Battery charging method and system

1. A method of charging a battery having a first voltage and a second voltage, the method comprising

in a first phase, applying a constant current to the battery;

in a second phase, applying current pulses to the battery, the current pulses being separated by a current pause during which the current is at or close to zero;

providing a fixed reference voltage;

repeating iteratively

sampling the first voltage during a current pulse to obtain a measurement of the first voltage;

sampling the first voltage during a current pause to obtain a measurement of the second voltage;

generating a dynamic reference voltage based on the fixed reference voltage and on a difference between the measurement of the first voltage and the measurement of the second voltage;

comparing the measurement of the first voltage with the dynamic reference voltage;

stop applying the current pulses when at least one of the measurement of the first voltage is equal to the dynamic reference voltage and the measurement of the second voltage is equal to the fixed reference voltage.

2. The method as claimed in claim 1 , wherein the method further comprises iteratively comparing the measurement of the second voltage with the fixed reference voltage.

3. The method as claimed in claim 1 , wherein the battery comprises a battery pack and wherein the first voltage is a voltage of the battery pack.

4. The method as claimed in claim 1 , wherein the dynamic reference voltage is the sum of the fixed reference voltage and a dynamic error voltage, the dynamic error voltage being determined as the difference between the last measurement of the first voltage and the last measurement of the second voltage.

5. The method as claimed in claim, 1 wherein iteratively sampling the first battery voltage during a current pause comprises one of

sampling the first battery voltage during each current pause; and

sampling the first battery voltage during the first available current pause after a predetermined time interval has lapsed.

6. The method as claimed in claim 1 , wherein the current pulses have a constant profile.

7. The method as claimed in claim 6 , wherein the constant profile is a square profile.

8. The method as claimed in claim 1 , wherein the amplitude of the current pulses is equal to an amplitude of the constant current applied to the battery during the first phase.

9. The method as claimed in claim 1 , wherein the duration of the current pauses is selected based on a relaxation time of the battery.

10. The method as claimed in claim 1 , wherein the duration of the current pulses is selected based on an average time required for the dynamic error voltage to vary by a predetermined amount.

11. The method as claimed in claim 1 , wherein the method further comprises:

applying a pulsed current to the battery prior to the first phase, upon identifying that the battery is fully depleted.

12. A charger for charging a battery having a first voltage and a second voltage, the charger comprising

a current generator to provide a current to the battery;

a voltage sensor to sample the first battery voltage; and

a controller configured to

in a first phase, apply a constant current to the battery via the current generator;

in a second phase, apply current pulses to the battery via the current generator, the current pulses being separated by a current pause during which the current is at or close to zero;

provide a fixed reference voltage;

repeat iteratively

sampling the first voltage via the voltage sensor during a current pulse to obtain a measurement of the first voltage;

sampling the first voltage via the voltage sensor during a current pause to obtain a measurement of the second voltage;

generating a dynamic reference voltage based on the fixed reference voltage and on a difference between the measurement of the first voltage and the measurements of the second voltage;

comparing the measurement of the first voltage with the dynamic reference voltage;

stop applying the current pulses when at least one of the measurement of the first voltage is equal to the dynamic reference voltage and the measurement of the second voltage is equal to the fixed reference voltage.

13. The charger as claimed in claim 12 , wherein the dynamic reference voltage is the sum of the fixed reference voltage and a dynamic error voltage, the dynamic error voltage being determined as the difference between the last measurement of the first voltage and the last measurement of the second voltage.

14. The charger as claimed in claim 13 , wherein the charger comprises a sample-and-hold circuit being operable in one of a first state and a second state; and the controller is further configured to control the sample-and-hold circuit such that

during a current pause, the sample-and-hold circuit is in the first state; and

during a current pulse the sample-and-hold circuit is in the second state.

15. The charger as claimed in claim 14 , wherein the sample-and-hold circuit is an analog circuit.

16. The charger as claimed in claim 14 , wherein the sample-and-hold circuit comprises a comparator having an output coupled to the controller; and the sample-and-hold circuit is configured such that

in the first state, the output of the comparator is a signal based on a comparison between the measurement of the first voltage and the fixed reference voltage; and

in the second state, the output of the comparator is a signal based on a comparison between the measurement of the second voltage and the dynamic reference voltage.

17. The charger as claimed in claim 16 , wherein the sample-and-hold circuit further comprises

a first capacitor, a second capacitor and a first buffer, the buffer begin coupled between the first and the second capacitor; and

a second buffer;

wherein

the first buffer, the first capacitor and the second capacitor are configured to sample and hold the dynamic error voltage; and

the second buffer and the second capacitor are configured to provide to the comparator a sum of the sampled dynamic error voltage and the fixed reference voltage.

18. A device comprising a charger for charging a battery having a first voltage and a second voltage, the charger comprising

a current generator configured to provide a current to the battery;

a voltage sensor configured to sample the first battery voltage; and

a controller configured to

in a first phase, apply a constant current to the battery via the current generator;

in a second phase, apply current pulses to the battery via the current generator, the current pulses being separated by a current pause during which the current is at or close to zero;

provide a fixed reference voltage;

repeat iteratively

sampling the first voltage via the voltage sensor during a current pulse to obtain a measurement of the first voltage;

sampling the first voltage via the voltage sensor during a current pause to obtain a measurement of the second voltage;

generating a dynamic reference voltage based on the fixed reference voltage and on a difference between the measurement of the first voltage and the measurements of the second voltage;

comparing the measurement of the first voltage with the dynamic reference voltage;

stop applying the current pulses when at least one of the measurement of the first voltage is equal to the dynamic reference voltage and the measurement of the second voltage is equal to the fixed reference voltage.