How to drive uphill? Prius battery drains quickly

Rent a different car for the Alps trip. You have a flat land car now- enjoy it for what it is. When the cost of rentals feels expensive, it's time to get a new battery instead.

B mode is a very good way to descend mountains. It places less stress on the battery and dissipates more energy as noise from the engine. The car can sustain that all day long. And of course it allows you to keep the service brakes from overheating.

 

I think it is just old HV battery. I had similar experience with my 2005 Prius. I could though manage to get another 2 years out of HV battery without codes. You could use Prolong system for extending battery life some. ( I did not use it. So I do not have any personal experience with that.)

What I did on similar trip later was to turn AC OFF on climbs and Turn it ON and FULL COLD on downhills and use B mode once the battery is full.. That way you can somewhat try to balance out battery usage. If you know you are going to have major uphills coming up, you could try to force charge the battery just before climbing..

 

stopping while hot is bad, unless it's over heating. it's better to drive faster with air flow than crawling (30khr is freaking slow!) and you're generating a lot of heat with b very little air flow.
user the HVAC so that the battery intakes cold air.

 

At the bottom of everything, you have a 57 kilowatt (76 horsepower) car. The battery can contribute some additional power for bursts of acceleration, short climbs, and so on, but that capacity is finite. On a longer climb, if you are climbing at a speed that physically requires more than 57 kW, the battery charge will drop, and when it drops to 40% actual (i.e. as shown by an OBD-II reader, not the dash bar graph), the car will stop using it, and you will complete the climb with only the 57 kW from the engine, which will limit you to whatever speed 57 kW can give you on that grade.

I've never driven in the Alps; my closest relevant experience is on Afton Mountain in Virginia, when I was driving my Gen 1 (which had a slightly less powerful version of the same engine, 52 kW).

When driven by me, the car could always complete that climb without any noticeable change of behavior. One time, I let my sister drive, and she took the same ascent at around 130 km/h. I was sitting in the passenger seat watching the battery SoC on the ScanGauge steadily tick downward, and it was just above 40% when we reached our exit. Another km and my sister would have found the car suddenly slowing and unable to climb faster than a 52 kW rate.

By the way, when the engine is at maximum output, it will sound loud, but it is in no danger. The engine control module knows its maximum output and its maximum safe rotation rate and will not exceed those. If you find the speed at which you can climb an Alp using only 57 kW, you can maintain that rate of climb all day, with the only concern being to watch the engine temperature, and back off or take a break if its cooling isn't adequate. You will at that rate be putting little net demand on the battery (and on the descent side, B mode and judicious use of the friction brakes should protect the battery from heavy charge rates).

In any non-plugin Prius, the battery is only there to buffer short-term peaks and troughs in power demand. For any longer known situation like a sustained climb, you'll have good results if you choose a speed where the engine is supplying substantially all of the power needed, and the battery is more or less resting at its preferred 60% state of charge.

 

I got one of these packs and it was a game changer: Toyota hybrid battery upgrade pack; NexPower Energy Felt like I was driving a brand new vehicle first time I did mountain climbing with it.

Also getting an obd2 reader and using a phone app like OBD Auto Doctor app so you can monitor the RPM and keep it at around 3500, which is where the most engine power / efficiency exists.

Here's buyers guide for an OBD2 device: Hybrid battery diagnostic and repair tool for Toyota and Lexus