Testing laptop battery: pinout, SMBus, charge capacity

As a result of visiting Ham fest, I ended up with a laptop to take apart – a fairly new Toshiba Satellite C675D with a broken screen. It’s not a Ham fest if you don’t bring home something to take apart of course! Today we’ll be testing the battery it came with to see if it’s salvageable. The date code says  it was made in 11/2011
Initial measurements
Label data indicates we are dealing with a 10.8V 4.4Ah pack. Since a typical Li-Ion cell is around 2-3Ah and 3.6-4.2V, that tells us the pack has 6 cells arranged as 3Series2Parallel (or 3S2P). The battery was fully depleted at the time of purchase and would not power the laptop. I could also not see any voltage present on any of its pins. Since we are dealing with a parts laptop here, an easy way to figure out full pin out is to look at the motherboard end of things:
It is pretty obvious that the larger blades are power and ground, and by measuring resistance to case ground we can tell that the leftmost two pins are ground. The next two pins (3,4) are routed through a SOT23 device that measures as a dual diode- most likely a dual Zener/TVS for ESD protection of comm lines. Pin 5 has a single channel TVS/Diode looking thing. That most likely means we have SM bus (a typical comm channel for notebook batteries) on pins 3 and 4, and a battery temperature on pin 5. Pins 6 and 7 remain mystery for now and pins 8 and 9 are the battery positive.
Testing laptop battery pinout, SMBus, charge capacityNext step is to try waking the battery up. We apply normal for it voltage (in the range of 3-4.2V per cell) and see if it “takes “any current. Nothing happens, no current is going in. Plan B-  put the battery back in laptop and observe signals on the pin. With the laptop powered by a bench power supply, we can see a notable current spike when battery is inserted, so something does happen. Voltage on the pins is now 12.6V, meaning the charger is trying to do its job but no current is actually making it in. Poking around with a scope, we can see activity on pins 3 and 4. And pin 3 definitely looks like an I2C clock, pulsing nonstop during the comm intervals, while pin 4 looks like an I2C data. Ok we’ve just confirmed 3 and 4 are indeed the SM bus pins and the battery is talking to the charger.
That seems to indicate it is alive, but too discharged to allow for charging- most likely staying in recharge mode with a large series resistor limiting input current to very low values. Leaving things alone for a bit and checking back in a few hours, the power supply current went up drastically from about 0.5A at 19V to 1.5A. That may be a good sign- battery is now taking current. Voltage check confirms that- we are now at 11V and rising, meaning the fast charge constant current phase is on, and the battery is finally charging. Well, might as well let it finish. Once the laptop’s charge indicator goes from yellow to green and power supply current drops, we can start the tests.

Read More: Testing laptop battery: pinout, SMBus, charge capacity


Leave a Comment

Your email address will not be published. Required fields are marked *