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Why Do Hybrid Batteries Fail?

Hybrid batteries are built to last a long time, but they don’t last forever and will ultimately shut down and stop working.  The number of miles and years of hybrid ownership are part of the equation, but the ultimate reason is a failed cell in one of the modules that make up your hybrid battery pack.  Depending on your make, model and year, your hybrid battery pack is made-up of somewhere between 20-40 battery modules and each module contains 6 separate battery cells. Each NiMH (nickel-metal hydride) cell has a voltage of 1.2 Volts.

For example, the hybrid battery for a Toyota Prius has 28 modules.  Each sealed module contains 6 numbered NiMH cells that are connected in a series.  When battery cells are connected in series their capacity remains the same, but their voltages add together.  Therefore, 6 cells times 1.2V gives you a 7.2 Volt 6.5 Ah (Amp-Hour)hybrid battery module. 6.5 Ah is the amount of power that the battery module can hold when it’s new.  The Toyota Prius has 28 modules connected together in a series, so 7.2V per module times 28 modules gives you a 201.6 Volt 6.5 Ah battery pack.

Heat plays an important role in the expected lifespan of any hybrid battery. When your hybrid systems charges and discharges the battery, heat is generated and we’re talking about a lot of heat.  To dissipate this heat, hybrid manufacturers designed special cooling systems with intake fans to draw cooler air from the cabin of your hybrid. These intakes are usually positioned on the passenger side of your hybrid next to the back seat.  The cooler cabin air flows over the top of your battery casing and through specially designed areas between the battery modules.  The intake air then exits through the bottom of the battery case and out of your car via hybrid specific air ducts.

Unfortunately, hybrid vehicle manufacturers have yet to develop a cooling system that cools the entire battery evenly.  The battery modules are stacked and connected in such a way that the ends of each module stay cooler than the modules in the middle.  Each cell of each module of every battery has an optimal temperature and their performance level slows down when they are heated or cooled outside of that optimal temperature.  This takes a real toll on center modules and cells over time and results in them being weaker and losing more of their original capacity that the rest of the battery pack.

This capacity imbalance will ultimately result in the failure of your hybrid battery pack.  Computer systems in hybrid vehicles are designed to monitor the voltage in each group of modules as they charge and discharge.  The center modules and cells will indicate full and empty very quickly because their capacity is so low, but the better cooled and higher capacity outer modules aren’t doing much in the way of helping.  The heavy loading on the weakest modules and cells eventually wears them down to having almost no cycling range left at all.  This is where rapid charging and discharging is most noticeable because the whole battery isn’t fully charging and discharging, it’s just the weakest modules and cell.

It’s the perfect scenario for hybrid battery failure and all you have to do is leave your hybrid parked overnight with a low battery charge.  The weakest cell in the weakest module will self-discharge enough in a short period of time to cause polarity reversal.  Polarity reversal will ruin that cell permanently and because the cell is in a connected and sealed module, it renders the entire module as bad.  Using the Toyota Prius as an example, it was designed to operate with six 1.2V cells per module for a total of 7.2 Volts.  The failed cell leaves the module with five 1.2V cells and total of 6.0 Volts.  The Toyota Prius computer system prevents battery use if the voltage difference between the highest and lowest cell in any battery module exceeds 1.2 Volts and in most cases, the entire battery will not work.

When this happens, you should receive a low block voltage trouble code of some type and your intake cooling fan may run on high all the time trying to keep your battery cool in it’s malfunctioning state.

If you’re concerned about the environment, gas mileage, performance and extending the life of your hybrid vehicle than maximizing your batteries potential should be at the top of your list.  We’ve been earning our customers trust and business daily for over a decade and if you’re looking for a top quality battery,  that you can depend on at an affordable price than you’ve come to the right place.

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