Unsolved Battery Drain (with detailed troubleshooting)

Semi-long story.

Background:
Earlier this week I noticed that the doors were having trouble unlocking. It would go through five sets of double beeps trying to unlock the doors. About 25% of the time, one of the drivers door wouldn't unlock. Odd I thought, did some searching and found that the Prius lock actuators appear to fail over time (see: Fixed: Door Lock Actuator in Gen II Prius | PriusChat). Others suggested 12V auxiliary battery issues. I just replaced the stock 12V with an Exide AGM FP-AGM51JIS in March (~6 months ago), so I assumed it wasn't the aux battery. I thought I'd dig into the locks further this weekend.

Today, when I attempted to start the car after work, I got the red exclamation light. I checked my ScanGuage and noticed the voltage was 9V. Odd, I thought as it always reads 12.X volts after sitting for hours. I shut the car off and restarted. Somehow it mustered enough power to kick on the electronics and start the engine. I drove home to get tools.

Troubleshooting:
My thoughts are either (1) that my battery has an internal short or (2) my car has an external short somewhere. I disconnected the negative battery cable and measured the current (car fully off, all dome lights off, ScanGuage disconnected). The measurement was 1.5-1.6A. This seems really high I thought. I did some searching on PC and found reports of the JBL amp being stuck on and drawing high current (see: Auxiliary battery keeps dying | PriusChat). I pulled the audio and amp fuses. No luck, still the same current draw. So I meticulously pulled each fuse in the driver's side foot well (2-3 at a time) and tested. Again no luck, same current draw. Also pulled all the fuses and relays (2-3 at a time) in the fuse box under the hood. Still no luck, same current draw. I did not pull the 30-100A fuses that are all tied together.

Battery voltage during the testing was between 10.7V and 11V. After some time testing, the current draw dropped from 1.5A to 1.0A, but this appeared to not be related to fuses, as it couldn't be repeated with the same circumstances. Regardless 1.0A seems very high. Others (such as Patrick Wong) report the "power-off draw" to be 20-50 mA. I assume these are utilizing the same method that I described via the battery ground.

My expensive multimeter won't power on (not the battery...ugh another issue). But I measured the current with two inexpensive Harbor Freight-style multimeters. Both measured the same current.

Also, no check engine light on the dash and no diagnostic trouble codes are reported by the ScanGauge.

Questions:
What am I missing? Are there any other fuses or relays to check? I would assume all relays are fuse protected.

What else could cause this large current reading? Does the car register a lengthy current spike for sometime when reconnecting the battery? I measured the current for up to 30 seconds when "completing the ground circuit" via the multimeter and the current seemed consistent.

I left the negative battery disconnected to prevent further battery drain and battery degradation.

If the battery truly is draining at 1.0-1.5A, it would drain 12-18 Ah when sitting overnight, which is probably consistent with dropping from 12V to 9-10V. The battery is reportedly 45 Ah.

Thanks to all for any guidance.

 

Thanks all for the assistance. Troubleshooting while stressed and at midnight is never a good situation.

Battery is on trickle charger to top it up from 10.5 V to ~12-12.5 V. I'll have to reinstall and continue testing tomorrow evening.

Patrick Wong - Good writeup. That is pretty much the the procedure that I used, but I tested at the battery versus the battery post under the hood. The one difference is I left the rear hatch open, which definitely kicked on some of the ECUs and electronic components. Testing in the front is probably easier since there is no hood switch in the factory alarm. As an alternative, presumably we can leave a door open as long as the door plunger switch is clamped in the closed position, correct? Is the door ajar only measured by the single switch per door?

Britprius: There was no voltage drop associated with the current drop. Voltage throughout testing was within 0.2-0.3 V range (approx 10.7 - 11.0V). Also the drop from 1.5A to 1.0A, was instantaneous, not gradual. It happened twice but could not be reproduced. I think it might have something to do with the various ECUs dropping to sleep mode as Drdiesel suggested.

Drdiesel: Good info. I didn't even think about the SmartKey being in my pocket. I'm surprised the ECUs take that long (20 min) to go to sleep. I have a battery post switch, but I'll have to leave the back hatch open to access it as I can't access it from a window. Since it's easier, I'll try Patrick's method first to see what the immediate current draw is upon reconnecting. This may not be the exact parasitic load, but it may be good enough to help me diagnose the problem. If it's 0.1A or less, then it may be a battery or charging issue and not a drain or parasitic load issue.

The Fluke multimeter is dead. It seems to be an issue with the LCD screen. I took it apart and this element doesn't really seem repairable. I'll have to pick up a new one.

 

Just an update on this issue. I 2A trickle charged the battery overnight until it was 100% charged (~12.5V). Later that day, I retested and it was down to 10.6V. I repeated the process and got the same results.

I returned the battery to the Exide distributer, as it was only 5 months old. They are a large distributor and near the Exide US headquarters. They had never seen one of the Exide Edge AGM batteries go bad (including all models). They tested it and got the same results. They were dumbfounded at the issue.

The battery had never been significantly drained and was driven far enough each day to fully recharge the battery. The previous OEM battery had no issues, but I proactively replaced it because it was almost 6 years old and had an overnight resting voltage of around 11.7V.

After a couple of days, overnight resting voltage when installed is normal (12.2V+ on the ScanGauge), so I think the issue was the faulty battery and not battery drain, but I'll give it more time to confirm.

Despite my apparently bad unit, I do think the Exide Edge AGM FP-AGM51JIS may rival the Optima for best auxillary battery for the Prius. The Exide is a direct drop in, has a longer warranty (48 months free replacement), and has higher specs (75 min reserve capacity and 45 Ah capacity). It's rated at 440 CCA, but mine tested at 705 CCA. It's also significantly cheaper. I purchased mine for under $120 + tax (after rebate), which is much cheaper that the $200 Optima.

 

Patrick - Haven't tested the quiescent current draw. I haven't had time to research and order my multimeter replacement yet.

nh7o - Are you referring to the temporary surface charge being above 12.5 V? I suppose that value depends on the charger, as float chargers can run as low as 13.5V and trickle chargers and speed chargers can run 14.4V+. My trickle charger was running at 14.4V, so you are right that a battery with a bad cell would report about 12.5V with surface charge and 10.5V in a rested, no load state.

 

The new Exide seems to be working well after 8 months or so. Resting voltage is over 12V after an overnight sit. I'm not sure what made the prior battery have a cell go bad. Based on the feedback and number of these batteries that the distributor sells, I think it must have been a fluke manufacturing issue.

For the price, I still think the Exide may be the best auxillary battery for our cars.

 

It's about 12.5V as measured with a ScanGauge, but this is with a slight load on the battery as some ECUs are active via the Smart Key system. I haven't recently manually measured it with a multimeter outside of the car or without the Smart Key present.