Bad starting battery cause poor MPG?

In some failure modes, the 12 volt battery can't ever get to 12 volts (one shorted cell?). The inverter will send a great deal of energy to it to charge it up to 12 volts. This is energy that would have moved the car, so MPG is lower. Other failures effect the capacity of the battery, not it voltage and do not effect MPG. Sudden death (usually an open circuit) does not effect MPG as the car does not start at all.

So yes, some owners will see higher mileage after installing a new battery, but it is not universal.

 

Yes, it's been shown that a bad 12v battery can have negative effects on MPG. The theory is the ICE needs to run longer (or continuously) to try to charge the battery.

BUT, your "short trips around home" probably have more effect on your MPG. What car did you previously own and what MPG did it deliver. My guess is that you get double the MPG in your Prius.

 

Yuuup!

 

Did you get a chance to measure the voltage of the old battery that was giving you the lower mpg's? My ScanGuage consistantly shows my battery at 11.xx volts at power up but before the ICE fires up. My mpg's are consistantly in the mid 40's and I just assumed it was my 17" tires. Now I am second guessing my thoughts after reading this thread.

Mike

Mobile on my SGH-i717

 

On my brand new HSD, Torque PRO shows consistenly 14,50 V @ on Power up before ICE ignition and the same when ICE shut-off in motion, and 13,50 V when ICE is on. 11,xx sounds of battery diying... i think 12,5 V before power up should be a minimum...

 

Thanks. I suspected the battery was not as good as it should be since new. I'll put a new one in there next spring when I take my car out of winter storage.

Mike

Mobile on my SGH-i717

 

ScanGauge II (Firmware 4.02) shows about 0.23V less that what it actually receives ( at least on mine). So, if the SG is showing 11.9V, the real voltage is probably more like 12.13 at the battery (which according to the manual is partially discharged IIRC). Also, have you ruled out your use profile as the cause of the low voltage issue (such as short trips not giving a chance for the 12V to be charged back up)? If it's your use profile that's the cause, then you could either put it on a charger overnight every couple of months or so, or keep replacing your 12V every 1, 2, 3 or 4 years as the case may be...

 

If you put it on a maintenance charger while in storage, you might end up with a 12V pretty close to being new (or not). Worth giving it a try IMHO, if you want to.

 

That is what I did last winter and kept it at 13.xx volts until I drove it last spring. The battery was still showing 11 something volts this summer when it was sitting for a few days. I will put my DVM on it this weekend and see what it is resting at. I last drove it 2 or 3 weeks ago. The maintainer is not connected right now so I suspect it should be hovering in the 11v range.

Does anyone know the Ah rating of the battery?

Mike

Mobile on my SGH-i717

 

You can test at the front hood jump points itself for an accurate reading. This will be the same as reading at the battery. Sounds like you don't use the car often; maintenance charging would be good in between uses. IIRC the original battery Ah is 36 or so. I believe the Exide battery for the Prius has a much higher Ah, even higher than the Optima.

 

Awesome thanks I have about 1450 miles on my car so far. I drive rentals more than my own vehicles. Right now I am driving a rental Prius. The 2nd one this month and one more rental on Sunday for next weeks trip. Sure keeps the mileage low on my Prius. It will be 2 years old in the spring with under 1500 miles ;-) Good thing the HV battery holds it's charge all winter. It went 7 months last winter and it was one bar from full last spring. I put it away with a full scale charge the previous fall. I'll top it off this weekend as snow is forecast and I will be leaving town Sunday.

Mike

Mobile on my SGH-i717

 

Considering it's a chore a get to the battery terminals, you might consider fitting something like this to your battery terminals with the connection end routed into the area under the hatch. Then all you would have to do is just open the hatch, hook up the charger to the connection end and close the hatch. The other option is to charge at the jump terminals, but if you are going to do that, I would strongly recommend using a charger with reverse polarity detection as otherwise a mis-connection could leave you looking at $$$$ in repair. In fact, I would recommend this in the other case as well (charging at the battery terminals).

 

Another option would be to get a disconnect switch that you can switch on and off either manually or remotely. You will lose radio presets, trip readings and some ECM fuel trim settings and such but shouldn't be a big deal.

 

Thank you I will check those links when I get home tonight. Tapatalk doesn't show any pictures :-(

Mike

Mobile on my SGH-i717

 

well if it sitting in a few days it makes sense to have it at high 11V ... If you drive short distances for days you may never actually fully charge the battery. It takes over an hour after a weekend standing.

What I can suggest that try to measure the battery next morning of a day when you drove the car at least 1.5 hour. If you can safely do you can simply leave the car powered on on your driveway (lock the door with the mechanical key) after a shorter drive to make up for the hour and a half. The car is charging the 12V with a low current so it takes quite a bit of time.

You do not need to get to the battery you can both charge and measure at the jump point under the hood if you have a garage.

The 12V battery in the 2012 Prius is 45Ah

here are pictures I took while doing the emergency generator project (1Kw inverter connected to 12V)

http://priuschat.com/attachments/img_1979_md-jpg.45339/
http://priuschat.com/attachments/img_1980_md-jpg.45340/