Recently the batteries in my shaver (an old, but working, Philishave 5890) and hair trimmer (an old, but working, Philips HQ-C280) died. I looked around online for replacement batteries but didn’t find any specifications. I opened the devices but they didn’t tell what to use for replacements.
The 5890 uses two serial connected batteries. I measured the original size for each battery to: 42.5 x 14 mm. A little bit too long for being an AA. But I took a chance and replaced it with two NiHM AAs (47 x 14 mm). You’ll have to shoe horn them in to fit but it does.
The HQ-C280 uses one battery. I measured the size to match an AA battery exact so I just replaced it.
All the replacement batteries are standard AAs (for mounting) 1,2 V and 1500 mAh. I charged them for 20 hours and it worked fine!