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TIDUEB2A July 2022 – July 2022

2.2.5 Battery Ideal Diode-OR

To discharge the two independent batteries in the design, an ideal diode-ORing circuit is used. In this circuit, Q3 and Q4 are driven by U5 and U7 to regulate the FET forward voltage drop to 20 mV. This allows for the efficient discharge of both batteries at the same time, while also preventing current from traveling from one battery to the other. This circuit also allows for passive battery pack voltage balancing to occur in the system by drawing more current from the battery with a higher voltage. Equation 2 shows an example calculation of battery discharge current based on voltage and equivalent series resistance (ESR). For this example, it is assumed that BAT_A is at 16.8 V and has an ESR of 50 mΩ, BAT_B is at 16.6 V and also has an ESR of 50 mΩ, and the system is drawing 8 A.

I_{L O A D} = I_{B A T A} + I_{B A T B}

V_{L o a d} = V_{B A T A} - V_{A f} - (I_{B A T A} \times E S R_{B A T A})

V_{L o a d} = V_{B A T B} - V_{B f} - (I_{B A T B} \times E S R_{B A T B})

V_{B A T A} - V_{A f} - (I_{B A T A} \times E S R_{B A T A}) = V_{B A T B} - V_{B f} - (I_{B A T B} \times E S R_{B A T B})

16.8 V - 20 m V - (I_{B A T A} \times 50 m Ω) = 16.7 V - 20 m V - (I_{B A T B} \times 50 m Ω)

Equation 2.

I_{B A T A} = I_{B A T B} + 2.0, w h e r e I_{B A T B} \geq 0

In this example, BAT_A supplies 2 A more current that BAT_B until the higher current discharge causes the voltage differential between the batteries to be reduced.