2009 Prius, started eating through 12 battery charge about a month ago. I measured around 1.065 amps of draw on the 12 volt battery after sitting for around ten minutes after a solid drive today. The battery had been charged up, disconnected, and left alone for around a week prior to today's travels, and the battery held that charge just fine (so I'm not thinking bad battery yet). While the car if off and sitting, I pull and replace fuses in the engine compartment until I hit one that instigates a substantial drop in current, down to around 100 mA (I feel like that is still a touch high, but, baby steps..). That fuse is labeled "DOME" and I've seen reference to it so I think to myself "awesome, we can live without the dome light, case closed!" Button the car up, test for power - nothing. No dash, no wake up, no functional car. I replace the 15 amp "dome" fuse, car starts up just fine. This car is also recently experiencing a display issue in which the MFD mostly just shows white with some vertical lines (but the touch functions still work if you manage to hit the correct spot) in case this might be related. Ideas?
First, how are you measuring the current? If the hatch is open that alone will cause hundreds of mA of draw. One way to prevent that is to stuff a carabiner of about the same size as the hatch latch loop into the slot until it clicks. Just don't forget to remove it (press the "open" button on the hatch) before closing the hatch. Second, you want to wait a long time before measuring this. Letting it sit overnight and with no key fob anywhere near it would be ideal. Even hours after being turned off the car can spin things up, and if you happen to be measuring then it will change the measurement. Lots of possibilities, some "common" ones: Type of failure in the combination meter. As I understand it, there is a circuit that causes it to draw power for a while after the car turns off. If that circuit fails the combination meter will continue to draw power indefinitely. JBL amp under the right seat can do this. Tracking device from used car dealer attached to ODB2 port. These are tucked up into the dash and the owner generally has no idea that it is present until they try to track down a parasitic load. Failed door/hatch sensor. Any aftermarket device wired onto the car. Note: if memory serves both the combination meter and the ODB2 port are on the Dome fuse.
Ah, I definitely had the back hatch open, with a key in proximity. Many thanks, it did not occur to me that the car would be this sensitive or "hyperactive". I was measuring current with a multimeter in current configuration serving as the negative battery strap, but yeah - it sounds like a rethink is in order. In reviewing the listed subsystems on what hangs off each fuse after yesterday's post, it seems "DOME" covers an awful lot more than lighting, and the car not powering up makes sense. Good idea, I think this should be my next step. The battery is dated 2020. If that passes I'll move on to more troubleshooting. Overnight will reduce the battery to around 7.5 volts, if it's left connected. This is an interesting lead, thanks! I'll search up "combination meter" on the forums if the battery is given a clean bill of health. This should be an easy test to perform. We've got a bluetooth dongle plugged into the ODB2 port, which I'm guessing is not yet a short circuited mess since it was used to scan for codes yesterday (no codes found), but easy enough to unplug to test if it comes to it. Would this throw a caution indicator on the dash display? Not applicable, at least to my knowledge. Many thanks to you both! Hopefully it's just a battery. I'd rather eat the cost of a battery than potentially burn days chasing some electrical gremlin around.
When you know how much current is actually being drawn (measured with all the "is it closed" switches for the doors and hatch in the "closed" position) it will be helpful. When that isn't the case the "door open" indicator on the dash will be lit. OBD2 dongles don't normally draw much power, but one might draw 40 mA, and on top of the 20-25 mA that is normal it might be enough to run a very weak battery down overnight, but it wouldn't be enough to make the dongle warm to the touch. Conversely, if the power draw is 1A, that would come out to 12W of power. If a dongle was dissipating 12W of power it would be hotter than an old style incandescent bulb nightlight. I suppose though that there is some possibility that the presence of the dongle might prevent some computer unit on the bus from shutting off. So definitely try unplugging the dongle. If there is a tracker attached to the OBD2 you will be able to see it by looking up under the dash. Normally there is a normal looking set of wires coming out the back (top) of the OBD2 socket. If there is a tracker there will usually be a splice onto one of those wires. Fingers crossed it isn't the combination meter. The dash needs to come apart to get to that. Note, the capacity of a healthy Prius battery is 45Ah. So a 1Ah load should take 22 hours to get down to a 50% SOC on the battery - if it was fully charged to start with. The small electronic battery testers can tell when a battery has certain electrical problems, but they are not at all accurate in determining the capacity of a battery. To do that a constant current (and ideally, constant temperature) discharge at 1/20C (2.25A) must be made for an extended period. Here is one on a battery that has very little capacity (incorrectly identified as S46B24R, it was actually the smaller Prius battery) IV discharge curves for a working Prius 12V battery? | PriusChat and here is one for a brand new (Walmart) S46B24R battery after it had been fully charged: IV discharge curves for a working Prius 12V battery? | PriusChat Both at 2.5A. (The Peukert correction for using 2.5A vs 2.25A is very small.) These 2 hour discharge curves give an indication of the capacity. The true measurement (to see 45Ah) requires discharging from 100% to 0% SOC, which will probably damage the battery. That is something manufacturers do on one or a few test batteries out of a lot, but it isn't something end users should do on the one battery in their vehicle.
