Will the hybrid battery charge the 12V battery when car is turned off?

The interest in 48V mild hybrids is cause that is the max voltage you can have before requiring isolating the battery on shut down.

So would replacing the traction pack. Like the traction pack, the 12V section shouldn't need replacing until well into the car's life.

 

My Prius 5 was parked at Brussels airport for 8 days last week. "Ready" immediately appeared when I pressed the start button.
The car is also only 6 months old, with 8200 km.

 

Mine was parked in the garage for 7 days recently. When I returned, the battery measured 12.2V and started with no problem. I keep it on a smart trickle charger that maintains a min. of 12.5V, max. 12.8V. The car is a year and a half old.

 

I wanted to share my experience with isolators versus battery combiners, which might shed some light on why your setup isn’t working quite right with a diode isolator.

I went through three separate diode isolators before switching to a Cyrix Battery Combiner, and the difference was night and day. The diode isolators kept failing or not operating properly, and here’s why I believe that happened:

1. Voltage Drop and Heat: Diode isolators inherently cause a voltage drop (usually around 0.7V per diode). This reduces the effective charging voltage reaching your auxiliary battery, which can cause undercharging or unstable charging behavior. Also, the diodes dissipate power as heat, which can eventually damage the isolator if it’s undersized or poorly cooled.
   
   
2. Current Limitations: The isolator’s diode bank has a maximum current rating. If your wiring or loads push beyond that, it stresses the diodes, leading to failure or inconsistent switching.
   
   
3. No Intelligent Control: Diode isolators don’t “know” when to connect or disconnect batteries other than blocking reverse current. So if the system voltage fluctuates or your loads vary, the isolator can’t adapt. This can cause the auxiliary battery to drain or the main battery to be overloaded.

By contrast, the Cyrix Battery Combiner is a microprocessor-controlled relay that monitors battery voltages and connects batteries only when one has reached a preset voltage level. This intelligent switching:

• Eliminates voltage drop because it uses a relay instead of diodes, so the full alternator voltage reaches the batteries.
  
  
• Automatically disconnects batteries when voltage falls below float level, preventing battery drain.
  
  
• Handles higher currents safely without overheating.
  
  
• Provides more reliable, stable charging and battery isolation.

Since switching to the combiner, my system charges the auxiliary battery efficiently, and the switching between batteries happens seamlessly without failures.

So in your case, the isolator’s voltage drop and lack of intelligent control likely caused improper charging and may have led to stress on the isolator itself. The combiner solves these problems by intelligently connecting the batteries at the right time with minimal voltage loss.

Hope that helps explain the difference!

 

This is the battery isolator I recommend. You can have a remote pushbutton to engage the second battery.



However if this was a requirement only for easy jump starting I would buy a lithium 12v with built in backup controlled by a wireless remote.

Will the hybrid battery charge the 12V battery when car is turned off? | PriusChat

Battery isolator manual

 

Let me ask you this... have you used it in a dual battery setup in your prius tho? if you haven't... how can you recommend it based on looks?

Did you read what I wrote about isolators?



https://black.jmyntrn.com/2021/03/16/new-isolator-installed-after-previous-one-died/

 

Except I'm not using a diode isolator. The smart isolator disconnects the auxiliary battery when it reaches a level of 12.8V, and cuts in when the charging voltage reaches 13.3V.

 

Thanks for clearing that up—I get what you mean now. Yeah, a smart isolator is definitely different from a diode isolator. It’s relay-based, so it doesn’t have that annoying voltage drop that diodes cause, which makes it a better choice for sure.

That said, it’s still an isolator at its core, just a more advanced version. Those voltage thresholds you mentioned—disconnecting at 12.8V and reconnecting at 13.3V—are exactly how smart isolators decide when to switch, and that works pretty well for a lot of setups.

But from my experience with my Prius and running dual battery systems, I found that a microprocessor-controlled battery combiner like the Cyrix really takes things a step further. It doesn’t just switch based on preset voltages; it actively monitors the batteries and uses smarter logic to decide when to connect and disconnect. This helped me avoid the glitches and hiccups I kept getting with isolators—even smart ones.

You know, those small relay-based smart isolators work well for a lot of people who don’t have a Prius or have simpler electrical setups with higher voltages and less complexity. But with all the delicate electronics and how electricity flows in a Prius, those isolators can struggle.

You might still have yours running fine now, maybe it’s been holding up for some time. But from my experience, it eventually fails. I put an isolator in, it worked for a while, then it went out. Replaced it, same story. After the third time, I said, “Enough.” I started digging into what would actually work reliably with a Prius.

That’s when I found the battery combiner—the one the Prius really loves. It’s smarter, more responsive, and designed to handle the Prius’s unique electrical demands without the headaches I had with isolators. Since switching to the combiner, everything’s been rock solid.

So while smart isolators are definitely better than diode isolators, I’d say for Prius owners who rely on their auxiliary batteries for camping, overlanding, or powering gear, battery combiners are the real game changers. They just handle the whole charging and switching process better in real-world use.

That’s just been my experience after going through a few isolators and finally settling on a combiner that’s been rock solid

 

I have been driving Prius Prime Gen 5 for about 2 years and don't pay attention to any of this. I charge the EV battery overnight when it is down. I don't drive every day, do a lot of local just on the EV, and a trip on gas engine of a couple hundred miles once a month or so. I use the radio, heater, AC normally. Live in an area with a real winter (sometimes). Have left the car at the airport, in the driveway for a couple or weeks. And I never do anything special about the 12V battery, never even check what the charge is. I just drive the car and it works. My question: am I just lucky, or living on borrowed time? I am not an engineer, am reasonably smart, but don't want to do the gymnastics some in this thread (which, admittedly, I have only skimmed) apparently do - kudos to them, but I just want to continue to happily drive the car.

 

Your golden; just keep doing what your doing. I would investigate and find the under-hood jump points; just in case. If the 12VDC battery gets weak; it's more likely to happen in the winter months - when most batteries go to heaven or Hades. At that point, all you really need is a jump start.