2005 HCH I, CVT, almost 53K miles (I have other Hondas) and original hybrid battery pack. I have had no problems at all from it and drive it often (work from home). My average has been about 44-55 MPG depending on cargo and driving habits. I use a Hybrid Automotive grid charger (installed 2015 when I first got P1600 and P1433) and run it about once every 2 months.

To the best of my knowledge, the ECM has NOT been re-flashed or upgraded by Honda. No other software updates have been done.

About 2 weeks ago the IMA and Check Engine Lights came on (P1600 and P1433, again) at the same time and stayed on. When I got home, I disconnected the 12v battery and pulled the IMA fuse for an hour. The lights went away; then, later came back at the same time.
FYI...
P1600 ENG - Integrated Motor Assist System Malfunction
P1433 HYB - Battery Module Deterioriation / Hybrid battery pack – deterioration

My friend rebuilt his Prius battery pack and offered to check mine out, using his scanning tools and load testers. So I pulled the pack and we tested each stick. All showed ranges from 7.81-7.86 volts. There was no corrosion on any contacts or cell damage.

I placed the pack back in the car and hooked up the grid charger, letting it run a good 24 hours for charging and balancing. After starting and running the car, the lights did not return until 77 miles later.

When that happened, the IMA status bar was about halfway, then dropped to about 3 bars. Around a mile later, it jumped up to almost a full charge.

My MPG hasn't suffered, nor has the car been behaving differently otherwise.

I tested the 12v battery when the car was off; it read as 12.6 volts. When started and running, it's 14.7 volts.

While the lights are on it still functions properly that I can see. If it weren't for these 2 lights, I'd never know something were wrong.

If the battery sticks test as good, what else could we look for? My friend suggested the Battery Control Module, or possibly a software glitch.

What else could we check next, before I feel resigned to buying a replacement pack?

 

Please understand your pack failed 7 years ago. You've been nursing a dead pack for 7 years. Eventually it stops working AND the process your using can make it worse over time.

The test results via your "Prius friend" are underwhelming. You simply measured voltage? You didn't even need to pull the pack for that. You can measure each pair through the back of the BCM plug while it's still plugged in.

The battery sticks did NOT test as good. You need to stop listening to your "friend." Success with a Prius pack in no way translates to expertise with a Honda pack. He's suffering from the Dunning Kruger effect in this regard. Note that I have tested and built hundreds of Prius packs. Even so, I will not do Honda packs except for my own use, and I've owned 5 Honda hybrids. Still have 3 (02 and 05 Insight, 03 HCH). I sold off my 2X 06 HCH2... total pieces of **** battery-wise.

Your friend confirmed:

1. Capacity
2. Internal Resistance
3. Self-discharge

Were all acceptable?

The battery gauge behavior you described is a negative calibration where one tap drops below low limit ("march" down to 3 bars) followed by a positive calibration, where one tap shows full before al the others ("march" to full). These are both signs of a failing/failed battery.

Use of a grid charger (low current intentional over-charge) exacerbates voltage depression, i.e., the "memory effect." Every time you use your grid charger, you lose a little capacity. Eventually, you will have little to no capacity left, and it will vary from pair to pair.

Have you ever discharged your pack? If "no", that's the next step. Given the year, I suspect your HA harness has a protection diode in it that prevents discharge. You can confirm by determining if you can measure battery voltage at the harness plug. If you can measure battery voltage at the harness plug, you can discharge through it. Make sure you wear insulated gloves when you check voltage. Small pin holes in tight spaces encourages mistakes.

Note that a discharge has 1 of 3 effects:

1) no effect.
2) improves.
3) permanently kills the dying cells and forces you to replace the pack.

 

I thought as much with the error messages popping up. The grid charger lulled me into believing the problem was fixed when, in fact, it was just a bandage.
Can you recommend a scanner for a Honda that will read hybrid codes?
When I had the codes read at Advance Auto, the P1433 code displayed as "Fuel level sensor" on their scanner.
On his car, my friend showed me how his scanner and the Dr. Prius app worked, and I wonder if there's a similar tool and software for a Honda hybrid.
Knowing all this now, can you suggest a quality Honda replacement battery pack? I've found so many different ones online; some use reconditioned cells, some use new ones.

 

You don't need any additional scanner. No consumer grade tool exists for HCH1 and even professional tools are very limited. NOTHING will report the block voltages like Dr. Prius does on the Prius. The data isn't available as the car only uses it internally. Even HDS (Honda Diagnostic System) at the dealership can't read that info, nor can my $2000 Snap-On Ethos Pro.

You have all the information you need. Your pack is deteriorated because it's reporting P1433. Discharge/charge the pack as I recommended or replace. No additional diagnostics are necessary.

Do not buy NEW cells. They are "D" cells made in China, and there's not much chance they're significantly better than quality reconditioned Panasonic cells that are 20+ years old. I'm not kidding.

Vigorously recommend Bumblebee batteries. Small shop that's been in business for 10+ years (recently bought by a MUCH larger organization with deep pockets). I have insider info on their reconditioning process (we have shared IP in the past), and completely trust their output. Nothing is perfect, but you won't get much closer than Bumblebee.

Full disclosure: I've known the former owner for a long time (he still works there). We talk regularly. I have never received any kind of compensation for recommending them.

BUT, before you sink that money into the car, I would DEFINITELY try a manual discharge first.

 

I appreciate the suggestions, and was about to ask opinions on new/reman battery packs. Bumblebee was at the top of my list, followed by HybridRevolt and GreenTec.
What would you suggest as the best way to do a manual discharge of the battery pack? I use a Hybrid Automotive grid charger, but didn't get the discharge feature. Would I need to remove the battery pack again for this?

 

No removal.

Easy mode would be to buy a discharge ready harness and a bulb charger assuming those are still options... Otherwise DIY.

Start with this:

https://hybridautomotive.com/wp-cont...00w__46760.jpg

The bulb sockets are wired in series, not parallel. wiring sketch as follows:

Battery (+)<-------------------------|- bulb socket 1-----------------------bulb socket 2 -|-------------------------> Battery (-)

The "|" are where you would attach voltmeter probes.

Rather than terminating in a connector, you could have insulated alligator clips at the end of each wire and then clip them to the same points the charge harness is attached (Battery + and Battery - in the sketch)

Start with a 30 hour soak charge.
Remove IPU lid.
Attach discharger leads.
Install 40-60W incandescent bulbs. Turn master switch on and discharge to 60V.

The discharge might take a couple days (46+ hours), but the low current/slow discharge should minimize the chances for damaging cells via polarity reversals.

Once you've hit 60V, turn master switch off, remove discharger, reinstall IPU lid (for cooling) and charge for 30 hours.

 

I'll check on that discharger, and hope HA has one available for this older model.
If not, I hope I don't get shocked.

 

If you do all your work with the master switch off and/or you wear quality nitrile gloves, and you don't do something to intentionally shock yourself, the chances of you getting shocked are very low.

The primary dangerous points are:
1) the main terminals when the car is on.
2) the harness connection points when the master switch is on.

If you have a voltmeter with an autoranging or 200V or higher VDC setting, check anything before you touch it.

 

Copy that. I'm always a cautious fellow when working with electronics, but a chance childhood lesson with electricity gave me a healthy respect (and fear) of it.

 

Little update: we pulled the pack to test the sticks further.
19 of the 20 showed voltages from about 7.9-8.1, with one reading 7.1.
We have time, so my pal is going to buy another stick if he can have this one for tinkering.
More testing will continue.