- See also: EV Charging Basics | EV Batteries – NMC v LFP | EV Range Variables
The Hook-up
Your EV’s battery is made up of many individual ‘cells’ containing chemicals that store a small amount of electrical energy. These cells are connected into groups, or ‘modules’, which are then connected together to form one big battery ‘pack’, the energy capacity of which is roughly equivalent to the sum of the capacity of all the cells in the pack. This potential energy is measured in Volts (V).
Remember the old starter batteries in ICE cars? The battery cells in those lead-acid batteries each produced around 2V. Connecting six of these 2V cells together created a 12V battery. Lithium battery cells are each around 3-4V, so an EV battery might have over 100 cells connected together (in ‘series’) to provide a combined voltage of 400V, and possibly hundreds more cells (connected in ‘parallel’) to increase the battery’s overall energy storage capacity (measured in kilowatt hours – kWh).
The technical details of individual EV batteries vary, depending on a range of factors, including the cell technology used in the battery (cylindrical, prismatic or pouch cells), the chemicals used to create the battery (LFP or NMC) and the storage capacity of the battery (40 kWh to 100 kWh). However, the bottom line is that EV motor propulsion batteries are comprised of a large number of individual battery cells connected together to produce a voltage of around 400V. (Some vehicle manufacturers, such as Porsche, install battery packs providing 800V of electrical energy.)
A Balancing Act
When a battery with multiple individual cells is discharged and recharged, the discharging and recharging process may not be exactly the same across all cells in the battery – depending on micro-differences in both cell chemistry and the electrical connections between individual cells and the charging source.
A battery’s overall State of Charge (SoC) will be defined by the SoC of the least-charged cell in the battery – similar to the old ‘weakest link in the chain’ scenario.
To overcome this problem, a Battery Management System (BMS) is built into EV batteries to electronically manage the battery. This management includes being able to ‘balance’ the charge of each cell to ensure that all cells are equally charged. This is good for overall battery health.
The battery cell balancing process in most EVs only occurs after a battery has been fully charged with an AC charger. You cannot initiate cell balancing when connected to a DC fast charger – DC fast chargers shut down when the battery reaches 100% charge.
| Reality Check…
With that ambiguity in mind, while it is generally good practice to regularly charge a Lithium Nickel Manganese Cobalt battery (NMC) to only 80%, it is also equally good practice to fully charge NMC batteries to 100% (on an AC charger) at least once every few months – and keep the charging cable connected to the vehicle after the battery reaches 100% SoC, until the charging device indicates that the battery is drawing zero current. Which might be some time after the battery is 100% charged. Not such a big issue for Lithium Ferrous Phosphate (LFP) batteries, since they are likely to be charged to 100% SoC more regularly than NMC batteries – but LFP batteries still need to remain connected to a charging source after reaching 100% SoC for cell balancing to be initiated. |
It is difficult to find a definitive generic answer regarding the initiation of cell balancing for all EV batteries. Lots of speculation, may-bes and could-bes for different manufacturers. It is likely that more recent Battery Management Systems (BMS) can better manage the even charging of individual battery cells as part of a regular charging regime, or possibly balance the cells ‘dynamically’. Maybe. For some brands. (And sadly, the sales and service staff at some car dealerships know very little about this sort of thing.)How do you know when the BMS is doing its cell balancing?
Check your charger’s app interface. When our EV is charging at full throttle through our wall connector the battery is drawing 7.0 kW. When the BMS is cell-balancing, the battery is drawing only 0.3 kW. When the cell balancing is complete there is no current being drawn, and the screen displays the message ‘Plugged in’. (Images below from the Tesla app charging a non-Tesla vehicle on a single phase Tesla wall connector.)
Depending on your charging setup, and the State of Charge (SoC) of the battery when you commence charging, multiple charging sessions may be required to fully charge the battery and complete the balancing process (during your cheap charging “window”) to the point where the EV is no longer drawing charging current.
The charging profile displayed in the above graph is using a 7 kW wall connector. A similar charging profile applies for a 3 phase 11 kW wall connector, but with a shorter time span. You can also fully charge and balance your battery with a portable 2 kW ‘granny charger’ – but it will take much longer, possibly over several charging sessions, to reach 100% SoC if only charging at night.
Going all the way?
All this discussion about charging your EV battery to 100% SoC always raise some concerns – should you be charging your battery to 80% or 100%?
EVs are powered by Lithium batteries – either an LFP battery (Lithium Ferrous Phosphate) or an NMC battery (Lithium Nickel Manganese Cobalt) .
Current ‘best practice’ advice is that LFP batteries can be regularly charged to 100%, with less effect on long term battery performance, while NMC batteries will have a longer lifespan if regularly charged to only 80%.
Do you need to charge to 100% to initiate cell balancing?
- For most cars, with both LFP and NMC batteries – Yes.
