ECU-B Parasitic Battery Drain Issue

Googling "Prius ECU B" I see a lot of results relating to excessive battery drain.

 

what does the battery charge up to, and hold overnight when disconnected?
what does it drain to when connected?
what is the spec for normal draw with car off?

 

I don't know what Toyota officially says about that, but around 20 mA has been reported here in the past----same as AllenZ mentioned with the ECU-B fuse out.
Coincidentally or not, 0.7A, the higher number AllenZ mentioned, is the same drain rate I've measured with a door ajar and all lights off.

 

Perhaps putting the correct battery witih the correct amperage wil resolve the problem?

 

700 ma is way too much assuming you waited ~15 minutes after shutdown. Typically it should be 20-30ma. The 30 ma is typical with a wireless fob enabled.

Fuse ecu-b is connected to several modules so isolating one module after another may be required. There are some junction connectors that might be easier in some cases, especially since a few are buried in the dash.

Two of the first suspects might be the combination meter assembly and the main body ecu. In reality a skilled diagnostician might make short work of this while a first timer could end up creating additional faults. Use of Toyota's Information System would help as well.

Some of Ecu-b's connections

 

Have to wait longer than 1 minute.

 

well excuuuuuuuuse me!

 

The parking pawl and all drive positions are electric. No mechanical. With that much parasitic draw something is staying on or partially shorted.

A Battery Tender Jr would be a short term solution to keep the battery at acceptable voltage. It would be plugged in overnight or anytime the car is not being used for more than a few hours.

While the size of your battery is not the cause, it has much less capacity meaning it won't last long under excessive parasitic draw.

https://bit.ly/BatteryTenderJrWalmart

 

I am having the same trouble with my 2008 Gen 2 Prius with 225K miles. I checked the current draw by disconnecting the positive connection on the 12V battery and using a multi-meter. There seems to be a continuous draw of about 0.7 amps, with all doors shut and interior lights off and key fob far away. It goes up to about 1.4 amps if I open the back door, so I am assuming the rear latch is not the problem. I thought at first I had an old 12V battery and replaced it, but it quickly died and was ruined when I didn't drive the car for a few days. That's when I realized there was a continuous drain, and I subsequently installed another new battery. Since then I have been leaving the car parked with the positive lead disconnected to protect my new battery until I can troubleshoot it more. Will post if I find out more.

 

Most clamp-on current meters read only AC current, but a subset of them contain Hall Effect sensors and can read DC current, too, via the clamp.

One of these Hall Effect current probes would be the best way to determine which wire terminating on ECU-B is the culprit parasitic current hog.

This is an example of a sub-$100 Hall Effect clamp on current probe: MA120: 200A AC/DC Mini Clamp Meter+Voltage Detector | Extech Instruments

 

Most of the Hall clamps I've seen are great for measuring large currents (that one seems to be a 200 amp unit, at least from the link text), and ok even down to currents on the order of several amps, but currents in the 700 mA ballpark talked about in this thread are less than a 200th of that rated scale, way down in the lowest end of the scale where I am used to the typical Hall clamps giving pretty darned vague readings.

 

Indeed: in the manual for the Extech MA120 (PDF), page 4 states the accuracy for DC current measurements as “± (2.8% + 8 digits),” and at the stated 0.1 A resolution, “8 digits” might mean ± 0.8 A, which isn’t great if you’re trying to measure 700 mA (0.7 A).

There are clamp meters designed to measure small DC currents. For example, the Kaise SK-7830 has a 4000 mA range, for which the manufacturer claims 1 mA resolution and ± 1.5% reading ± 5 digit accuracy (from 5 mA), and the Amprobe LH41A, recommended by Honda Motor for dark current measurements, has a 4.000 A range, with claimed 1 mA resolution and ± 1.3% + 5 digit accuracy. For even smaller currents, the Kyoritsu KEW 2500 has 20 mA and 100 mA ranges, but a list price of ¥62,000 (at this writing, about $460).

 

So loop it through a few times and divide the reading by the loop count.

Doesn't work everywhere but it helps.

 

That works well when the wiring has enough slack to make loops.

For some other situations, an ordinary inline ammeter and a Fuse Buddy can be the ticket.