Think I might 12V battery problem.

my 12v battery started to fail me around 48k miles.. given time and similar mileage.. yes, it may be dying. the car will start unless the voltage drops below 11.5 volts or something similar... it isn't happy.. but starts..

 

Don't trust the MFD battery check. At the age of the battery, I'm with efusco. Replace it.

 

It sounds like you are checking voltage while the Prius is READY. If this is correct, then you are measuring voltage produced by the DC to DC converter. This is not the same as the voltage provided by the 12V battery, which you can assess after the car has been IG-OFF for a few hours. The point of waiting a few hours is to let the surface charge on the battery dissipate.

So the question is why the DC to DC converter is producing voltage at 13.4 instead of 14.1. BTW, whenever I measure battery voltage with the Prius READY, it is pretty close to 13.8.

It could be that the system thinks the 12V battery is fully charged, and has dialed back the charging voltage. There is a SENSE wire attached to the positive terminal of the 12V battery, that allows the DC to DC converter to assess the voltage at the 12V battery. Or it could be that the DC to DC converter is starting to overheat, which causes the voltage being produced to decline.

Do you see fluid turbulence in the inverter coolant reservoir? Can you hear the inverter coolant pump running when the car is IG-ON (not READY)? IG-ON is the operating state where the instrument panel warning lights are on. The pump is located immediately behind the driver's side headlamp assembly.

If you can't hear the pump running and do not see fluid turbulence in the reservoir, then I'd suggest having that pump replaced immediately. If the pump is working OK then I'd suggest keeping an eye on the voltage but no other action required at this time unless you see warning lights appear.

 

Here is a photo of the 12 vdc battery at the right rear of the car



The battery posts are circled with blue circles. The positive post is the closest one in the photo, it faces towards the front of the car. It's usually covered with a giant red plastic PITA thing to get off

The post furthest away in the photo, with the wire and rubber band hooked up to it, goes to the unitbody as a ground.

Before we start, make sure you're comfortable working around circuits. If you were to cause a dead short across the battery posts - like dropping a wrench on them - the battery could explode, causing serious injuries or worse.

You can set the DMM for VDC, put the leads on the battery, red to pos and black to neg. Doesn't really matter though, the DMM could care less if the polarity is reversed. Instead of displaying something like "12.9" it will display "-12.9."

Make sure the DMM is set to VDC. If it were set to amps, the DMM fuse will blow or you could destroy the meter. It would be like a short across the posts. The better DMM's, such as the industrial/commerical Fluke, are intrisically safe. No sparks, no fuss, just change the fuse

You can also check by popping the hood and using the underhood jump point. I've never noticed any difference in voltage if I checked at the underhood jump point

Whenever I checked my Prius while in Ready, the battery was at 13.8 vdc. Fully off and resting, around 12.9 vdc

Given the age of the car, first thing I would do is back off and snug up the bolt that holds the battery ground wire to the unitbody. With age there will be corrosion. I used to back off and snug up the bolt twice a year

It's entirely possible your battery is on its last legs anyway. Given the age of the battery, I'd remove it, fully charge it, then take it to a Pep Boys, Autozone, Napa, etc to do a load test on the battery. You may need a battery anyway

Remember that if you disconnect the 12 vdc battery in the Prius, all memory presets will be lost (Radio, etc). Also, the power windows will have to be initialized (Power window about halfway down, then hold up button for about 5 secs after window closes ).

The rear hatch is electric release. If you close the hatch with the battery out, you will have to crawl into the back to use the emergency release.

 

Battery voltage is only one small measure of battery health. A 12v battery fully charged, (at rest) should read ~12.7vdc. The reality is that battery voltage changes with lots of variable. The best way to measure battery voltage is then the battery has sat several hours neither charged nor discharge.

Contrary to other opinions, 12v batteries can last a long time if properly cared for. The loading on the Prius battery is fairly light so I would expect it to out live the average battery. The biggest enemy of 12v batteries is excess heat and excess vibration. The Prius battery life is helped by the fact that the battery is in the cabin rather than the engine compartment.

The real test of battery health is a hydrometer test to check acid specific gravity (tough to do with a sealed battery) and a load test.

