ZOLL SurePower™ Battery Well Spacers are installed in the back of the charging bays. Use these well spacers if you will be charging only SurePower™ Battery Packs as the spacers improve the fit of the SurePower™ Battery Packs in the charger bays and prevent older ZOLL lead acid batteries from being used with the charger. Compatible with AED Pro, E Series, M Series, R Series, CCT