I am considering moving to LiFePO to replace a 660Ah bank of deep cycle gel.
This is on a sailboat where I have dc motors in the bow and stern thrusters, electric winches, and windlass that run from the battery bank. I would like to avoid the complexity of keeping lead acid batteries for the motor loads and I have no worries about scaling the Lithiums to provide the occasional peak currents.
My question is one of the compatibility between the Lithiums and older dc motors.
My understanding is that when a dc motors spins-down after discounting power it momentarily act as a dynamo. The relatively high internal resistance of lead acid batteries limits the peak current returned to the batteries. However, with the low resistance of LiFePO this momentary current can be high enough to damage the windings or brush connections in the motors.
I know this from experience. When I replaced the wet cell batteries on the boat with gel (a per battery internal resistance drop from about 20mΩ to 5mΩ) I destroyed a €7k electric winch. I hadn’t seen the service bulletin about incompatibility with this battery technology and didn’t consider the effects of the very low resistance of having a bank of six batteries in parallel.
With no voltage regulation the motor/dynamo could momentarily put 15V across the batteries. If the operating motor had been drawing 400A, even healthy lead acid batteries could drop below 11V therefore before they “chemically” recover there could be a 4V over-voltage in to a mΩ and a resulting a 4kA transient.
Has anyone come across or solved this issue?
I am thinking that a correctly rated diode/resistor combination in the connection to each motor would limit the return current.
