A new method for maintaining the resettable circuit of a lithium-ion battery is introduced.
The rapid increase in the use of high-rate discharge lithium-ion batteries has created a demand for cost-effective circuit protection devices capable of supplying continuous currents of up to 30A+ at DC voltages up to 30V. To meet this demand, a hybrid device combining a bimetallic retainer with a polymer positive temperature coefficient (PTC) device in parallel has been developed. The resulting MHP (Metal Hybrid PPTC) device provides reconfigurable overcurrent protection and the low impedance characteristics of the PPTC device, thus preventing arcing of the bimetallic retainer at high currents.
Figure 1 shows the shape and dimensions of the 30A rated holding current MHP device, which is the same size as a typical bimetallic holder with a rated current of only 15A. Furthermore, the device is arranged at a flat angle on one side, allowing it to be tightly mounted between lithium-ion batteries with a diameter of 18 mm in the battery pack.
MHP circuit maintenance equipment provides battery planners and manufacturers with a method to optimize space, reduce costs, and improve safety, and can be used in a variety of applications. Equipment with higher voltage and continuous current is under development, and future planning considerations include battery maintenance for wireless power tools, e-bikes, scooters, light electric vehicles (LEVs), lithium-ion battery packs for backup power, and non-battery applications such as motor maintenance.
Planning concept
Test data shows that typical bimetallic protection units integrated with polymer PTCs, such as the polywitchRXEF050 device, can provide similar interlocking for both ceramic PTCs and bimetallic protection units, jointly providing enhanced power supply and arc suppression capabilities.
Because of the low contact impedance of bimetallic contacts, current flows through them under normal operating conditions. When an abnormal event occurs, such as during the measurement of a power tool rotor, a large current flows through the circuit, causing the bimetallic contacts to flip and their contact resistance to increase. At this point, the current is diverted to the low-impedance PPTC device, which also heats the bimetal, flipping it and holding it in a certain position, thus helping to prevent arcing between the contacts.
As shown in Figure 3, to reduce the size of the device, the MHP device uses a procedure called double-make or double-break as the touch mode. Compared to a typical bimetallic retainer, this technology has several advantages: the device impedance is very low due to the short current path; heat can only occur at the contact point, allowing for precise thermal activation through thermal control; and it promotes the use of small maintenance equipment.
