Why the Yaris IV kills its 12V battery

Tl;dr: too small battery, a charging algorithm that never fills it, and low quality OEM batteries that lose capacity quickly. Each would be manageable on its own. Together they made 12V failures one of the best-known Yaris IV problems.

Based on my posts on Toyota Owners Club, 6 Jun 2024 – 7 Nov 2026.

Three causes

Small battery capacity (35Ah)
A poorly chosen charging algorithm (not charging fully, resulting in about 20-25Ah of useful capacity)
Poor quality OEM batteries, which lose capacity, halving what’s remaining with every ignition failure (ie. the state when you need to charge or boost the battery because its voltage fell too low)

So after the first failure (because you left the car for eg. 3 weeks) you end up with an effective capacity of 15Ah. A second failure and you have 7Ah.

None of the above is that dramatic if you have a proper base capacity. It’s just harder to kill the battery: you need more time (a month or so), and how many people leave their car for that long? Those who do accept the possibility of it failing to start and introduce prevention, like cutting off the battery, or putting on solar or trickle chargers etc.

The car never charges to 100%

The DC/DC in the Yaris is NOT charging the battery to 100%. Sometimes after a ride the voltage drops, meaning that more current was taken from the battery than was put back into it. This is by design, probably to save some fuel on WLTP test because I don’t see any other reason not to charge the battery at 14.4V all the time (maybe except conditions like rapid acceleration, where you need ALL the power at the wheels).

You can charge the battery to 12.7V, drive the car for 100 miles, let it rest, and after 30 minutes you will read 12.5V. Meaning that this drive effectively discharged your battery.

Long term voltage monitoring with my BM6 showed that the maximum charging voltage is somewhere around 14.2V, with drops to 12.75V. There were many reports of the charging voltage going down during a drive. This means that the battery is not charged with the maximum possible current all the time.

The Yaris will try not to exceed 80% SoC, and that’s wrong, because the lead-acid battery needs a full charge from time to time to regain its capacity, reduce potential sulfation and build up the electrolyte density. You can’t do that with 12.8V (which is applied instead of 14.3V when the car decides that the battery is full enough). The car tries to keep the battery in the 60-80% charge range. That’s not enough to regenerate the battery chemistry (dissolve the sulphate crystals from the plates).

Charging often stop after only a few minutes at the start of a drive, even when the battery starts at 50–60% SoC. I can see this really happen kinda randomly. Regardless of external temperature, initial voltage or the current. The car will never charge the 12V battery to anything more than 12.8V in the long term. Even if I charge mine to 13V, it drops the voltage to <12.8V after a few rides.

What decides when it charges

We did the reverse engineering by measuring voltages and the behaviour of the DC/DC converter responsible for charging. And there is nothing indicating that the car measures the state of charge according to a programmed capacity. The only thing it relies on is the voltage. And it will work perfectly fine with any (!) battery capacity, because it limits the charge rate (this is documented in the user manual) and controls the voltage. The decision on charging or not charging the battery is solely based on the initial battery voltage ( show the threshold: from about 12.31V up the car decided not to charge, from 12.21V down it charged).

IMO the reason is that they think, based on the rather high voltage of the battery before the start of the journey, that the battery is indeed full and there is no need to charge it further (which from a chemistry point of view is kinda bs, because the battery won’t suffer from this). They just replenish what was lost on startup (and even that not fully), and there goes your charging.

Time parked seems to matter too. There were reports that the battery voltage is increased (by the DC/DC converter, which is responsible for the charging) to a greater value (meaning that the charging current is higher) after the car is started when it was left turned off for a longer period (more than 24h), when compared to a shorter period of being left unused. Meaning that the smart charge algorithm may take into consideration the time when it was left parked. This happens despite the fact that the initial battery voltage was higher after 24h+ parking, which means that this is not directly dependent on the battery voltage, at least not only.

One more quirk: the maximum charging voltage is present in P and R gears only. You don’t have to actually drive. That’s something counter-intuitive, yet it’s a fact. [P]arked and Ready is better for charging the battery. And 30 minutes should be enough.

