On 2/28/2013 9:23 AM, Ian Field wrote:


"mike" wrote in message
...
On 2/27/2013 12:09 PM, John Fields wrote:
On Wed, 27 Feb 2013 16:17:04 -0000, "Ian Field"
wrote:



"John wrote in message
...

My request was about having you describe the dynamics of a
light-bulb-in-series-with-a-battery current limiter which, no matter
how loudly or how long you squeal excuses about why you can't, you
can't, of course.

The bulb trick isn't that great because of its pronounced PTC
characteristic - as the filament voltage decreases the current doesn't
decrease in due proportion.

---
???

If a you want the current to change linearly with voltage, then a
simple resistor will do the trick so, obviously, you don't understand
how the bulb trick works.
---

I did try to dumb it down for you - I do hope I didn't use too many big
words for you!

---
The words are no problem but it seems, for you, the theory _is_.

Would you like me to explain how the bulb trick works in terms even
you could understand?

I understand how the light bulb works, but I'm very interested in
hearing YOUR explanation.


When the battery terminal voltage is low and the drop across the
filament is high; the bulb does its best job of current limiting - just
when high charging current would be no bad thing. As the battery
terminal voltage approaches or exceeds the required voltage, the
inherent PTC characteristic of the filament means as it cools its
resistance falls, its much less effective as a current limiter - just
when the (possibly overcharging) battery needs it most.

The bulb as a current limiter is less than ideal for battery charging -
but better than nothing if you don't have a float capable charger to hand.

I didn't bother to dumb it down for JF - you can draw pictures for him
as a homework excercise.

So, you're saying that a light bulb can work very well for current charging
as long as you pick the voltage and the bulb correctly.

Also, a light bulb can work well to limit the maximum charge current
to stay within the limits of the battery and charger, even in the event
of shorted cells or other faults...as long as you pick the voltage
and bulb correctly.

You can do better. You can abuse any charge method.
IMHO, the bang for the buck of a light bulb current limiter is
second to none when properly applied.