Just a few days back, I get one pack work with chipset S8254AA (primary protection )+bq2060 gas gauging ic. 2060 is a very old chip, but it is very comman in those old days pack, where we see plenty of them before, but it is really rare that it can fail. I am thinking of taking it picture, but the pack have already been posted back to customer, have forgoten to do that.
After replacing the cells of the pack, the pack neither charging nor able to discharge, I am begining to worry and about to tell the customer that your battery fail to be repaired.
But after some while, I am begining to realize that that could occur only if two possible, 1. S8254AA is out of order. 2. S8254AA is not being enabled.*Note: It could also be power mosfet failure, but it never been before that power mosfet fail in original pack although it sometime fail on some third party pack. It could also be the problem of bq, but I have check it funcationality through software, all activity pass.
A little bit hard work, manage to trace out that the control pin of S8254AA is actually control by Charge FET Control(CFC) pin of the bq2060, and CFC is measured a 10Ohm to ground, which mean a shorted could occur there. The real circuit is a little bit more complicated that above one, the picture above is for conceptual purpose only.
Unsoldering the bq2060 and measure the CFC pin with respect to ground, guess what? It is really shorted. The NMOS junction inside the I/O have probabaly subjected to permanent break down already. That is what make the current flow meter measure an aprox -10mA eventhough the pack is under static condition.
Guess what? Just replace the fail bq with a working one, it live again! Fortunately the replaced chip have pretty much the same property as the fail one, not noticable diviation of voltage, current and temperature as the old one. It save some time to recalibrate those parameter to correct one.
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