But they are cheap and you will only let one undervolt before you realise what you need to do

Charging needs safety but follow the instructions and it's safe, I charge mine unattended because I'm that confident - so confident I risk £135,000 (~$205,000) of house. I do use a fire proof charging bag mind you, I'm not that stupid

Go with LiPo, I cannot recommend them enough.

as ive said, I normally put my trust into ni-mh batteries. I cant entirely trust lipo batteries as I feel that just using, then charging, then using, then charging without any extra supplies or work is normal. lipos as you said I need a fire-proof bag, and I can't allow under voltage(what is that by the way) at all. I don't feel that its worth my while to deal with these batteries.(or time bombs, what ever you think.)

Under voltage is when you run the battery too long and the voltage drops below around 3v (I think it's .7 or 2.8 actually, I work on 3v to be safe). Basically, when you use a battery the voltage will drop when it's discharging, the lower the voltage the lower the power left in it. So LiPo start off at 4.2v per cell, over their discharge this drops, which is how you can measure the charge left in it.

If you use the script and circuit I did the tutorial for you will know when you need to charge, throw in the auto disconnect circuit (I haven't tested it yet but the theory should work and I have seen similar circuits on other forums) and you're safe. Use a decent charger with safety cut offs (most chargers unless you buy very cheap from dodgy places) and charging is safe, I didn't used to use the charging bag, I only use it now because it come free when I bought my last 2 LiPos. It's now part of Melvin so it's no extra hassle.

FYI, short out a NiMh battery, +ve to -ve with no load and tell me they are safe - Do not do that really!

@techno pro , under voltage means you run the battery till its completely dead. With a lipo its found to be best for long life ( hundreds of charge cycles) to not let them completely go dead before they are charged again. the recommended voltage is 3.7 volts per cell. 4.18-4.2 volts is considered full charge. A lipo requires the same safety and respect you "should" be giving a NiMH battery when charging. So please don't buy a NiMH battery just to ignore the basics in charging / discharging safety.

3.7v? I thought it was 2.7v or 5.4v for a 2S...

The "2s" designation indicates the amount of cells. 2x 3.7 = 7.2 volt battery base voltage 8.4 charged. 3s is 3x3.7 volts =11.1 volts base voltage and so on..

I may have misunderstood. When you said 3.7v is that the lowest voltage to take it to? Before it starts to damage the cells? Because I thought it was around 2.7v when it undervolted. If its 3.7v I need to change my scripts.

At 2.7 volts you have already reduced your batteries overall life. The goal is never under 3.7. 3.4 its not the end of the world but also not the healthiest level for your battery. I believe turnigy considers the cells permanantly damaged and should not be used in a aircraft at risk of failure at 2.4-2.6 volts. The lower you go under 3.7 the worse it affects the ability to retain charge. After even one significant under volt scenario you have reduced the mah capacity forever. It has to do with the lithium beginning to form crystals.