- The Battery Management System (BMS) in some (more expensive) cars is claimed to ‘dynamically balance’ the cells in the battery. How do you know if this applies to your vehicle’s battery? Firstly, read the manual. Second, try it and see…. charge the battery to 100% SoC (on an AC charger) and see if the charge rate then drops to 0.3kW for 30 mins (or more) after it has reached 100% SoC. If it does, then this would suggest that the cells required balancing, and may not have been balanced by the BMS under normal driving conditions.
- For the cells in both LFP and NMC batteries to be balanced, leave the AC charger connected until after the battery has completed charging to 100% SoC at the full rate of charge (2 kW, 7 kW, or 11 kW). Keep the charger connected – the battery will then draw a much lower rate of power (around 300 W) for an extended period of time (30 mins to over an hour), until it does not draw any electrical charging current at all. (The charging source must still be supplying power.)
- It is OK to charge NMC batteries to 100% every now and then when required, so long as they are not left at 100% charge for long periods of time. Charge to 100%, then go for a decent drive in the next day or so.
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No maximum charge setting?
This is typical in early MG ZS EVs, and no doubt other vehicles. If your wall connector has a mechanism to read the SoC of the vehicle, and set the cut-off SoC, then read no further! In not, you will need to do some maths to calculate how long to leave the charger connected for the battery to reach 80% SoC – based on the size of the battery and the rate of charge, and thus the time it will take to reach 80% SoC – then adjust the charge connector’s ON/OFF time accordingly. Let’s say you have a 44 kWh battery, and the car indicates a remaining range of 50 km. You know that when fully charged it has a range of 260 km. So 50 km is near enough to 20%. 20% of your 44 kWh battery is around 9 kWh. 80% of your 44 kWh battery is 35 kWh. So you need to charge from 9 kWh to 35 kWh – that’s 26 kWh of electricity to bring the battery to an 80% SoC. If your wall connector is charging at the rate of 7 kW, it will take about 3.7 hours to add 26 kWh of energy, which will bring your battery’s SoC to 80% – from a range of 50 km to a range of 208 km. So, set your charger to run for 3 hours 45mins, and it should turn off when your battery is at around 80% SoC. (Applying the same maths, using a 2 kW granny charger will take 13 hours to reach 80% SoC.) Or you could just keep an eye on the charging display on the dashboard!
But keep in mind that having the charger turn off when the battery reaches 80% SoC will not initiate cell balancing. You will have to let it go all the way to 100% every now and then for cell balancing to occur. |
What if your EV with an NMC battery has no control option to set the maximum battery SoC, and you want to set the maximum SoC to be only 80%?
How low can you go?
While not directly related to cell balancing, this topic leads to further discussions around Depth of Discharge (DoD) – How deeply should you discharge your battery before recharging? Does it make any difference to battery life?
Firstly, let’s keep in mind that the BMS in your battery will prevent it from completely discharging – maybe no lower than 10% SoC, even when the dash display might be displaying 2%. Ditto for the maximum charge, which is likely to be only 90% – 95% of actual capacity when the dash display indicates 100% SoC.
Also remember that it is early days for EV battery technology. Much of what we know is from lab experiments, rather than long term real-world use – we will know more in 10 years!
With all that in mind, most studies suggest that recharging your NMC battery more frequently over a narrow charge range is better for the long-term health of your battery than deeply discharging before recharging. These studies suggest that keeping your battery SoC between 50% and 80% will lead to an improved long term capacity when compared to batteries whose SoC regularly ranges between 10% and 90%.
For NMC batteries, don’t let the battery deeply discharge regularly – top up the SoC when you can.
On the other hand, recent studies suggest that LFP batteries may have a longer life if regularly charged / discharged over a wider range. Regularly charging to 100%, and discharging to 20% SoC is ‘beneficial’ for the long term life of LFP batteries.
But we need to remember that these studies have been conducted in laboratories at higher temperatures, to stress the battery and accelerate cell degradation to see trends and obtain results more quickly. So who knows how this may apply to daily driving routines!
Reality? Don’t stress too much about this stuff. However you charge your battery, it will probably outlive the life of the car anyway. Use the car however you need to use it. But, for NMC batteries, every time you come home, plug it in – avoid regular deep discharges.
The Bottom Line
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Note: The information presented here is fairly generic. The specifics of battery charging vary depending on the chemistry of the battery and the characteristics of the BMS fitted to the battery by the manufacturer. Check what type of battery is in your car, and the manufacturer’s recommended charging, discharging and balancing practices for your EV’s particular BMS and battery chemistry. (Read the manual!)- See also: EV Charging Options | EV Batteries – NMC v LFP | EV Range Variables
More reading…
- EV Battery Composition (Samsung) >>>
- EV Battery Voltage (EV Hub) >>>
- Tesla BMS and Battery Calibration Info >>>
- Battery Discharge for EV Owners >>>
- NMC Battery Science & Charging Best Practice (Video) >>>
- LFP Battery Science & Charging Best Practice (Video) >>>
- Managing your battery charging (Kia Canada) >>>
- Depth of Discharge – EV basics >>>