The mere fact that its from YT is dubious:

"STRONGEST

arguable debatable equivocal fishy hesitant improbable problematic questionable reluctant shady skeptical suspicious uncertain unclear unconvinced unreliable unsure

WEAK"

chancy diffident disputable dubitable far-fetched fly-by-night iffy indecisive moot mootable open perplexed suspect touch and go trustless unassured undecided undependable unlikely untrustworthy untrusty wavering

Cheers

2 hours ago, Ray Proudfoot said:

Sounds like an American magazine. Surprised Honda was below Toyota. One of their EVs is having problems with the 12v battery draining in a couple of weeks. Customers are not amused. UK, perhaps not US.

Quite a few hybrids and BEV's have the 12V battery issue. Ive not heard of a Toyota drainingin two weeks though, unless you mean left undriven for two weeks and a less than top condition battery.

The 12V battery is just there to power the infotainment, lights, locks, computers, all the low voltage electrics in a hybrid or BEV. So manufacturers spec low power batteries. Trouble is, if you don't drive the vehicle for longer trips, just short journeys, the high voltage traction battery doesn't have time to fully charge the 12V battery.

Toyota recommend for hybrids, that if you're sitting in your car, you leave it in "ready mode" so that the hybrid battery is charging the 12V battery, rather than auxiliary mode. The engine might fire up for a couple of minutes but then shut down.

Washed my car yesterday and left it in ready mode untill I'd finished washing. You have to make sure it's safe to do so of course, and on private land, as it's not legal on a public road.

Something else to do, if you have keyless entry, is make sure your remote is stored a fair distance from the car so they arent constantly talking to each other and firing up the computers and draining the cars 12V battery.

Of course, living on a small island it's mostly short trips, so I have a quality Noco jump pack in case I ever get caught out. Solar trickle chargers are also an option.

One of the Hyundai models had an issue with excessive server polling that was draining the 12V battery.

Edited August 17Aug 17 by martin-w

22 minutes ago, martin-w said:

unless you mean left undriven for two weeks and a less than top condition battery.

People left their car at airport car park and battery was flat two weeks later. Being a Honda owner I’ve never heard of ready mode. Toyota only I guess. Which? Magazine reported the issue.

11 hours ago, G-YMML1 said:

Hondas are very popular among car thieves in the U.S. :-) Here in Northern Virginia, we used to have nighttime raids targeting Hondas—everything from wheels and tires to airbags

That sounds terrible. I would move.

10 hours ago, Ray Proudfoot said:

People left their car at airport car park and battery was flat two weeks later. Being a Honda owner I’ve never heard of ready mode. Toyota only I guess. Which? Magazine reported the issue.

Oh yes, thats it. Common to many cars, not just Toyota. If you just do short trips the 12V doesn't get fully charged, couple that with leaving it un-driven for a fortnight and a flat battery is possible. Toyota say they have beefed up the batteries since then.

You have ready mode too. If, in PARK, you put your foot on the brake and hit the start button you should see a green READY on the instrument panel. The engine might fire initially but then drop back to traction battery mode. In this state the traction battery charges the 12V battery.

‐---------------------------------------------------

"On a Honda Civic Hybrid, a flat 12V auxiliary battery makes the vehicle completely inoperable. Even though the car has a large hybrid traction battery, the small 12V battery is required to power the onboard computers and close the main high-voltage contactors. Without it, the car cannot enter "Ready" mode.

Why the 12V Battery Goes Flat

• Accessory Drain: The 12V battery runs all electronics like keyless entry, security systems, and computer monitoring while the car is parked.

• Short Trips: Short drives under 20 minutes do not give the DC-to-DC converter enough time to replenish the energy used during startup and accessories operation.

• Inactive Periods: Leaving the hybrid parked for more than two weeks without driving or trickle-charging will deplete the 12V battery."

8 minutes ago, martin-w said:

Oh yes, thats it. Common to many cars, not just Toyota. If you just do short trips the 12V doesn't get fully charged

It does actually. I bought a battery monitor which attaches to the 12v and using its app I can see the battery state. It only takes a few miles to charge it to 90%+.

With ICE cars you could tell the battery state on engine turnover. With hybrids that’s not possible. Very useful device. Plus I have a trickle charger for overnight charging if required. Benefits of a garaged car. 😁

35 minutes ago, Ray Proudfoot said:

It does actually. I bought a battery monitor which attaches to the 12v and using its app I can see the battery state. It only takes a few miles to charge it to 90%+.

Depends on the 12V charge you started from. Usual advice for hybrids is a continous drive of 30 minutes once a week.

If charged in only a few minutes, then it was probably already well charged to begin with, due to a previous drive.

The 12V issue appears gradually over multiple short trips of less than 20 mins. It doesn't give the DC to DC converter time to replenish startup and accessory drain.

Depends on battery quality and size, too.

