Battery Health Testing

i'm no battery expert, but i did stay at a holiday inn express last night.

 

In other vehicles where I've installed a "always-on" voltmeter, I've noticed that the battery's voltage after sitting overnight would gradually decrease with the age of the battery. It might be 12.6V when the battery was new, and decrease to perhaps 12.3V for a battery about to fail. The voltmeter that I used was a VDO unit that had a current draw of about 70ma. For the Prius I would used an LCD unit (fused near the source, of course).

I'm about to put a data logger on my 12v battery for about a week and will post the results.

(Actually, with a one minute sampling interval I can get 22 days of readings.)

 

There are things that go wrong with Prius 12V batteries that the tester will not find. They cannot measure specific gravity of the electrolyte or individual cell voltages for example. A good test would be an 8 hour test discharge to determine the capacity, compared to what it was when new. But the dealer does not have the time, personnel, or equipment to do that and it is cheaper just to put in a new battery.

My car is 3 1/2 years old and I wouldn't be surprised to have the same problem any time now, although some people have gotten 5 years or more out of their 12V batteries with the Gen2.

 

Whenever I have a left-a-light-on event, resulting in a near-dead battery I immediately charge the cr*p out of it, using my CTEK 3300. Any similar "intelligent" charger with amperage under 4 will do. Then drive for a day or two, then check voltage with a digital multimeter. If it's around 12.6 or higher, you're out of the woods, but your battery's likely going to die sooner than later.

 

I only get about 3 1/4 years out of a battery. I know mine is on the low end so I'm shopping for one now. A discharged battery can read "good". I just went through this in a PepBoys. I would have asked the dealer to show me the voltage readings before swapping out the battery.

Charge up the battery overnight & see what it holds. With 2 deep discharges your original, IMO, was on the weak side. I suspect your dealer's idea of dead is that the car won't start and the battery won't hold a charge.

 

Your old battery may have been badly sulfated from the days long total discharge. A sulfated battery will have less capacity than a healthy one, and be slower to charge, as the sulfate interfers with recharging.

I have a BatteryMINDer charger/ desulfator / maintainer permanently installed on my Prius, as I often do not drive it for several months in winter. The normal parasitic electrical drain in a Prius will discharge even a new battery in about two weeks. If I know I will not drive my Prius for a week or longer, I plug in the batteryMINDer to keep the battery fully charged and to remove any sulfation buildup on the battery plates.
I have had good luck using the little BatteryMINDer charger to not only maintain battery charge, but will slowly remove sulfation over a period of weeks / months. Over a period of months I have had good success bringing even badly sulfated batteries back to like new health. They are no help for a worn out decade old battery or one with a shorted cell, but will over time remove sulfation damage from an otherwise mechqanically sound battery.

Some cheaper charger/ maintainer/ desulfators are not made for a deep cycle AGM battery like what is in your Prius, but the BatteryMINDer unit is .
BatteryMINDer Charger/Maintainer/Desulfator System — Model# 1500 | Battery Maintainers| Northern Tool + Equipment

The Battery MINDer comes with both battery clip cables for temperory use, or ring terminal cord for permanent mount like I use.
The battery cord plug hangs in the below floor package tray. When I want to use it, I plug the charger cord and pigtail together, run the cable through the notch where the RR D ring is located, run the cord under the hatch and plug the unit into an extension cord. When not needed, the charger is unplugged from the battery and stored in the package tray under the floor.