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Excessive Charging Voltage Kills 12V Battery On Prius 3?

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A year ago the 12V Battery needed replacement on my Prius 3 after just 2 years' service. It showed 10V and that caused starting problems. The Battery was replaced under warranty. When I began to get the same starting problems just before my warranty ran out at 3 years I asked my garage to check the charging arrangements and, at the same time, fitted a Scangauge. The garage came back with a clean bill of health. The Scangauge has told a different story!

One of the built in gauge options is Battery voltage and this can be monitored when the car is running and for just a few moments before and after power is on.

When the power is first turned on the voltage is around 12V and this increases to 14.4V as soon as the inverter kicks in after a second or two. After power off the voltage is around 12.8V and this drops rapidly to about 12.2V over the 15 to 30 seconds before the Scangauge powers down.

The more interesting and revealing part is during running!

As I said above once the power button is pressed and the inverter kicks in the Battery voltage jumps up to around 14.4V. It stays at that level for a mile or two and then, if lights are off, falls back to around 13.5V where it stays for the remainder of the journey. This is exactly what should be expected - the initial 14.4V is usually needed to pump some energy back into the 12V Battery and the drop to 13.5V denotes a fall back to trickle charging. Obviously the time taken before the drop to 13.5V will reflect the stage of charge of the 12V Battery.


My researches on the web into the requirements for Lead-acid Battery charging have led me to the following conclusion:

It is recommended practice for lead-acid batteries that a charging voltage above 14V should only be maintained for a short period. At voltages above 14.34V, lead-acid batteries begin to gas producing hydrogen gas by electrolysis of the water in the electrolyte. Danger is avoided by the escape of the explosive gas through the vent tube and the venting arrangements in the boot. In many batteries the resulting loss of water by this electrolysis can be rectified by topping up with distilled water but this is not possible with the sealed type of Battery as used in the Prius. Thus, over a period, the level of the electrolyte will fall and this will lead to reduced capacity and premature failure.

I shared this information with my garage who agree with the sentiments but have said that, although they do not fully understand, the vehicle is within specification.

I have also ascertained that on the Series 2 vehicle the charging voltage falls to the safe level of around 13.5V irrespective of the load that is applied.

I therefore raised the problem with Toyota Customer Services and specifically asked them why, on the Series 3 but not on the Series 2, they ignore the advice that batteries should not be charged for long periods at voltages above 14.34V, together with all the information in this post.

The only response that I get is that the vehicle is within specification and that the reason for applying what I regard as excessive charging voltages is not available to the public domain.

As far as I can see, the only long term solution to this is to replace the Battery with one that can be topped up to replace the water that is lost.

Does anyone else have this problem? Does my analysis sound right? I wonder if Toyota increased the voltage on the Series 3 to help avoid the "flat if left for a fortnight" syndrome that occurred on earlier versions?

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Would fiting A GEL type Battery help in the problem.

Dont know anything about GEL batteries, but had them on two Mondeos i have owned and no problems.

The Battery charger i have has a switch for standard or GEl. so not sure if they can be fitted to any car.

But then Fitting another Battery is another expense you should not expect to pay for.

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If one is available in the Prius size, a Silver Calcium Battery would be better able to withstand higher charging voltages.

Ford vehicles with the "Smart charge" system have to have Silver Calcium batteries to withstand the higher charging rates put out by this system.

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  • 2 weeks later...

I am interested to see that at least one other person has had this problem (though on a Gen 2 vehicle) - see


Should these topics be linked in some way?

Later edit: They have this problem in the USA as well! See:-


And the picture there of the 12V Battery shows the warning label which specifically prohibits fast charging. Isn't 14.6V fast charging? I need to check my battery's label.

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