Humming bird Ni-Mh battery charging

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Gazza - 18 Apr 2005 17:32 GMT

Hello,

I have a hummingbird heli that came with the 7 cell 600mAh battery. I
use the charger that I also got with the unit which just looks like a
wall plug PSU delivering 12V / 300mA. This charger takes 1.5 hours to
charge the battery - well thats what it says in the manual for the
heli.

My first question is that as there are only 7 cells is 12V not too many
volts for the battery pack or is this OK?

Secondly I have just bought a charger where i can pick the rate at
which I charge the battery but it looks like if I choose 300mA then the
time takes longer than 1.5 hours, is this because of the voltage
difference? If so what limits the max voltage that can be applied and
how do you choose a current voltage combination and now how long to
charge for?

Thirdly (lots of questions - sorry but new to battery charging) I have
bought some 750mAh AAA batteries that I intend to wire in series and
use to get more flight time out of the Heli but they say to charge them
at a very low rate (even when they describe it as fast charge) for
several hours. Can I charge them faster at a higher current than it
says on the pack as I am able to do the birdseed in 1.5 hours?

I guess I am also asking if there are any good articles that anyone can
recommend on battery charging (voltages and currents etc?)

Many thanks in advance.

Gary

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Jonathan Hodgson - 18 Apr 2005 19:02 GMT

> I have a hummingbird heli that came with the 7 cell 600mAh battery. I
> use the charger that I also got with the unit which just looks like a
[quoted text clipped - 4 lines]
> My first question is that as there are only 7 cells is 12V not too many
> volts for the battery pack or is this OK?

In order to make a cell accept current, you have to apply a higher
voltage than the cell itself is producing. 12 V across seven cells
sounds a little bit high, but not necessarily excessive. I don't have a
lot of experience with NiMH but nicads, when fast charged (around 2C -
twice the capacity rating, so about half an hour) peak around 1.5 - 1.6
volts per cell.

> Secondly I have just bought a charger where i can pick the rate at
> which I charge the battery but it looks like if I choose 300mA then the
> time takes longer than 1.5 hours, is this because of the voltage
> difference? If so what limits the max voltage that can be applied and
> how do you choose a current voltage combination and now how long to
> charge for?

If the cells are 600 mAh and you charge them at 300 mA, simple maths
says that it's going to take at least two hours to fully charge them
from flat.

I would trust this proper charger more than the basic one. The
'correct' way to charge nicad and NiMH cells is constant current, so
that the cell voltage steadily increases, and then peak detection to
stop the charge when the voltage starts to fall back again.

I've recently found a few, cheap, setups (like walkie-talkies) which I'm
fairly sure are charging cells at a pseudo-constant voltage, and simply
letting the current tail off to nothing - so you never quite get the
cells fully charged. Your basic 'PSU' charger /might/ be working on
this basis.

If in doubt, use no more than 0.5C (300 mA) and keep checking the
temperature of the cells - if they get any more than warm to the touch,
stop charging.

> Thirdly (lots of questions - sorry but new to battery charging) I have
> bought some 750mAh AAA batteries that I intend to wire in series and
> use to get more flight time out of the Heli but they say to charge them
> at a very low rate (even when they describe it as fast charge) for
> several hours. Can I charge them faster at a higher current than it
> says on the pack as I am able to do the birdseed in 1.5 hours?

Again, I'd stick to 0.5C (375 mA) or therabouts, and keep an eye on the
temperature.

It's possible to charge cells quite a lot faster, but there is a real
risk of blowing the cells up if you don't stop charging them at the
right time.

<Disclaimer> I'm an R/C car racer, so if any of this sounds wrong to the
heli people then listen to them first! </disclaimer>

HTH,
Jonny

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Jennifer Smith - 18 Apr 2005 22:09 GMT

> Hello,
>
[quoted text clipped - 3 lines]
> charge the battery - well thats what it says in the manual for the
> heli.

Could be, because you'll hardly ever exhaust the batteries to zero voltage.
With these amps charging should take about 2 hours for a full charge.
Chances are it's more like 1.5 hours.

> My first question is that as there are only 7 cells is 12V not too many
> volts for the battery pack or is this OK?

Nah thats OK. It's a little on the high side, but NiMH are fairly forgiving.
LiPo on the other hand.. :)

If it charges at the exact voltage for a 7-cell-pack, you might be able to
go to 1C for your batteries - depends on how it charges. There's pulse
charging, normal DC charging with all kinds of curves and whatnot. Good
chargers and good batteries are usually fine for fast-charging, but it does
in my experience always diminish the lifetime of your batteries.

If you really want longer flight times, I'd go for LiPo batteries. They are
lighter weight with higher current. Also, they keep a constant voltage for
the most part, whereas NiMH voltage decreases with depletion.

There is one big disadvantage to LiPo batteries: They are very very
sensitive to damage, and you need to ALWAYS be around while you charge
them, because if they are damaged or overcharged they tend to explode
rather violently.

> I guess I am also asking if there are any good articles that anyone can
> recommend on battery charging (voltages and currents etc?)

Yes... use LiPo batteries.

J

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Model Forum / Radio Controlled / Land Models / April 2005

Humming bird Ni-Mh battery charging