EV Battery Cell Balancing

<<< EV Charging Options

The Hook-up

battery cellsYour EV’s battery is made up of many individual electric ‘cells’. These cells are connected into groups, or ‘modules’, which are then connected together to form one big battery ‘pack’.

A regular 12V lead-acid car starter battery, for example, is made up of six cells (2V each). Lithium battery cells are each around 3-4V, so an EV battery might have over 100 cells connected together to reach a combined voltage of 400V, and hundreds more cells to increase the battery’s overall energy storage capacity.

Details of individual batteries vary, depending on the cell technology used in the battery – cylindrical, prismatic or pouch cells – resulting in an overall battery voltage of over 400V. Some vehicle manufacturers, such as Porsche, have battery packs outputting 800V.

A Balancing Act

Balanced and unbalanced battery cellsWhen a battery charges and discharges, the electrical current flowing into (and out of) the battery cells may not flow equally to all cells – depending on micro-differences in cell chemistry, how the cells are connected to each other and to 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 the Battery Management System (BMS) built into EV batteries is able to ‘balance’ the charge of each cell to ensure that all cells are equally charged. This is good for battery health.

EV battery cell balancing 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.

Cell balancing doesn’t kick in until the battery is fully charged – usually 100% SoC.

Note: The requirement for 100% SoC to initiate cell balancing may vary from brand to brand. Or it may not. No-one seems to be sure! The BMS fitted by some battery manufacturers might commence cell balancing when the battery reaches the maximum charge limit set by the user – say 80%. Regardless of the % SoC, for cell balancing to happen the charger must stay connected for some time after the main charging has completed – 20 mins to a couple of hours, depending on how out-of-balance the individual cells might be.

Some vehicles may also give you a dashboard alert when the battery cells require balancing.

Reality Check…

Balancing charge - maybeI have not been able to find a definitive answer regarding State of Charge (SoC) and the initiation of cell balancing for EV batteries. Lots of speculation, may-bes and could-bes.

With that ambiguity in mind, I consider that it is generally good practice to charge a Lithium NMC battery to 100% (on an AC charger) at least once every few months – and keep it connected to the charger after reaching 100%, until the charging device indicates that the battery is drawing zero current.

Lithium Iron Phosphate (LFP) batteries can be charged to 100% SoC regularly.

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 Tesla 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 ‘Charging completed’, ‘Plugged in’, etc. (Images below from the Tesla app charging a non-Tesla vehicle on a single phase Tesla wall connector.)

Battery charging and balancing in Tesla app

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 to the point where the EV is no longer drawing charging current.

Graph of battery charging and cell balancing

The example above is using a 7 kW wall connector. A similar charging profile applies for a 3 phase 11 kW wall connector. You can also fully charge and balance your battery with a portable 2 kW ‘granny charger’ – but it will take much longer, usually over several charging sessions if only charging at night.

Going all the way?

Charge percentage dialShould you charge 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) .

LFP batteries should be regularly charged to 100%. NMC batteries will have a longer life if regularly charged to only 80%.

Do you need to charge to 100% to initiate cell balancing?

  • For cars with LFP batteries – Yes.
  • For cars with NMC batteries – Maybe. The BMS in some cars with an NMC battery might initiate cell balancing once the battery reaches its set limit – say 80%. How do you know if this applies to your NMC battery? …… Try it and see: if the SoC reaches the set limit of (say) 80%, and the charge rate drops to 0.3kW for 30 mins or so, then this low power charging period is most likely a cell balancing operation, as below…
  • For the cells in both LFP and NMC batteries to be balanced, leave the AC charger connected until after the battery has completed charging at the full rate of charge (2, 7, or 11 kW) – it will then draw a much lower rate of power for a period of time (say 300W), and eventually 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 if necessary, 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.

No maximum charge setting?

CalculatorWhat 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%?

This is typical in early MG ZS EVs, and no doubt other vehicles. If your wall charger 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 – 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 26 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.

Or you could just keep an eye on the charging display on the dashboard!

Charged to 80%

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.

How low can you go?

Battery charge cycleHow 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 15 years!

With all that in mind, most recent 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%.

Keeping the SoC high is better for long term battery cycles than regular deep discharging of the battery down to 10% – 20% SoC.

For both NMC and LFP batteries, don’t let the battery deeply discharge regularly – top up the SoC when you can. (LFP batteries should be charged to 100% SoC.)

Regular charge range

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 every time you come home, plug it in – avoid regular deep discharges.

The Bottom Line

  • Balancing: At least every few months, regardless of battery chemistry, charge your battery to 100% and leave it charging after that limit is reached, until the charger indicates that the battery is no longer drawing any electrical current. Then, for NMC batteries, go for a decent drive the next day, to reduce the SoC to 80%.
  • Charging LFP batteries: There is no problem regularly charging LFP batteries to 100%, and doing this regularly is good for LFP battery health.
  • Charging NMC batteries: NMC batteries can be charged to 100% every few months for balancing or longer trips, but (depending on the vehicle) cell balancing may still occur if the maximum charge is set to 80%, and you leave the charger connected until the battery is drawing zero current.
  • Discharging / Recharging: Your long term battery health will be improved if you top up the battery regularly, rather than deep discharging before recharging. Lots of small charges over a narrow charging range is better than infrequent long charges over a wide charging range.

Balanced battery cells and charge rangeNote: 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!)

More reading…