cells used have a higher nominal voltage of 3.35V at 99% up to 40% of the "BMS SOC", and 3.28V up to 10% of the "BMS SOC", with an approximate average of 3.25V. This means that there is more usable capacity not communicated by the manufacturer. This silence is probably to mask the perceived 5% degradation in the first year that everyone is talking about.
In reality, does the battery really lose this 5%? Hard to prove without a full discharge followed by a full recharge. This is what I have done several times. After a year of use, my estimated SOH was down to 95%. By completely draining the battery and recharging it, I gained +0.80% each time, until I got this estimated SOH back up to 97.11%.
Even after a year and 23,000 km, I still discharge 50.80 kWh. Today, my car is 2 years old and 44,500 km, and I still discharge the same capacity. This is due to the generous usable capacity that was initially defined but not officially communicated.
Raw capacity revealed
When I go down to 0% displayed on the dashboard, this corresponds to a BMS SOC of 4.5%, which means that there is still energy left. At 0% BMS SOC, the cell voltage is 3.10 V, so the battery still has juice. By using consumers like defrosting and ventilation (about 750 W), I pushed the discharge to 2.88/2.99 V over a period of 2 hours. I thus recovered an additional 1.5 kWh.
Photo of a 0% dashboard discharge, after the dashboard counter has stopped counting
By fully recharging after this deep discharge, the electric meter recorded 59.2 kWh, and my terminal indicated 55.05 kWh injected into the battery at 32 A.
An LFP battery cannot be used at 100% of its gross capacity, as it is limited to about 87% to preserve its lifespan. The gross capacity of the MG4 LFP cells is 173Ah, or about 58.5kWh. This value can vary depending on the cells.
If we apply the 87% rule, this gives a theoretical usable capacity of about 50.89kWh. In my case, by pushing the discharges deeper (to 2.88v/2.99V), the CCU reset the zero point, allowing more capacity to be exploited. However, this setting reverts to 3.10V each time the CCU or BMS is updated at the dealer.
Therefore, no significant loss of autonomy due to estimated degradation is observed. Additionally, the recently announced 49kWh battery is simply the same battery as the 51kWh, with a more realistic advertised capacity based on the dischargeable energy between 100% and 0% of the dashboard (without considering the BMS SOC).
