Deep discharged lithium-ion-batteries: Recovery?
If a lithium-ion battery (also known as Li-ion or LiPo) no longer supplies any power and can no longer be charged, this may be due to a so-called deep discharge. In most cases, the only option is to dispose of the battery properly. Some instructions on how to revitalise deeply discharged batteries can be found on the Internet. As the risk of fire is particularly high with lithium-ion batteries after a deep discharge, we explain here:
- What a deep discharge is.
- How a deep discharge occurs and how you can avoid it.
- Why you should not recharge deeply discharged batteries.
- In which cases you can revitalise deeply discharged batteries.
What is deep discharge in batteries?
When charging and discharging lithium-ion batteries, in simple terms lithium atoms are separated into lithium ions and electrons. Both then travel along different paths from one pole to the other – in one direction when charging and in the other when discharging. If the supply of lithium runs out when the battery is discharged, the voltage continues to drop until it falls below the so-called cutoff voltage. This can lead to further undesirable chemical reactions that cause lasting damage to the battery.
A deep discharge can be recognised by the fact that the voltage is below the cutoff voltage even when ‘idling’. This voltage depends on the cell type and is around
- 2.5 volts for classic lithium-ion batteries (Li-ion)
- 3.3 volts for lithium polymer batteries (LiPo)
- 2.0 volts for lithium iron phosphate batteries
When a deep discharge occurs and how you can avoid it.
Ultimately, deep discharge occurs in batteries when they are discharged too much. Common causes are
- A battery is stored in an appliance for a longer period of time. Most devices continue to consume power even when switched off. Batteries from power tools, motorbikes, lawnmowers or e-bikes should therefore be removed or at least disconnected before they are stored for the winter, for example.
- Even disconnected lithium-ion batteries lose a few percent of their stored capacity per month. You should therefore never store batteries (almost) empty and charge them in the meantime if they are to be stored for a long time.
- Modern devices – or the battery packs themselves – are equipped with battery management systems that switch off the battery before it becomes completely discharged. However, this requires that you only use batteries and devices that are compatible with each other. Otherwise, the battery management system in the device may assume an incorrect cutoff voltage and keep on draining an empty battery.
Why you should not recharge deeply discharged batteries.
A deeply discharged lithium-ion battery is usually permanently damaged so that it will no longer achieve the capacity or output of a new battery. It is therefore usually not worth revitalising a deeply discharged battery anyway. Recharging is also associated with risks:
If a deeply discharged lithium-ion battery is recharged, so-called dendrites can form from lithium or other electrode materials such as copper. These can be imagined as a small metallic spur that continues to grow with each charging process and can also pierce the membrane between the electrodes. If it reaches the other electrode, a short circuit occurs, which can lead to thermal runaway. This means that the battery spits fire under high pressure or even explodes.
Deep-discharged batteries also pose challenges for retailers and other collection points for old batteries and accumulators. The batteries are potentially dangerous and should therefore be stored safely. RETRON helps commercial customers to handle and dispose of lithium-ion batteries safely. Get in touch with us.
In which cases ‘deeply discharged’ batteries can be revitalised.
If the open-circuit voltage of a battery cell is significantly below the cutoff voltage, it can be written off. Normally, however, the voltage is not measured directly at the cells (e.g. by the charger), but at the connections. If a battery pack has its own battery management system, the cells may be disconnected from the connections before the cutoff voltage is reached in order to prevent deep discharge. In this case, the seemingly deeply discharged battery can be revitalised.
A battery that only appears to be deeply discharged has an open-circuit voltage of 0 V at the connections. However, this does not mean that you can be sure that the cells are really not deeply discharged. If you want to be sure, open the battery pack and measure the individual cell voltages. If they are all above the cutoff voltage, you can recharge the apparently deeply discharged battery:
To revitalise the lithium-ion battery, you must apply a voltage to the connections during the charging process that corresponds to a battery that is not deeply discharged – either via an external voltage source or by connecting an identical battery in parallel. As soon as the battery management system has reconnected the cells and the charger has started the charging process, the voltage source can be removed again.
We advise non-expert users not to recharge deeply discharged lithium-ion batteries. Even opening the battery pack can be dangerous. And charging a battery that has really been deeply discharged can cause an explosion.
