Removed from the old battery, after screening, pick the factory date is closer to the appearance of good, and then use a special meter to test the internal resistance of the battery, choose a small internal resistance can continue to use, if the ~ lead-acid battery multiple batteries are used in series, as long as the capacity is the same problem is not big, if it is a ternary lithium battery or lithium iron phosphate battery, series parallel use has strict requirements, There are strict internal resistance and capacity requirements for each battery, and there must be a control board to balance the charge and discharge of the battery, so as to ensure the performance of the battery.