Prius Gen 4 12v Battery: losing charge

[Peter Snoopy]

After a 300 mile journey, my 12v Battery charge reduced from 100% to 25%. After not driving my car for 3 weeks, my 12v Battery charge reduced from 100% to 50% (or less). My key fob is protected by a Faraday cage and no lights have been left on. The 12v Battery is almost new.

Please, does anyone have any suggestions what's wrong ... or is this behaviour normal for a Prius Gen 4?

Many thanks.

P.S. I have just read RJParker's excellent post on Priuschat in February 2024. This might explain the "Parasitic Draw" when the car is not driven for 3 weeks but might not explain why the battery seems to lose too much charge when driven.

[skidlid]

Not enough info. How are you measuring Battery charge? After a 300 mile trip I would expect any 12v Battery to be 100% charged and showing approx. 12.7 volt after standing overnight.

[BhxTrev]

I also have a 2016 Gen4 Prius, which I’ve owned from new. I’ve never experienced the problem you’re having & I don’t think the behaviour your experiencing  is normal.

If the Battery is almost new, it must have a guarantee. I bought my last Battery a couple of years ago from Halfords & it has a 5 year guarantee. That being so it’d be worth going back to where you’ve bought the Battery & have them check it out since it could be faulty. Besides that if they check it out & say it’s okay they might well tell you what the problem is.

[Peter Snoopy]

@skidlid - thank you for your insight. I am only using a Battery charger to give an indication of the 12v battery's charge. I appreciate this may not be accurate. However, the Toyota garage also said the 12v charge was low (even though I'd fully charged it a couple of days before).

@BhxTrev - also, thank you for your helpful advice. Yes, maybe my new Battery (from Halford's too)  is faulty. This would be annoying because I replaced the old one due to this charging issue.

[BhxTrev]

Hi Peter,

Thanks for your reply. I forgot to mention that like you I frequently leave my car parked for up to 4 weeks but I’ve never experienced any problems.
 

The reason I bought a new Battery a couple of years ago was because I didn’t use the car for a few weeks in a particularly cold spell in winter & while checking my tyres with 12V inflator, before I used the car again, I flattened the Battery. I wasn’t sure what had happened as the car was totally unresponsive & I did call the Toyota breakdown service who got the car started through jump leads.
 

As the Battery was past its sell by date anyway, I thought it best to replace it, which I did. The battery I had off Halfords has been trouble free so far though I’ve still got the receipt just in case it does fail before the end of the 5year guarantee! 
 

Trev

[Peter Snoopy]

@BhxTrev Many thanks [again]. Unfortunately, I'm not as organised as you - I may have lost my 12v Battery receipt. However, I'm tempted to ask the garage to run a set of electrical tests on the car e.g. Battery condition, 'parasitic drain', DC to DC convertor and anything else they can think of. If there's anything conclusive, I'll post a message.

[Paul john]

7 hours ago, Peter Snoopy said:

After a 300 mile journey, my 12v battery charge reduced from 100% to 25%. After not driving my car for 3 weeks, my 12v battery charge reduced from 100% to 50% (or less). My key fob is protected by a Faraday cage and no lights have been left on. The 12v battery is almost new.

Please, does anyone have any suggestions what's wrong ... or is this behaviour normal for a Prius Gen 4?

Many thanks.

P.S. I have just read RJParker's (https://priuschat.com/members/rjparker.41651/) excellent post in February 2024. This might explain the "Parasitic Draw" when the car is not driven for 3 weeks but might not explain why the battery seems to lose too much charge when driven.

Parasitic draw does not apply to 2016 gen4 as the services that cause this are not installed on that model

[BhxTrev]

Hi Peter,

Thanks for your reply. Please keep us updated as your experience might just prove to be valuable to me & others.

Trev

 

[TonyHSD]

Perhaps the Battery is dead already before the long journey and not possible to hold charge anymore.
I had experience that in other petrol car and after a long journey similar to yours my Battery had died totally and the car couldn’t start.  
You need a new Battery battery replacement most likely. 

[Peter Snoopy]

11 hours ago, Paul john said:

Parasitic draw does not apply to 2016 gen4 as the services that cause this are not installed on that model

@Paul john Many thanks for this information. I didn't know.

[Peter Snoopy]

8 hours ago, TonyHSD said:

Perhaps the battery is dead already before the long journey and not possible to hold charge anymore.
I had experience that in other petrol car and after a long journey similar to yours my battery had died totally and the car couldn’t start.  
You need a new battery battery replacement most likely. 

@TonyHSD Many thanks. I had not thought of this scenario. Maybe, as @BhxTrev also suggested, my new Battery is faulty.

[loz8]

6 weeks ,4weeks, mine would be deader than Pythons parrot. It's a historic Toyota problem,no official suggested fix. Maybe more cash for a Dealer.

