HV battery drains to 1 bar quickly on cold start; some thoughts and questions

My HV battery is obviously old (250,000km), and has a lower capacity. It isn't out of balance, however, and often has a voltage delta of 0.08 volts between the blocks, which I think is quite good.

My commute involves a very steep driveway, which uses a lot of power, followed by a fairly flat road, then a big downhill stretch. My battery was going down to 1 bar if I didn't have 'car sympathy' (actually battery sympathy), and just drove it after letting the engine warm up for maybe 5 seconds (given the engine doesn't get used initially, anyway). I looked at the live data, and it is quite interesting. The SoC can be 58%, but drops to 40% quickly. It then starts charging, but, despite charging, the actual SoC can drop to 28% even with charging occurring.

This is a sign of a dying HV battery but I do believe it has a lot of life left in it. What I have found is that if I wait for 1 minute, until the first warm up cycle occurs, then the battery isn't stressed at all. This is what I am now doing and the rapid discharge problem has completely gone away. So, if people are getting this problem (because the car runs everything off the HV battery even with the engine running for the first minute), consider doing a stationary warm-up for a minute first.

I would be grateful for any thoughts or if anyone else has come across this and this 'fix'?. Also, I would be keen to hear thoughts on an experiment I did; I drained the battery to a 16% SoC, then disconnected the 12v, so it reset to 60%, then drained it to 12% SoC. I ensured that (with slow discharge, car in neutral to prevent charging), no modules ever went down below 14 volts (e.g. when under a low load). I also charged it downhill to 80%, pulled the 12v, so it went to 60%, then charged it again. Then I did the drain cycle again. This was a few days ago... And now, this morning, I tried to get it to 'one bar' while recording the screen on my phone, but the battery performance has improved, so that it only got to 38% SoC even though I deliberately tried to make it drain down. So, it seems that an unorthodox charge cycle might have improved capacity? (I don't recommend this as a usual procedure of course!).

 

I never use the air con during start up, and actually have to be mindful to run it 15 mins per month to maintain it. At the moment I commute to work with the windows open, 9 degrees centigrade! I always have the rear window open to allow cold air to blow on the battery vent as needed

I will actually look into a prolong charger, especially as I have two other Prius batteries that also need some TLC, and given the improvement that occurred after my unorthodox deep charge / discharge!

I don’t need to do forced charge - it isn’t that bad yet - but will do a 1 min warm up. I took phone screen recordings of commutes where I just drive, and where I wait, with loads of parameters up, to show the difference... in short a 1 minute warm up dramatically dramatically reduces daily strain on the HV battery. Especiallly in my case, where I lose 2% charge on my Leaf simply getting up the driveway, which is steep enough that most small vans such as a Toyota Hiace have real trouble getting up even in dry conditions!

 

That is what I was thinking; basically, many people don't warm up their car before driving; the Prius protects the engine by using the battery, but it puts strain on the battery; another thing is big hill climbs; this can draw huge amps from the battery. My battery is worn, has delta 0.08-0.12 normally, but the delta went to 0.70 when I experimented with flooring it up a large mountain - this was an instant change showing a cell (or more) is weak.

I was also thinking it is quite possible that, if someone had a marginal battery, it could be that warming the car up before driving off, avoiding driving electric-only, and not using full throttle (besides emergencies) - could allow a failing battery to last longer before error codes keep getting thrown. This could buy time to save up for a proper new battery.