| In article , | "Six_O'Clock_High" wrote: | | > I was at one of the flying fields Sunday and one of the guys showed me a 4 | > cell NImnH pack he had made up from AA cells purchased from Harbor Freight. | > He said they cost him about $2 and cycled repeatedly at 772 mAHr. | > | > Is this reasonable for that type of cell? | | If all you're counting is $/mAH, it's not bad. If you figure $/(mAH*gram), | it's not great.
Well, the batteries he's referring to are almost certainly NiCd rather than the NiMH cells he said they were. Given that additional bit of information, they seem a good deal better than Red suggests.
But cost, capacity and weight (parts of $/(mAH*gram)) aren't really the only things to consider. There's also resiliancy, internal resistance and physical size (of course, if you're only looking at AA cells, you've got the physical size fixed.)
I've not seen any real empircal evidence of this, but from what I've heard NiCd cells are more resiliant than NiMH cells, handling overcharges, reverse charging and vibration better than NiMH cells.
NiCd's lower internal resistance (for a given physical size, AA vs. AA for example) is easily verified. And their self discharge rate is lower -- also easily verified. And they can apparantly handle more charge/discharge cycles, but I've not verified this myself.
The larger capacity NiCd packs for a given size seem to be more fragile than the smaller capacity ones -- like 1100 mAh AA cell vs. 600 mAh AA cell. Which makes sense -- they replace the internal components with smaller/thinner parts, leaving more room for electrolyte, which is where the energy is actually stored.
For a receiver pack, especially on a glow plane where weight isn't quite the concern, NiCd packs are preferred to NiMH packs by many people. The lower internal resistance is good for times when all servos are struggling at once, and the increased resiliancy (?) helps prevents problems. Or it may just be that people are being conservative (which isn't a bad thing.)
For a power pack for an electric plane, NiCd cells can dump power faster, making them better for a high performance plane. NiMH cells are probably better for a gentle flier. Of course, both are being replaced by LiPo at a high rate, making the question more and more moot. And there's a lot of overlap -- NiMH cells are getting better at dumping their power quickly (but not the AA cells -- they generally have a high internal resistance.)
But for a transmitter pack, especially on a computer radio, I'd say NiMH cells rule. The only reasons I see for not replacing a NiCd transmitter pack with a NiMH pack are 1) you can get NiCd cells cheaper, or 2) you don't have a charger that can charge the pack in a reasonable time.
| I recently bought 50 Sanyo AA NiMH cells, 1800 mAH with solder tabs for a | buck apiece from
Check them out.
Not a bad price. I've been getting 2000 mAh AA cells from Frys when they're on sale -- either a 4 pack for $5, or a 10 pack for $10 -- but they're not always on sale. And not everybody has a Frys near by. (You really don't need solder tabs if you're careful.)