TPL33,

technologh primer: NiCd and NiMH are basically nickel derivative batteries. Both are good for decent flying, safe to use and give good life. However, they have a low discharge rate (sometimes also called C rate). You will struggle to find anything over 2500mAh. Thats a technology limitation. They are both 1.2V nominal. Make a note that when fully charged, they will show a higher voltage. For example, NiMH 6V pack when fully charged may show 6.5V or so. That will drop quickly when load is applied. They can be plugged direct into the RX. One queer thing to remember is that they suffer from "charge memory" i.e. if you keep them fully charged and do not discharge for long time like in the off-season, they will slowly lose charge. This is because they have high internal resistance. In this condition if you check them in, they will show a full charge but discharge almost immediately. For this reason, they need to be kept charged and discharged or regular basis. This is called cycling. You could do it by puttinh your battery under load (even moving your servos around in the airplane), letting it discharge fully and then charging. Many chargers do this automatically for you. The adapter you got with your TX is not one of those, you will need to but one seperately. NiMh chargers typically send a small current through the battery to charge it, nothing complicated. But as the battery charges, the internal voltage increases. As current flow is inversely proportional to voltage, as the voltage increases, the current flow into the battery drops. The battery stops charging when the charger voltage and battery voltage is equal. Since, during discharge, battery current varies as voltage, these can be monitored with a voltwatch

Lithim Polymer or LiPo batteries are more advanced than NiMH / NiCd. These have to be charged by the constant current constant voltage method. They also have to be thermally monitored because there is a risk of fire if they get too hot. LiPo chargers come with all these features and therefore are expensive. The batteries cant take any abuse at all. As a rule, anything high performange spoils faster. If case of LiPo, the internal chemistry generates hydrogen making them explode sometimes. You will see this clearly- the rectangular battery will become a barrel. The resulting lithum fire burns at a high temperature and does not extinguish easily. Thats what makes the battery risky to use. I have seen a LiPo battery start to burn on its own in a heli and the owner took the risk and threw the battery onto the ground. Inspite of using a fire extinguisher, it burned most of the day in the corner of the field. Hence not recommended unless you know what you are doing!. They are high performance though and 5000mAh and 50C is not uncommon. Each cell is 3.7v nominal, so they are 7.4 and 14.4V though, so you will need a regulator. They have a flat discharge curve (current and voltage remain constant as the charge drops) so cannot be monitored with a standard voltwatch. There is no charge memory problem. Due to their voltages, they are mostly used on electric airplanes where the voltages are designed to use these batteries

Lithiun Iron Phosphate (LiFePO4) are much more stabe than LiPo. They also need to be cc/cv charged, so a dedicated charger is required but can take a standard charger (dont test that claim!). Like LiPo they are available in high capacities. They also have a flat discharge curve with a sharp drop at the end. Again a voltwatch cant be used but some monitors can count the total mAh use and thet can be used to monitor. These are 3.3V nominal per cell, so 2S is 6.6V, meaning direct plug into Rx. Though more stable that LiPo, you have to remember that these are lithium batteries, so more care has to be taken and the battery shouldnt get hot (over 65Deg C). Since the electric models are used to the LiPos LiFes can be used there

My recommendation of a 2100mAh was based on pricing. In Hobbico, 1900 / 1800 / 1300 are priced similar as they geared to small airplanes, transmitter use etc. 3200 is fairly priced but very heavy and I believe 2200 is priced similarly. 2100 lies in the sweet spot.

As a corollary, I have some NiCds, specifically one from my 9CAP, which though only 800mAh, is unbelievely resilient. When I got the 9C, I replaced the low capacity 800mAh stock with a Hydrimax 2900mAh. I didnt use the 800 at all for over a year. Then expecting it to be a gonner, I still tried it with a electric pump and it had full capacity after charging. These original batteries are good. My 2200mAh and 1800mAh battries I had purchased for my 6c conked off in under a year. Just FYI

Ameyam