I thought I would post this as a reminder of the importance to your MPG that the 12v battery has on the FEH. I looked through the archives anddidn’t see much about the 12v battery. I even read someone post that the FEH12v battery doesn’t effect your MPG. Most seasoned FEH owners know thatreducing your electrical load (head lights, blower motor, radio, etc.) willhelp your MPG, which is very true. Remember that the less RCM or reservecapacity your 12v battery has, the more frequently the ICE has to come back on to recharge itonce depleted again. It would be a good idea to have your 12v battery testedand make sure that it is in at least 95% health or better.

 

Would it be a concern since the battery just power electronics, not the starter? The starter is powered by traction battery.

 

Besides the concern akfonzy described, there are two other, perhaps more important aspects of a weak 12V battery in the FEH.

The first is that as the battery approaches end of life, various systems will go awry. They may not appear to be related to the battery, and so may result in costly but futile repairs.

The second is that although you are correct that the 12V battery does not start the FEH's engine directly, it has to have enough charge to close some relays that connect the hybrid battery so the start sequence can begin.

The fact the FEH does not display weaker starting as the 12V battery ages, disguises the battery's poor condition until it shows up as a more serious problem such as failing to start. So a pre-emptive replacement isn't a bad idea.

 

I juat ran into this situation hopefully someone has the answer. Yesterday my 12 volt died truck would start then die. Replaced 12 volt all afternoon it was showing 14-15 volts on the dash. Then I get in the morning and it won't get over 13. Checked it with battery checker while running no charge.

Is this normal? if not what could be some causes for this? Thanks for any help you can provide.

 

Peak charging is around 14.5V. Once peak has been hit and maintained until the current drops below a defined threshold, the battery drops to float, which should be between 13.2 and 13.8V depending on the vehicle/battery.

I suspect your battery was sitting on the shelf for at least a few weeks and likely lost 10-20% of its charge. Thus when you first started it, it needed to charge it to full. Now that it's full, and the 12V doesn't have serious loads like a traditional starter battery, it is immediately showing full as soon as the car tries to charge it.

 

there is no starter in the traditional sense - isn't that correct;
mg1 (do they use the same terminology as toyota) connected to the ice starts the FEH.

 

Correct.

I don't know if they use the same terminology, but I do.

HV battery spins MG1 to start engine.