Hi, Just found Eneloop batteries at Wal Mart in the photo dept. They are AA size, 4 with a charger for $19.97. I'm sure if you asked, they would get them with out the charger.
Rayovac has also come out with their version of the Eneloop, it is called the hybrid.
Kodak has their version also. They are just marked pre-charged rechargable and will store for a year between charges.
This question is for Red Schofield (sp?).
On RCGroups, a fellow keeps stating that Eneloop batteries are fine for transmitters, but would be bad for receiver packs because the internal resistance of the cells is 25 milliohms. I can't get him to explain why he says this. That's not very much resistance.
What is the internal resistance of a regular NiMH? What does this all mean anyway?
| This question is for Red Schofield (sp?).
Well, I'll take a stab at it.
| On RCGroups, a fellow keeps stating that Eneloop batteries are fine for | transmitters, but would be bad for receiver packs because the internal | resistance of the cells is 25 milliohms. I can't get him to explain why he | says this. That's not very much resistance.
AA NiMH cells are known for having high internal resistances, so people flying larger planes or planes with lots of digital servos aren't terribly fond of them. AA cells in general are bad, and NiMH is generally worse than NiCd. subC cells are generally very good, which is why they're used for R/C cars and for the battery packs on big planes with lots of servos.
I think 0.025 Ohms is pretty typical for AA NiMH cells, actually, but I don't know if that's for high or low discharge rates. I'd guess low, however.
In any event, a high internal resistance limits the amount of current your servos can draw, or more practically causes a larger voltage drop when drawing a large current. This slows things down, and under extreme conditions the voltage can drop enough that your servos or RX don't work properly anymore, at least until the current drops and the voltage goes back up again.
| What is the internal resistance of a regular NiMH? What does this | all mean anyway?
Every real-world battery has some internal resistance, and we generally approximate it with one fixed value. This resistance goes in series with whatever load you put on the battery, and this I^2*R (I = current, R = internal resistance) power goes into heating your battery rather than powering your plane.
If you want tables of internal resistance values for certain cells, get it here --
As for where to get them, they're $20 for a pack of 8 at Amazon --
| In any event, a high internal resistance limits the amount of current | your servos can draw, or more practically causes a larger voltage drop | when drawing a large current. This slows things down, and under | extreme conditions the voltage can drop enough that your servos or RX | don't work properly anymore, at least until the current drops and the | voltage goes back up again.
Let me give some real world examples -- they'd make this point better.
Suppose you've got a full house 3m sailplane (because I was setting up one on Saturday rather than flying :\ .) Let's say it has a V-tail, and six servos. Probably six digital servos, because these things tend to be expensive and flown in competitions and so people tend to put in expensive servos.
When you activate crow/butterfly, this tends to move every single servo at the same time.
Let's make a guess that at the maximum, the current draw of all of this, just for a fraction of a second, is 4 amps. (The real figure depends on a lot of things, of course, but four amps is not unheard of.)
Let's suppose this is all is powered by 4 Eneloop AA NiMH batteries, with an internal resistance of 0.100 Ohms (as measured by Stefan, though he measured at 1.2 amps rather than 4 amps.) For four cells, that's 0.400 Ohms. Multiply that times 4 amps, and that gives us a voltage drop of 1.6 volts. So if our four cell pack starts out at 5.6 volts, now we're at 4.2 volts. Which is probably OK for now, but as the surface charge goes away, that voltage will drop, and soon we're going under 3.6 volts when we deploy crow. At which point our RX may cut out, servos may stop working, etc. Some RXs even take a few seconds to come back to life once the voltage drops like this, and right before you land is usually a bad time for that to happen.
Now, 4 amps is probably a bit high, but it's not unheard of. In any event, a fifth cell, another pack in parallel or using a subC pack is probably appropriate in this case.
Using 4 Sanyo 2500SCR sub-C cells with an internal resistance of
0.0034 Ohms, the voltage drop is only 0.05 volts instead of 1.6 volts. Of course, there's voltage drop in your battery leads, RX and servo leads too ...Now, to be fair, I don't know if it's fair to compare that 0.0034 Ohms vs. Stephan's measured 0.100 Ohms figure. But using the sub-C battery pack certainly does make a big difference -- it's quite noticeable, even if your voltage doesn't go so low as to make things freak out..
Ok, I'm understanding this better now. I've only ever used 4 standard analog servo's in a plane. (My trainer and a Brightstar) I have been getting by with a 4 cell nicad pack rated for 700mAh. If I use the Eneloop's for an rx pack, I'll make it a 5 cell pack. I just don't want to use regular NiMH's because I might not get to fly for several weeks at a time and didn't want to mess with high self discharge problems.
Thanks, Vance
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.