Mr.Pibb

OK, guys...a COMPLETE newbie question...

So my wife got me this for my birthday:
http://www.hobbyzone.com/rc_planes_h...rd_freedom.htm
That was yesterday...I haven't even flown it yet.

The geek Electrical Engineer in me immediately looks at the 8.4v 1000mAH NiMH battery pack and says "Even seven standard AA NiMH batteries should give me around 2000mAH, why can't I just make a pack out of those? It would be easier to charge, too, since I can just charge a bunch of them in advance"

OK, beat me up, tell me why I wouldn't want to go down this path. You guys are way more advanced in this area than I am, and I still don't know all the terminology used on this forum.

Chocks_Away

Hi,
You are quite right, 7 standard AA's will do the job at 2000 mah, with the added bonus of twice the flight time..

Now heres the bad bit

I don't know what sort of cells are used for the original pack, but it doesn't really matter.
What does matter though, is how much it weighs.

The original pack may weigh in at maybe around 120 grams.
If you make up a pack of AA's of 2000 mah each, they weigh something in the region of 30 gramms each.
Add 7 of those and voila, you have 210 grams of flight battery.


Now, some models may be able to cope with the extra weight with a small performance loss, but others, will just be too heavy to fly properly if at all.

It's a question of can it take the extra weight ?
My best estimate would be er, maybe not

Plus, depending upon where the battery is located in the plane, there is also the issue that if you put in a different weight of battery, you may move the centre of gravity position, which will affect its flying ability.


If you wanted to add more capacity to a flight battery without added weight, then using lithium batteries is generally the only route to go, but on a model like this, it's not really practical for numerous reasons.


If you did decide to make up a battery pack to use, then I would check first to see if it could handle the extra weight by means of test flying it with lead weight added in stages to make the plane the same weight as it would be with the heavier batteries in.


Hope this help you.
Craig [sm=thumbup.gif]

Mr.Pibb

That was VERY helpful. The weight issue was something I was worried about as well. I haven't had a chance to weigh their battery pack and compare it to the wieght of 7 AA's. The battery is located in front of the wing so I think extra (or lower) weight would cause a balance issue.

Their batteries are "shorter" and "fatter" than AA's.
http://secure.hobbyzone.com/catalog/...s/HBZ1013.html

What came with the plane says 1000 mAH, not 900 as the web page shows.

Chocks_Away

The way to check if it has affected the balance is to check it with the original battery in first, by means of balancing the plane with two fingertips (one under each wing)
The balance point is where the plane will sit perfectly horizontal, note this position for later.
This is usually about a third of the way back from the front edge of the wing with your fingers on the underside of the wing near the fuselage side, not the outer wing tips.

Try balancing it again with the heavier battery pack, if the balance point is in a different position, then it will affect the flight.

If anything, you are more likely to get away with the balance moving further forward than rearward, but only by a certain amount, which differs from plane to plane.
If it has moved the balance by only a few millimetres either way then it shouldn't matter, its only when it gets shifted by + or - 10mm or more.

coolbean

Also you need to check if your standard AA's can discharge fast enough. Most standard batteries are not designed to dump all their power in the span of 5-10 minutes. You'll experience voltage loss and may damage the batteries.

Mr.Pibb

Considering how quickly the stock batteries charge (10 minutes), I'm guessing that they can transfer power more quickly than standard batteries. I know that if standard NiMH AA batteries charge even close to that quickly they would be super hot, and the stock batteries weren't even warm. Maybe I'll just purchase a second set of batteries instead

coolbean

That might be a good idea. Besides, making your own packs doesn't always work. There is a little plastic cap right under the positive end. That cap has a little hole in it to let gases escape. If while your soldering you get it too hot and the hole closes up so the gases have no place to go and so the battery doesn't discharge properly.

Chocks_Away

Yep, it looks as if you would probably be better off using the proper ones in the long run.
At least you know that they are designed for the job, and you can buy extra packs to keep you in the air longer

Mr.Pibb

Thanks for your help guys. Now I'll have to focus my hacking energy on modfying my 2.4GHz wireless X10 camera to fit on the bottom of the plane

Chocks_Away

Your welcome, and good luck with the flying [sm=thumbup.gif]

Scrape

Bump lol

Mr.Pibb

lol...wow, how things change over time.