HPS10 won't run (start) on batteries

I have an HPS10 that has operated as expected for years using either rechargeable NiMH batteries or an external power adapter. Recently after a few months without use it will not start on the batteries. It will start on the external power supply and continue running on battery power when the power supply plug is removed from the HPS10 (I’ve let it run that way for up to 30 minutes). But turning it off and trying to restart on the batteries alone doesn’t work.

I know that the batteries are good and are making good contact with the battery terminals. They are fully charged, each one can deliver at least 1000 mAH of charge at a 250 mA discharge rate, and each maintains >=1.3 V output voltage at that current. I know that they are making good contact with the battery terminals both because the unit runs fine on the batteries once the power supply is unplugged, and also because I measured 6.7 volts DC at the terminal connections in the battery compartment (so there is continuity across all 5 cells).

I tried resetting the device using the reset button on the front, and that did not correct the problem.

Otherwise the unit seems to be working normally both on AC power and battery power. Wave forms and frequencies match the signal generator output settings. Voltages are about 15% off, but they’ve always been that way. It sometimes picks up what seems to be a small amount of line voltage noise when running on the AC adapter, but signals are smooth when operating on battery power.

Unfortunately I need to be able to use the unit over a full day at sites where AC power is unavailable. So I need to be able to run on batteries, but battery life is too short to last the whole day without turning off the unit between uses. I need to be able to turn it off and restart it on batteries alone.

What could be the problem and how can I correct it? I saw a thread discussing a similar issue back in about 2012, but there was no resolution in that thread.

A new bit of information. At the suggestion of GadgetUK164 on YouTube (thanks) I tested using fresh alkaline batteries, and it worked. Not a practical solution as the HPS10 eats batteries even when off, so single use ones get wasted unless they are removed after each use. And that is not practical.

Any thoughts why a scope that worked just fine for years on rechargeable batteries will no longer work with them. Even with a fresh set of fully charged rechargeables? As I wrote in my original posting, there is no sign of corrosion anywhere in either the battery compartment or inside the case.