Kept having the "p=lock error" appear on my screen and could not leave my garage. I forgot having the smartkey function turned on can drain the battery and it was indeed low. Put it on an 8A charge for several hours and it was fine again. Then a couple days later, as I was leaving the post office it did it again. I checked the battery and somehow did not have the negative cable at the post tightened completely, tightened it and it was fine again. This morning it did it again. But then I discovered something. There was 12.5v at the battery itself, 12.5 volts at the jumper point under the hood, and 12.4 volts at the headunit when going into the diagnostic menu, but my OBD scanner showed only 11.5 volts at the port, and when I checked the voltage at the transmission control unit, it was also only 11.5 volts as well. If I remember correctly, the p-lock error can also occur if the voltage falls under 12.1 volts. I dug up the wiring manual and checked both ground locations for the TCU and both were fine, no resistance or rust/corrosion.I thought a bit of corrosion at the connection was causing this, but cleaning it didn't fix it. I'm just disappointed at myself because I'm good with cars, heck I even rebuilt the motor in this thing myself, but there over a dozen computers in a Gen2 Prius and hundreds of feet of wiring, and dozens of added failure points from all that. I even put a booster pack on the battery hoping it would trick the TCU, but that didn't work either. I'm not sure what's causing a voltage loss of 1 volt going into the computers, but it's hot as heck outside and I spent 4 hours sweating my butt off and made no progress.
I'm not sure what you're remembering, but what I've seen of the voltage thresholds for most of the ECUs that flag low-voltage conditions are a lot lower than that. I don't have access to a gen 2 repair manual at the moment, but in gen 3 the threshold for the transmission control ECU setting the C2310 code is given as "8 V or less".
a load test is important with a 12v. if you charge it fully, and it keeps dying, it's either bad, or something is draining it when the car is off. are you getting any trouble codes? sometimes corrosion at an ecu connector is a problem
Have you tried this? Maybe you're in the early stages of this problem. P Lock C2310 Fix Fixed Solved Solution | PriusChat
I did, there was no corrosion or anything on that plug at the front jump/fuse box location. When I took the lower panel off which covers the TCU, I noticed the wires going into it were awfully tight, meaning the entire harness was pulling on the connection. If I wiggled it, it started up even more error codes, including evap and ABS codes. But there is something causing voltage loss, oddly when the car was running, it showed the correct charging voltage on the OBD2 connector and at the TCU. But not when it was off. I've read other posts where there was maybe 0.1-0.3 volts of loss between the measurement at the front jumper location and the OBD2 port, so an entire volt is concerning. The only place the voltage source from the TCU and the Datalink/OBD port are even remotely near each other is the underhood junction block. I'm not going to have Toyota do to much if it's going to be expensive, but hopefully they can at least find the actual issue.
The maximum charging current for an AGM is usually given as 0.3 C, on this battery if brand new 45*.3=13.5A. Somewhere or other Toyota says to only charge this battery at 4A (a little less than 0.1 C), although that limit may be only from the jump post. Given the tiny amount of fluid between the plates and the unknown capacity loss of an aging battery I would err on the side of caution and try to keep it closer to 4A. (I think the car itself may try to keep it below that, which goes a long way towards explaining why it has such a hard time keeping the 12V fully charged.) Since temperature, depth of discharge, and I think high discharge currents reduce life, I would expect that there is some curve that shows battery lifetime decreases with increasing charge current, although the decrease between 4A and 8A might be only a few percent.
That 4 amp limit is written on an information sticker on the TrueStart battery itself. The actual wording is: Never add water nor charge quickly. When you charge battery, charging current should not be higher than 4.2 amps and charging time should be within 10 hours. Never take off the upper cover.
