Effect of the 12-Volt Auxiliary Battery on Prius Gas Mileage

Effect of the 12-Volt Auxiliary Battery on Prius Gas Mileage

Optima Yellow Top Battery

I forgot (again) and left my lights on at the RC flying field.  My 2010 Prius is a sort of smart car and will turn the lights off if you forget; unless you leave a door open.  Its computer considers the hatch a “door” and that is normally left open for hours during a flying session.  I drive with my lights on during the day to improve visibility (a habit from motorcycle riding).  A few years ago I had to get jumped because I left my lights on at the field.  I had been good about remembering to turn the lights off since then until recently when I got lax and it slipped my mind.  I had to get jumped again.  When I got home I charged the battery up fully and then planned to load test the battery with my Harbor Freight 100 amp load tester.  Perceiving the location of the auxiliary battery was not conveniently located, I did some internet research first.

If your Prius has a multi function the display (like the screen in models with a GPS) you can access a maintenance menu which will give you a battery voltage readout.  If you then leave the engine off but turn on the accessories including the lights, you can place a low to medium load on the batteries to see if it has an abnormal drop in voltage.  I tried this and the voltage dropped from 12.7 volts to 10.9 volts, which I considered an excessive drop; indicating a weak battery. (after later removing the battery I confirmed the “weak” state with the 100 Amp load tester).

I called my local Toyota dealer for a battery price, with and without installation.  They wanted $210 for the battery + $40 core charge if you didn’t exchange the old battery.  The price including installation would be $265.

I got the Optima Battery on Amazon.for $138.74 (including free Prime Shipping and no core charge).  However, a week later at the time of this writing, the price is now $202.99.

Optima Batteries 8171-767 (DS46B24R) YellowTop Prius Battery

There are good how-to replace the Prius Auxiliary Battery videos on YouTube.  Although it looks like a lot of steps and the access on first glance looks awkward, it is actually very easy and the steps go very quickly.

I had read some reviews on the auxiliary battery and heard mention that some people had observed improved gas mileage following replacing their auxiliary battery, however, I dismissed them as bullshit.  But, immediately after replacing the auxiliary battery I noticed a dramatic improvement in gas mileage.  Depending on the type of driving, we are talking about an additional 10 miles per gallon.  Holy crap!   (I got 62/57 to/from MPG on a trip to a town 15 miles away after changing the battery)

Although it happened, I didn’t understand why. I did some more research and what I found does make sense.  To explain it, let’s first review how the Prius Hybrid gets high mileage.

1. Lightweight
2. Aerodynamic frame.
3. Efficient gas engine
4. Electric “traction” engine
5. Regenerative brakes (the brakes act as electric generators to recharge the batteries
6. Computer control algorithms that continuously monitor the state of the vehicle and appropriately optimize the use of available energy.

So, for example, if the batteries are sufficiently charged and less energy is momentarily required, the gas engine use is minimized or totally turned off and the car runs more (or totally) on electricity.  If a lot more power is needed and/or the batteries are in a relatively low charge state, gas is used to charge the batteries in addition to powering the gas motor.

The computer programing is based on normal fresh batteries that are working as expected.  When the auxiliary battery weakens, its maximum peak voltage drops some, and the change of voltage over time with the application of a charge current is also affected.  The result is that the computer spends more time attempting to charge a battery that can’t accept a complete charge.  Much of that time with a normal, fresh battery, the gas engine would have been decreased or turned off while running on electricity.  This wastes gas.

So I did some calculations to estimate how much that would cost and I calculated that I could buy one battery per year for the money wasted on excess gas purchases.  If you consider the drop in gas mileage a proxy for a weak auxiliary battery I estimate the drop in mileage occurred 2 years ago.  My Prius is 7 years old.  I was still on my original auxiliary battery.  What many Prius owners may not realize is the 12-volt battery is not used for the starter motor.  It only runs the electronics.  However, if the electronics don’t run, you still won’t be able to start the car.  The result is until you see the red triangle and your car is totally dead, you don’t get a prior warning (weak starter motor).