Odd battery behaviour!

1. It looks like you only drive 8K miles per year which is very low usage, not enough to keep the 12V battery in the car charged.
2. The new battery at 12.7V open circuit was not fully-charged to start with.
3. What was the voltage of the old battery, one day after it had been charged? That voltage should then be compared with 12.57V two weeks later.
4. If the voltage of the old battery, one day after it has been charged is not at least 12.9V, maybe you need a new charger that is intended for use with AGM batteries.
5. Then, both the new and the old batteries should be periodically charged to keep both in top condition.

 

Also, the voltage on the MFD is convenient, but not necessarily accurate. To be certain, one must measure directly at the battery posts. A large discrepancy might indicate a poor connection.

 

I second what Fred_H said. Having tested a number of different gen2 Prius I have found the MFD can read up
0.5 volts low on some vehicles. This is a very large discrepancy on the state of charge of the battery. A volt meter on the battery terminals is the only accurate method. Any discrepancy can then be factored in when using the MFD in future.

John (Britprius)

 

Wow that's a big dependency John. I've crossed checked my MFD measurements with my voltmeter and it's only off (low) by about 0.1 volts. It looks like everyone should try checking their readings against a voltmeter at least once if they've got one available.

*Goldfinger*, it sounds like that battery might have benefited from a charge before it was fitted, especially if you're only driving a few hours per week. One other thing about the battery test: When you enter the service mode menus you can't see the climate control screen, and a lot of people accidentally leave the fan running which lowers the second "ig-on" reading somewhat. Mine can drop an extra 0.2 to 0.3 volts with the fan on medium and an extra 0.5 volts with it on high.

 

As I have pointed out a number of times in other posts the MFD can, and does read low on some vehicles "not all", possibly due to a small amount of resistance between the battery at the rear of the car and the point at witch the MFD measures.
The fact is that 0.016 ohms resistance at 30 amps drops 0.5 volts. So it does no take much to give a low reading on the MFD. The resistance can be in the positive line "aluminium cable" and connections, or in the negative steel bodywork return path. Even 0.1 volts as in your case is significant.

John (Britprius)

 

I would also guesstimate an accuracy of about 0.2 V for the MFD plus about 0.2 V drop at minimal load due to the long distance from the battery, for a total of around 0.4 V. A voltage drop of 0.5 V might also be OK, but at 0.6 V discrepancy, I would be double checking the resistance and voltage difference between the negative terminal and ground, and between the positive terminal, cable, and fuse box. Again, that's just a guesstimate from taking a few measurements on my own car.

 

Yes I believe you John, and I know about voltage drops, that's why the very first thing that I did way back when I learnt about the MFD service menu method was to cross check the results with my DVM. I was fortunate in that the results agreed to within about 0.1 volts.

I know it's not perfect, but you can allow a little for the extra volt drop and still have a very convenient way of regularly checking for deterioration of your 12V battery health. I found this extremely handy last winter when I my OEM battery started deteriorating. It was really the changes in voltage over time that I was more interested in than anything else.

I'd expect that any resistance based voltage drop would also effect DMV measurements taken at the front jumper terminal too. Has anyone confirmed this?

 

1. Remove the 12V negative battery cable where it bolts to the body. Clean off the cable end and the body attach point, to reduce the resistance below 1.2 ohm which is way too high.
2. Check the 12V battery voltage one day after you've charged the battery and see what it becomes.
3. Your previous four cars presumably were not Prius? The Prius uses a lower voltage (13.8V) on the 12V bus compared to other vehicles (may be 14.4V or more), so it takes much longer to charge the 12V battery.

 

That 1.2 ohms is most likely just the resistance of the contacts/leads (and/or offset resistance) of the meter. Try shorting the two leads directly together on the meter to measure the offset and subtract it from your measurement.

In any case it is extremely difficult to get milli-ohm scale measurements from a standard consumer type multimeter. Not directly using the resistance ranges anyway.

 

A resistance reading of 1.2 ohms is much to high and possibly the reason your MFD readings are so different to those at the battery. Cleaning the battery negative cable and the bodywork where it attaches should reduce that figure. This will affect the charging voltage at the battery, and thus the charging rate. A test of the current drawn when the car has been shut down for at leased 15 minutes should show if you have a problem in that area.
I strongly believe your problem is in the ground return path. (1) Giving false low MFD readings. (2) Reducing the charge rate at the battery, reducing the voltage available but required to reach full charge.
I agree it is not your lack of millage that is causing your problem unless you do long runs, and then do not use the car for a long period. I only do 40 more miles per week than you and have never had a problem with either the OEM battery or the battery I now have, "although this is a very high capacity battery at 62 AH".
Your readings at the MFD and battery are "although higher" are conformation of the sort of numbers I have noted on a number of Prius vehicles. The highest discrepancy I have measured being 0.5 volts difference.

John (Britprius)