hovercraft

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

B Frey - 21 Aug 2004 09:01 GMT

I have a R/C hovercraft and I got a NiCd (8x)AA 700 mAh 9.6V battery pack
when I bought it.

The problem is that the power only lasts for about 5 minutes...

- is it possible to use a more powerful battery pack?
- it is the mAh that should be higher, right?
- how big can you get those battery packs?
- I have seen that you can also get a NiMh battery pack - what is the
difference?
Can I use both NiCd and NiMh in for the hovercraft?

- What would you suggest?

Hoping for help :-)
thanks..
B Frey

Reply to this Message

Colin Dawson - 21 Aug 2004 09:53 GMT

>I have a R/C hovercraft and I got a NiCd (8x)AA 700 mAh 9.6V battery pack
> when I bought it.
[quoted text clipped - 13 lines]
> thanks..
> B Frey

Hi.

I'm no expert in modelling battery packs, but I think I can help a
little....

1. it depends on what you mean be more power - higher voltage with run your
motors harder, giving them a speed boost. There's always the risk of
overheating things though so be careful not to raise it too far. On the
other hand if your talking about longer running time, that's a different
story....

2. Increasing the mAh will increase the running time, the more power stored
in the battery the longer it'll take to drain.

3. I've got some AA's that hold 2300 mA. That should give you some idea.

4. The difference between NiCad and NiMh is the chemicals used in
construction. In real terms, NiCad's have a problem known as the Memory
Effect, which will degrade the performance of the battery over time. After
a while you won't be able to put a full charge into the battery, shortening
the running time. This problem can get so bad that it's impossible to
charge the battery.

NiMh batteries as alot better in that the memory effect is not as bad.
Contry to popular belief there is still a memory effect in these batteries,
but it's so small that it's hardly worth noting. I'm not sure, but I also
think that they're capable of delivering more power. In short NiMh's are
alot better than NiCad's.

Regards

Colin Dawson
www.cjdawson.com

Reply to this Message

B Frey - 21 Aug 2004 10:16 GMT

Thanks for the answers.

So you think I can buy a 2300 mAh NiMh battery and use it instead of the
NiCad battery I got with the hovercraft?
If NiCad batteries are poor why do they sell them with the hovercraft? -
because they are cheap?

Regards
B Frey

> >I have a R/C hovercraft and I got a NiCd (8x)AA 700 mAh 9.6V battery pack
> > when I bought it.
[quoted text clipped - 47 lines]
> Colin Dawson
> www.cjdawson.com

Reply to this Message

Colin Dawson - 21 Aug 2004 11:46 GMT

> Thanks for the answers.
>
[quoted text clipped - 5 lines]
> Regards
> B Frey

I just bought a Ni-MH 1850mA pack (5 Cell). So I know these are available.
The 2300 are standard AA type, which I use in my Digital Camera.

Ni Cad's are cheaper to produce and easily available. Ni-MH are normally
sold in Electrical shops as opposed to your local hypermarket.

Regards

Colin Dawson
www.cjdawson.com

Reply to this Message

Kevin R - 21 Aug 2004 12:23 GMT

if its just a longer run time you after go for NiMh they have a better
capacity for the same size but the initial power(current) is less than a
NiCad but the power level remains more or less constant I believe its down
to a high internal resistance of a NiMh cell which limits the maximum
current , the initial power high from a NiCad drops off to a level roughly
the same a NiMh, I have a set of NiMh batteries in a plane the performance
is double or I could have used a battery that's nearly half the weight

> Thanks for the answers.
>
[quoted text clipped - 62 lines]
> > Colin Dawson
> > www.cjdawson.com

Reply to this Message

B Frey - 21 Aug 2004 11:42 GMT

Okay - thanks.

Now the final question.

I have a 700 mAh NiCad battery pack (8x AA 9.6V).
The run time of the hovercraft is about 6 min with those batteries.

I have found two battery types:
- 900 mAh NiCad
- 2300 mAh NiMh

Does anyone have any idea how much the run time will be with those types?

Which batteri would you suggest?

