I have replaced the batteries and the charger of my Philips FC6149 Mini Vacuumer because the batteries were worn out 1 year after purchase. Only after I started hacking I realised it was still in its 2 years guarantee period, so I might as well have returned it for a new one. My mini vacuumer had always been on it's charging base.

I have tried to rehresh the batteries by running them all the way down and recharge a few times, but that didn't help. It was time to open it up. 

I found ten 1.2V Ni-MH batteries in series. The charging circuit basically exist of a 18V/200mA external power adapter and a series resistor. What? Oh my god. That is so bad. This charging circuit is designed so cheap, no wonder after 1 years of continuous charging the batteries wear out. Philips, this is so bad!

I have replaced the batteries with two 18650 Li-ion cells in series, a 1A 2S Li-ion charger and a 10A/2S battery protection/balancing board. And the 18V/200mA power adapter was replaced by a 12V/1A adapter.  It took a while to remove some internal plastic to make place for these bigger cells. 

Even though the battery voltage (7.4V) is lower than before (12V) it works great again.