Once you start messing, you may as well go the whole hog and start changing things. Inside the hard metal case of the supplied battery I'd bet you would find a standard LiPo cell, which suggests the supplied battery may be rechargeable using an appropriate LiPo charger program.
Since electonics can't tell what battery you're using you could replace the manufacturer's battery with any other that has the same voltage, cell chemistry and similar capacity.
I'd recommend using a single cell lithium polymer 750mah cell as supplied in model shops for use in small electric helicopters. Availability is pretty much global and cost is about £5. These are definitely rechargeable (with appropriate charger) and will require some silicone sealant or double sided foam tape to fix. Also since this is weightweenies they will be lighter since they use heatshrink for protection rather than a hard case.
No, you are wrong, LTC-7PN-S4 is not a LiPo or even a rechargable battery.
EaglePicher Keeper II LTC-7PN-S4 datasheet: http://www.epcompower.com/images/stories/ltc7pn-1.pdf
LiPo would be a bad choice for this application as it has a higher self discharge rate and therefore wouldn't last as long when left unused. And without a charge plug, using a recrageble battery make even less sense as the energy desnsity is less and the battery probably expensive.
The battery is a Lithium Thionyl Chloride (Li-SOCl2) primary cell (primary cell = non-rechargable and secondary cell = rechargable).
The good thing about Lithium Thionyl Chloride (Li-SOCl2) batteries is the very low self discharge rate (< 1% per year). This means they will last for many years if not used.Do not try to charge these non-rechargable batteries. You might start an explosion or fire by doing so.
And you should also be very careful charging LiPo / Li-ion battery cell. It's very critical how they are charged and temperatures etc. should be monitored while charging. There's a reason why end customers can't buy bare LiPo / Li-ion cells, since it's dangerous to charge these. The LiPo / Li-ion batteries you buy for cell phones, laptops etc. has built in charging protection circuit to avoid fires or explosions. It think most people have heard of all the recalled defect SONY laptop Li-ion batteries. These were recalled because some exploede or caught fire.
Do not even try to charge any bare LiPo / Li-ion cells unless you are absolutely sure what you are doing and know a lot about electronics. A lot of people have exploded LiPo / Li-ion cells trying to charge their own bare cells for RC models etc.
And if Li-ion / LiPo cells have been discharged too much, you can't just recharge them again, which is why power cuts off in e.g. a laptop or cell phone before the charge voltage drops so low that the cells can't be charged again.
Bare NiMH and NiCd cells on the other hand are sold to end customers and there's very little risk charging those as opposed to LiPo / Li-ion. Charging and using LiPo / Li-ion is much more complicated than charging NiMH and NiCd.http://www.epcompower.com/content/view/53/7
LiSOCl2 (Lithium Thionyl Chloride)
As the first and still the only producer of a prismatic LiSOCl2 cell, EaglePicher is truly an innovator in this chemistry. LiSOCl2 cells and batteries have been in production at EaglePicher for over 15 years. Our line has expanded of to include cylindrical cells from a ½ AA up to D sizes as well as wafer designs, which means our LiSOCl2 line of cells can now handle any application you can dream of. The applications this chemistry powers enjoy the high energy density offered by a 3.6- volt chemistry that can be packaged in a small area. Add the 15+ years of confirmed shelf life, a flat discharge curve and wide operating temperature range (-55C to +85C) and it’s hard to pass up EaglePicher cells when designing a new product or replacing an old.