12V Battery Fluctuations

That is your charging voltage out of the Inverter. Wide variations sometimes in battery charging. Doubt there's any issue. 12.7 sitting no load is about right.

True test of battery health is done here on the second test the load test. Do this after the car has sat overnight.
You may have seen this already but it works:

Weird stuff happening? MPGs dropping? Test The Battery | PriusChat

Btw, its a very good thing if your concerned about the 12 volt battery health to put it on a battery charger once a month for a full charge. Its good for the battery to see a maximum charge periodically. Alot of us on here do this once a month.

Best part about that is you can keep an eye on it how long it takes to reach a max charge.
My newish Yellow Top charges to the max real fast as I would expect.Not sure how old yours is.
A slow charging battery is a very good barometer if the battery is starting to get old or sulphating. Once that starts taking a long time to charge up you know the battery is about to give up the ghost. Use a charger with a meter not LED's as its a little more accurate usually.

 

Did you fully charge the battery before you installed it ? All new 12V batteries need to be properly charged before use.

 

Charging systems aren't a constant voltage. They very widely depending on load. If you hold the brake and put the vehicle in gear. the brake lamps are drawing voltage as well as all the other systems that are working. As long as it's
over 12.3 volts, I wouldn't worry about it. The scan gauge is reading voltage interpreted by the ECM. Not the batteries
voltage at the battery posts.

 

Have to put a load on it. Press power button twice without foot on brake. measure then.
Sounds like your battery is fine though.
Don't worry about charging voltage. Its all over the place.

 

The ECM is off when the car is power off so the OP is not referring to the PID read by ECM. The built-in PID in ScanGauge, VLT, is actually reading the voltage feed into it's microprocessor, close enough to the voltage at the battery posts.

Vincent

 

Do you have a schematic so we can verify it's reading input data and not interpreted output ?
I'll bet it's an output value given to the Scan Gauge by the ECM
I say this, because the Scan Gauge is reading the ECM information the same as a scan tool would through the OBDII connector.
That's all interpreted data output by the ECM. The only way to read input voltage is at the ECM's
connectors on the voltage (directly) input circuits.
You could also monitor both and see the difference the OBDII data outputs.

Pin #16 on the OBDII connector is direct battery voltage. Pin #4 is the ground circuit.
With the proper schematic, we could see what data the Scan Gauge is reading. If it's reading pin#16 it's actual direct battery voltage. Otherwise, it's ECM output data on the CAN circuit. Either way, you can't see the input voltage to the ECM's processor with the Scan Gauge. Only the ECM output information or actual battery voltage straight from pin #16. It's not the same as the ECM's output voltage reading will be.

 

This is a known "feature" of the gen 2 Prius 12V charging system when it's not too cold out. Other ways to move it back up to 14 V include turning the headlight or A/C on.

So maybe you had the headlight or A/C on this time?

 

No, the SGII schematic is proprietary but it is still connected to the OBDII port which is std.



When the car is Off, the ECM is also Off so it can't be sending interpreted data on the CAN bus. ECM can only communicate when car is in Ig-On or READY.

I've made an adapter for SGII before and the VLT will show the battery voltage even not connected to car, just connected to a 9V battery. The 9V battery is connected to pin 16 and pin 4 only, all other pins are not connected.




The PID read by ECM is blank when the car is Off. OTOH, SGII is able to show the battery voltage when the car is Off. When the car is in Ig-On or READY, then the PID read by ECM will show up and it is usually slightly different from the SGII's reading.

Vincent

 

That means the Scan Gauge is reading ECM interpreted output information and not input data. Otherwise it would be reading pin #16 and it would show up without the key on.

 

I found some PID data to show the difference in voltage specs.

 

As 2009Prius said, this is totally normal!

Many of us regularly see exactly this happen. It's basically just a two stage charging system (though with a slightly quirky implementation). When the temperature is warm enough, and after you've driven for about 25 minutes or more, then it reduces the charging voltage from around 14 volts down to about 13.4V.

I know it's seems strange that shifting to park (or turning on the aircond or blower) puts it back to 14V, but that is just the quirky way it's implemented. Your car is behaving exactly as normal.

 

With my adapter connected to 9V battery and not to car, it reads about 9V. So, yes, it is reading from pin 16. If it were to read about 12V, then I would have agreed with you.

Anyway, I suggest you borrow a SGII and see it for yourself.

Vincent

 

Mine will be here tomorrow. If it's reading pin#16, it's reading battery voltage from the chassis feed circuit
that's feed through the main fuse block and not through the ECM.

 

This leaves me intrigued, because I have consistently noted about a 0.23V difference between that I read at the front jump points and that shown to me by SGII... In your 9V test above, did you simultaneously compare the SG-shown value to that shown by a digital multimeter?