Battery Chargers

[Al Skierkiewicz]

Quick correction, last years rules specified the NP18-12 which is also an AGM design. For most types of lead acid batteries the charger is a voltage type. This means that the charger adjusts the output voltage as a method of controlling the charge current. An 8 amp charger uses a higher output voltage than a six amp charger. The Enersys data sheet recommends..."• Cyclic use: Apply constant voltage charging at 2.40-2.50 VPC. Initial charging current should be set at less than 0.25CA." Stated another way, 17.2AH * 0.25 = 4.3 amps and 2.5v * 6 cells is 15 volts. It is my opinion that 6 amp smart chargers will not stay at the full 6 amp charge currents for very long when charging these batteries making 6 amp chargers OK for our use. In discussion with MK engineers several years ago, they felt that higher charger currents (and the implied higher charge voltages) might cause internal cell arcing between the plates. This might lead to permanent damage to the cell(s) in the form of a internal short. There are a variety of factors that play into this so testing at higher charge voltages would not reveal consistent results but the specified max charging current is what MK felt was safe. For our purposes, most teams using 4-6 amp smart chargers have very little problems in charging or battery life. The critical factor for our use is the deep discharge and current demand we make on these batteries during competition. Under normal FRC usage, I tell teams to expect no more than about 400 charge/discharge cycles before the battery can no longer supply current for competition matches. If your robot is regularly depleting the battery in one match, life expectancy is even less. About 80 % of the failed batteries I see at competition are due to mishandling (dropping).
The charger shown above can be used but I would not recommend (nor was it legal in 2012) to charge at 8 amps.