Newbie Q: Battery charging.

[KLM]

I just bought a sub-C battery pack for a RC car. Its rated at 7.8
volts. I didn't want to spend money on a charger as I am up to my
eyeballs with plug-in power adaptor units from PC equipment and other
electronics gadgets. So I used a 9 volt adaptor to charge this sub-C
pack. It charged up alright but the battery felt hot. I know this is
not good since it boils off the electrolyte.

What do you guys do other than drop more money into a hobby specific
charger? Come to think of it what exactly is the output voltage and
amp rating of the hobby specific charger?

I am thinking of adding a timer to reduce the charge duration or
adding a power resistor in series to lower the voltage , say to 8 V.
Or should I use a smaller 9 V charger? What characteristics should
I be loooking for in a charger. I am quite capable of building the
circuitry if I know what to build for.

I also have a number of 9V sealed lead acid batteries I bought for a
song from the surplus store. Their size and weight is ideal for the RC
boats I am building. Same question. They feel hot during charging.
Same converns as in para 2.

[Justin Mahn]

If you really want to, you can use a temp sensor to cut off at 120 degrees
F. I'm pretty sure that you'll never get a full charge quickly using a
voltage limit.

--
Justin Mahn
remove Panties to e-mail


"KLM" <klmok88@yahoo.com> wrote in message
news:t8mic0187rbgffq3ke8i25u1io6ii6ps0d@4ax.com...
> I just bought a sub-C battery pack for a RC car. Its rated at 7.8
> volts. I didn't want to spend money on a charger as I am up to my
> eyeballs with plug-in power adaptor units from PC equipment and other
> electronics gadgets. So I used a 9 volt adaptor to charge this sub-C
> pack. It charged up alright but the battery felt hot. I know this is
> not good since it boils off the electrolyte.
>
> What do you guys do other than drop more money into a hobby specific
> charger? Come to think of it what exactly is the output voltage and
> amp rating of the hobby specific charger?
>
> I am thinking of adding a timer to reduce the charge duration or
> adding a power resistor in series to lower the voltage , say to 8 V.
> Or should I use a smaller 9 V charger? What characteristics should
> I be loooking for in a charger. I am quite capable of building the
> circuitry if I know what to build for.
>
> I also have a number of 9V sealed lead acid batteries I bought for a
> song from the surplus store. Their size and weight is ideal for the RC
> boats I am building. Same question. They feel hot during charging.
> Same converns as in para 2.

[Jonathan Hodgson]

On Fri, 11 Jun 2004 09:51:02 -0500, Dean wrote:

> But seriously, it will be cheaper to buy a quality peak charger after
> you ruin 2-3 packs using this rube goldberg!

Copy that. Even if you're going to use cheap cells, invest in a
reasonable-quality charger for them - peak detect cut-off is a *must*
for Ni-Cd and Ni-MH cells. Note that if they're Ni-MH, you should
make sure that the charger is suitable as they require a more
sensitive cut-off (easy to overcharge them).

Jonny

[KLM]

On Fri, 11 Jun 2004 22:16:43 +0100, Jonathan Hodgson
<usenet@jphMUNGEDdevelopments.org.uk> wrote:

>On Fri, 11 Jun 2004 09:51:02 -0500, Dean wrote:
>
>> But seriously, it will be cheaper to buy a quality peak charger after
>> you ruin 2-3 packs using this rube goldberg!
>
>Copy that. Even if you're going to use cheap cells, invest in a
>reasonable-quality charger for them - peak detect cut-off is a *must*
>for Ni-Cd and Ni-MH cells. Note that if they're Ni-MH, you should
>make sure that the charger is suitable as they require a more
>sensitive cut-off (easy to overcharge them).
>

I know I can buy the equipment. But I also like building my own
equipment and this is my hobby long before my interest in RC toys.
Now there is nothing high tech in a hobby battery charger that I
cannot easily duplicate in hardware including the 12V converter, the
timer and the cut-off circuitry. $20 or $30 max in parts should get
me there plus the satisfaction of a project accomplished. Rather than
guess at the features or buy one to open it up to see what make it
tick I thought I would ask you guys what is desirable or available in
hobby battery chargers that you pay big bucks for. If I don't think I
can duplicate those features or its not worth my while to re-invent
the wheel then I'll buy one.

