Why does my hybrid 12v battery keep going flat?
My wife and I have had several Toyota hybrids since 2011. Our current car is a Yaris Cross which we got in Dec. 2023. It has done almost 18000 miles. Just over a week ago I got a message form the Toyota app on my phone. The 12v battery was low. The car started and I took it for a thirty minute run. During the following week we used the car about three times, but ten days after the warning on the app it refused to start. I was advised by Toyota that the car has a 35amp battery as opposed to a 45amp and if the car is not used for 2 or 3 days the battery will go flat because of current being used by the security systems etc. A recovery technician got the car started and on his advice I took it for a run. It seems that I will have to take the car to our Toyota dealership for the battery to be put on charge for 4 hours. It will then be decided whether the battery will be replaced under warranty. Do you know if Toyota have changed the 12v batteries in the Yaris Cross for 35 amp instead of 45 amp. I find the situation quite puzzling and fairly ridiculous that the car can't be left for 3 days without the battery going flat.
Asked on 18 March 2026 by Chris Harper
Answered by
David Ross
The situation you're describing with your Toyota Yaris Cross is, unfortunately, a very well-documented issue that has become a major talking point with Honest John readers. Did Toyota change the battery size? Well yes and no. It isn't that they downgraded your car specifically, it's that the Yaris Cross (and the standard Yaris Hybrid) was designed from the factory with a 35Ah (Amp-hour) battery. Larger models like the Corolla or the older hybrids you likely owned typically used a 45Ah or larger unit. Because a hybrid doesn't need a massive burst of power to crank a starter motor (the high-voltage battery does the heavy lifting), Toyota opted for a smaller, lighter 12v battery to save weight and space.
The reason it goes flat in 3 days is that modern Toyotas are always on. Even when parked, the car is running with things like data communication modules (DCM) which talks to your MyT app, tracking location and status plus keyless entry and security systems.
A 35Ah battery has very little reserve capacity. If the battery isn't at 100% charge when you park, these background drains can pull the voltage below the critical level (roughly 12.1V) in just a few days. The Catch-22 of hybrid charging is that driving the car for 30 minutes is often not enough. Unlike a traditional car with a high-output alternator, a hybrid charges the 12v battery via a DC-DC converter from the big hybrid pack. This happens at a lower, steady rate. Toyota officially recommends leaving the car in "Ready" mode for at least 60 minutes a week just to maintain the 12v battery.
Toyota dealers are currently required to follow a specific health check protocol. If the diagnostic shows the battery is defective (won't hold a charge), they will replace it under warranty. If it shows the battery is healthy but discharged, they will charge it and tell you to "drive it more."
The good news is that because of the sheer volume of complaints, many owners in 2025 and 2026 have successfully lobbied dealers to fit a 45Ah battery upgrade under warranty as a "goodwill gesture." It is a direct fit and provides that extra 25% buffer you need to leave the car for a week. When you go to the dealership, don't just ask for a recharge. Explicitly mention that you are aware of the "35Ah vs 45Ah" issue and that the car is not fit for purpose if it cannot be left for three days. Ask for the upgraded battery that will remedy these problems.
The reason it goes flat in 3 days is that modern Toyotas are always on. Even when parked, the car is running with things like data communication modules (DCM) which talks to your MyT app, tracking location and status plus keyless entry and security systems.
A 35Ah battery has very little reserve capacity. If the battery isn't at 100% charge when you park, these background drains can pull the voltage below the critical level (roughly 12.1V) in just a few days. The Catch-22 of hybrid charging is that driving the car for 30 minutes is often not enough. Unlike a traditional car with a high-output alternator, a hybrid charges the 12v battery via a DC-DC converter from the big hybrid pack. This happens at a lower, steady rate. Toyota officially recommends leaving the car in "Ready" mode for at least 60 minutes a week just to maintain the 12v battery.
Toyota dealers are currently required to follow a specific health check protocol. If the diagnostic shows the battery is defective (won't hold a charge), they will replace it under warranty. If it shows the battery is healthy but discharged, they will charge it and tell you to "drive it more."
The good news is that because of the sheer volume of complaints, many owners in 2025 and 2026 have successfully lobbied dealers to fit a 45Ah battery upgrade under warranty as a "goodwill gesture." It is a direct fit and provides that extra 25% buffer you need to leave the car for a week. When you go to the dealership, don't just ask for a recharge. Explicitly mention that you are aware of the "35Ah vs 45Ah" issue and that the car is not fit for purpose if it cannot be left for three days. Ask for the upgraded battery that will remedy these problems.
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