Honda Pilot - Honda Pilot Forums banner

Mystery battery drain -

827 Views 4 Replies 3 Participants Last post by  WiiMaster
So the girls going over 3 batteries swap in 2 months: something definitely wrong.

The thing we checked: all interior lights are off, and all headlights are off.
Thing possibly wrong: AC relay ( manually turn off both front/rear AC)
  • Things that newly installed since 2022: towing wiring hardness (w/relay) from etrailer.
  • Thing that add-on since 2014 : side step with lighting. I think the electrical goes under the passenger side.

So the routine is voltage measurement (engine off =12.x ) and engine on ( 13-14.x V)=> alternator is charging !

Then connect the DMM inline, power everything off and start check for mA drain ( anything over 25mA) .

I will need to print out fuse boxes and relay diagram to start to pull out the suspected drain .

Is there a common source for the tow wiring hardness elimination?

1 - 1 of 5 Posts
There have been some reports of the memory and CANBUS keep-alive circuit causing a similar drain. That's Fuse 23 (10A) in the main under-hood fuse and relay box, the one on the passenger side. It's labeled "Back Up" because it "backs up" critical memory items and allows for some pre-start intelligence. A module that fails to shut down correctly on key-off might still draw memory power through this fuse. If you see a whole lot more than a 30mA drop when you pull that fuse, you get to go after individual smart modules.

That's said, taking parasitic-current measurements is a bit of an art form as much as a science experiment. If you are using a DMM as your ammeter, you'll need to maintain the current flow path continuously when you get your measurements, without actually opening the circuit after you shut down the engine. A technique that works for me is to connect one probe securely to the negative battery post, the other securely to a body ground point that is not the bolt for the battery ground cable. S Configure the meter and meter connections to read DC current. Door lights are off, headlight switch OFF, driver's window open. Start and then shut down the engine through that open window, keeping the doors closed. Remove the key from the ignition after you stop the engine. Now unbolt the negative battery cable from the body, and your meter will show the parasitic drain current. I measure about 30 milliamps (0.030 amps) on my Touring, which may have more modules to keep alive than an LX might. Wait five minutes or so after stopping the engine and removing the key to get your readings. Removing that 23 fuse should drop the number to almost zero.

Other favorite causes of parasitic drains --
-- The AC clutch relays are known to stick, and may offer a high-resistance current flow path through what's effectively a weld blob bridging the contacts inside. Use car when removing relays for testing, as you must pull them from the bottom and not by the shell. Pull on the shell and you might see the contact damage, not really the best way to diagnose this problem. The fuse protecting that circuit is fuse 20 (7.5A) in that same main under-hood fuse and relay box. It's labeled "MG Clutch" for the magnetic clutch on the AC compressor drive.

The trailer primary circuits are piggybacked off the primary lighting circuits. the actual lighting module gets power through a circuit protected by fuse 3 (30A) in the aux fuse and relay panel, the rectangular box by the battery. There's a separate fuse 17 (20A) in that same box that's used for a trailer battery charge circuit. That space is normally vacant unless you decide to put a fuse in to charge a trailer battery while you drive. Isolation relays prevent flow through those fuses when you aren't driving, so generally these are not a worry when chasing parasitic drains.

I found a really nice clamp-on DC meter that reads down to very small single-digit milliamp levels. It's so sensitive that it needs to be zeroed while placed in the same rotational position it will be in when measuring, as it reads changes in the earth's magnetic field based on it's position. I could probably use it like a compass. Anyway, it was under $50 on Amazon and it makes measurement a little more interesting.

TL;DR --
Battery math is always fun. The car comes with ~~65AH battery capacity. That's the amount of current that the battery can supply under defined conditions. The drain is even over a 20 hour period, with specific temperature conditions and weasel requirements, and it's for a specific total voltage drop of something like 2.5. Bottom line though is that you can divide that total rated capacity (65,000 milliamp-hours) by the parasitic losses you measure in milliamps, and find the expected time in hours that your parasitic drain would take to drain the battery to ~~10V at the terminals. Using my 30mA measured parasitic drain for instance, 65,000/30=2,167 hours. A parasitic drain that would kill the battery overnight would be closer to the "amps" range rather than "milliamps" with a new fully charged battery.

Keep in mind as you do the math that a regular flooded-cell car battery gives up about 5% of its -remaining- storage capacity each time it's drained below about 10 volts. Think 0.95^[number of discharges] times 65 original rating amp-hours to find what's left for capacity. 20 discharges leaves you with about a third of the original capacity. That doesn't mean it won't start the car after it's charged up, just that it won't support cranking the starter for very long. Think about this when you decide to sit with the radio on and engine off, or even parking lights. There are extra drains that happen if you leave the key in the ignition while parked, even though the key is in the off (0) position. So if you are sitting in the car, maybe camping or you leave the keys in the ignition at soccer practice or even at home parked in the garage; Take the keys out and save some extended battery life. Each discharge steals life, and each discharge is more likely to steal more life as remaining battery capacity is depleted. The term "death spiral" might be a good way to describe the whole thing.
See less See more
1 - 1 of 5 Posts