A bit of information about Li-Ion batteries that may answer some questions: 1) Difference between 18V and 20V max batteries (also covers 36V/40V). There is no difference between 18V and 20Vmax batteries, other than how their manufacturers market them. All Lithium Ion (Li-Ion) batteries are made up from a number of individual cells welded in series and usually featuring a protection circuit within the battery housing. Each Li-Ion cell has a nominal (working) voltage of about 3.6V and a maximum of 4.0V when freshly charged. 18/20V batteries have 5 cells connected in series, so the voltage is 5x3.6 = 18V nominal or 5x4.0 = 20v Maximum. As soon as current is drawn from the battery the voltage drops to the 3.6V per cell and stays there until it is about to become discharged, so there is no difference in the actual power of the cells, irrespective of what it says on the outside. A protection circuit either built into the battery or into the device it is powering prevents the cell voltage dropping much below 3.6V to prevent irreversible damage to the cell. It is really not worth trying to squeeze the last bit of power from one of these batteries as it could reduce its life. Common combinations are 3 cells - 10.8V/12V max, 4 cells - 18V/20V max, 5 cells 21.6V/24Vmax and 8 cells - 36V/40V Max. 2) Interchangeability of 18V/20V max batteries - Erbauer, Titan, WORX, Guild. I only have experience of Erbauer, WORX and Guild and interchange these batteries freely. Over 18 months I have found no issues with charging or excessive discharge. I understand that Titan are a more recent addition to the Screwfix range and are also compatible, but would value independent confirmation. As far as I can ascertain all these products have a common manufacturing source and differ in branding and specification (eg some batteries have built in charge indicators, some devices have built-in charge indicators and some have neither). Providing the battery will slide into the charger or device without being forced then you can be pretty sure they will work together. 3) Charging. Batteries are charged at a given rate and the resulting battery voltage and temperature are constantly monitored by the charger. If either exceeds the set limits then charging is reduced or stopped automatically to prevent damage. Batteries can generally be left in their chargers for extended periods without harm to the battery, although it is good practice to remove the battery once it has achieved full charge. These batteries retain their charge almost indefinitely, unlike NiCd and NiMH batteries which run down after periods on non-use. 4) Temperature effects. Li-Ion batteries work far less well at extremely low temperatures. I found this was a particular problem some years ago when working in sub-zero temperatures outside, but have found it far less of a problem recently, so I guess the technology is improving and internal structures and chemical formulations in Li-Ion are less susceptible to cold. 5) WARNING. These batteries hold a lot of energy and if they are abused the energy can be released rapidly with spectacularly dangerous results, resulting in fire and/or explosion. Treat them with respect and they will reward you with lengthy and safe service.