Thanks so much for your comments flecc, billadie and d8veh. They were very useful. I think I’ve now solved the problem, but first I’d better give you the “egg-on-face” result, for which I feel a bit embarrassed. It’s revealed a couple of things that might be useful though.
First, I made a mistake about the ‘usual’ full charge being 38V. That’s a figure I remember from lead-acid days and when checking my available batteries after your responses, the not-so-old and still functioning lead acid still gives me 38V, but my second LiPo4 battery is, as you suggest it should be, 41.5V. So, I’m happy about that.
Then I STARTED to take the advice about sliding off the bottle end-caps to measure individual cell voltages, but got no further than realising the lower end cap was already departing the body of the battery! There was a gap of around 1 cm! Also the fuse, which had unscrewed to a good 0.5 cm! With it in this state I’m not surprised the power had started cutting out!
The explanation is simple and it’s that I rarely ride on anything that might be considered a smooth surface (pretty well all my outings are off-road) so vibration has done this. It was a useful lesson, kicking me into checking whether anything else had worked loose. Nothing critical had, but I’d lost a nut from a light fitting and I needed a spanner on a few other bolts. It was a useful warning.
As to the problem of the shortened charging time and loss of ‘distance run’, I should explain that I’m a spinal patient and rely almost exclusively on power from the motor. For me, e-biking is a million times better than sliced bread in that I’m still able to enjoy bike runs. With 2 batteries I’m good for about 15 to 20 miles in the Peak District (where I live) so the short ‘usual runtime’ is expected because I’m both battery driven and have some pretty big hills to get up (and without the ability to get off and push)!
You might say none of this explains the shortened time to full charge, but I’ve been doing a bit of reading and came across this article – which explains what is happening. Over the weekend I checked it out.
This is the link and the relevant bit.
Charging Lithium-Ion Batteries
Li-ion does not need to be fully charged, as is the case with lead acid, nor is it desirable to do so. In fact, it is better not to fully charge, because high voltages stresses the battery. Choosing a lower voltage threshold, or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime. Since the consumer market promotes maximum runtime, these chargers go for maximum capacity rather than extended service life.
It's worth looking at the diagram in that article.
When charging the battery on the occasion that caused me to panic, I’d not done it in one continuous charge, but because I’d arrived home late had put only around 30 min charge into it (I considered it better to do that than leave it flat(ish) for perhaps some time). When I then completed the charging later it was “full” in a very much shorter period of time (by nearly 1 hour)
So, over the weekend I’ve had a couple of runs to flatten the battery (well, it was glorious weather) then with one re-charge I did it continuously, whereas the other re-charge I did in two stages. The same thing happened again with continuous charge taking around 50 min longer than one in which I paused the charging after the first hour and then completed it 2 hours later.
It all now makes sense to me (I hope it does to you too) and my only problem is now the one of choice, to opt for preserving the battery with an interrupted charging and have less runtime, or to opt for maximum runtime and so reduce battery life.
But – big BUT – as you’ve implied that it’s easy(ish) to replace individual cells in a dying battery, then for me I’m going to opt for the maximum runtime.
Next question, where’s a recommended place to buy the individual battery cells?