MG S5ev: battery matters

 Most people that worry about EVs (usually non-ev owners) are concerned about range

...and therefore its the traction battery that comes under scrutiny.

But, possibly rather surprisingly, its the 12Volt system battery which seems to cause most breakdown issues.

In this post I hope to provide some more information on both the high voltage traction and low voltage system batteries fitted to the MG S5ev.

 

ev Traction battery

The S5 comes with either a 49kWh LFP traction battery (standard range models) or a 64kWh NMC (long range models). These 2 battery types have different characteristics, and should be treated differently when charging.

Both battery types need electronic 'balancing', not overcharge 'equalization' which is a term that only applies to lead-acid batteries. The battery management system (BMS) monitors individual cell or module voltages and balances them by diverting small currents (or adjusting charge timing) so each cell group matches the rest.

However, BMS balance does occur at a high state of charge. In many MG EVs (including S5, ZS EV, MG5 EV) the BMS only attempts a balance cycle after the HV battery reaches close to 100% SoC on AC charging.

When balancing begins, the charger current drops significantly, and finishes automatically when the BMS determines cells are balanced.

I put together this table, largely from data generated by chatGPT:-

Feature or characteristic	LFP 49kWh battery	NMC 64kWh battery
Cost	Lower	Higher
Cycle life	Higher (maybe 2 or 3 times NMC)	Lower (~1,000–2,000 cycles)
Range (per kWh)	Lower	Higher
Usage best practice	Urban: great for daily use and deep cycling	Ideal if you often need longer range
Safety / thermal stability	Very good	Good, but slightly more thermal sensitivity
Daily charge limit	100% OK	70–80% best
Full charges	Encouraged: frequent 100 % SOC	Occasional: 100 % SOC can stress cells
Cold charging	Weak	Better
‘Healthy’ battery range	20 – 100% SoC	40 – 80% SoC

 

12V System battery

The system battery powers the electronics; e.g. the system controllers, displays, anti-theft system, key-less entry & so on. Even if your ev has plenty of charge, if the 12V system battery is flat, you're not going anywhere!

Batteries fail after a few years in traditional cars; petrol/diesel. The main difference is that batteries get a bit of a hammering in a traditional car, because the starter motor draws a very high current each time it spins up the engine.

With an ev, the car is either in one of its stand-by modes (low current) or in a Ready or Drive mode (moderate current). So on the face of it, you might expect the battery to last much longer in an ev, than in a traditional fossil fuel burner.

The traditional car battery (...that many of us have grown up with) is generally a 'flooded lead-acid' battery (FLA); basically, lead plates sit in a chamber containing sulphuric acid.

These batteries are designed such that they tolerate short duration, deep discharge use. But they don't perform so well, or last so long, when installed in an ev. One reason is that these batteries often run in a partial state of charge (PSoC) which can result in imbalance between individual cells; a 12V FLA has 6 x 2V cells.

With continual PSoC, FLAs also suffer from sulphation; lead sulphate crystals grow on the lead plates. Another issue is acid stratification; dense acid sinks while water-rich electrolyte floats nearer the top, which can cause corrosion to the lower plate surfaces.

Another type of lead-acid battery holds the electrolyte in a glass-fiber mat sandwiched between each pair of plates; Absorbant Glass Mat battery (AGM). These perform better & last longer when used in PsoC.

I had (mistakenly) believed all present day EVs had ditched FLA for AGM batteries. 

 

Most ev manufacturers use either AGM or lithium 12V batteries, but unfortunately, MG are still using FLA.

You don’t want to spend time faffing around with battery chargers, having shelled-out tens of thousands of pounds on a nice, shiny new electric car.

But it looks like the system battery could be the weakest link, on our MG S5ev.