Because the scan gauge only shows 13.X volts at any given time may not mean much, depending on the state of charge, and the loading. For example in a conventional car, turning on the lights wit the engine off may drop the static voltage of the battery to say 11vdc, that doesn't mean the battery is dead, just that it is under load. The opposite is also true, a battery being charged at 14.4 volts isn't necessarily good either.

For reference,, for those that are interested:Car and Deep Cycle Battery FAQ, Battery Manufacturers and Brand Names List, and

Icarus

 

We're not seeing that, however. The Prius 12 vdc battery doesn't appear to last any longer than a regular car battery, perhaps it expires sooner

I'm wondering if the charge system really isn't up to the task? The first winter I had my Prius, driving from the heated underground condo parking to the heated parking at work, around 15-30 minutes depending on traffic, I had problems

Winter, temps down to -40, headlights on, rear defrost on, max heat, etc. By the end of January, the battery was at the point when I powered up in the underground parking lot, the Auto headlights would be dim, and just about go out before the relay closed and the dc-to-dc supplied power. The coolant transfer pump also made a low groaning sound

I got a VDC Battery Minder and, just before a long business trip, removed the 12 vdc battery and put it in my condo suite. Left it hooked up to the Battery Minder for 3 weeks. When I returned, hooked up the battery, and on power up the headlights barely flickered.

So I wired the Battery Minder directly to the battery, and got in the habit of plugging in the Prius everytime I returned to the condo. That seemed to fully restore the battery

I wonder if we could get feedback from Prius taxi operators? Those Prius are usually in Ready 24x7, could be under that situation the system is able to better charge the battery

 

My battery regularly reads as low as 8.4V and will still allow the car to start. I'm still waiting for the car to fail to start so that I can call Toyota recovery as more ammunition to get my battery replaced under warranty.

 

The Prius battery may be lightly loaded, but it is loaded most of the time. The Prius draws power even when sitting for all of the fancy little SKS circuits and such. Added to that, as Jayman points out, the Prius 12V charger appears to be on the anemic side. These two factors conspire to kill the 12V battery about as fast as on a normal car.

Tom

 

Jay, Tom,

You are both right but, (aint there always a but?)

The biggest cause of premature deaths in car batteries having them sit under charged for periods of time. In the RE/Solar world, the rule is, a battery that sits below ~80% state of charge for more than 3 days MUST be brought to 100% S.O.C or sulphation will occur significantly shortening the life span of the battery.

I suspect that the biggest culprit causing headaches with Prius' is the smart key and the battery energy require to keep the electronics/stereo/alarms lit when the car is not being driven. I suspect, (but don't know) that battery problems are more endemic with cars that are drive less frequently and or short distances. I do know that when we left ours for 3 weeks, even with the SKS system turned off, the 12v battery was essentially dead. I now keep a 5 watt solar panel on it if we leave it for more than a few days.

Clearly the battery is small (by design) and as long as it is used every day it is fine. The problem is when the battery is allowed to discharge and not be recharged (fully) for days at a time. I suspect that some amount of complaining about the charging capacity of the Prius is really an issue of a tired battery requiring more current (AH) to charge.

It would be curious to put a ammeter or a recording battery meter like we use on PV solar batteries and clock how many AH are drawn out of the battery every day it sits in stasis, both with the SKS on and with it off. It would also be curious to see what kind of charge rate the car is capable of.

Icarus

 

That's where deep cycle gel cells or AGM batteries have a big advantage. While it's never good to let batteries sit around at a low SOC, the gel cell and AGM deep cycle batteries handle it a lot better.

Tom

 

Try heavy drinking, that *always* works for me

I agree

I've noticed the Prius charging system doesn't deal very well with a Prius driven short trips, especially with heavy loads (Headlights, rear defrost, max heat, etc) in bitter cold temps

Once I put the Battery Minder in, all the problems went away.

 

Heavy drinking?

Just kidding!

Check the charge on the Optima, make sure it's properly charged. Then store in a cool dry place, eg basement or wherever convenient.

Optima Product Support : Maintenance & Storage

Optima recommends checking the charge twice a year. I'd check the charge once a month or so.