Everything You Need to Know About Power Tool Batteries

Once you have made the decision to go cordless and you know the category of drill you want then your next consideration should be the battery. Here are the things you should know.

Please remember we’ll be talking about rechargeable batteries as opposed to disposable ones.

And although we’ll be referring to them as ‘batteries’ they are actually ‘battery packs’. This is because batteries are comprised of groups of individual cell pairs (e.g. 5 cell pairs in an 18-volt pack) bound together within a plastic structure that is commonly referred to as the “battery”.

Battery Chemistry

The first rechargeable batteries were composed of Nickel Cadmium or NiCad and have been around since the late 1950s.

The next generation, introduced in the late 1980s, were made from Nickel Metal Hydride (NiMH).

The latest generation, Lithium-Ion (Li-Ion) have only become popular in cordless tools over the last several years. Only in the last 3 or 4 have their prices reduced enough for them to become a realistic alternative to NiCad and NiMH batteries in the power tool market.

These days if a cordless power tool is supplied with a battery (i.e. it is not supplied ‘bare’) the more expensive ones tend to be supplied with Li-Ion batteries.

More basic ranges come with NiCad or occasionally NiMH batteries.

Let’s look at the ins and outs of these 3 types…

Battery Chemistry Characteristics – Cycles, Self-Discharge, Memory Effect

The 3 types of battery – NiCad, NiMH and Li-Ion – are made up of different chemical elements and each provides the battery with different characteristics.

These include

  • the number of times they can effectively be recharged (cycles)
  • the length of time they can hold the charge whilst left idle (the loss of charge is called ‘self-discharge’) and
  • their ability to consume the same amount of charge when continuously recycled. ‘Memory effect’ is the term used to describe any resultant degradation and it comes into play if you do not charge /discharge the battery in the appropriate way.

NiCad batteries can be recharged over 1,000 times and last many years.

However they are susceptible to self-discharge (about 20% per month), and high memory effect – they need to be fully discharged once a month to prevent it.

NiMH batteries can also be recharged about 1,000 times. They suffer less memory effect than NiCad batteries but still need a full discharge every 3 months to prevent it and also have a high self-discharge rate (30%).

The lifetime of Li-Ion batteries is less than that of NiCad or NiMH batteries – maybe 500 charge cycles – but Li-Ion batteries have no memory loss and minimal self-discharge – maybe 5-10%.

In line with the above, there are techniques to consider when charging the battery in order to maximise its lifetime. NiCad batteries should never be allowed to discharge below 70%, NiMH below 30% and Li-Ion batteries below 20%.

Battery Susceptibility to Heat and Temperature

Batteries are susceptible to heat and temperature extremes. When in use heat builds up within them and once used they should be allowed to cool before being charged.

When not in use they should be stored in a cool place and out of the sunlight.

The charging process itself also generates heat within the battery.

These days most chargers are sensitive to this danger and stop the charging process if the battery shows any signs of overheating. Some chargers are now fan-cooled.

Battery Cell Protection

Furthermore, battery manufacturers have introduced electronic cell protection to ensure the battery itself is protected, not just from overheating but also from voltage spikes and current overload, especially when being charged.

Battery Capacity

A battery’s capacity or run-time is described in Ampere/Hours (Ah), a measure of how much current can be stored.

Typical values in the lower ranges are 1.3 Ah -1.5 Ah and recently batteries with 6.0 Ah capacity have been produced.

NiCad batteries can only store just over 2.0 Ah.

NiMH can extend this to 3 Ah.

Anything greater is only found in Li-Ion batteries.

You do see some higher-powered batteries (we discuss power and voltage later on) with a relatively low 1.5 Ah rating but more often higher powered batteries feature greater capacity.

That is until you get to 36-volt batteries which would be too heavy to manage with 5 or 6 Ah capacities so they tend to max out with 2.5 Ah capacity. If 36 volts is required to power a tool (e.g. a chainsaw) then 2 18 volt packs are used instead.

