Power Tool Batteries – Roger Bisby Investigates

Power Tool Batteries – Roger Bisby Investigates

Roger Bisby takes a closer look at power tool batteries – and reveals some interesting quirks…

The first cordless tools were fairly puny affairs, but they have evolved and much of that evolution has been in the batteries. We went from Ni-Cad, which had to be run right down before recharging, to nickel metal Hydride, which had more energy density and could be re-charged at any time.

Eventually we got to Lithium Ion, which has twice the energy density of the NiMH batteries and, until the next big breakthrough, seems to be the holy grail of the technology – but it was not without its teething issues.

Lithium Ion was widely used in mobile phones, laptops and other low drain applications, but adapting it for power tools presented a few problems.

If you are to prevent it from bursting into flames, drawing a high current through a Lithium Ion battery needs to be done with care and that care came in the form of a computer chip. The chip can be in the battery, in the tool, and in the charger.

Some manufacturers go as far as putting a chip in all three so there is, in effect, communication between the three intelligent components.

This obviously adds to the cost, but it also gives a high degree of cell management that prolongs the life of the battery. That is just one of the reasons why cheap batteries don’t last. Another early problem with Li-Ion batteries was deep discharge or ‘the point of no return’.

This is where a battery became so depleted that it was impossible to re-charge. This could happen within weeks of using the battery, and lead to a huge number of warranty claims. These days most batteries and tools switch off when the voltage drops to a critical level, and again this is something that is done by the cell management technology.

Without a doubt the biggest enemy of Li-Ion batteries is heat. Anything over 65°C can cause permanent cell damage and, in the great scheme of things, 65°C is not that hot.

It is worth remembering that daily temperatures are taken in the shade. A solar panel on a roof can easily top 140°C so a carpenter who leaves their lovely new drill lying in the sun could find it cutting out as soon as it is used and assume that it is a fault. However, the most common cause of a battery over-heating is drawing excess current through it.

batteriesThis can be done simply by trying to drive a larger auger bit with the drill set on second gear.  Not only is there a risk of damaging the battery, you may also damage the motor.

If you are experienced or intelligent enough to stop and change to a lower gear you can prevent damage, but manufacturers realised that this is asking a lot of some users, so it is now all about building in electronic cell and motor protection to make the tools idiot-proof. Better batteries now have a heat sensor which stops the tool working if the chemicals approach 65°C.

With some makes this requires a return to the charger to monitor the cells, cool the battery by blowing air through it and then re-setting it. Only when the battery falls below the critical temperature will the charge cycle begin.

The monitoring of battery cells is becoming more sophisticated every year. The latest move is to allow the monitoring of pairs of cells, so if a single cell is over-heating, but the whole pack is not yet critical, the management system will adjust the output of the battery to prevent damaging the healthy cells.

In an ideal world rapid charging of a battery is to be avoided, but customers seem to demand it. When the battery is low it will take a reasonably rapid charge but, as it reaches full capacity, the charging cycle needs to slow down to prevent over-heating.

It can actually take longer to charge the last 20% of a battery as it did to charge the first 80%. So when the ready light comes on the battery may still only be 85% full. Cheaper, less sophisticated chargers are unable to trickle this last bit of energy into the cells.

For this reason some batteries never reach their full capacity. 4 Ah batteries, for example, may typically be working with a capacity closer to 3.6Ah charge – not the end of the world, but not what you thought you were getting.

There is a trend now towards ever higher Ah. This requires additional rows of cells which produce more heat. Battery run times have crept steadily upwards over several years but we are now on a very steep curve, with 6ah, 8ah and even 9ah batteries in the market.

If you have a specific requirement for a battery this big that is all well and good but where will all this will stop? A bigger battery takes longer to charge and costs more. If it fails you are also saying goodbye to a significant investment. There is also the argument that adding weight and bulk to the battery negates some of the benefit of having a portable tool.

As a parting thought I would just like to add that, at present, there is no way for the end user to replace individual cells in a battery. To me it is unacceptable that a whole battery is rendered unfit for service just because one cell has fallen down on the job. It is also highly wasteful to ditch all that sophisticated computer wizardry just because the cells need renewing. Since much of the value of a power tool now lies in its batteries I think it is time that manufacturers started offering the kind of cell renewal that is currently only offered by after-market organisations. If not, there is a danger that end users will be tempted into using back street battery refurbishments companies, who may not have the same quality controls as the major manufacturers.

Batteries have come a long way in the last ten years and the future will see us using more and more battery driven devices, from electric vehicles to ever more powerful tools. There are still some bench tools, such as chop saws, where a battery offers few advantages over the mains-powered equivalent but even that will change.


  • Electronic Cell Protection (ECP) – Manages deep discharge and the tool simply stops.
  • Current overload – Tools stops in drilling telling you to change to gear 1.
  • Current Management – We did not really go into this but it manages the current onto the job face. It gives just enough current to do the job without wasting any.
  • Electronic Motor Protection (EMP) – A heat sensor monitors the motor heat. If it’s too hot – perhaps you kept the trigger pulled – the motor shuts down.
  • Coolpack – 100% longer lasting cells!
  • Individual cell or cluster monitoring – If the cells get too hot the battery shuts down.
  • The higher the AH the longer it takes to charge or the faster the charger you need.


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