[JSH]

Originally Posted by Isaac Zachary

Of course it also comes down to longevity.

Part of starting this thread is the assumption that an HV battery is going to quit at about 15 years. I think you, JSH, commented that the average may be more like 18 or more years. My insignificant experience was I bought a Prius that had lost it's battery at 213K miles and 14 years.

You bought a vehicle with more miles than 98.5% of the vehicle sold in 2021. That is what I keep coming back to. A vehicle that old or with that many miles has reached useful life. Actually past useful life as defined by regulations and industry (150,000 miles). It has almost no remaining value and anymore years of use is a bonus. Manufacturers could make light-duty vehicles with a useful life of 450,000 miles like a commercial Class 8 truck but none of the stake-holders have any reason to do so and cars would be much more expensive.

1. Manufacturers care about their customers - the original owner. They also have no financial incentive to make cars that last decades past the original owner. They also have no incentive to make cars that last longer than customers really want to drive them. Sure some people are happy driving an air-cooled bug but most drivers want modern features.
2. The initial buyer has no financial incentive to pay more for a car so that the 3,4, or 5th owner can drive more miles on the cheap.
3. Regulators have no incentive to keep cars on the roads for decades. Regulators want to refresh the fleet periodically to get cleaner, more fuel efficient, and safer cars on the road.

Originally Posted by Isaac Zachary

If there were another part of any other vehicle, say a CVT or 9 speed transmission, that was known to fail at around 200K miles or 15 years then the vehicle would probably lose value towards that point, especially if it were dead expensive to repair to the point of being destined to the junk yard only.

There are lots of other expensive components on old cars that are known to fail at 15 years / 200K miles. That is why old cars have depreciated by 90 - 95%.

I sold my 2003 Jetta Wagon TDI when it was 10 years old with 245K miles for $3,000. Nothing wrong with it at the time but that was only 10.5% of the original purchase price. If it had been a Jetta with the gas engine and an automatic it would have been worth about $1500 with those miles.

Originally Posted by Isaac Zachary

Something else that hasn't been brought up is what can a person do to make their batteries last on an old hybrid? With an engine you can change the oil often and drive it moderately, for an example. You can do the same with a gear transmission (I'm not sure what makes CVT's fail).

You can change the fluid in a transmission all you want but you aren't going to replace the friction material in the clutches (a 10 speed has 6) If you are talking manual transmissions well those are about as rare as unicorns today. Manuals are down to 1% of new vehicle sales.

Originally Posted by Isaac Zachary

But what about a battery? I've heard of people drilling into their Prius batteries and topping off the electrolyte with distilled water or some special chemical solution and then sealing those holes back up. There are also people who put their hybrid batteries through a deep cycle. But the thing is that batteries are made to be maintenance free.

The biggest enemies of battery life are cycles, heat and time.

With cycles you want to keep them as small as possible. For example cycling a battery to 10% you can get 15,000 cycles mile cycling a battery 100% will drop that to 600 cycles. (For a LiPO4 battery) This is why the 2nd Gen Prius only allowed 40% of battery capacity to be used. You also want to stay away from the bottom of the discharge curve and the top - you want to use the capacity in the middle not the extremes.

This is why the EV button was mentioned as a battery killer - EV miles on a hybrid are much harder on the battery than hybrid miles. Want to kill a PHEV battery fast - cycle the battery fully twice every day. With an EV about the worst thing you can do to the battery is exactly what the cannonballers recommend to make time quickly an EV - only use the bottom 50 to 60% of the battery capacity and quick charge it every stop. For long EV battery life you want to use the middle capacity and slow charge every night.

Originally Posted by Isaac Zachary

What about better battery cooling? Some have tricked their battery cooling fan into staying on full time. Would setting the battery in a bath of mineral oil and pumping that through a radiator be worth it? Or adding a duct of some sort from the A/C unit to the battery cooling duct?

As I mentioned - heat is the enemy of a battery so cooling is important. Some early hybrids took this to the extreme. The early Ford Escape hybrid only allowed the battery to use the 40 - 53% SOC and had an A/C circuit for the battery. It had rock-solid hybrid battery life. Later models ditched the A/C circuit and relied on a fan to pull cabin air through the battery. Not as much air flow and most owners never changed the battery air filter so it clogged up and blocked off airflow. 2nd Gen Prius batteries also relied on fans and cabin air to cool them. They were also know to have clogged vents. Don't let the vent get clogged.

Of course ambient temps matter too. Don't let the car sit outside in Arizona in an uncovered asphalt parking lot so the car heat soaks to 140 / 150 degrees all day. Park in the shade, in a parking garage, in a garage at home. If you have to park outside vent the car and use sunshades. Some hybrids have powered sunroofs to keep the car interior cooler. Once you are in the car and driving use the A/C instead of rolling down the windows to lower the interior temperature and push cooler air through the battery.