Time to replace the aux battery?

Thanks for that thoughtful post. That agrees with my experience. Over the years, lead-acid technology hasn't changed much, except that longevity is much reduced, at least in the bigger brands. It's easy to be misled by "feeling" our fuel economy is improved or reduced; a huge after-market exists based on those perceptions.

I try to look at actual numbers. I measure the actual amount of fuel used over an actual distance,and often it disagrees with my "feeling" about fuel economy. I'll buy a story that is based on real data, but I'm full of questions when it's purely anecdotal.

Realistically, it's better to look at the physics of the situation. The total amount of power being used by the Prius drive system is massive compared to the piddly power of the 12V system.

 

Yes, I'm not saying that shorted cells don't happen. And when they do there is a possibility of the DC-DC converter working overtime and that may have an impact on fuel efficiency. It's just that (in my experience) I mostly see low capacity and higher resistance rather than shorts in batteries under this type of usage pattern.

BTW. With the battery on lab power supply, did you happen to notice the maximum voltage that it reached? Typically with lab power supplies the current setting will limit the voltage, so that if it was sitting on its 3A current limit setting then its 14.3V voltage setting probably wasn't being achieved. If the voltage was under 13V then that would likely be a shorted cell. If however the voltage was up around 14 to 14.3, then it would probably be more like a high leakage condition than an actual short.

The way most shorts occur in traditional LA batteries is that over time some lead can be dislodged from the plates and form a sort of lead detritus in the bottom of the cell. This is particularly the case in batteries that are worked hard with large current pulse demands. Over time the lead detritus can build up enough to form a short circuit in the base of the cell. Another case that is also related to high load demands is where the battery gets worked so hard that the plates heat up and buckle. This can also cause shorts. I wouldn't think that this would happen to often with the demands that the Prius makes of its 12V battery though.

 

Probably around 12V.

After the charging current was removed and the battery had a chance to rest, the open-circuit voltage was around 11V which is consistent with the voltage that you would expect a charged, 5-cell battery to produce.

 

A normal healthy battery is around 12.7v and "depleted capacity" charging rate is around 13.7v, anything outside of this is not the way they and the charging system have been designed.

 

I don't know if you noticed the age of this thread, but it was many years ago when Ashyukun posted that. He was last seen on PC on May 2, 2015. I'm only replying as the information posted was open to misinterpretation. While I would consider your 13.7 V to be a bit light, any voltage up to 14.4 V is an acceptable voltage for "working as designed". However, any voltage of 13.9 V or above would be cause for investigation as that would indicate a very "depleted capacity" battery, high internal resistance, or a shorted cell among other things. The net result is the charging system is working very hard to try and get charge into the battery. So if it is any of these things then yes, it is time to replace the 12 V battery.