My girlfriend just got a new 2010 gen III prius. I have a 2009 gen II prius. I first checked the gen III 12 volt battery with system in ready. It was 14.8 and stays there till the car reaches crusing speed (35 or 45mph) then is reads 13.8.. and when stopped at traffic light it goes back to 14.8. This varation seems consistant 13.8 volts moving 14.8 volts stopped. My gen II is very constant at 13.8v except when battery is low, then voltage increases to 14.2 (when heavy current charging )due to remote sense voltage regulation wire that is connected to 12volt battery in rear compartment via a small 20 guage wire. Anyway the gen III doesn't have the remote sense voltage regulation wire connected to the 12volt battery (very disappointing). It would seem the remote voltage sense wire is connected at a point where there is a 1 volt voltage drop when the car is stopped in ready mode. Thus the dc-dc converter regulation compensates by increasing its output to 14.8 volts. The battery is overcharged reading 13.9 after siting overnight with all systems off. Does anyone know where the remote voltage sense wire for the dc-dc 13.8 volt converter is actually connected. While 14.8 might be desireable at the ignition coil for hotter spark and better fuel combustion it will shorten the headlamps life and shorten the 12volt battery life. And possibly shorten other 12-13volt components.. The car was delivered right of the truck at 2 miles odometer and we know have 170 miles and 3 days ownership. Voltage monitored at the cigarette lighter with plug in volt meter. and voltages cross checked with handheld voltmeter under hood and at battery terminals directly. 14.5 volts vechicle stopped. and no sense wire at battery. But is see what looks to be a temperature sensor mounted at the battery hold down bracket..
I can confirm the 14.8 V and it does seem excessive. However, I want to measure the voltage at the battery terminal before getting too concerned. It might help to read out the battery temperature too (over charging heats even lead acid batteries.) Bob Wilson
13.9V reading from the battery seems too high for normal flooded battery, but it could be normal for a special type of lead battery (ie. gel with modified electrodes). Also, the 14.8V charging voltage doesn't strike me as excessive, especially in winter with lower temps. Charging voltage goes up while temp goes down. This sounds like only slightly higher than normal charging voltage in conventional cars. Maybe the charging system is more robust in gen 3 in response to failing batteries in gen 2? It would be interesting to compare battery types in gen 2 and gen 3. I will read mine when I get home.
Update: I measured my 12V battery after overnight rest and it reads 12.68V, normal for fully charged AGM lead battery. The measurement was taken at the jumping post. I believe OP is mistaken with his 13.9V reading. The charging while Ready is indeed 14.8V. I compared it to my battery charger (automatic) and it outpts slightly more than 15V, so the charging Volts are not excessive. Also, At idle, most charging systems will produce 13.8 to 15.3 volts according to http://www.aa1car.com/library/charging_checks.htm Like I said, the Prius gen 3 charging is probably more robust to prevent battery discharge. Should not harmful for electronics but could somewhat shorten light bulb life indeed. BTW, the battery went down to 12.45V after one cycle of Ready and keeping driver door open for a couple of minutes, so the battery capacity is endeed low.
I have watched mine with the Scan Gauge and it varies from 13.5 up to 14.8 while driving, Scan Gauge reads about .1 Volts lower than Voltage at the battery. I never checked the 12 Volts on my Gen 2 but a quick search of Gen2 topics reveals that most of them run about 13.8 or so. Most European and American vehicles I have owned, as well as my old Toyota pickup, use about 13.8V. But I have had two Hondas and both ran at 14.8 to 15. Interestingly the batteries in the Hondas seem to last as long as the ones in other cars but the light bulbs burned out more quickly. I think the "sealed" batteries we use are still lead acid batteries and should require the same charging current as the more common "flooded cell" type. Go down the page to "Charging System Checks" in this link: Battery Testing
You perhaps meant to say charging voltage, as the Prius AGM lead battery has charging current limitations (4A IIRC) unlike regular flooded car batteries that can vent freely gases without permanent damage (except water loss). EDIT: I researched charging AGM lead batteries more and apparently the Prius battery charging current limitation have nothing to do with the AGM type and everything to do with the battery size. Lead batteries should not be charged at higher currents than Capacity:8. That is 4.5A in case of 36 Ahr battery.
Well actually I did mean current because when charging a battery I consider that it is really the current that matters. The Voltage is what we can measure easily and if it's a good battery, no hi or low impedance cells, the voltage will be a fairly good indication. However the battery is not like a big resistor, it is a Voltage source so it's state of charge is also a factor. Fortunately, as you probably know, batteries are kind of self regulating anyway so if we apply a constant Voltage to a partially discharged battery the initial current will be hi and as the state of charge increases the current will decrease due to the increase in counter EMF. So maybe I should have said Voltage. I suspect the 4 Amp limit is not so much to protect the battery as a limitation of the converter's output. I think the battery could safely be charged at a slightly higher rate, at least for a short time.