[Peter Snoopy]

@loz8 Many thanks for your interesting [but worrying] thoughts.

[Paul john]

Cant say i have seen the “historic toyota problem” in 500k of Prius driving across a variety of models in the last 19 years. 
This may be due that i treat the 12v as a disposable item, like tyres. If it gets worn out after long term use , just replace... 

I changed one at 120k miles and another at 90k miles (different car). 

[Peter Snoopy]

2 hours ago, Paul john said:

Cant say i have seen the “historic toyota problem” in 500k of Prius driving across a variety of models in the last 19 years. 
This may be due that i treat the 12v as a disposable item, like tyres. If it gets worn out after long term use , just replace... 

I changed one at 120k miles and another at 90k miles (different car). 

Many thanks again @Paul john. I replaced my 12v Battery a few months ago but the issue persists. Maybe it was a faulty replacement.

[TonyHSD]

3 hours ago, Paul john said:

Cant say i have seen the “historic toyota problem” in 500k of Prius driving across a variety of models in the last 19 years. 
This may be due that i treat the 12v as a disposable item, like tyres. If it gets worn out after long term use , just replace... 

I changed one at 120k miles and another at 90k miles (different car). 

+1 👌

My 12v Battery lasted more than the hybrid Battery. Both over 13 years and 267k miles, not bad at all. 
We both new batteries the car drives really good. 

[Peter Snoopy]

43 minutes ago, TonyHSD said:

+1 👌

My 12v battery lasted more than the hybrid battery. Both over 13 years and 267k miles, not bad at all. 
We both new batteries the car drives really good. 

@TonyHSD Wow. 267 000 miles. That's over 10 times round the earth 🌎.

[BhxTrev]

15 minutes ago, Peter Snoopy said:

@TonyHSD Wow. 267 000 miles. That's over 10 times round the earth 🌎.

My Toyota garage told me that they have a Prius customer who’s car they service & it’s done well over 300,000 miles. My 2016 Prius has only just done 50,000 miles after 8 years, so on that basis should be good for another 40 years!🤔

[Peter Snoopy]

1 minute ago, BhxTrev said:

My Toyota garage told me that they have a Prius customer who’s car they service & it’s done well over 300,000 miles. My 2016 Prius has only just done 50,000 miles after 8 years, so on that basis should be good for another 40 years!🤔

@BhxTrev Wow. I'm way behind you guys. My 2016 Prius has only done 30,000 miles. Mines good for another 72 years!

[BhxTrev]

Hi Peter,

I thought mine was a low mileage for an 8 year old car, but your low mileage is impressive. Looks like our cars are going to outlive us! 😂

[Peter Snoopy]

1 hour ago, BhxTrev said:

Hi Peter,

I thought mine was a low mileage for an 8 year old car, but your low mileage is impressive. Looks like our cars are going to outlive us! 😂

👍

[Paul john]

2 hours ago, Peter Snoopy said:

@BhxTrev Wow. I'm way behind you guys. My 2016 Prius has only done 30,000 miles. Mines good for another 72 years!

That’s barely run in 😂.

i looked up my first gen2 the other day (the one where i changed the Battery at 120k) and at 269k its still going strong. Just front shocks replaced at 200k

so i agree with @TonyHSD a good brand.

well done on such a low milage gen4. 

[Peter Snoopy]

7 minutes ago, Paul john said:

That’s barely run in 😂.

i looked up my first gen2 the other day (the one where i changed the battery at 120k) and at 269k its still going strong. Just front shocks replaced at 200k

so i agree with @TonyHSD a good brand.

well done on such a low milage gen4. 

@Paul john Wow re. "front shocks replaced at 200k". I had to replace my rear shocks at 20k. I blame the speed bumps around our estate.

[BhxTrev]

7 minutes ago, Peter Snoopy said:

@Paul john Wow re. "front shocks replaced at 200k". I had to replace my rear shocks at 20k. I blame the speed bumps around our estate.

On the subject of shock absorbers, I remember our holidays in Malta a place we enjoy visiting & have done so many times over the years, where the roads, though not so bad these days, used to be so bad it was common to have to replace car shock absorbers on a yearly basis.
 

I also had a rear shock absorber go on one of my cars & it became positively dangerous to drive especially when negotiating bends. Sorry to stray off topic!

[Peter Snoopy]

1 minute ago, BhxTrev said:

On the subject of shock absorbers, I remember our holidays in Malta a place we enjoy visiting & have done so many times over the years, where the roads, though not so bad these days, used to be so bad it was common to have to replace car shock absorbers on a yearly basis.
 

I also had a rear shock absorber go on one of my cars & it became positively dangerous to drive especially when negotiating bends. Sorry to stray off topic!

@BhxTrev 👍