Take a look at this melted Nimh battery from a Spektrum DX8 transmitter.

This one belongs to Terry. Paul A. is also familiar with this situation because he had one melt too. In fact his battery looked worse than this.

Having looked around on the internet, it’s a common problem.
The cause? Overcharging. It turns out that the DX8 charging circuitry has no means to shut off the charge current as the battery reaches capacity and starts to heat up.
Overheating is not normally a problem at the low charge currents (typically about 100-200mA) used by the little wall plug chargers. The reason being that the battery can easily dissipate the heat as it is charging.
That is until you factor in the shrink wrapped battery pack, surrounded by foam packing that is inside a plastic box. Then heat dissipation becomes much more difficult.

The foam packing is in there because Spektrum also sell a larger battery for this TX, which is a 4000mah LiPo not a 2000mah Nimh. When installing the LiPo, the foam has to be discarded to make way for this larger battery. The foam simply stops the smaller pack from rattling around in the TX.

So basically, the message is, if you are doing a long charge on one of these, watch out for this problem. Check the voltage of the pack (say by turning on the TX every hour or so) to see when it’s nearing completion. I would be extremely wary of leaving this on charge overnight in the house while you’re sleeping.

At £30-£35 the Spektrum branded LiPo replacement battery is expensive.
(example: http://www.sussex-model-centre.co.uk/shopexd.asp?id=33559)

An alternative to the Spektrum branded version of the LiPo is HobbyKing’s one, at just about £12. Here’s the link:

I’ve been using the Spektrum LiPo for over a year now. I highly recommend it because it only needs charging every two or three months and doesn’t seem to suffer from the heating problem. The irony here is that LiPos are normally the scary batteries. Nimh batteries have always seemed tame.