2008 Prius. I've had it for nearly 3 years and it drives beautifully as my daily driver. It has a NexPower Sodium V3 battery in it (the two-pack system, not the regular blade-style), which was a replacement of the V2 NexPower Lithium that failed. I drove it Monday and Tuesday, then stayed home Wednesday, then it threw P0AFA on Thursday. The car turns on fully and the ICE stays running. The LCD states "Problem" and the RTOD, VSC, and (!) dash warning lights are on. Car shifts into all gears fine and drives but the ICE stays on. Battery voltage on block 1 and block 14 fluctuates from positive to negative every few seconds. I checked the 12v battery and also tried clearing codes and it came back. Today I took the battery compartment apart and checked all the terminals and connections, as well as to the ECU. No corrosion anywhere. The two main nuts connecting the bus bars back to the cutoff seemed to be a little loose when I took them off but I tightened them back fully. After putting it partially back together I started it up again and P0AFA came back. I'm wondering if it's the battery ECU? All help is greatly appreciated. Thanks!
A negative voltage for a hybrid battery block indicates a problem with the voltage sensing wires somewhere or a problem with the battery ECU. It could be a wire issue at the battery end, at the battery ECU end, or somewhere between.
Thank you, Brian. First I tested the voltage at the main connection into the ECU and it was good, then checked between the pins and found an inconsistency between the red and black wires. So I took the whole battery assembly out and took apart the two sodium battery modules. I traced and tested all of the balance leads between the cells and the battery control board. When I located which module the issue was in I then tested the individual leads thinking there was a loose connection on the battery. All of those tests came back normally. I then did the same with the second module. Again, all good. I re-tested some of the leads and then thought to check the connectors to the board itself - Voila - an issue! It turns out that one of the leads on the bottom side of the first battery module had short-circuited and blown. See black marks in photos. In Jack's reply to my previous email he mentioned that this was identified as a failure point in some cases of the 2-pack sodium style and was a contributing factor to them discontinuing the 2-pack module. I emailed Jack with an update and asked if they had any of these control boards leftover. He replied within minutes and said Yes, they do, and that he would send it to me. Soon after I received a shipment notification! I should be able to install it Wednesday or Thursday and will know if this solved the problem. Since everything else tested good and this is definitely bad I believe (and HOPE) and that this will solve my problem. Interesting side note: When I took the battery assembly cover off I noticed that the batteries were covered in light droplets of condensation. Jack had mentioned that this happens on some models and that the short on the board is related to the condensation. I'm not really sure why, but thought I would mention it here for anyone else who might run across the problem.
The circuit board consists entirely of the following circuit, repeated once per cell in series (QTY35 copies per PCB): -a comparator (on the other side) that outputs high above ~3.5 volts, and low below ~3.5 volts. -the comparator output drives an N channel MOSFET (on the other side). -When the MOSFET is driven high, a ~33 ohm load (visible on this side) is placed in parallel with that cell. -that's it... there's nothing else. Things about this circuit that could cause the failure in @JasonMRC's picture: -there's zero overcurrent limit. -there's no temperature measurement (that gap pad got HOT). -there's no thermal fuse in the BMS circuit (e.g. a PTC thermistor in series with the input). -there are numerous creepage and clearance issues on these high voltage PCBs. -there's no conformal coating to prevent surface contaminants from shorting. -the circuit is functionally identical across all NexPower products, including V1, V2, and V3. @JasonMRC, can you send me the PCB for failure analysis?
I received a pair of new PCBs from Jack on Wednesday. I used nail polish to paint over the traces to help protect the boards from future condensation causing shorts. Reassembling the units was relatively easy after having taken them apart to troubleshoot. I put the battery pack back together and hooked things back up to test and voila - it worked! Car started fine with no errors. Ran Torque to check codes and it found nothing. Voltages were consistent and regular across all cells. I then turned it off and put the rest of it back together. Yesterday I drove my usual commute and had no issues. Hope it remains that way for a while. TLDR for future users searching: Sodium battery voltage error P0AFA was caused by a short circuit on the battery PCB computer board caused by condensation. The solution was a set of new boards.
I propose the solution is to conformally coat the PCBs, and also to properly space the high voltage traces... ...but glad you got back up and running. How did you deduce that condensation was the instigator for this thermal event?
Well, yes, it seems the solution to preventing the issue would be conformal coating. The solution to my already damaged boards was new boards. If anyone reads this before their sodium 2-module pack has an issue I'd recommend coating the PCBs as a preventative and hopefully you won't run into this issue. I don't know for sure that condensation caused this specific issue, however, when talking with Jack he said that other 2-module packs had shorts on the PCBs and condensation on the batteries, which is what I found on mine. So it is logical to assume that somehow condensation got on the PCB traces. Still stumped as to why the batteries condensated...
Condensation happens when the object is colder than the dewpoint of air. So at 3:30 today the dewpoint of our outside air was 78f. Meanwhile the battery blower fan is pulling in cabin air which may be 75f. You stop the car and outside air quickly surrounds the cooler battery and condenses liquid water on it. The answer is a qualified design engineer when you are trying to produce aftermarket batteries especially something with high energy content and fire potential.
I only see condensation inside my vehicle is winter, when the windows can be cold enough to "steam up" from the passengers' respiration plus however much (not much) humidity is in the outside air. Probably in part because where I live the humidity is usually very low, whereas at the OP's location today the national weather service reports humidity of 92%. With that much water in the outside air seems like one might want to make a habit of running the A/C in recirc mode at not too low a temperature (not below the dew point of the outside air) to condense that water out of the air in the evaporator and let it drain harmlessly out the bottom of the car. Where we are the outside air is typically lower humidity than the inside air (after people have been in it for a while) and replacing inside air with outside air reduces the amount of water in the car. While the dewpoint argument is technically correct it leaves out an important factor - the pack is considerably warmer than the rest of the interior of the car while the car is being driven, and for some time after that. So I'm not so sure about the atmospheric air "quickly" condensing on the pack, although it could certainly do so once the pack cools off later. However I am sure that if the car's interior has been cooled to well below the dew point by the A/C, when the passengers exit the car and the air exchanges with the outside there will be rapid condensation on at least the metal surfaces from that moist air. Prius vehicles in humid areas have been reported to be prone to corrosion failure of ECU in the pack. Starts out or is promoted by condensation from atmospheric moisture? Probably. Does the OP's car perhaps also have a water leak? They are common for this car and having a puddle in the 12V battery well will provide an ample source of water to condense elsewhere later.
Traction battery cells have huge thermal mass. They cool down every night. If you don't drive during the day, the battery temperature is going to trail the cabin air temperature. That's a recipe for condensation: whenever a surface is cooler than the dew point, water vapor from the surrounding air will condense. Washington, Georgia is deep in the humid south... so it's easy to see how condensation would form on the cells during the day, particularly when the car isn't driven (because the cells won't heat up due to their high ESR).
