12 volt Battery Issues

i'm thinking more of an excess drain when the car is off.

did you get warranty on the 3 y.o. battery? what battery is in her now?

i would get a smart charger, disconnect the negative from the battery, charge the battery to full, take a reading after a couple hours to lose the surface drain, then read it again in the morning.
if it's not holding, it's bad. if it is, try again hooked up to the car. if it's not holding, something is draining it.

15.2 might be because it is trying too hard to charge the battery.

 

3 amps should be fine

 

Interesting. I have 3 older Priuses - 2004, 2005, 2008. In each car there appears to be some constant drain on the 12v bat. I Mickey Moused a "solution" by disconnecting the battery every single time I leave the car. I'll be curious to know if anyone has found a "typical" constant drain.

 

A minor point of clarification:
When the car is running (READY mode), then the voltage that you are reading is NOT the battery itself but the charging voltage.
Those numbers you are reporting seem perfectly "normal" under the circumstances.

And yes, the higher charging numbers indicates that the system is trying to make up for a half dead battery but apparently you don't drive long enough for that process to complete.

IF your "normal" driving really isn't much, you might need a supplemental charge for the battery every week or so.
Or a battery tender.

Allowing it to be constantly sitting at 11.9, which is a bit more than half discharged, will greatly shorten it's life.

 

I've seen more than one Prius get less than 2 years out of a new battery. I suspect because the battery was never fully charged and Prius only driven on short drives means it never had enough energy going into it to keep it stable. The pandemic seemed to make this problem even worse. So first step, by a legit smart charger that goes up to at least 8 amps and make sure the battery is fully charged periodically. It also might be too late and you'll have to get a new one.

 

The max draw on the 12V bat is pretty low - it only really powers up the various computers and relays. The highest current flow is almost certainly when the 12v bat is being charged. I had considered, like you, putting a switch in series. Instead I just disconnect the battery every time by pulling the big white plug on the positive terminal (gen 2 and 3). Of course I have to open the driver's door with the physical key. And/or have easy access to the battery by either leaving the trunk blocked open. Also I have the back seats folded down/removed so I can easily access the bat via the right side back door. It's not pretty. I also don't latch the hood fully so I can access the jump point with a cheapo Costco battery jump unit.. Hope you find the offending fuse/circuit ;-)

 

The normal parasitic draw when off should be 25 ma with a keyless fob enabled and around 15 ma without a fob enabled. If you are reading 800 ma (0.8 amps) after 30 minutes you do have a seriously excessive parasitic draw that will drain a battery fast. Computers in the vehicle take time to shutdown which means measuring parasitic draw immediately after shutdown is invalid.

DC clamp meters can pickup abnormally high parasitic draws like 800 ma but are normally ineffective at 25 ma. At low ma the series connected amp reading is necessary. For fastest checks without waiting for computer shutdowns, a meter must be inserted into the battery circuit without disconnecting. Disconnecting requires the longest wait times.

We have seen aftermarket radios, navigation, amps and obd2 readers add significant parasitic draws. Sometimes it turns out to be damaged equipment on the car like the electric power steering or an ecu that never shuts down. Normal draws include hatch lights that don't switch off or map lights manually engaged.

A fully charged battery voltage is about 12.7 v. In order to charge any battery, the charging system must raise the voltage while still carrying the Ready state load. This is basic battery charging. A normal lead acid battery may charge at 13.5 to 14.5 vdc depending on its state of charge. A Prius charges an AGM battery which can accept up to 15 vdc. After a charging system shuts off, battery voltage will quickly revert to its steady state, somewhere between 10 vdc if its still severely discharged up to 12.8v if its fully charged.

One method to series insert a meter without disconnecting the normal offstate loads.

 

At least on my cars - 2004, 2005, 2008 if you remove the red plastic part (be careful but it can be done without breaking it) under that on the left hand side of the remaining black part is a large white plug (on the right is a small white plug. I used to disconnect that too, but it made no noticeable difference). Again, being careful, the big white plug can be unplugged. This disconnects all power to the car (except whatever goes through the small white plug). I filed down the big white plug "latches" to make it easier to remove. With this plug removed I have left my car for months and the 12v bat loses virtually no charge. Like I said, it's not pretty, but it's a lot quicker than unbolting the ground/negative cable at the bat or chassis.. Of course every time I start my cars I have to go through the steps required if I disconnect the 12v bat.

 

Hi Alan, Kudos to you for digging into this so deep. When the 12 v bat is reconnected after being disconnected it takes a finite time for the various computers/devices which are powered by the 12 v bat to "calm down". (I've never noticed or waited 30 minutes) When I reconnect my 12 v bat, in order to start the car I push the power button, wait for about 20 seconds, then push and hold it until the Ready light comes on (In my case because of other faults, the ICE engine also starts). I mentioned the rear hatch being open/unlatched only because the easiest way to access the 12 v bat is via the rear hatch, and without 12 v connected the hatch can only be opened by accessing the latch from inside physically (the fob will not work). As far as I know the rear hatch being open or closed makes no significant difference to current draw when the car is running. With the 12 v bat connected (but the car not running) the hatch being open will probably illuminate the little light you mentioned, and maybe the "door open" warning and will also not allow the immobilizer to work. (ie the doors to be locked with the fob). Good luck.

 

Restarting the car after a disconnect is not a time consuming issue although two tries are often needed. Trying to get an accurate low level parasitic draw reading is a problem after a disconnect because of wait time.

The rear hatch light being on for a couple of days is enough for a no start because of reduced 12v voltage. The hatch is fairly easy to leave slightly ajar. In case of a discharged battery, the best solution is to jump start it from the front using a protected lithium jump box.

The worst damage is when someone accidentally uses another car and jumper cables, especially if they are momentarily reversed. This point is the reason you want to fix an excessive parasitic draw. It reduces the opportunities for a bad jump.

 

If you charger said 14.7 volts and then when you took it off the charger it right away said 12.8v I'd be concerned about battery health. But I don't know how many amps you're charging at or how much time it took to drop down to 12.8v or if you've even fully recharged the 12v.

If you're using a trickle charger, you haven't fully charged the 12v and you need a better charger.

As for systems on your car, if you have an sks system w/key FOB on your Prius, that's the main voltage drain on the car when it's turned off.

 

Hi rjparker, Are there any diodes you know of which limit current flowing from the front jump points to a discharged 12 v bat in the back? If not, I suspect a severely discharged 12 v bat would "suck up" the current from any low power lithium or other jump box attached to the front jump points. I know I've had problems trying to jump my cars this way - with a very low 12 v bat in the back. For this reason I charge my 12 v bats only when they are disconnected from the car.