Also, battery ‘kits’ often come with two batteries, one with a lower rating of e.g. 1.5 Ah and the other with a higher rating of e.g. 4.0 Ah.

This is again due to a significant weight difference between the two and such a combination will provide better options and flexibility when working on different jobs.

Charging time relates to how long it takes to ‘refuel’ the battery’s capacity. (It has nothing to do with the battery’s power or voltage – see below).

The greater the capacity and the more depleted the charge is, the longer it will take to replenish.

Charging times are quoted assuming the battery is fully discharged. Chargers themselves can also supply the current at a fast or slow rate.

As an example a 4 Ah Li-Ion battery can be recharged in 60 mins whilst a 1.5 Ah version can be recharged in just 15 minutes.

Battery Power

A battery’s ability to deliver power is denominated in Volts.

The voltage in the battery should not be the only criterion used to assess power in a power tool.

You should combine battery power with motor design (e.g. brushless) and transmission to get the full picture.

But in general higher battery voltage equates to greater power, and as you might expect light-duty tasks can be accomplished with lower-powered batteries. Heavy-duty tasks will require more voltage.

So Cordless Screwdrivers are commonly quoted with 3.6 or 4 Volt batteries,  for example, Drill Drivers with 12 Volts and Impact Drivers and Hammer Drill Drivers with 18 Volts.

Battery Shape and Appearance

What does the battery actually look like? They are commonly described as ‘stick’ or ‘pod’ when they ‘insert into’ the ‘bare tool’ (or charger), or ‘slide’ when attached to the tool or charger via a pair of metal ‘rails’.

Battery Weight

Battery weight is a very important consideration. Li-Ion batteries are significantly lighter than NiCad or NiMH ones, assuming the same voltage and capacity.

For example, an 18 volt 3 Ah NiCad battery can weigh 1.5 lbs, whilst its Li-Ion equivalent can weigh just 0.6 lb.

An NiMH equivalent would weigh 0.9 lb.

Of course Li-Ion, being the lightest, is also the most expensive.

For batteries with the same chemistry:

  • Those with greater power will be larger and heavier than lighter powered ones, and
  • those with greater capacity will be larger and heavier than those with less capacity.

In other words for the same weight and battery chemistry, you can choose between

  • higher voltage / lower capacity and
  • lower voltage / higher capacity

Any choice will be influenced primarily by the tasks you want to perform. Heavy-duty tasks will require higher voltage in the first place.

Advances in Battery Development

Today it is quite clear how the introduction of Li-Ion batteries (initially too expensive for most of us but now relatively affordable) with their greater capacities and run times have revolutionised the way we can now work.

Most recent advances in battery cell development have taken place with Li-Ion batteries and several manufacturers have introduced ‘compact’ battery pack versions providing the same voltage and capacity with smaller footprints than their predecessors.

Such reductions in the size and modifications to the shape of the battery pack also reduce the weight of the tool and have a significant effect on its balance and feel.

Furthermore, improvements to battery chargers have been kept in line with those in the battery packs themselves.

Battery and charger work in conjunction with each other for optimum protection.

A fan-cooled charger will keep the battery cooler while charging, and the battery’s electronic cell protection will safeguard it from overheating or overload whilst the tool is in use.

Battery development, in general, has become a major focus for manufacturers in attempting to make gains in the marketplace.

Apart from tweaking mechanical components (e.g. advances in brushless technology) and tool ergonomics, there isn’t much left apart from battery technology for the manufacturers to improve on.

Whereas designing smaller lighter cells for battery packs, increasing cell protection from overheating and malfunction, and reducing charge times, all have mileage.

Take a look at the latest innovations on manufacturers’ websites to appreciate this first-hand.

If you are wondering where we will be in 10 years’ time it’s quite possible that solid-state technology, where sodium replaces lithium in the battery’s chemical makeup and increases Li-Ion run times by 300%, will be the norm!

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