Thanks
B Frey

> if its just a longer run time you after go for NiMh they have a better
> capacity for the same size but the initial power(current) is less than a
[quoted text clipped - 72 lines]
> > > Colin Dawson
> > > www.cjdawson.com

Reply to this Message

Kevin R - 21 Aug 2004 13:04 GMT

forget the NiCad 200ma will only give you 2 mins extra possibly, go for the
NiMh and looking at the rate on your old battery 20 mins run on the NiMh ,as
long as they are high current cells that is ,as your are looking at about 5
or 6 amps discharge current to flatten a 700mah in 6 minutes you will need a
battery that will stand that current drain
Kevin

> Okay - thanks.
>
[quoted text clipped - 96 lines]
> > > > Colin Dawson
> > > > www.cjdawson.com

Reply to this Message

Colin Dawson - 21 Aug 2004 13:12 GMT

> forget the NiCad 200ma will only give you 2 mins extra possibly, go for
> the
[quoted text clipped - 6 lines]
> battery that will stand that current drain
> Kevin

I'll second that. I've not heard about the discharge current limit before,
it's worth checking out if possible. If the batteries arn't up to it, either
your hovercraft won't run right, or something will go bang. More than
likely everything will be OK though as rechargables are supposed to be
better for high current applications.

Regards

Colin Dawson
www.cjdawson.com

Reply to this Message

Henry Springer - 21 Aug 2004 21:13 GMT

All other things (especially the number of cells) the run time will be
proportional to the battery capacity (mAhr0 however the weight of the
battery is also proportional to the capacity so if you double the
capacity you will roughly double the weight. I don't know much about
hovercraft but I suspect that weight and balance are critical and you
may find that performance is unsatisfactory with higher capacity
batteries.

Reply to this Message

Kevin R - 22 Aug 2004 00:57 GMT

> All other things (especially the number of cells) the run time will be
> proportional to the battery capacity (mAhr0 however the weight of the
[quoted text clipped - 3 lines]
> may find that performance is unsatisfactory with higher capacity
> batteries.

not true that a higher capacity is a higher weight if you look at different
types of battery the same capacity the weight of a Nicad over the weight of
a LiPo battery, the Nicad will weigh possibly 10 times the weight of the
LiPo a NiMh should not weigh more than its Nicad counter part due to its
higher charge density
Kevin

Reply to this Message

Chris - 22 Aug 2004 22:18 GMT

Hi there

I built a scrat-built hovercraft some time back, and getting the
battery pack right was a real problem. It's a lot more difficult than
I imagined. "Dizzy" as my hovercraft is called, was simply an
experiment (and my first r/c sratch build that was not a boat).

It's easy to get into a vicious circle with batteries on hovercrafts,
because bigger batteries may give you more power, but then you need
the increase in power to create more lift, to lift the heavier
batteries.

Also, it's not just the added wight and the effect on lift that's the
problem. You also have the issue of balance.

To get around the balance issue, I eventually ended up having a C sell
3000 mAh pack made up. 7 cells, in a 3-1-3 configuration. This meant
that I could put the two packs of three cells either side of the
vertical lift turret, with one cell in front of the turret. This
effectively meant that the weight was relatively evenly distributed
around the central turret, with the centre of gravity in the centre of
the turret. It made the craft relatively stable (for a hovercraft).

Heavy hovercrafts are more difficult to turn, as the momentum keeps
them travelling for longer in the original direction.

When I was prototyping dizzy, I jused a tiny 7 cell battery pack (just
during the tuning-up process) and the whole thing was much easier to
turn and steer than with the heavier battery pack in place.

Of course, with a pre-assembled craft, maybe the room for
experimentation is more limited.

Overall, I found that flying (is that the right word?) a hovercraft
was a bit more stressfull than I needed for a "Sunday morning" hobby,
and went back to boat building.

Here's a picture of Dizzy, on a maiden voyage

http://homepage.ntlworld.com/christopher.shilling/photogallery/dizzy2_way_out.JPG

And a link to my site, where there's a write-up on the building
process, and photographs of the prototype.

http://homepage.ntlworld.com/christopher.shilling/

Chris

>> All other things (especially the number of cells) the run time will be
>> proportional to the battery capacity (mAhr0 however the weight of the
[quoted text clipped - 9 lines]
>higher charge density
>Kevin

Reply to this Message

Model Forum / Radio Controlled / Water Models / August 2004

hovercraft