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

My OEM GS Yuasa 45-Ah flooded-cell battery was close to four years old. I had left it at standby for six weeks last year, and it was OK, but this year, it didn't take a four-week standby, possibly due to some battery degradation and/or the SOC not being close to 100% when I left it.

The battery was still OK after recharging it, likely with some degradation though, but I decided to upgrade to an AGM battery. AGM batteries are superior to flooded cell batteries in virtually every way:

• charge five times faster (internal resistance five times lower)
• much lower rate of self-discharge
• less degradation in deeper discharges
• much longer cycle life
• much higher cranking current
• spill-proof
• less gassing
• etc.

The only drawback is that, in the EN LN1 size, they are about 7-lb heavier because of thicker electrodes, but that shouldn't be a big deal.

Uplus is a Singapore-based OEM that sells to major auto makers, and they make very high-quality batteries. They are selling directly to customers on Amazon and currently with very deep discounts; so, their AGM batteries are actually cheaper than flooded-cell batteries out there. So, I got one.

Uplus EN LN1 50-Ah AGM battery on Amazon

The battery ships from a large importer warehouse in Azusa, Los Angeles County, California, and comes in a very well-padded box.





It also comes with a barbed plug to plug the unused vent hole for Gen 5 trunk installation.



It is very important to install an AGM battery with a full charge, as otherwise, the charging system won't recalibrate itself properly and the AGM battery may never get optimized and initialized. For that purpose, I bought a Noco Genius 5 charger. It took 42 hours to fully optimize the battery, as the last phase of the charging is a constant-current phase and the charger doesn't stop until 14.80 V is reached. When it was over, the slowly flashing green LED became solid. Note that you can get the cheap Noco Genius 1 as well.

I also bought the “classic” BM2 battery monitor, which shows the battery voltage on your phone through Bluetooth. It is a very useful device.

“Classic” BM2 battery monitor on Amazon

I installed the AGM battery I fully charged using the Noco Genius as well as the BM2 battery monitor. There is a hold-down clamp that you can remove with a 12-mm socket with an extension or spinner handle and you need a 10-mm socket for the terminals. Note that you need to wiggle the terminals to remove them.

I also installed the barbed vent-hole plug on the negative-terminal side, even though it wasn't needed for Gen 4. That will reduce the possibility of a hydrogen-gas explosion resulting from sparks generated when you disconnect or connect the negative terminal. To do that, you need to press the barbed vent plug into the vent hole with a socket handle or such, as it is impossible to press it in with your finger. Of course, I left the vent hole on the positive-terminal side unplugged. In Gen 5, there is a vent tube that goes into the vent hole in the negative side; so, the vent hole in the positive side should be plugged.

And, to install the battery monitor, I had to slowly and carefully bend its U-shaped terminals into the correct angle. I installed it directly onto the battery so that the voltage reading would be accurate at high currents. The drawback of that is that the battery sensor doesn't see the 1.5 mA drawn by the battery monitor, but that shouldn't be a big deal. I verified using a precisely calibrated multimer that the battery monitor was precisely calibrated to 0.01 V as well.

Initial observations:

It's been about three days. The open-circuit voltage of an AGM battery is supposed to be around 12.89 V at 100% SOC. Noco Genius was aggressive in optimizing the battery; so, it started at around 13.10 V, and after three days, it is down to 12.95 V. I don't know whether it will drop to below 12.89 V (possible) or stabilize around there.

Here is how the charging system works:

When you turn on the car, it will charge at about 14.10 V for about five–ten minutes. It will then stop charging and the voltage will hover in the 12.85–12.92 V range. Occasionally, it will rise back to about 14.10 V. And, interestingly, it will sometimes rise to about 13.53 V and charge there instead, in particular, when you slow down to a stop and especially when you put the car in park. So, there is definitely truth in that putting the car in park initiates some charging (at ~ 13.53 V), but I have also seen the charging stopping when at park.

