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crwdns2944351:0crwdnd2944351:0Philips QG3040 Trimmer - Battery Replacementcrwdnd2944351:0crwdne2944351:0

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Conclusion
To reassemble your device, follow these instructions in reverse order.
Use a brush to clean the inside of your trimmer before re-assembling it. After so many years of service, hairs have surely gotten inside of it.
To re-assemble the trimmer carefully place the top part over the bottom one (make sure no wires get in the way) and press down until the to parts snap into place. You'll have to use a bit of force. Also, make sure the charging port at the back is placed properly.
In the end there will be some marks left on the plastic even if you used plastic cards to help protect the case but nothing as bad as using metal tools for the job. I don't believe that Philips intended for this to be repairable.
On choosing the right rechargeable replacement:
I went with a 800mAh NiMh. Sure, without the plastic casing around the battery you could fit an AA size rechargeable in there. However, keep in mind that while a bigger battery will hold a charge for a longer time (more trimming sessions) but it will also take a lot longer for your included charger to actually charge it.
Philips says that their original battery takes ~10h to fully recharge. The 800mAh will take roughly 16h.