Note: Use our solar panel size calculatorto find out what size solar panel you need to recharge your battery. I've seen many ways to calculate the battery runtime online. Which are easy but least accurate. So I'm gonna share the most accurate and difficult method. Formula #1 (Best For Large Capacity Batteries): Battery runtime = (Battery capacity Wh × battery. Calculating how many hours your battery will last while running a load is not an easy task. There are so many factors to consider for an accurate value. You can use our lithium battery run. Rechargeable batteries are designed to be charged/discharged at a limited current rate to increase the battery lifespan or life cycles. Lithium batteries.