Then there are details on battery types that I am not familair with
yet since I don't use batteries for my stuff. I know I can read the
manufacturers' specs on the Internet but the feedback from hobbyists
who actually use the stuff is a lot more useful.

I tried asking the local hobby shops. No good as they don't know any
technical details. They only know what to sell.

[Jonathan Hodgson]

On Fri, 11 Jun 2004 22:04:23 -0500, Dean wrote:

>>I know I can buy the equipment. But I also like building my own
>>equipment and this is my hobby long before my interest in RC toys.
>>Now there is nothing high tech in a hobby battery charger that I
>>cannot easily duplicate in hardware including the 12V converter, the
>>timer and the cut-off circuitry. $20 or $30 max in parts should get
>>me there plus the satisfaction of a project accomplished. Rather than
<snip>

> If you have that much knowledge, then all you need to do to charge the
> battery is to design a circuit to put 5amps DC into your pack, and
> have it shut off after a preset time. Voltage will be 8-11 volts
> approximately depending on the pack condition.

No, no, NOOOO!!! For Ni-Cd and Ni-MH you *must* use delta-peak
cut-off - *never* a timed charge.

You need a constant current (depends on the type of cells and the use,
but for model car sub-C cells we generally charge between 4 and 6
amps) and you monitor the pack voltage. The voltage will rise as the
cells charge, then it will peak and start to fall again.

For Ni-MH you should stop the charge once the voltage has fallen back
from its peak by 5 to 7 millivolts per cell (i.e. 30 to 42 mV for a
6-cell pack); for Ni-Cd, use a slightly greater fall-back.

http://www.rcracechat.com/vb/showthread.php?t=74

After this, many/most chargers switch to a trickle charge which is
generally ideal in a racing situation, although the cells shouldn't be
left on this indefinitely.

A friend did tell me that you can buy chips, or small circuits, off
the shelf to control a complete peak charge cycle - but I don't know
any details, such as current or voltage limits.

Ni-Cd can also be charged using a temperature probe placed on one (or
ideally all) of the cells - IIRC 40°C was supposed to be about right -
but this method has been out of fashion for several years now.

For any other cell chemistry, you'll probably need something totally
different - for example, lead-acid needs a constant voltage with a
current-based cut-off, followed by a different constant voltage as an
indefinite 'maintenance' charge.

HTH!
Jonny

[Jonathan Hodgson]

On Sat, 12 Jun 2004 08:27:44 -0500, Dean wrote:

> On Sat, 12 Jun 2004 13:47:08 +0100, Jonathan Hodgson
> <usenet@jphMUNGEDdevelopments.org.uk> wrote:
>
>>No, no, NOOOO!!! For Ni-Cd and Ni-MH you *must* use delta-peak
>>cut-off - *never* a timed charge.
>
> AH is AH no matter how you put it in... If you know *for sure* that
> the battery is fully discharged and you know the capacity of the
> battery, a timed charge is OK.

.... except that you need to put in more Ah than you expect to get out,
since some of them will be wasted as heat in the cells during charge.

And you'd need to do a calibration charge on each new battery, to find
its capacity, not to mention repeating the exercise as it aged, and
then adjust the timer for each individual pack, adding an allowance
for efficiency as above...

> Any other situation and it will ruin batteries, you are correct.

Timed charging is just not worth it. Aside from any cost issues,
exploding cells are said to be rather dangerous :-) Plus if you want
optimum performance, you need to get the cells exactly to 100% charged
every time and the only sensible way to do that is with some sort of
sensing device - either peak or thermal.

Jonny