36 V, 4.0 Ah Lithium-Ion battery. The AK 20 powers tools within the STIHL AK System. With an energy capacity of 144 Wh, the battery has an integrated LED charge indicator, clearly displaying as and when the battery needs charging. Ideal for homeowners working in their garden or, the AK 20 is versatile, long-lasting and durable, making it ideal for demanding gardening tasks. This battery has a cold start feature, which protects the cells by warming them up in a controlled way before delivering full power and allows them to work down to -20° Celsius. AK System batteries feature intelligent battery management system - this clever technology gives a constant level of power whether the battery is 100% charged or 1% charged. Compatible with the AL 101, AL 301 and AL 501 charger unit.