Customer question - Is it bad to fully discharge a lithium ion battery?

May 22 , 2019

Customer question - Is it bad to fully discharge a lithium ion battery?

We get questions from our customers and one question has been asked many times so we thought we would answer it today.

Q: Is it bad to fully discharge a lithium ion battery?

A: YES!! it is bad to fully discharge a lithium ion battery!!

Let’s look into this a little further and find out why and some measures to avoid it.

From a chemical side, there is a few different types of lithium ion batteries.  In general, it is bad to fully discharge any lithium ion based battery chemistry, but, because MEGALiFe Battery is based on LiFePO4 cells, we will place our focus there.  We will also classify ‘fully discharging’ a battery as taking it all the way down to 0v.

Lets look at LiFePO4 chemistry at a single cell level.  Lithium Iron Phosphate cells will usually have a fully charged voltage level of around 3.65v and a fully discharged voltage of 2.0v.  It might seem obvious that over charging is putting an excess of energy into the cell resulting in excess heat causing damage to the cell and, in the worst of cases, the cell may catch on fire.  What is not too common knowledge about lithium ion cells is that over discharging can also create a situation where the cell becomes

So if fully discharging a lithium ion cell can be a potentially dangerous exercise, what can we do to keep ourselves out of trouble?  First let’s look at the available lithium based cell chemistries and how they compare on safety.  The table below shows some comparisons and is a simple and clear way to see that lithium iron phosphate (LiFePO4) cells carry the highest safety of all currently available technology.  A LiFePO4 cell does not have the ability to thermally runaway under its own power in either charged or discharged state.  They will be damaged by an under voltage event but the damage is performance based (capacity and power output will be reduced) not a safety based.

Electrical batteries can be defined as a connected group of cells.   The type of connections (e.g. parallel or series) and the numbers of these connection can vary greatly but we use a 4s configuration to get to our nominal 12.8v batteries.  When cells are connected in this way, unless properly managed, it is possible to have the cells out of balance.  For example, 4 cells in series start at 3.6v-3.6v-2.5v-3.6v, a total of 13.3v.  If we are to discharge this battery enough to reduce the total voltage by 1v we would have a battery charge of 12.3v on not considered fully discharge….BUT……we may be in the position where the cell voltage level is 2.6v-2.6v-1.5v-2.6v giving our number 3 cell an under voltage (<2.0v LiFePO4 minimum charge) and subsequent damage causing reduction in the overall battery performance!  This type of imbalance can also be seen if we were to charge the example above with unbalanced cells potentially causing an overcharge of the cells if not properly managed. 

To avoid these situations, a battery management system (BMS) can be used to balance the cells correctly to ensure they discharge to equal levels.  There are many ways to ensure the cells are balanced starting right from the production line where batteries should be assembled with matched capacity and state of charge cells.  There is also other common methods with passive, active, bottom and top balancing being common place among battery builders.  MEGALiFe Battery has an internal BMS on all our batteries that takes care of the balancing function using the proven method of passive, top balancing.

For more information on lithium ion batteries, their application and how MEAGLiFe Battery Australia is using LiFePO4 technology in their products, jump over to our site www.megalifebattery.com.au or check out all the action on our social media.  You can browse our extensive tech article library, see some of our success stories and even order online with free shipping Australia wide on our batteries!

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