Quote from @martin-w post:

"On a Honda Civic Hybrid, a flat 12V auxiliary battery makes the vehicle completely inoperable. Even though the car has a large hybrid traction battery, the small 12V battery is required to power the onboard computers and close the main high-voltage contactors. Without it, the car cannot enter "Ready" mode."

Also the Toyota hybrid...as I recently found out myself. 5-year old Toyota Corolla hybrid - runs smoothly like a sewing machine; key fob batteries replaced in past 6 months; serviced by Toyota to their schedule, etc etc.

I unlocked car as normal with the fob button and WOW!!...pretty much every warning light and alert on the dash intermittently flashing; boot locked and wouldn't open; back doors and passenger door locked. Key fob wouldn't lock or unlock the doors or shut down the system. Only way of stopping it all was holding dashboard start button. This shut down the system but still couldn't lock the door. Basically, full hybrid failure.

It was Sunday before a long and important journey - Toyota closed; local garage closed; my car breakdown service didn't cover 'at home'; user manual checked but makes no reference to this. So I Googled it. One post talked about a flat 12v battery doing this. I got the emergency 12v jump lead pack out and connected to the 12v car battery - and the car behaved completely normally. I disconnected the booster pack and shut down the system. I tried to open the car with the fob - but was back to total illuminations and random car locks once more.

So I started the car with the booster pack and drove straight to a tyre/battery outlet and got a new 12v battery fitted - and it's been all normal since.

There is NOTHING in the manual about this. The thought of this happening at, say, a motorway services with kids now locked in the back of the car gives me the shivers...

Edited August 18Aug 18 by AJZip2

48 minutes ago, Ray Proudfoot said:

I bought a battery monitor which attaches to the 12v and using its app I can see the battery state.

Yep, ive seen them. BM2 device. A great idea. Not expensive, either.

2 minutes ago, AJZip2 said:

Also the Toyota hybrid...

Yep, any hybrid or BEV.

Do you do a lot of short journeys? 5 year old battery is getting on a bit in a modern car.

I recall it is mentioned in my handbook. The dealer went through it too, advising against accessory mode for extended periods, that doesn't charge the 12V battery.

I think when hybrids and BEV's arrived the manufacturers deemed a less powerful 12V battery enough, as there's no starter motor etc. As they've become more sophisticated with more electronic wizardry going on and communicating with servers, its more of an issue.

Toyota say they've beefed up the batteries in new cars. Given the driving we do on the island is by necessity short trips, I guess I'll find out. 😄

Solar trickle chargers are a good option. You can get them now that connect to the OBD2 port, so a bit more convenient

Edited August 18Aug 18 by martin-w

19 minutes ago, martin-w said:

Depends on the 12V charge you started from. Usual advice for hybrids is a continous drive of 30 minutes once a week.

If charged in only a few minutes, then it was probably already well charged to begin with, due to a previous drive.

The 12V issue appears gradually over multiple short trips of less than 20 mins. It doesn't give the DC to DC converter time to replenish startup and accessory drain.

Depends on battery quality and size, too.

The car was bought on 1 April 2025. Charge went from 40% to 90% in around 7 miles on a dual-carriageway. I do that in winter to keep the battery and engine in good nick.

The 12v also gets charged from the hybrid batteries when stationary on my drive with ignition still on and engine not running. Very clever system.

5 minutes ago, Ray Proudfoot said:

The 12v also gets charged from the hybrid batteries when stationary on my drive with ignition still on and engine not running. Very clever system.

That's "ready mode" as I mentioned. 🙂 The petrol engine will fire up from time to time to charge the traction battery... so dont leave it in an enclosed garage like that. 😃

Not on a public road either as its not legal. Private drive only.

Will your car lock in "ready"? So it can't get nicked. Not sure about mine.

@martin-w , only left in that state on my drive. The engine did start after ignition on for several minutes when I was working out all the options in the infotainment system.

Not sure about last question. In any case it goes in the garage and is locked to turn off the multiple sensors.

My best option seems to be what I've been doing, wash the car once per week in ready mode, so the 12V battery gets a good charge.

Edited August 18Aug 18 by martin-w

"Here are practical steps to keep your hybrid car's 12V battery charged and prevent sudden failure from short trips.

Maintain the Charge

• Use accessory mode less: Avoid running the radio or climate control when the hybrid system is off.

• Take weekly highway drives: Drive for 30 consecutive minutes once a week to let the system fully top off the battery.

• Hook up a smart charger: Connect a dedicated AGM or lead-acid battery tender if the car sits for days.

• Unplug extra cabin gear: Remove dashcams, phone chargers, or OBD-II scanners that pull power while parked.

Optimize Vehicle Settings

• Lock your vehicle doors: Locking the car forces onboard computers to enter a deep sleep mode faster.

• Keep smart keys away: Store key fobs at least 15 feet from the vehicle so they do not continuously wake up the car's computer systems.

• Turn off welcome lights: Disable automatic puddle lamps or exterior lighting delays in your vehicle's dash menu. "