Does actual capacity of traction battery decrease linearly with age?

Dxta · May 27, 2019

David Beale said: ↑

A quick response to whether storing a new car "in the hot Florida sun" would damage the traction battery is NO. The battery -might- reach 20F above ambient, with no gas pressure inside. When in operation it can run hotter than that and with gas pressure. Of course driving it with gusto off the lot when it's that hot -can- cause problems. Keep in mind, these cars are operated in very hot places such as Texas, Arizona, and Nevada. How hot do you think the traction battery gets when it's 120F outside and you are hammering up the mountain pass between Arizona and California?

The 12V battery, if not recharged at least once every few days (by the car running or by a "battery tender" of some sort), will sulphate when it discharges too much. Discharge can be from two sources, the car (parasitic losses keeping the computers in standby), and internal self-discharge losses. If not taken care of the 12V battery can be expected to fail after 3-5 years. The main failure mode of the 12V battery is caused by loss of electrolyte, because you cannot add water (it's a "maintenance impossible" battery). IF you peal off the label, add distilled water, and charge it up with a proper charging technique you can get 6-7 years of life from it. I'm cheap, and I do this. Many can't be bothered and replace it after 3-4 years.
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Problem is sealing the modules after refilling with distilled water.
How do you do that?

Mendel Leisk · May 27, 2019

Put some fresh tape on?

Dxta · May 27, 2019

What sort of tape you're talking about?

Mendel Leisk · May 27, 2019

What sort of battery are you talking about; that could be causing confusion? I meant the 12 volt (and any sort of tape, say electrical), maybe you're talking about the big battery?

Dxta · May 27, 2019

Mendel Leisk said: ↑

What sort of battery are you talking about; that could be causing confusion? I meant the 12 volt (and any sort of tape, say electrical), maybe you're talking about the big battery?
Click to expand...

I'm talking about the traction battery.
If you go through the comments of Bill, at the beginning of the thread, you'd see where he talked about his experiment with a traction module, where he added distilled water, and the cells came back to life.
The problems a lot of people had with topping up the modules, were how to seal the openings back.

Dxta · May 27, 2019

Mendel Leisk said: ↑

What sort of battery are you talking about; that could be causing confusion? I meant the 12 volt (and any sort of tape, say electrical), maybe you're talking about the big battery?
Click to expand...

bwilson4web said: ↑

The ordinary Prius use Nickel Metal Hydride (NiMH) chemistry for the main traction battery. The Plug-In Prius uses a massive LiON pack shown in the Toyota Dismantling Manual. My interest has been with the NiMH batteries.

The NiMH battery works by converting between two nickel hydroxides and storing hydrogen atoms in the metal hydride:

Ni(OH){2} <=> NiO(OH) + H{2}O + e-

H{2}O + M + e- <=> OH- + MH

What ages our NiMH batteries is the loss of H{2} and O{2} from the electrolyte.

During normal charging, a small amount of water electrolyzes into H{2} and O{2} gas. Sealed, the metal hydride can recombine the H{2} and O{2} into water, a good thing. But the weak seals of the earliest 2001-03 battery modules could leak a little of the gas as they got hot and the gas pressure built up. Charging is the risk because it generates heat and gas pressure against the terminal seals:

The left photo shows rubber "O" ring is partially melted on the right side, the black smear. You can also see the nickel compound has changed color from the normal silvery color of the other electrode.

Now as the H{2} and O{2} leak out, the electrolyte, a saturated solution of KOH, dries. This gradually reduces the conductive area in the plastic mesh separator, increasing the resistive heating to a smaller and smaller area. When the heat becomes hot enough to melt the separator, it forms an internal short and that cell is failed:

But the melting doesn't happen instantly. First the reduced electrolyte increases the internal resistance of the cell. This reduces the capacity, the Ahr, and charge-discharge efficiency. So the obvious solution is to add water.

There is an excellent Toyota-Panasonic patent on refreshing the electrolyte and some of us have tried different approaches:

I have successfully returned full capacity to worn, battery modules by adding water and cycle charge-discharge . . . replicating the Toyota-Panasonic patent. But I sealing the modules turned out to be a problem.

I tested stainless steel inserts and epoxy:

The stainless steel nut plates even with heat did not form a gas-tight seal. Epoxy would not adhere to the plastic and the different thermal expansion rates would not be reliable either. I tried plastic welding like the Toyota-Panasonic patent only to realize we would have to use salvage case material and a mold fitting and still it would be marginal. But I could never figure out a solution for melted or weak "O" rings. So how does this relate to Prius efficiency?

Thanks to Patrick Wong, the early traction battery capacity loss occurs in the first 5-6 months:

So unless someone is buying a new Prius or replaced the traction battery with a new one, this short interval really doesn't come into play. Most of the time, the Prius battery is only doing small, fractional charge-discharge cycles UNLESS the driver chooses to put a heavier load on the battery.

Remember, charging is the risk:

Here I am doing a forced charge recording the pack temperature. It generates:

heat - that weakens the "O" ring, terminal seal

gas - increases the pressure to leak it out

The best path to a long battery life is to monitor the energy flow and AVOID battery discharge. So when climbing a hill, back-off when the energy flow shows a discharge. The same is true for acceleration, avoid drawing charge from the battery. If not discharged, the car won't have to charge the traction battery and heat it up. But sometime, we have to descend a large hill, 500 ft or so. In those cases, use "B" so the engine will absorb the excess energy that would over-charge and heat the traction battery. But this is just background information about our NiMH batteries: Prius Battery Photos

The Prius shop manual describes a way to force-charge the traction battery to maximum, 80% SOC. Then shifting the car into "R" and holding a accelerator down, the battery is discharged to the minimum. This technique is used to identify weak modules or a marginal traction battery. However, it can also provide a rough, battery Ahr measurement. It also 'stresses' the traction battery which a seller may object.

Personally, try the car and use the original battery to learn how to drive it. IF the battery fails, either buy a quality rebuild or new Toyota replacement. Then use your 'lessons learned' to let that battery last until 'the wheels fall off.'

Bob Wilson
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Thais is in reference to the quotes above by Bob.
So, @Mendel Leisk . tapes or electrical tape can never work.

Mendel Leisk · May 27, 2019

Apologies, I'm not following this thread closely. I saw this:

And jumped to conclusion your question was regarding the 12 volt.

bisco · May 27, 2019

same. actually was more of a step than jump

Dxta · May 28, 2019

bisco said: ↑

same. actually was more of a step than jump
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Yeah, .y mistake too, Mendel! So sorry.
A whole lot of mix up here then.

Dxta · May 28, 2019

bisco said: ↑

same. actually was more of a step than jump
Click to expand...

Yeah, my mistake Mendel, a lot of mix up on my part.

Does actual capacity of traction battery decrease linearly with age?

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