Weak 12V battery wont necessarily hurt fuel economy.

My 12V battery is starting to get a bit weak, 12.1 volts (service menu displayed) with no loads in accessory mode, and 11.8 volts with no added loads in Ig-On mode.

Obviously I'm going to have to replace it soon, but with so many people reporting poor FE when their 12v is weak it got me wondering, how much is this battery's condition impacting my fuel economy?

So I did a little test the other day to estimate how much energy the battery was taking and found that, despite the battery's apparently poor condition, it was fairly negligible.

Procedure

Leave the Prius off and (using an external charger) measure the amp-hours required to bring the battery up to the approx 14.0 volts that the DC-DC converter usually holds the battery at when in ready mode. Then to measure the energy requirements to hold the battery in this state.


Measurements

- Starting conditions, battery at 12.3 volts (digital multimeter, measured with car turned off, car hadn't been driven for few hours).

- The initial charging current was 2 Amps, tapering off to 0.7 Amps in the approx 3 minutes that it took the battery to reach 14.0 volts.

Results and Interpretation.

- The energy required to initially bring the battery to its 14.0 volt nominal operating voltage was only about 3kJ (equiv to approx 0.07 Amp-hour from the battery). To put this into context, its about the same energy as running your headlights for 25 seconds.

- The energy required to hold the battery at 14.0 was no more than 10 Watts.

Conclusion.

Despite the weak condition of this battery it's impact on the cars energy consumption is negligible in the context of the other energy requirements of operating the car. It is highly unlikely to have any measurable effect on the cars net fuel efficiency.

 

Yes that's correct. The reason for this post is that I've noticed lately that many people here assume that if the 12V battery is a little weakened that it's a guaranteed MPG killer. I just wanted to make the point that it isn't necessarily the case.


There are three main ways in which an aging battery deteriorates.

1. Reduced amp-hour capacity.
2. Increased internal resistance.
3. Increased self discharge.


Points one and two will generally act to reduce the voltage of the loaded battery, whereas only point three is likely to have any impact on fuel economy. From my experience with batteries, the high self discharge is particularly an issue with old nicads, especially ones that have been "cooked" by overcharging them.

In my case what I'm saying is that my aging 12V battery is probably exhibiting (1) and (2), but not so much (3). I think this situation is not uncommon for lead-acid batteries that are weakened because they're old, but otherwise haven't been too badly abused.