Parasitic Power Drain

[inventor111 28]

My local Honda dealer here in the UK has been investigating an issue with my six month old Prelude that results in failure to start the engine after just a few days of non-use.

While diagnostic tests report that the parasitic drain on the auxiliary battery is "within spec" it does not explain why, after a journey of 200 miles plus, the car will not start without rescue assistance after 3 days of non-use.

Personally I had suspicion the DC-DC converter is not charging the 12v battery sufficiently or the parasitic drain was well above spec, but the garage was convinced the battery was the cause.
Also anecdotal comments I've come across suggest the TCU may be drawing more power than anticipated if the mobile network connectivity is poor where the car is parked.
Here in the UK I have the MyHonda+ App which uses the Honda EU API and keeps track of my Prelude status. Other regions likely have different variations on connecivity.

The garage eventually got a fault code generated on the battery test and I've had a replacement under waranty.

I'm still not convinced the root cause has been idenified so I will be monitoring the auxiliary battery performance over the coming months and certainly don't expect to be rescued yet again via Honda Assistance (aka The AA, in UK).

Researching the subject, it seems to be a common problem with Hybrid cars in general because the manufacturer deliberately incorporates a small capacity 12v battery for various reasons.

In our 2026 Prelude I found in practice that if the battery was only slightly depleted (through the inevitable parasitic drain, particularly caused primarily by the TCU) the voltage drop on cranking was enough to kill start attempt, requiring a jump start or booster to get moving again.

 

[OldCoupeGRGuy4 3]

I found this with my Yaris Cross.
I did a lot of research and it wasn't just hybrids. Full EVs were having the same problem.
I just don't understand why the engineers at these companies can't figure out a solution. There will be one.
My niece is constantly calling out her dad for a jump start on a relatively new EV.
I had the Yaris Cross nearly 3 years without a problem but I did buy a CTEK conditioner/charger and fixed that on the wall next to the car.
I do this with my GR Yaris and that is fine.
I buy the connector that fits onto the battery and negative bolt so I snap connect it in seconds.
I have had my prelude 3 months and it gets put on the CTEK overnight about every 2 weeks as it is so easy.
I did it the week I bought it and it took a few hours to get through its full cleaning and charging routine probably because it was a few months since the battery was made then fitted.
I did it again last week and it quickly went through all stages and showed a full battery.
I would advise every new car to have this done to clean the battery of the sulphor build up between the lead plates that built up in those months before you drove it off.
I also have a NOCO jump starter in the glove box of both cars. Only ever used it with other peoples dead cars.
Apparently the battery only needs to drop into the mid 11v range for the build up to start so that delivery time will do that so an evening on the CTEK can only be a good thing.
My GR Yaris lives on it in the garage and at the 5 year service the battery test came back at 100%
My advice is buy one and use it maybe once a month. It isn't difficult.
The pictures show my GRY on it and fully charged.
The prelude isn't on it but you can see the cable into the + terminal and the negative on a bolt with the connector near it.