Thinkpads Forum

I was recently given a T420i with what appears to be a nearly dead battery: 42T4791 (55+) that Linux reports as having only about 9 Wh capacity when full, when the design capacity is around 56 Wh. So I bought an OEM 45N1001 70+ battery on eBay (I know, bad idea maybe, but I've had good luck there before), and it's even worse: around 4 Wh capacity when full. Even Windows says "consider replacing the battery".

Is it possible that this T420i has some system board problem that causes both batteries to be reported as having less capacity than designed? Unfortunately I don't have a way to test this hypothesis, not having another compatible ThinkPad on hand.

If you make a battery extension cable you can try the battery in other Thinkpads: viewtopic.php?p=680106#p680106

That's a great idea, making an extension cable, thanks! I think I may have the parts here for that.

So I got halfway through making my extension cable, but then the piece-of-crap Weller soldering iron stopped working after I got one of the connectors hooked up. A Hakko is on order.

In the meantime, I decided to check the continuity of what I've got so far. Interestingly, the two pins that are by themselves to one side of the key finger (not sure what the technical term is) seem to be shorted together. This seems to be the case on both of the connectors I clipped from a T61 system board and a T60p battery. I'm going to assume this is right, but it would be nice to see a pinout diagram for this connector; I just don't know where to find one.

Battery pinout (from T6x upwards)
--- Ground
--- Ground
-- ??
-- ??
-- ??
==== plastic separator
--- Positive
--- Positive

RealBlackStuff wrote: ↑

Wed Mar 10, 2021 11:15 am

Battery pinout (from T6x upwards)

I found the following pin-out online for this T6x+ connector (which matches yours):

+, +, (plastic-separator), SCL, SDA, Thermistor, GND, GND

SCL and SDA are the two wires for SMBus protocol, embedded controller in Thinkpad will use that to talk to the batt. Thermistor.. probably tells Thinkpad the battery has overheated and it's time to stop charging?.. Not sure

Thanks, folks! Now I don't have to worry that I've done something wrong.

The extender cable worked great. I was able to determine that the eBay battery was bad and that the T420i was fine. Then did the same test on an R61 with a bad battery.

Thanks again.