MPG and the 12 volt battery

people have been reporting that the 12v has caused mpg issues here forever. others disagree, and the only proof is that mpg recovered after replacement.
it is unfortunate that the dealer battery tester is not always accurate, or is incapable of finding the actual problem.
glad you got it resolved.

 

I generally agree with that although, if the battery was bad, I would have expected that you would have seen DC/DC output in the high range, that is 14.2 to 14.7 V.

I have a few general comments from my long-term observations of a voltmeter I installed in my car temporarily in 2016.

My observation is that a 12 V battery that will impact fuel efficiency is permanently pegging the DC/DC converter to supply 14.4 V constantly in an attempt to charge a failing battery.

With a good battery installed, I very rarely see the converter charging above 13.8 V with a range of 13.2 V to 13.8 V which I interpret as low to medium charging.

I sometimes see 12.8 to 12.9 V which I interpret as no charging. I see this most often on medium to long distance drives (i. e. greater than 20 minutes) after the 12 V has been fully charged externally with a battery charger while the car has been sitting unused.

I have a sealed vented flooded battery in one Prius and a sealed vented AGM battery in the other and notice very little difference between the two in terms of the charging behavior. In saying this, I more often drive the Prius with the sealed vented flooded battery.

The AGM battery is ten years old, at this point, and the flooded battery is seven years old. Both are still going strong. The AGM Prius is driven more regularly and is externally charged less often.

I'd be interested if you have any observations you could share.

 

Picture? Where does it attach to "12V"?

 

Call around and find a shop that still uses the old fashion carbon pile load tester - I bet'ch it'll fail that test. Those small hand-held battery testers has a very small load resistor in them that can't place a large enough load on a borderline battery - so the battery gets a pass.

Poor test equipment = poor diagnostic & troubleshooting.....

Hope this helps...

 

I bought a cigarette lighter adapter which contains a voltmeter and two USB charging ports. Amazon: B07K2XL6VX. Yesterday it was plugged into the port under the dash and it was readable from the normal driving position. It read 14.1 V, on the first trip, both before and after the one stop, about 20 minutes total driving time. On getting home checked the voltage at the jump post while the car was still running with a good quality voltmeter and it read 14.00. So the plugin voltmeter seems to be good to about .1 V.

That value is between the two ranges you cited above.

This 12V was charged externally about a week ago and the next morning it read 12.72V. Other than cracking the windows open and popping the hood 5 hours ago it has not been used today (or since the 14.1/14.0 readings). It just read 12.61V at the jump point. Despite what the little Viking battery tester says, I believe this battery only has about 1/4 of its full capacity, so it discharges and recharges quickly. It doesn't seem to get all the way up to the 12.8V or so of a fully charged AGM, but that could be the charger's problem. MPG has been around 40 lately because it has been hot, trips have not been long, and the AC runs all the time. On the few long highway trips recently it was giving 49 MPG or so, which is typical. Oddly, given how hot the weather is, it does seem that the car is taking a little longer (5 min more?) to warm up to the point where the MPG is optimal, That might have nothing to do with the battery though.

So I think this 12V may be in battery limbo. It isn't so "bad" that it has a dramatic effect on MPG, but it is weak enough that the car needs to charge it more than a "good" 12V.

Note, there is no large parasitic load when the car is off. Last time I checked it was still 20-25 mA draw when the car is off, which is normal.

 

One other observation - the Viking battery tester

12V Digital Battery and System Tester

always fails this battery when it is attached at the jump point (ground lead to a bolt on the inverter, test lead clamped onto either the screw or jump pad under the red cover). It typically shows a low voltage (like 12.1V), SOH less than 20%, and low capacity in AH, like 7 or 10. Seconds before my voltmeter showed 12.54 V when measuring at exactly the same test points.

Today after seeing yet another horrible test result that tester was moved and attached directly to the battery, which was still attached normally to the car. Other than opening the hatch, and then sticking a carabiner in the latch slot to make it look closed to the car, the car did not change state. When attached at the battery the tester read (8.19.24 / 8.11.24 right after charging [Viking "4A" charger] and a 20s headlight discharge / 8.11.24 before charging):

SOH% 80/100/88
SOC% 67/94/90
AH 42/45/44
Volt 12.41/12.57/12.54
milliOhm 8.8/7.8/8.4

12.57V is the test result immediately after 20s headlight discharge to remove surface charge, and so includes some voltage depression from that. Voltmeter read 12.72V before the headlights were turned on. The battery is losing around .15V a week, the way we are driving it. Each week includes a handful of <10 minute drives and one longer drive of two 15-20 minute segments.

I think the most likely reason the tester fails to work accurately at the jump point is that it doesn't make a low resistance electrical connection there for some reason. Clearly nothing very wrong with the car's wiring as it runs just fine with all 12V power delivered through that location. Test results on the clamps on the battery are a completely different animal, as shown above. I agree with Biomed1 that the little electronic testers do not pull a large enough current for long enough to trigger a voltage collapse on a marginal battery, and the AH values are derived from unrealistically small current draws.