No takers so far

Go ahead and design this modification yourself. Once you have this working in your Prius, come back to this thread and provide all the details about your design. I think you will be the first person to do something like this.

 

So, I guess you will have to live with your Prius the way it was designed by Toyota or go buy a new car with this design.

 

Okay, how about this response.

No one in their right mind would try to make a modification like you are suggesting because of safety concerns.

 

Not very helpful either.
How about something like:

‘A setup as you describe leads to significant safety issue due to [enter safety concerns].’

I know BEVs are designed to charge the accessory/starter battery. I would think the design is robust and would never want to try to create it as a DIY project.

Explaining WHY it is a safety concern, or won’t work would be more helpful.

To the OP, trying to tap, or modify a high voltage battery is, in my opinion, extremely dangerous.
Trying to use a high voltage battery to charge a low voltage battery seems full of potential for bad things to happen.

I am also not an electrical guru, so I don’t know specifics, but I would steer clear of the idea.

 

Well, now you are responding in kind, with ad hominem attacks.
Let it rest, and perhaps someone with more knowledge will give you the details you are looking for?

 

The "mod" you want currently does not exist for said vehicle. You don't know how to do it...and neither does anyone here. Why are you taking these facts personal?

 

I’m confused as to why you choose to do this?

Personal experience with finding my car would not start one morning was resolved by removing my Bluetooth Scanner plug, and not letting the car sit more than two weeks undriven. I did install a Bluetooth battery monitor which constantly tells me the battery is low, but it has not failed to start in the last six months.

 

Hybrids aren't new.
They've been around in cars for a quarter century and in other vehicles for a lot longer than that.
There's a pretty good reason that in all these years there is not much of a(n intelligent) demand for cross connecting the Traction battery with the Aux Battery. It's because the traction (big) battery is fairly expensive and the aux (little) battery is not.
People like to tinker with their car's electronics by adding all kinds of electronic accessories and at the very least you can cause some parasitic drains which would turn your car into a 3200-pound piece of lawn art........or worse.
I'm just guessing, but "jump starting" a 220v Li-Ion battery that's been 'flat-lined' "may" be a little more difficult than booting up the car with a 12vdc source.

Who knows?
It might be a little safer and faster too......

Priuses are fairly notorious for having an undersized aux battery, but it really only has ONE job:
To boot up the car so that the traction battery can start the car.
EVEN SO......most people do not have to use secret squirrel technology to keep the aux battery from flat-lining every month or so.....

UNLESS:
They've 'modified' their car's electronics.
Their car has a manufacturing defect.
Their car has a major design flaw (plant based wire insulation, for example.....)
Their car has been in an accident, and incompetently repaired.

If you fall into one of these categories then you REALLY should consider using a small jump-pack to boot the car up so that the traction battery (isolated from whatever is draining the start battery) can start the car, and you can get on with your life.

Or?
Buy a better car.

I used to poke holes in the ocean and travel for Uncle.
I would not buy a car that I could not park for several months without a reasonable expectation that it would start up when I got back.

....but since 'life' happens, I'd still keep a $50 jump pack on hand in case it does not.

 

or you could just say there would be very limited times this would even help you. To start, the 12v battery is required to be in reasonably good condition just to get the car to power up. For your scenario, it would need to be in good enough condition to power up the car, and allow you to activate your new auxiliary relays. If it's strong enough to do that, then it's strong enough to just energize the car normally, so what do you gain? If the 12v battery is so weak that you need to charge it in order to get the car to do anything, how do you propose to connect the HV battery to the 12v system? The power button won't do anything for you if the 12v is so weak it won't even power up the car electronics.

 

What I should have written is:

No competent engineer would try to make a modification like you are suggesting because of safety concerns.

If you truly want this modification, you will need to hire a consulting engineer who is willing to take on such a project. This will become very expensive in a short time period. So, are you willing to pay for engineering help to get this modification in your Prius? Do you know how much a consulting engineer charges per hour?

 

seeing that the prius has had a 12v problem for over 20 years, i have to think this is an engineering problem that even toyota has not be able to over come, or is too expensive to implement.
my bolt has the same problem, albeit a larger more robust 12v battery. chevies answer is to leave it plugged in when not using, and i have read of other cars with the same problem over the years

 

But there is no "mod" that applies at this time. Your kicking a dead dog.