Obviously, it hasn't been long, and I will report my findings over time. Meanwhile, here are the initial screenshots of the battery-monitor app. Note that the voltage data is stored every two minutes and the time units are in hours. The straight sections are the standby sections when the car is not being driven:








The ~ 14.10-V charging voltage:



The ~ 13.53-V reduced charging voltage when the car is in the park gear etc.:



Nightly ~ 6–8-minute-long parasitic drain that happens after seven hours of standby (telemetry or traction-battery check?), the open-circuit voltage falling by 0.01 V after it is completed:



General observations on the Gen 4/Gen 5 Prius/Prius Prime 12-V battery and charging system:

• Gen 4 and Gen 5 use identical 12-V batteries, even though Gen 5 got a central-vent lid for trunk installation.
• Gen 4 an Gen 5 12-V charging systems are likely identical as well.
• There is mainly one significant instance of parasitic drain that lasts for about six–eight minutes everyday roughly seven hours after the car is driven. I don't know what the car is doing during that. Telemetry? Checking on the traction battery? The open-circuit voltage is falling by 0.01 V after it is completed; so, I am guessing it could be consuming up to about 1% SOC. So, if your 12-V battery is degraded and/or not fully charged and you leave it at standby for a few weeks, it could deplete.
• I haven't done any AC charging yet, and that will be interesting to see what it does to the 12-V battery and if there is any parasitic drain with the car plugged in. I will report on that later.
• I think the 12-V charging system in Gen 4 and Gen 5 is very smart, and there should generally be no issues. However, this doesn't mean that there will be no issues. The main issues I've so far identified are as follows:
  • Toyota should have opted for an AGM battery instead of a flooded-cell battery. As I stated earlier, an AGM battery charges five times as fast a a flooded-cell battery and has many other advantages as well. It is 7-lb heavier in the EN LN1 size, but I don't think that is a big deal.
  • While the charging system is very smart, the car wants to prioritize fuel efficiency and limit charging (reduce the DC–DC-converter voltage) as much as possible. This could possibly result in the SOC drifting lower over time, and at one point, the SOC could be too low for the charging system to be able to keep up with it. Moreover, lead–acid batteries don't like low SOCs, which cause battery degradation through sulfation and stratification.

My recommendations:

1. If you replace the battery out of warranty, definitely opt for an AGM battery.
2. Invest in a Noco Genius (the cheap Noco Genius 1 is sufficient) or similar smart charger/battery maintainer. If you are a little technically inclined, get the “classic” BM2 battery monitor as well so that you could observe the SOC and manually charge the battery when it drops to a too low level. Otherwise, you can charge the battery every few months for precaution and extend its lifespan.
3. If you own a Gen 4/Gen 5 Prius Prime, don't leave the car plugged in for longer than overnight, as parasitic drain is a known issue mentioned in the owner's manuals in that case. As I said, I will report on this when I gather data.

More reports will follow when I gather more data.

 

There is no core charge or hazardous-material fee. You can return your old battery to AutoZone for a $10 gift card. It ships nationwide through FedEx Home Delivery.

 

Yeah, sure, if it is under warranty, then that's free vs. $125.99.

There are about a dozen different CCA standards (BCI, SAE, EN, DIN, JIS, …). The OEM battery's CCA standard is EN. I don't know what the CCA standard for the Toyota TrueStart is, but I am sure it is apples vs. oranges to compare them. In any case, AGM batteries have a much higher CCA if that matters.

I will take pics of the installed battery as well as the BM2 battery monitor.

 

A very interesting observation on the fourth day after the installation of the AGM battery:

The DC–DC-converter standby voltage, meaning the battery voltage when the car being driven but the battery not being charged, was 12.85–12.92 V on the first three days. On the fourth day, it was around 13.10 V throughout the drive. (See the first screenshot below.) It appears that the charging system convinced itself that it was an AGM battery and raised the standby voltage by about 0.20 V. The charging system indeed seems to be very smart. Therefore, this is great news that the charging system will be able to charge the AGM battery properly.

