Batteries for the classics.

[Carlos Parellada]

Modern batteries have a lot of capacity, but classic calculators often need more than 3.4 V to work. The red low battery diode on my HP-67 lights up when the battery is still 85-90% charged. If the voltage drops to 3.4 V, we have the red diode, if it drops to 3.1 V the calculator does not work. I have assembled three batteries in parallel of the type 14500 Li-ion 3.7V with 1600 mAh capacity each. This 15% of 4800 mAh gives me about three hours of calculations without plugging in the adapter. It is the most I have achieved. Does anyone have something better? Or is something wrong with my HP and it doesn't feel like running with low voltage?