When there will be 12-V-battery issues, when not, and what to and not to do

I am creating this thread because many (though still a small fraction—and in no ways most) owners have had 12-V battery issues; yet, they have not been able to pinpoint the underlying cause because there has been no experimental investigation. I and another poster have investigated it using a battery sensor that plots the battery voltage as a function of time, and I am fairly certain at this point that I have been able to identify the problem, which I will explain below.

First of all, Gen 4 and Gen 5 Prius/Prius Prime use the same flooded-cell lead–acid battery (with a central vent in Gen 5), and they have virtually (if not identically) the same battery-management system (BMS).

What is a BMS? It is something the older Prius generations and older cars don’t have. It has a voltage, current, and temperature sensor attached to the battery, and it computes the battery state-of-charge (SOC), state-of-health (SOH), and starting performance (SOF). It uses a computer and a programmed algorithm to do that using the sensor readings. Moreover, it controls when and how much the battery will be charged and when and how much the battery will be discharged. It does this by controlling the DC–DC-converter voltage charging (or discharging) the battery, which is the equivalent of the alternator/voltage-regulator voltage in older cars.

Regarding the battery type as in flooded-cell vs. AGM, I have verified that the BMS automatically detects the battery type and calculates the SOC correctly. So, if you swap the OEM flooded-cell battery with an AGM battery, the SOC calculated by the BMS will be correct and things will work just fine, despite an AGM battery having a voltage about 0.25 V higher than a flooded-cell battery. The AGM battery will fare somewhat better than a flooded-cell battery due to its higher quality, performance, cycle life, and robustness, but both types will work.

Unfortunately, the BMS is very miserly in charging the battery, which is the main cause of the 12-V issues. This is by design by Toyota. While older cars always charge the 12-V battery, Toyota has chosen to program the BMS to minimally charge the battery in order to improve the fuel economy. This is explained in the Toyota new-car features manual. What the BMS does is like a thermostat—it aims to keep the battery SOC in the 70–100% range, and it won’t charge it much until it falls to 70%, and then it will charge it to 100%, and then wait until it falls to 70% again, and the cycle will repeat. What I mean by it won’t charge is that it will keep it at a float voltage (of about 12.89 V) or slightly discharge it while driving the car, and the battery voltage and SOC will then drop due to parasitic drain and self-discharge while the car is not driven.

The parasitic drain and self-discharge is about 2–3% a day—similar to other modern cars with advanced electronics. While many here thought so, the parasitic drain is not the root cause of battery issues. Toyota telematics is not a source of significant parasitic drain either.

Another contributing factor is cars already being delivered with a battery with a low SOC from the dealer due to spending long wait periods in dealer lots. The BMS and charging system will not fully rectify a very low SOC.

Plug-in owners: While this is not a root cause of battery issues, leaving the car plugged in for too long will increase the parasitic drain. One improvement in Gen 5 is that the 12-V battery is charged if the traction battery is being charged until the traction battery becomes fully charged. Gen 4 only keeps it at a float voltage during charging. In either generation, you should not keep the car plugged in unnecessarily, as it will increase the parasitic drain after the traction battery becomes fully charged.

It takes about 45 minutes for the BMS to increase the SOC of the battery by 25%—if it decides to do its most aggressive charging, which is done at a DC–DC-converter voltage of about 14.3 V.

So, if you have read the detailed information above, you can ask what the root causes of battery issues and what the solutions are if you haven’t already guessed them. Here they are:

1. The battery already coming with a low SOC from the dealer. If that is the case, you should definitely manually charge the battery with a smart battery maintainer like Noco Genius. If not, have the dealer do so using their charger. This is because the BMS is not capable of rectifying a low or very low SOC like in older cars with an alternator. To measure the SOC, you need to get a voltmeter accurate to two decimals (or a battery monitor) and measure the battery voltage with the car not driven for at least half a day and the doors having been kept closed and key fob having been kept 20 feet away from the car for at least three minutes. You can then look into the following table. Note, again, that this will only work with a rested battery and doors kept closed and key fob kept away for at least a few minutes, as you need to measure the rested open-circuit voltage, the latter meaning no (or minimal) current drain.



2. The parasitic and self-drain is about 2–3% SOC a day. Given that it takes the BMS about two minutes to put about 1% SOC of charge into the battery, if you are driving the car only once a week for fifteen minutes, the SOC will keep getting lower and lower and you will eventually end up with a dead battery. Therefore, if you are not driving the car frequently and/or long enough, you need to manually charge the battery with a smart battery maintainer like Noco Genius. In fact, the BMS algorithm is somewhat buggy and may not be happy with only ten-minute trips, and you might want to regularly hook up a battery maintainer if you are only doing short trips as well. By the way, a “trip” does not necessarily mean that you need to drive the car. You can turn the car on to the “READY” mode (the hybrid system is on) and sit in it for 30 minutes of longer, and the BMS will charge the battery if it thinks that the SOC is below 70%. That would be an alternative to hooking up a battery maintainer for those who don’t drive the car regularly and don’t have a garage.

If the battery comes with an SOC greater than about 70% from the dealer and if you drive the car regularly, you do not need to hook up a battery maintainer.

Here are some details on mine and others’ tests:

AGM battery for Gen 4/Gen 5 Prius/Prius Prime and observations on the 12-V charging system | PriusChat

Good luck!

 

It has been widely reported here that both the Gen 4 and Gen 5 do aggressive 12-V-battery charging at around 14.3 V throughout the trip when the weather is very cold. I can’t observe this behavior in LA because it is almost never very cold here.

 

Read my original post.

Chances are that your SOC was already very low, and using the accessory mode for a few hours was the final nail in the coffin.

At this point, unless you replace the battery, you must manually charge it with a smart battery maintainer (like Noco Genius), as the BMS cannot fully charge a (nearly) fully drained battery. If you do not have a garage, remove the battery and charge it at home.

 

Again, read my original post.

You don't need to disconnect the battery (or just its negative terminal) unless you want to use the repair mode of the smart battery maintainer (like Noco Genius), which may apply a too high voltage. The repair mode might help reduce the sulfation caused by deep drains though.

 

When you poke around the car with an ammeter with the key fob in your pocket etc., of course you will see a large parasitic drain, even when the car is off.

You cannot measure the parasitic drain randomly with an ammeter. You need to monitor the battery with a battery monitor 24/7 to observe the parasitic drain. You can then infer the daily SOC loss due to parasitic drain and self-discharge from the daily resting-voltage drop. You can also see when large parasitic-drain events happening through the inverted peaks on the voltage vs. time graph.

As I stated in my original post, the parasitic drain in the Gen 4/Gen 5 Prius is no different than in any modern car with advanced electronics, and it is not a culprit in the battery issues. Moreover, the Toyota Connected services over the mobile data network is not a significance source of parasitic drain with or without an active subscription.