Question:
We are using MAX1757 in our charger module to charge Li-ion 2- cell battery configuration (2S1P-NCR18650B). The full charge condition for the battery pack is 8.4V, we are charging the battery with 1.5A constant current at initial stage, the issue lies when the battery pack charges till 90 % of its full charge potential, the full charge indication goes high, we would like to show this full charge indication when the pack voltage reaches till 95%-96% of its full potential. We tried many setting as per calculations given in the data-sheet but the full charge indication is going high before expected time. Kindly suggest some steps to overcome this issue.
Answer:
This behaviour is expected and cannot be changed. The part is programmed like that. The state machine asserts the FULLCHG output low when the device transitions from constant current to constant voltage charging, and then deasserts FULLCHG output high when the device enters the Top-Off charge mode. After the Top-Off timer expires, the device enters the Done state, however there are no logic output indicators that change state for this transition from Top-Off to Done.
The charge status indicators FASTCHG and FULLCHG can be logically NANDed together to indicate the actual completion of the constant voltage charging state where the MAX1757 enters the top-off charge state. In this way the NAND gate output will drive low when both the FASTCHG and FULLCHG outputs are high, and the NAND output can sink current for an LED that indicates the battery has completed both constant-current and constant-voltage charge states.