HP-29C Low-battery warning, help needed please!

[AndyWalter]

I started doing some tests yesterday. I'd already swapped the Orange 47 microFd cap for a MLCC suface mount one on prongs in case that was the culprit, but it tested fine & the swap didn't solve the problem. So I've replaced it now for originality. I've already swapped the 22 microFd electrolytic cap at battery-input to psu board as electrolytics tend to have a short life (40 years so far!), and that swap will stay.

I've always thought the original Nicads were 1.2V, and on that basis the 2.441V I was measuring at battery-connection to the power-supply board meant my cells were fine. I was using Duracell 2650 mAh nimhs and they aren't that old. The way my flashing low-power warning was getting progressively worse made me think it was the calculator, not the batteries. And if I left it for a few more minutes, the display would start to break up & obviously the cpu etc was struggling. Battery pack contacts, springs etc were all clean, and I've had the batteries in & out & back in the pack, so I didn't think it was connections.

Since swapping the 2 batteries, it's charged ok (it always did seem to charge ok), but most importantly it's now holding charge overnight & not flashing at me the moment I power-on. So it's deffo the batteries, and the 2.441 I was seeing would not have flashed the led, but I was testing without the display+keys board attached as it's easier to measure that way! No doubt it would have dropped below the 2.27 Jim found, if I'd attached the display.

I've now charged the dodgy batteries overnight in separate charger, also 2 others, and am now running the set of 4 in series on a torch bulb to stress them a bit & see what happens. So far they charged to 1.493 & 1.484V, and the set is happily putting 0.470A through a bulb so looking good, but it's only been running 25 minutes.

So exactly what is/was wrong about the batteries I don't quite understand. More info if/when I have some.

Very many thanks to everyone for the immense help & support here - now I must go & recharge my super-large-and-reliable battery using my solar panels 'coz the sun's up, courtesy of the Chevvy Volt Mk1 I drive (& love to bits!). Known as Vauxhall Ampera here in UK. Rather like the best HP Calculators, they're rare, very wonderful, and you can't buy them new any more! But that's another story!!
Andy