Oddity with 12V battery voltage measurements andtranmission

Finally managed to get some current measurements with the car in the high and low voltage states. Voltage measurements on the battery, current measurements with a clamp ammeter on the positive cable that runs to the front of the car:

Transmission in "D":
13.6V with current varying mostly between 30mA and 70mA (sometimes a little higher) into the battery.

Transmission in "P":
14.1V, on the transition from 13.6V the current went briefly over 2 A (too fast to read more than the first digit) then it fell back in seconds to somewhat over 1A, but varying. Again, into the battery. Let's call it 1.2A with huge error bars.

The current at 13.6V is very similar to what I have measured for a BatteryMinder 1500 on the same car, once it is in float mode. The current at 14.1V is at the high end seen during the absorption phase with that charger. I did not wait long enough to see if it continued to behave like the absorption phase. If it did it would have fallen very slowly (over hours) down to 200-300 mA.

Turned the car off, opened and closed a few doors and then locked the car, then waited 5 minutes, and the battery was reading 12.65V. Before the drive the car had just come off the charger and it was also at 12.65V. Seems to me that this Prius is somewhat overcharging the 12V battery for no obvious reason. The battery is fully charged as far as a battery charger is concerned, but the car often pushes it to a charging voltage anyway. Admittedly that is only ~1/40C at worst, and probably ~1/150C (300 mA) at best. Still, float is ~1/600C and down.

One other odd observation (so many...). Drove the car or had it sit in READY for about 35 minutes. That entire time it was stuck at 14.1V. Stopped at a store for 10 minutes where the car was turned off. After turning it back on in a few seconds it was showing 13.6V in "D" again, and it stayed like that for the 20 minutes it took to get home, and 10 minutes more while I made those measurements. Almost like the logic got stuck on the first part of the trip somehow, or power cycling is needed to let it reexamine the state of the 12V.

 

It is a 4 year old Toyota OEM battery. S46B24R. Not the Yuasa version with the hex shaped cell caps but one that looks like the Walmart S46B24R battery, with the pinwheel "tamper proof" cell caps. It has always seemed weak, even when new, in that the voltage seems to drop too much for the amount of current it is supplying. Despite the HF battery tester claiming it has ~45 Ah capacity. The OEM battery before that was the Yuasa battery (with a Toyota sticker), and it lasted all of two years. A sustained 13.x volts from the battery (not the converter) just from driving around in the car was a pipe dream with both of those batteries. When charged externally on float for a while of course when they were unhooked they remained briefly at the float voltage, which was indeed 13.x V (or thereabouts). But that was in large part surface charge. Turn on the headlights for a few seconds, turn them off, wait at least 5 minutes for the battery to relax, and they would read somewhere around 12.75V. On a good day, more commonly now it is more like 12.65V but sometimes it is over 12.7V. The battery is in the car, so it will also lose ~.03V every day from the normal 20-25 mA parasitic load and self discharge.

I recently picked up an electronically programmable load tester to measure how much the voltage drops for known test loads, and to be able to discharge the battery a known amount at C/20 and see where the resting voltage ends up. Funny thing about battery capacity specifications, to actually measure one seems like it would trash the battery. That is, take a brand new fully charged battery, draw C/20 out of it until it is near 0% SOC, and that is the capacity. Except doing that will permanently damage the battery (at least a lead acid one). Even a deep discharge battery isn't supposed to go below 20% SOC. Makes me wonder if the battery manufacturers really measure capacity like that - and then just toss the damaged test batteries. I'm thinking for a start, take it down by 5Ah and compare the relaxed voltages before and after. Iterate, keeping the voltage after discharge above 70% SOC, and then recharge immediately.