New 12 volt battery improved MPG

That is good news! I hope your MPG stays high now.

 

The AC runs from the high voltage system. If the old 12V battery really was the cause of all that it must have been in bad shape, perhaps a shorted cell.

 

The resistance of the wire to the front of the car will make no difference as voltage drop through a resistance is current dependant. Your DVM draws so little current as to make the voltage drop not measurable by the DVM.
DVM= digital volt meter.

 

Sounds like you REALLY needed a new battery.
>> Second tankful after Optima install, now we have much better MPG too.
Road trip at 65+ MPH and we are reading 50 MPG.
Seems like I could get home on this tank which would be 500 mile round trip. Old orig battery was getting 43 MPG, so this is non-scientific, but more like what we were used to.

 

Trying to figure out if I need a new 12V battery. Just got the car a couple of weeks ago.
Parked on the street with hazard lights on for about 45-60 minutes and when I was back and pressed the power button, the battery was dead. Red triangle was on and the car would not start. I asked someone to jump start me. Drove around for 15 minutes and it was fine again.

I did enter the system mode after driving for 30 minutes, the battery was at 12.1. Turning headlights gets it to 11.6.

Oh, and my MPG is 40-42 in mixed cycle with really bad and old tires. Driving mostly on highway at 75 mph. The car has almost 200K miles. Hybrid battery was replaced to rebuilt one last year.
I guess it's OK?

 

A new 12v will give you optimum mileage, as yours already died, and is suspect, check the build date, if it's 4-5 years old get a new optima, charge it before installing.
Your rebuilt traction battery will be an issue, save up 2,500$ plus labor 4-5 hundred more and you'll have a practically new car, just watch the brakes.
Not what you wanted to read, what you needed to read.

 

Thanks,
I'll try to find out the year.
But is it supposed to hold charge for a long time with hazard lights on?

 

hard to say, but probably longer than an hour. if you are going to diy, get a volt meter and trickle charger. charge it up and test it after the chargers been off for a few hours. if it drops below 12.4, replace it.

 

The Prius batteries don't have that much reserve. I'm always a bit sceptical of posted voltage readings involving activating the car and reading it in-dash. 12.1 volts (if accurate) is very low. Try checking with the car off with a digital multimeter?

Better would be to get a dealership or battery retailer to check it with a digital load tester, see what they say. To see for yourself, you can get a relatively cheap prosumer tester, Solar BA5, for around $60.

I'd second charging: get a decent smart charger, around 3~4 amp level, use it regularly. Most importantly: read up, on the do's and don'ts, Zen and the Art of 12 Volt Maintenance.

We're low usage, car will sometimes sit for a few days, and I leave a smart charger on pretty much constantly. It's safe, won't overcharge, just keeps it topped up.

 

I've found the in-dash readings to be repeatable, reliable, and accurate for what they are, as long as you do not expect them to be the same as a measurement at the battery itself.

While britprius was right in #43 to point out that the resistance of the wires and fuses in-circuit won't drop the voltage noticeably if everything is off and your meter is the only load ... when the car is in ACC or ON so you can use the voltage readout of the MFD, that isn't the case.

If you just get in the habit of taking the MFD reading and adding a good fraction of a volt (if you want, compare it one time to a simultaneous DVM reading at the battery, and remember the difference), most of the "horrors! your 12V is on its last legs!" reactions would be a bit more muted.

-Chap

 

Is there a fairly consistent difference. I can't check: our Canadian Touring doesn't have a MFD.

Edit: There's probably too many variables, ie: radio, Heat/Vent on or off.

 

As consistent as you make the load conditions. You'll probably see variation if one time you measure in ON with the headlights, wipers, defoggers and seat heaters blasting, at the same time the brake pump is working, and the next time in only ACC....

-Chap

 

People scratching their heads over the 12 volt condition tend to have squat to measure it with, a perfect storm.

 

I cannot tell you how many times I was fed the line, "your car is the only model that uses this part" from a dealer...........enough said.