Bought Battery, It Sat on Shelf 17 Months, Installed Today: Readings are 11.4 / 11.1 / 14.3 V

Never a good idea to let a lead acid battery just sit for long periods of time.

Too bad you didn't test it with a voltmeter before putting it into storage and again before putting it in the car. In theory AGM batteries for the Prius are supposed to self discharge at 1-3% per month. Would have been good to know what the rate for this brand actually was. (It also varies with temperature, more discharge with more temperature.) Anyway, lemons, but let's make lemonade anyway.

Vmonths=Vstart*exp(months*factor).

From date of manufacture that one is 12 + 7 = 19 months old. We will have to estimate Vmonths from the 11.4 value as 11.6V, since that power mode usually pulls down the battery voltage by around 0.2 volts. (That's with a fully charged battery, it probably comes down more for a significantly discharged one, which means the discharge rate might be slightly less than calculated because of this error.) We also have to estimate Vstart, let's use 13V.

ln(11.6/13)=19*factor
factor=ln(11.6/13)/19
factor= -.005997 (which is hardly any different from...)
factor=-.006

One month percent loss is exp(-.006) = .994, or 0.6 percent. That is a really low self discharge rate, even for an AGM battery.

On second thought, the preceding is the wrong approach.

What decays is the fraction of the capacity which is available, not the voltage. They both go down, but the latter is a function of the former. Capacity starts at 41 Ah (for that battery) and would be 0Ah when the voltage is around 10.7V and the battery fully discharged. It doesn't really matter what the starting capacity is (41 Ah, 45 Ah, or %SOC) though because the formula is the same and a ratio wipes out the units. What does matter is that it decays to no capacity, whereas the voltage doesn't really mean much of anything once the battery is below 0 charge. 10V and 2V are still both 0 charge. So using %SOC Cstart=100, Cmonths=45 (45% is roughly %SOC at 11.6V in the table I consulted) and the formula for capacity:

Cmonths=Cstart*exp(months*factor)
ln(Cmonths/Cstart)=months*factor
factor=ln(45/100)/19=-0.042
One month percent loss is exp(-.042)=.959, or 4.1%. The average absolute monthly loss over 19 months is 55%/19=2.89%, which is lower because that .949 gets multiplied by ever lower remaining capacities. And 2.89% is between 1% and 3%, but 4.1% is not.

Which makes me wonder exactly how battery manufacturers define "self discharge rate".

 

Should of listened to ChatGPT... Only problem is your trickle charger is inadequate and you needed to put in on a smart charger of up to 10amps or greater and that 12v would of been good as new.

 

You should have a jump box for those cold Kansas winters. You'll need to take it inside with you, because lithium batteries don't like the cold either.

 

when I bought my 2007 Prius with a bad HV battery, the 12v battery has been sitting for the whole winter (Duracell) When I arrived to buy it the owner just did a quick charge so that I can hear the engine and move it to a towing dolly. Once I got home I used the harbor freight smart charger and the battery is still working fine after 3+ years. The problem with these smart chargers is that if the battery is too low it won't charge at all, it needs a jolt of 12v to go back alive and start smart charging.