Hypothesis (not tested yet): I think that one of the triggers to lower the voltage is a low current measured on the negative battery terminal by the sensor, while the voltage is set high by the DC/DC converter. It would make sense to assume that if we are pushing 14.2V into the battery and the resulting energy flow to the battery drops to, let’s say, 1 amp (so 20% of the standard charging current, according to the user manual), then we can assume that the battery is already charged to 80%, so we can drop the voltage to stop charging completely. If that’s true, then a higher capacity battery will regain much more energy (potentially even reaching a higher SoC than a 35Ah battery), because the moment when the charging current drops to 1 amp will happen later. If anyone has a clamp ammeter and wants to monitor both the charging current and the voltage (to detect when it drops and whether it’s correlated with the current), feel free to take part in this experiment.

Why older cars didn’t have this problem

We don’t see batteries failing in older (15+ year old) cars. In newer cars the problem with failing 12V batteries is also present; being active on Ford’s forum, I’ve seen a reported issue with a 3-year-old Ford Focus where the battery failed to start the car after a few days, because Ford used the same stupid pseudo-eco-friendly charging algorithm, keeping the voltage low instead of pumping in as much current as possible, which is crucial for survival on short trips. And the battery there was 50-something Ah IIRC.

A small capacity battery would have been sufficient in most cases if the charging kept a constant 14.5V all the time, pushing in as much power as possible (of course limited to the max values stated in the user manual, ie. 15 amps (fast charging) and 5 amps (regular charging)). This is a self-regulating mechanism, because with the battery being charged using the constant voltage method, the current drops as the SoC rises.

And, fun fact, when the 12V BMS was introduced with the Ford Focus Mk3, people reported the lifespan of their OEM batteries dropping to 3-4 years. They incapacitated the smart system by pulling the responsible fuse, which caused the voltage regulator to fall back to regular 14.4V charging, and boom, magic, suddenly the lifespan of batteries was reaching 5-6 years. We are struggling with the same situation, but without the possibility to do anything. Mutlu would perform much better if only Toyota didn’t ignore good engineering practice to chase after dropping 2 grams of CO2 emissions instead.

For some reason the charging algorithm in the Yaris doesn’t always use the maximum voltage, dropping it to 12.7V. Could you imagine what the charging current is then? How long would it take to not only charge it but restore the correct electrolyte density? In no way can you restore what’s lost during parking. And because of that, to keep the battery in good shape, regardless of the manufacturer, in a Toyota Yaris IV you need to charge it manually from time to time. Otherwise it will degrade over time and fail sooner than it should.

Three reasonable decisions, one bad result

A few years ago, when production of the Yaris IV started, there were very few options to get an LN0 size battery. Today you can even get AGM. At the same time LN1 was an established and quite popular standard for the “smallest” battery, and even the naming tells you that they didn’t expect anything smaller than LN1. So yeah, that’s a valid question: how much did Toyota save with LN0? I’d say they tried to lower the weight to maximise the MPG in WLTP tests. Same story with the charging algorithm, which was the final nail in the coffin for 12V battery reliability, because I’m quite sure that these batteries would do OK if only they were charged to full every time.

I have a feeling that this was caused by the decisions of 3 independent design teams.

The first team, responsible for charging and energy management systems, was ordered to optimise the charging process so it gets better results in WLTP. And they used the solution of this 60-80% SoC target.

The second was responsible for deciding on the capacity of the battery for the new Yaris. And after they did some calculations, they decided that 35Ah would be fine.

And the third team analysed the costs to profit and dropped the battery tech from AGM to a regular flooded lead acid battery.

By their powers combined… they resulted in failure, and it’s not a single team’s fault. All these decisions made sense. They just didn’t work together.

Same thing with the gauges software devs and the multimedia software devs. GPS sync for the clock settings used to live in the multimedia system. With the 9″ screen they dropped it, but didn’t reintroduce it in the gauges menu. Why?

What you can do about it

The charging algorithm is faulty and there is nothing we can do about that. We can swap to a larger capacity battery, but this only slows down the process of battery degradation (it buys us time though, so it’s still worth doing IMO). So doing a full charge once in a while allows the battery to regain its full capacity.

That’s why I suggest doing the full charge cycle with any battery charger at least twice a year, preferably 4 times (every 3 months). If it has a recondition/desulphation mode, it may be used too, especially if we are low mileage users (the 12V battery should be disconnected if you want to be on the safe side, as the voltage in this mode often reaches over 16V).