I am supposed to use NiMH batteries for one of my RC plane project.
I have never used anything other than LiPo before. I am completely clueless about NiMH.

It would be very helpful if some of you could guide me in this respect.

What are the specifications of NiMH batteries and how to choose the right ones?

Some facts you need to know:

1. NiMH are 1.2v per cell so multiply to get the right voltage, for a 12 v use 10 batteries in series. Your battery pack will be heavy
2. LiPos have better power to weight ratio.
3. LiPos will be able to deliver better currents for your motors (in lesser weight) (c rating)
4. NiMH takes longer to charge ( You will need about 12-14 hours to charge them completely) at C/10 rating
5. NiMH lose current faster than LiPos, you will notice that a NiMH battery not used for a week will be down to almost 20 - 30%, unlike LiPo which will still be good
6. NiMH has memory effect (although not as pronounced as NiCd), for prolonged life NiMH will need to be completely discharged before charging
7. In a battery pack, all NiMH do not get charged equally so it is useful to match batteries while creating a pack. There are generally no balance plugs or any chargers available with balance plugs to individually charge NiMH.

So all in all, LiPo is a better chemistry if you're planning to power your electric plane with it. Using NiMH will be tracking back in the technology timeline. All the above *does not mean* that NiMH are bad. They're pretty much used in all nitro setups because they add to the safety as NiMH has lesser chances of going infernal. However, with careful use, nitros can be powered with LiPos equivocally.

Hope that helps

GS

Ok that helps, thank you for the information.

Do we have anything like c- rating in NiMH to calculate max current like in case of lipos ?

http://www.rcgroups.com/forums/showthread.php?t=1127160
http://www.ultimaterc.com/forums/showthread.php?t=140189

Thats turnigy LSD (Low self discharge) which means they do not lose charge as fast as other NiMH batteries when stored.

can i replace NiMH with Lipo

Yes you can as long as the voltage matches and so does the current discharge. A single nimh cell gives 1.2v and a lipo gives averaged 3.8v so do math. For eg. 10 nimh cells will give u 12v and 3s lipos give you 11.4v so they are close. Similar with current discharge. In general lipos have higher current discharge rating than nimh. You will also need a computerized charger for lipos

Friends

If planning for NiMh  1.2v go for ENELOOP by sanyo best in market.Its available in many online store excellent life and less discharge.

Note:
Get White(silver) eneloop not the black with double XX.

A NiMh battery will charge fully in more than 7hours and there no conformation that they are fully charged, they are very heavy also 

thanks girish... i had 4cell Nimh 500Mah that means i can use 3.7v lipo