My wife has a 2004 Civic, 67k miles. A few months back the IMA and CEL would come on for a few days, then disappear. It did this several times, but there seemed to be no drop in performance or mileage. Anyhow, with all this "stay home" Covid stuff, it's been just sitting in the garage. I think we may have drove it once about 3 weeks ago? Other than that, it's been sitting about 6 weeks. I know, that was dumb, so much else going on, I didn't even think about it...

Today when we started it, it idled high, and the IMA + CEL was on (like I said, it did that sometimes), but the red 12v battery light was on too. If I raised the RPM above 1500, it would go off. We ran a few short errands, but I could tell the battery was depleting. The wipers were slowing down, etc. The red light no longer would go out at any RPM, it stayed lit.

When I got back home, I put my meter on the battery, with the vehicle running, it had about 11.9v. If I shut it off and start it back up, I see a 13.X blip on the meter and it drops back to 11.X.

Where do I start? I hate to spend $120 on a 12v battery if it isn't the problem. Will the Civic's charging system give up that quickly on charging a dead battery?

Right now I have the battery on a slow-charger, I'll try it again when it's topped off (if it will top off).

 

So slow-charging the 12v battery to completion, disconnecting the negative and allowing it to reset for a few minutes, and firing it back up seemed to resurrect it! No red light, and the IMA began to charge at idle. I drove it around and it was back to normal. The IMA and CEL lights came on, but the system still seems to be functioning.

Now, feeling like I may have dodged a bullet, can somebody point me to a primer on grid charging? I would like to squeeze some more life out of my hybrid battery. I don't quite understand how a grid charger works verses how the pack charges in the car. I have an electrical/electronics background, but I haven't done much with battery charging systems.

 

When the IMA battery triggers a severe failure, it is isolated from the system. When it is isolated from the system, the DC-DC converter can only charge the 12V between about 1500 and 3500rpm. Outside that range, the IMA motor output is either too low or high voltage. That's what you observed.

While it feels horrifying, it is actually recommended in the manual to re-charge the IMA battery. Start and idle the car until it it to operating temperature then rev and hold 3500rpm. You will see the IMA battery charging and the IMA battery gauge will increase. Once the battery is full or near full, charging will stop. While this feels and sounds stressful, it's only pulling about 13hp out of the engine, which is about the same as highway cruise at 65mph, so you're not doing any damage.

My '03 HCH has likely thrown its CVT belt, so every week or so, I do the 3500rpm charge.

67K is absurdly low meaning it hasn't been driven enough to ensure the Hybrid battery is balanced. That causes cell drift where the 120 individual cells can be at wildly different states of charge and damage can occur.

hybridautomotive.com offers hardware and complete guides for grid charging.

 

Thanks for the response. I'll check that site out.