Just something that uses AA batteries but says rechargeable ones aren't recommended; why might this be? I don't like using disposable ones as wasteful IMO.

1.2V vs 1.5V perhaps. Try them and see!

Beat me to it

Well that's the only thing I could think of – just wondering if there's any potential for damage but can't think why.

Tiger – in my experience, some stuff just doesn't work properly with lower voltage rechargeable – our baby monitor doesn't like them and a few other items (e.g. often LED torches) won't work properly either.

Most stuff does work fine though – I run my Hope Vision 1 on them and they last ages – try and get high mAh ones that last longer and a decent charger – makes all the difference.

It's this thing – what's the worst that can happen?!

http://www.mothercare.com/Tomy-Winnie-Pooh-Lullaby-Dream/dp/B000JFD8Z0/sr=1-42/qid=1277800530/ref=sr_1_42/276-2736073-4194633?_encoding=UTF8&m=A2LBKNDJ2KZUGQ&n=42822041&mcb=core#productInfo

It's because the nominal voltage is lower – ie the one that's written on the side. 1.2v for rechargables vs 1.5 for normal.

It's more complicated than this though (although I'm sure TJ will disagree with me) because the voltage at the terminals will vary depending on how much load you apply, and with alkalines this voltage will drop more easily than with rechargables (because the internal resistance is much higher).

So if you connect up an alkaline battery to say a torch, it'll be about the same as a rechargable since you are drawing a lot of current and the voltage of the alkaline will drop to similar to that of the rechargable.

If it's something like a clock, which has a tiny current drain, then an alkaline will have a higher voltage at the terminals.. and the appliance is designed for this.. which is why clocks never seem to work at all with rechargable batteries in my experience.

Any journal links to back up your wild assumptions molgrips?

And because they don't hold their charge for as long

The battery companies pay the manufacturers to write that on their products to keep you buying batteries!

molgrips is correct, but it's even more complicated than that, as batteries don't maintain a constant voltage as they discharge. 1.5V is the initial voltage for alkalines, whilst 1.2V is the mid charge voltage for rechargeables. Both will discharge down to 1V to get full use of them, whilst rechargeables will start at 1.4V under low load. Yes there are things that won't work with rechargeables for this reason, but IMHO any such device is not fit for purpose given it will also stop working when your alkalines are barely used.

Whatever you do, get a BL700 charger. It'll make the most of any rechargeables you have – a brilliant little device.

Tiger, we have the same thing for our little one and use re-chargables and we've not had any issues with them.

batteries don't maintain a constant voltage as they discharge

True, and the discharge characteristics of rechargables are better ie the voltage curve is flatter. This is why my first digicam lasted for a fair while on its NiMHs but only about 10 pictures (seriosuly) on brand new alkalines.

Rechargeable batteries have lower voltage. However the voltage remains pretty constant over the life time, whereas with the disposable batteries, the voltage drops significantly over time (IIRC the phys chem practical labs, voltage goes from 1.5V to 1.4V very fast and then has another drop around the last few days of the battery).

I've never had a problem with rechargables in anything
I've got a load of Uniross Hybrios right now that are great as they always [pretty much] hold their charge till you need them

I haven't, apart from clocks.

btw – in case you haven't come across them, there's some low self-discharge cells available now (ie they retain charge longer than normal nimh) – eg sanyo eneloop.
Lower capacity than standard nimh, but normally I find they run out of charge sitting in the drawer rather than in use.

Yes.. normal NiMHs lose 10% in the first week apparently. Although I've kept them in the camelbak for months and they still power my rear lights for a while.

This explains it quite well

Being a geek, I have different types for different jobs – high mAh Ansmanns for stuff like bike lights and torches, Hybrios (hardly self discharge at all) for general use. I still use normal batteries for my RC car transmitter, my Petzl Myo XP head torch and our baby monitors as well as my Tesco AA LED Cree torch – they just don't work as well with rechargeables.