12v Battery Is Discharging, Re-Charge Now !

Ok thanks, but I don’t understand why there wasn’t a fairly significant increase in the voltage after the EVSE charge. I had roughly a .25v increase from my drive yesterday. I was hoping to get close to a full charge from the EVSE charge. The good news is that I received the Battery Tender today, so tomorrow I will use that to bring it up to a full charge and start maintaining it.

 

Ok thanks!

 

Eeek! In 77.75 hours it will be down to 0 volts! I just did the math.

 

But it's odd that the manual states that the 12v will charge when the EVSE charger is charging the traction battery. Maybe they mean a trickle charge. The way I understand it is that the 12v is supposed to be charged when the traction battery is charging, but not just when the EVSE is plugged in but no charging of the traction battery is taking place. Is that correct.

Also found this tidbit in the manual this morning (I added the bold):

"Open voltage at 68F (20C): 12.0 V or higher

If the voltage is lower than the standard
value, charge the battery. (After charging
the battery, turn on the high beam headlights
for 30 seconds with the power switch
off, and turn the headlights off.)

Charging rates 5 A max."

This seems to indicate it is not necessary to charge the 12 volt battery unless it drops below 12.0 volts. Maybe this is the threshold for the system not starting. Even so, given what's been posted on here, it would probably still be better to maintain the battery at a higher voltage for the sake of battery health.

Any thoughts?

 

OK, 6th day after a full charge. 2020 PRIME almost brand new 12v battery is back to a pre-full-charge level of SOC at 12.56v 90%. During the 24 hours period, it changed from 12.62v to 12.58v, 0.04v decrease.

I think I will stop the parking test for now and will take the car for my weekly drive today. I will not charge the 12v battery today and see the change in 12v SOC before and after the drive and belief charging with L1 for 2 hours to bring it back to 35-45% traction battery SOC.

 

Yap, I agree. From @Mark57 comment earlier, 11ish volt is no go for Prius. So yes, 12v maybe is all needed for the car to start-up.

 

Besides starting vs not-starting, an overtaxed battery maybe dies sooner?

 

Yes.

 

What do you folks think of the statement in the manual to turn the high beam headlights on for 30 seconds after charging the 12v battery? I have never seen that recommendation made for any other car.

 

I agree with this statement but take it a step further.
If you want the best battery 'maintenance charger' go for one with the 'Anti-Sulfation' function.
Do some research on this subject.

All large UPS systems in office buildings use Anti-sulfation functions in their maintenance. These can be systems with 10's of thousands of dollars worth of Lead Acid Batteries. They don't mess around with old tech on these systems.

Voltage readings are a very simple measurement of SOC, but battery temp should be factored in with those simple charts.

You can't walk away from an old school 10Amp battery charger. It can send your battery north of 15.5V.
You only need a 1 - 2 A 'Tender' for 'maintenance'. The kind you can set and forget,, while the battery is plugged in.

I really recommend BatteyMinder brand. They cost more because they do more.
They make Aviation approved chargers. The charger I have has an external temp sensor that can be extended to where the actual battery is located.
I have this one: BatteryMINDer 12V Maintainer for Sealed AGM Lead-Acid Batteries

I'm convinced all the batteries around the ranch are lasting longer than I ever experienced!
Yep, every 1-2 months, year round, my batteries get a week or so of the Ol' BatteryMINDer.
(The difference between the AGM version and the regular version is a minimal difference in topped voltage. I use mine on all Lead Acid batteries.)

 

that's to remove the surface charge and get a true reading, same as leaving the battery overnight and testing in the morning.

 

do you have a day to day comparison rob? that looks higher than your original post a week ago.

 

Thanks! It might work for removing the surface charge to get a true reading, but in the owner's manual it was not presented in the context of checking the battery again after charging. But maybe this was the intention of the statement, just not clearly stated.

 

can you post the section, or quote it?

 

I quoted it in my post #107 above. It's in 9-1. Specifications, Electrical system (12-volt battery) on page 724 of the 2020 manual.

 

confusing, maybe out of context? but i think they forgot to add, 'before rechecking the voltage'

odd, toyota has never recommended letting the voltage get down to 12v before charging, iirc.

 

Yeah, it is odd. I haven't seen anything about turning the headlights on after charging in the sections about charging the 12v battery.