Volts, amp hours and watt hours on tool batteries

Volts: nominal and maximum

Lithium-ion tool cells are about 3.6 V nominal, which is the average while discharging, and about 4.0 V when fully charged. Five cells in series make 18 V nominal or 20 V maximum. UK and European makers usually quote the nominal 18 V; DeWalt and others sell the same packs in the US as 20 V MAX.

The same logic applies to other sizes: three cells are 10.8 V nominal, sold as 12 V max; ten cells are 36 V nominal, often sold as 40 V max. DeWalt FlexVolt packs switch between 18 V and 54 V by rearranging their cells.

Amp hours: how much charge

Amp hours (Ah) tell you how much charge a pack holds at its voltage. A 5 Ah pack holds roughly two and a half times the charge of a 2 Ah pack of the same voltage, so it runs a tool for longer. Higher Ah packs usually contain more cells in parallel, which makes them heavier and larger but also lets them deliver high current more comfortably.

Runtime still depends on the tool and the load. A circular saw ripping wet timber drains a pack far quicker than a drill driving small screws.

Watt hours: the fair comparison

Watt hours combine both: energy (Wh) = nominal volts x amp hours. That lets you compare packs across voltages and brands. An 18 V 5 Ah pack holds 90 Wh. A 12 V max (10.8 V) 3 Ah pack holds about 32 Wh. A 36 V 2.5 Ah pack holds 90 Wh, the same as the 18 V 5 Ah pack.

Always convert a max rating to nominal before multiplying, or the result will be inflated.

• 18 V x 2 Ah = 36 Wh
• 18 V x 4 Ah = 72 Wh
• 18 V x 5 Ah = 90 Wh
• 10.8 V x 3 Ah = 32.4 Wh
• 36 V x 2.5 Ah = 90 Wh

Try the battery energy calculator →

Twin battery tools

Some tools run on two 18 V packs at once. Makita's DSP600 plunge saw and DHR263 SDS drill connect two LXT packs in series for 36 V. Einhell's 36 V mitre saw does the same with two Power X-Change packs. Total energy is the sum of both packs. That is a good way to run larger tools without buying a new battery system.

Separate high voltage ranges, such as Makita XGT 40 V max, use their own packs.

Which battery size to buy

For drills and drivers, 2 to 3 Ah packs keep the tool light. For saws, grinders and SDS drills, 4 to 5 Ah or larger gives more work per charge and handles high current. Many people own a couple of small packs for light tools and one or two large packs for saws.

Our combi drill kit guide and twin pack guide list the packs included in each kit.

Charging and battery life

Lithium packs do not need to be run flat before charging and do not suffer the memory effect of old nickel batteries. They do age with time, heat and full charge cycles. Keep them cool, avoid storing them fully charged for months, and charge at room temperature. See storing tool batteries in winter.

Note:Use only genuine or maker approved batteries and chargers, never mix packs from different systems with adaptors you are unsure of, and recycle damaged packs. Follow the tool and charger manuals.

Frequently asked questions

Is a 20 V drill more powerful than an 18 V drill? Not because of the label. 20 V max and 18 V nominal are the same five cell pack. Compare torque and watt hours instead.

Does a bigger Ah battery make my drill stronger? It mainly increases runtime. Larger packs can also hold voltage better under heavy load, which some tools use for more output.

What does Wh mean on a battery? Watt hours, the total energy stored. It is nominal volts times amp hours.

Can I use a 5 Ah battery in a tool sold with 2 Ah packs? Yes, if it is the same brand and system. It will run longer but make the tool heavier.