HV battery drains slowly in the morning...

First, I realize there have been some topics similar to this but I felt neither of the ones a search pulled up fully hit the mark for my situation, including the answers.

I'll begin with a bit of history. I have a 2005 Prius with about 159,000 miles on it. Around the 120K mark the HV battery started acting funny and my wife, who drove the car at the time, took it to the nearest Toyota dealer in limp mode. The diagnose was the battery was bad and that it needed replacement. It will be $5000, thank you very much. Fortunately she called me before doing anything and my response was the usual "Hell no!" when large sums of money are involved. To cut the story short, we got the Prius home, took out the HV battery pack, checked each cell and yes, one cell was very low and wouldn't hold a charge. So we took it out and replaced it with a used cell bought on eBay. We didn't have any way to charge or balance the battery but got lucky because that cell's voltage was within 1-2 volt of the rest of the cells.

Since that I've taken over the car because I drive looong stretches for work (project engineer) and have put the 39K+ miles on it. In other words, our swap out worked nicely. However, the other day I parked outside a building in Mobile, AL to wait for a realtor and did so with the AC on because it was hot. Yeah, I know, dumb move. The battery drained, all kinds of warning lights popped up and ever since then it has been a little bit of a dork in the mornings. So this is the current situation:

It drives fine. Battery charges and holds a charge just fine. The charge will drop if getting stuck in traffic, hit 2 bars then go back up again when the engine turns on. A/C is not its friend in those situations. However, in the morning, it will show an almost full battery (green -1) then when I start driving slowly drain all the way down to 1 bar while kind stutter or have hick-ups. Not massive ones, but more like a hesitant deal. After about a minute or so the battery fills back up and all is well again.

Since I know how to get the HV battery out quickly I did and checked all the battery cells. I did it in the morning to see if there was a drop. All cells were nicely balanced around 7.75 to 7.77 volts except one that was 7.82 and one that was 7.72. Not sure if that last one is enough to worry about? I don't think so. Bottom line is that I feel the battery is good. Or?

The 12V battery is also good, showing 13 volts. It is quite new, less than a year old.

Is this behavior normal, maybe because of the battery being cold in the morning? Or is it an omen of some kind? I am tempted to put the battery back in and say f it but will wait and see what you all think.

Thanks.

 

Hi and thanks for your reply.

No, I don't have either but after your post I am seriously considering buying a Mini VCI. Sounds like quite a handy tool to have when dealing with this Prius of mine. Not that I have anything to complain about. Apart from regular oil + filter + spark plug changes the only mechanical work that has been done on it is changing the water pump (started leaking at about 100K, easy as heck to change, cost me $150 or thereabouts), that one bad HV battery cell ($45 on eBay) and the 12V battery. That's it. Almost like magic that.

Anyway, back to topic. I left the battery sitting on my workbench all day and when I got back I took another quick voltage reading. Most of the cells keep the charge pretty well and have only dropped a few volts since this morning. Two of the cells however, are dropping faster. The very first one and the very last one actually. One dropped 10V from 7.77 to 7.67 while the other 5V from 7.72 to 7.67. Hmm. Those are the likely culprits then. However, they are not gone to battery heaven just yet so I will keep using them until they do. Mostly because I have to get the car ready again tonight and hell, it's already 7:16PM central. I always carry the tools I need plus extra battery cells so if it does break down, I can fix it reasonably quick.

If anyone else wants to chime in, please do. I will definitely appreciate it. Thanks.

 

No, it is not. I marked the one I replaced last time.