Like coolant, batteries only have a limited lifespan, and few will last longer than five years. For many it will be much less. If yours is nearing the end of its life, be sure to replace it before it fails. If you do flatten it, however, you could try to bring it back to life with the CTEK Multi XS3600 charger (£49.95). It can revive batteries from very low levels, and also be used to keep power packs on rarely used vehicles topped up.