Re: Lipo Battery | Volt & Amps | Newbie questions

Geekay wrote:

(top posting fixed)

Tim Wescott wrote:

Geekay wrote:

Folks,

For a electric newbie, its always confusing to understand the
difference between volts and Amphs in a Lipo Battery Pack and how to
relate that to motors. I have put together some questions. Pls let me
know your thoughts.

1. Longer flying time:
Can longer flying time be acheived by increasing the voltage or amphs?
or both? or neither? :)

2. Parallel / Serial
What does it mean to connect two batteries in serial versus parallel.
What will be the final volt and amphs in serial mode and parallel mode?
What is the end result of connecting them in serial and parallel mode?

3. If the battery power (Does power means volts or amphs?) is more than
what is required, will it toast the motor / speed contoller or
receiver?

Thanks
Paul


0:

Hey Red! Do you have a Really Basic FAQ for the newbies?

1:

There's three things you need to worry about: voltage, current, and
capacity (amp-hours). Often newbies confuse the Ah rating of a battery
with the current capability -- they're two different things.

Voltage is just that -- the volts that the battery supplies.

Capacity is what they put in big letters on the pack. It's a measure of
how much electrons you can pull out of the pack before it's discharged,
and for batteries it's expressed as amp-hours (1 amp for 1 hour, or 1/10
amp for 10 hours, etc).

Current capability is the amount of amps (_not_ amp-hours) that the
battery is capable of delivering without damage to the pack. You'll
often see this expressed as xC, so a 5C pack can deliver 5x it's
amp-hour rating (presumably for 1/5 of an hour, but practically less), a
20C pack can deliver 20x, etc.

2:

That's series and parallel -- you've been working on too many computers.

Connecting batteries in series increases the voltage that they deliver
-- 1.2V per cell for NiCd and NiMH, 3.7V per cell for LiPo. So you
decide what voltage you want and start stringing batteries together.

Connecting batteries in parallel increases the amount of current that
they can deliver and the amp-hour rating of the pack. BUT! For
charging you can't just connect two multi-cell packs in parallel; you
should either charge separately or connect individual cells in parallel
then connect those paralleled packs in series. Furthermore, you should
only do this with cells that are as identical as you can get them --
doing this with dissimilar cells is a Bad Thing.

3:

The motor & speed controller will (within limits) take as much current
as it can get from the battery; if it takes more than the battery,
wires, or speed controller can handle then yes, you'll let the smoke out
of those delicate electronics. Some speed controllers will limit the
current, but it's been a long time since I've shopped for one.

If you can get it, a speed controller that lets you set a current limit
to what the battery is capable of would be a good thing -- generally a
motor will suck more current when it is stalled, and sometimes it's hard
to remember to flick the throttle down when you crash. LiPo's, in
particular, don't like to deliver too much current.

> So is it correct to say that higher the capacity the longer the flying
> time.
>
> Ex, a 2100mah lipo pack will flying longer than 1500map (Both at say
> 15C).
>
> This bring another question, For a speed 400 brushless, what is a
> preferred C?
>
>
The higher the energy the longer the flying time, all else correctly adjusted -- energy being voltage * capacity, more or less.

For the same number of cells, certainly a 2100mAh pack will fly longer than a 1500mAh pack (LiPo, NiCd or any other cell chemistry).

Unless you're just pounding a poor set of NiCd cells into the ground, as long as you get a pack that can supply current to the motor you'll be in the air for a satisfactory flight. Before I remodeled the airplane's nose with dirt, my brushed speed 400 had a 600mAh NiCd pack. With throttle management it flow longer than I wanted to (now it sits in a box, awaiting some glue).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/
.

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