Third day with new car, dead 12V battery, second day in a row :(

Ok, but at no point did I say to put the car in the accessory OR the ready state. Either of which may trip that 8A limit.

The negative side is the simpler side to go inline with since the positive side has that distribution block on it.

All you're doing is putting the multimeter in line then closing the hatch/doors and locking the car then walk away. Might even be a good idea to just lock the doors manually. My 10A fused meter handled my old junk car just fine. Though I also didn't throw it into accessory or ignition while it was hooked up. Cause that would've been silly.

 

A lot of people on the Lexus NX forum are having 12 volt battery problems mainly due to two things: leaving ACC on too long and 2. Digital Keys parasite battery drain.. There are some band aid fixes, trickle charger or bigger 12 v battery. Don't count on Toyota/Lexus to come up with a fix anytime soon going on 2 years with NX problem.

 

It seems that the charging regimen of the Prime is quite different from that of the regular hybrid. Have you asked Toyota if the pattern you're seeing is normal? Perhaps they can explain the "logic" behind it.

I have a voltmeter plugged into my accessory socket and also have a similar bluetooth monitor. When I'm driving my LE the voltage increases to well over 14v while the battery is charging. I mostly do short trips (generally total of ~15 miles or so) with several stops along the way. So it's almost always in charge mode unless I take a longer drive. When I get home the voltage is usually in the range of 12.5 to 12.6v.

I recently didn't drive the car for 5 days. When I drove it again the monitor reading was ~12.08v. That's sitting in the carport before I unlocked and opened the door. It started right up. During colder weather (cold for Florida) I went 6 days without driving and voltage was about 11.95v. Started right up.

Can you switch your monitor to display voltage instead of state of charge (SOC)? I wonder if the pattern will be the same. My monitor only graphs the voltage. It shows SOC but the % shown doesn't coincide with a table someone posted a while back so I don't really trust the SOC%. The voltage readings on both devices are accurate though and agree with a multimeter.

I only do spot checks and only rarely at this point since I've never had a problem. Just checked and it says 12.15v, "battery normal", "electricity 92%".

From what I keep seeing here, the Primes are much more likely to have 12v battery issues. What is it about the Prime that would require the 12v charging regime to be different than the regular hybrid?

 

“My monitor only graphs the voltage. It shows SOC but the % shown doesn't coincide with a table someone posted a while back so I don't really trust the SOC%.“

So how do you figure soc by the charts when the voltage is 14.5? I am sure the monitor isn’t just taking voltage, then calculating soc. We use soc all over the place, phones, cordless power tools, etc, and accept the magic inside. When I manually charge it goes to 100% soc. It doesn’t stop at 99, and doesn’t go over 100.

 

I totally ignore SOC for the 12v. All I care about is whether or not it has enough voltage to start the car.

Monitoring SOC makes sense with devices like phones, iPads and laptops. That's because they are expected to be able to operate all the way down to 1% SOC, after which you recharge them. Their usable range is extremely wide. And, unless you keep them plugged in, they are not designed to recharge while in use.

A 12v car battery is not expected to drop down anywhere close to 0% SOC. I assume that the bluetooth monitors use some sort of software calculation to extrapolate a SOC figure. Apparently different software comes up with different figures.

That's why I asked if @onthesalt 's monitor could also display voltage. Voltage is voltage, it's not a calculated figure that's linked to the specific needs of a particular device. Two devices might both say they are at 1% SOC yet the voltages might be quite different depending on the device. Or, to look at it another way, a certain voltage might equate to a 10% SOC on one device and 25% on a different device.

A complicating factor is that the 12v in a Prius doesn't start the car the same way as a 12v used in a normal ICE car. The ICE car requires a certain amount of amperage / "cranking power" to run the starter motor. The 12v in the Prius just needs to have enough voltage to power up the car's computer. This doesn't require much amperage, which is why their 12v batteries are much smaller. As far as I know, the BT monitors are not specifically designed for hybrids so it's likely that their SOC figures reflect battery conditions that are required to start ICE vehicles, which make up the vast majority of cars on the road.

Here's the chart that someone posted elsewhere. My monitor says 12.15v is "electricity 92%" but the chart indicates it would be ~55% SOC. Whatever the SOC is, I've had no issues even with a voltage as low as ~11.95v.


When I first got the car I found a post (not sure which forum) by an owner who said that they only had issues when the voltage dropped to ~ 11.7v on their earlier generation hybrid Prius. The car's computer started exhibiting odd behavior and would not go into Ready mode so they replaced the battery, which was about 10 years old at that point.

I'm not sure what the minimum voltage threshold might be for the Gen 5 hybrid. Apparently, the Prime is less forgiving.

Regardless of what the car is doing during 12v charging - driving, sitting in Ready mode or plugged into the wall - the battery should measure higher than the nominal 12v while it's charging. As the battery is charged, the voltage will drop down to nominal voltage. On a longer trip, my voltage will eventually drop to the 12.5 to 12.6v range after it's charged.

 

The voltage listed in the table is the open-circuit voltage, which means that the battery must be disconnected to measure it. If that's not practical, then you need to make sure that everything is off so that the current drain from the battery is minimal. Otherwise, there will be a significant voltage drop across the internal resistance of the battery due to Ohm's law, subtracting from the open-circuit voltage.

 

I just reposted the chart since I mentioned seeing it previously. I don't consider it particularly useful.

The bluetooth monitors seem to be the most practical method of regularly measuring the battery with "everything off" as their current draw is minimal.

How much voltage is required to put the car in Ready mode? That's what really matters. I know that 11.95v is sufficient for my LE but I don't know how much lower the actual threshold is. It may be different for the Prime. Hopefully some Prime owners will report their voltages after the car fails to start.

 

I probably have seen those charts for 40 years. Soc viewing is the same for anything. I charged my Prime 12 v overnight last night because it was at 70 % soc. I charged my phone because I saw 38%.
My gen 2 was not good to the 12 v battery either. I never used anything but a volt meter to check it, or the dash display, Don’t need to play around with that now.
The sensor on the battery is the same for prime or hybrid and some other toyotas. Im thinking they need to revise the ecm program, but so would many other makes of hybrid and electric cars. It’s actually sort of strange why this is so common a problem.

 

Was reading the reddit Prius board and saw this comment:



So Toyota has updated charging ECUs for the prime? I assume this is some kind of software update?

 

There have been complete replacement charging units for early cars - there's a replacement part number for the whole assembly.

That's not related to any 12V battery issue though - some sort of fan or fan sensor problem. That could be what he's getting - he's got it fixed in his mind that his 12V battery issue is a charger problem, and the dealership may have checked that his vehicle is eligible for that replacement.

But apparently there is a TSB for a batch of bad 12V batteries (both Prius and other models).

 

Yeah, I heard about the TSB. Hopefully it is available online at some point. I’m curious as to what it says…