Hi Trevor, 8.4v is the max voltage for two cell pack in parallel. ( Each battery rated at 3.7v and can have no more than 4.2v across them) So the battery can be rated 7.2v . Have a look at the following link at a 4 cell battery for bike light. https://batterydoctor.com.au/store/7-2v-5-8ah-high-current-liion-battery-with-2-1mm-dc-jack-and-fuel-gauge-bike-light-battery.html The chargers suitable are in the description and as you can see they are rated 8.4v output.