The open-circuit voltage after rest (and after the nightly six–eight-minute parasitic drain) is still at 12.92 V; so, the battery is still at 100+% SOC. (See the second screenshot below.)

The only drawback of the AGM battery is that the higher standby voltage (by about 0.25 V) and heavier weight (by about 7 lb in the EN LN1 size) slightly hurt the fuel economy, but then you gain some fuel economy from lower charging losses in an AGM battery; so, I doubt it makes a measurable difference in fuel economy, whether it is a slight loss or a slight gain overall.

 

Here is my first observation on what happens to the 12-V battery during traction-battery AC charging and with the car still plugged in after the traction-battery charging is over.

The Level 1 (120 V) charging started shortly after 12:00 at 5% SOC and ended after 17:00 at 100% SOC. THe 12-V battery was at 12.90 V just before the charging started. There was a dip in the 12-V battery voltage when the AC charging started for a few minutes, meaning that it was being discharged. After that, the DC–DC converter kept the 12-V battery at 12.97 V throughout the charging. When the charging was completed at 100% but the cable was still plugged in, the 12-V battery voltage was 12.93 V, indicating that it slightly charged. However, in four hours, it dropped back to 12.90 V. It is not clear what would happen if I kept the car still plugged in instead of driving it, but it looks like keeping the car plugged in may indeed be draining the 12-V battery faster. Surprisingly, there are no isolated current draws but apparently a continuous current draw when the AC charger is plugged in.

The long story short, you might want to promptly unplug the car after the charging is completed to prevent the 12-V battery from draining.

 

I don't think the BEV mode vs. the HEV mode makes any difference at all in charging from what I've seen.

It is true that the 12-V battery charges rarely during driving—it doesn't matter how long the trip is. There is a brief period of 14.10 V at the beginning, which then very infrequently kicks in. You can sometimes see a 13.53 V of charging, and @Mr.Vanvandenburg was correct that the car charges at 13.53 V (not at 14.10 V) when the gear is in park.

After the traction-battery AC charging is finished, there is a slow but continuous drain on the 12-V battery as long as the car is plugged in. It appears that a 12-V subsystem that monitors the plug is in continuous operation when the car is plugged in. Therefore, it is important not leave the car plugged in. You could easily deplete the 12-V battery in a week if it already has a low SOC.

The advantage of an AGM battery is that it charges five times as fast as a flooded-cell battery (it has 1/5th the internal resistance). That somewhat overcomes the issue of infrequent charging when the car is being driven. So far, mine has been hovering near 100+% SOC (12.89–13.00 V open-circuit, rested), and we will see if it will stay there or I will start having the low-SOC issues you are having.

 

It's the same flooded-cell battery on all Gen 4 and Gen 5 Prius and Prius Prime trims (with the Gen 5 trims having a central-vent lint for trunk installation). There is no AGM OEM battery in Gen 4/Gen 5. (Gen 2 and Gen 3 OEM batteries are all AGM.)

So far, what I am seeing is that the car virtually never charges the 12-V battery regardless of how long the trip is. I guess it charges it once every 20 driving hours as the new-car features manual says. LOL We will see how low my SOC will go in three weeks from now—perhaps since it's an AGM battery, it will manage to stay charged with minimal charging activity.

 

This is the rested-battery open-circuit voltage for the last seven days:

day voltage

18 13.05 V
19 12.98 V
20 12.95 V
21 12.92 V
22 12.90 V
23 12.87 V
24 12.87 V

AGM batteries are supposed have 100% SOC roughly at 12.89 V; so, perhaps mine has settled around 98% SOC.

So, it seems to charge well. There is significant drain when the car is off, which is much higher if the car is plugged in, and the latter drain is also continuous.

Yesterday, the car hardly ever charged when being driven. Today, it almost always charged. Go figure.

In any case, if you have an out-of-warranty replacement, an AGM battery is a no-brainer.