Recharging NiMh cells

[Animal]

Always on trickle charging works well with Ni*, but does require a slow 24hr+ charge rate. Vape cells are another option, can't charge those as easily though.

[Paul]

Example of a couple PP9s in the back of this. Looks like a major battery hog. Like a boom box almost.

The general discussion here, is to use a non-SMPS mains supply (linear).

One thing they mention here, is the PP9s don't stay at 9V for very long.

You could just buy four 4-cell rechargers, and recharge the whole lot of them in one go :-)

A device like this, can tell you how well they match one another. It includes a battery analyzer.

The radio would not have a feature, to stop drawing power from the rechargeables when they're exhausted (UVLO). For example, falling asleep listening to radio, one of the cells could get reverse biased if there is no cutoff circuit.

When testing the capacity of a NiMH or NiCd battery a cut-off voltage of 1.0 V per cell is normally used, whereas 0.9 V is normally used as the cut-off voltage of an alkaline cell.

Devices that have too high cut-off voltages may stop operating while the battery still has significant capacity remaining.

16 * 1.25 = 20V 16 * 1.0 = 16V

But if you look at the curve at the bottom of this page, a somewhat higher cutoff should be selected.

16 * 1.18 = 18.9V [ With so many cells in series, don't want to go so far down the knee ]

The effects of temperature are noted here. Picking a cutoff is more complicated than it looks. Between 20C and 40C, it's looking pretty good. A wider temperature range, might take a low power micro or something.

But at some point, the more cells you put in series, you almost have to monitor cells individually to detect discharge-knee-approach. One cell could fall over the knee before the others (mismatched cells).

Maybe 12 cells would slice the PP9 characteristic better, 6 cells per PP9 replacement.

16 * 1.40 = 22.4 [ Fresh off charger ] 16 * 1.25 = 20.0 16 * 1.18 = 18.9 Stop

14 * 1.40 = 19.6 [ Fresh off charger ]

14 * 1.25 = 17.5 14 * 1.18 = 16.5 Stop

12 * 1.40 = 16.8 [ Fresh off charger ]

12 * 1.25 = 15.0 12 * 1.18 = 14.16 <=== PP9 pair spends most of its life at this voltage, or a bit lower

( )

Your circuit could do this:

+--- V++ or this: +---- V++ | | PP9 PP9 | | PP9 +---- Vmid (SPKR return) | | +--- GND PP9 | +---- GND

and the first one is going to be a LOT easier to design the UVLO (Under Voltage Lock Out) for than the other tree.

( UVLO is terminology from SMPS design, protecting an SMPS from drawing high current, when the input voltage is abnormally low. Which is not what we're protecting against here, it is battery damage due to not having load cutoff behavior available. )

Paul

[Cursitor Doom]

Wow! A lotta useful info there, Paul, well done! That certainly clears up a lot of incomplete aspects to this issue and gives me the necessary details I was lacking. Many thanks indeed for that sterling effort!