Do rechargeables behave similar to capacitors?

[RipplesOfLife]

(A DC circuit)

In that, once a capacitor is fully charged. The current flow will stop. (At least the way I understand it, it will stop)

I don't remember ever reading about over charging a capacitor, except applying too high of a voltage, but that's a different matter.

Now, if I took one nimh battery, and applied 1.2 volts (from a constant source) across it. Once the battery is fully charged, would the current flow not stop?

I notice that after being charged in a commerical charger, the voltage is almost 1.5 - 1.6 volts. Which means the voltage applied across is about that high.

If I made my own charger which supplied about 1.3 volts, would I have to care about how long I leave it charging? Or maybe put a diode in to bring the voltage down to 1.2 volts and keep the current moving one way only.

Thanks.

 

[BobVA]

Nope.

Capacitors store electricity in an electrostatic field between two or more plates separated by an insulator.

Secondary (rechargeable) batteries store electricity by using it to make a chemical change in the electrolyte (i.e., the goop between the battery plates). Energy is drawn out by letting this chemical reaction reverse.

Fundamentally different processes.

Different battery chemistries require different charging schemes; poke around in CPF and you'll see a lot of discussion on this, or consult the battery makers documentation for the final word.

As a general rule, all batteries require some type of charge termination function to prevent damaging them, and all eventually wear out.

The approach you presented would work fine for a capacitor (assuming you limit the current) which is why some of the advanced caps being developed for battery replacements are so interesting. Easy to charge, impossible to overcharge and (theoretically) unlimited lifespan.


Cheers,
Bob

 

[VidPro]

yes you potentially could charge a ni-mhy to a specific voltage, leaving it 80-90% charged, and never reach an overcharge state with it. that would be very good for the battery.
but if you were wrong in your calculations, or the battery began to act different, AND if it did reach an overcharge state, the Vdrop would increase the power going to the battery.
in other words, you could do it and if you did it right it would work fairly well, if you screwed up it would get worse than other methods.
like say if you choose ~1.4V max, the charge would taper quite a bit near the end and float down to a topping charge.
or if you chose a ~1.35v max it would charge and taper off before fully charged, and top off very low.
if you chose ~1.3v max it would never fully charge, but easily float like that forever. <--- i think this is what your actually referring to.

if you chose something around 1.45V you could possibly reach overcharge and get a Vdrop. which would change your voltage differential and start charging faster at the worst possible moment. a VDrop can be as high as 1V when it occurs.

anyways that is the IDEA, you would have to work it out in reality by applying it.
a capacitor doesnt actually fully stop either, just like this situation would not, a capacitor does slowly loose its charge, and just like the both of them the voltage applied to it would keep the voltage on it at what it is set for.

generally older lesser crappier batteries have a tendancy to reach overcharge and v-drop at a HIGHER voltage, so although things will completly change based on your battery and its resistance and all that other good stuff. chances are a system like that would handle aged dying battereis. but then again, if it did not, you would want to insure the current didnt exceed overcharge specs for the cell.

i have attempted to discuss this possibility of treating a ni-mhy like you would treat a li-ion, the idea of doing it is worth testing for sure.

there would be some issues with chemical vibrancy, as there would be some chemicals that never underwent conversion (cycling), but that would be at your high end, that basically you would not be using if you stopped prior to full charge..

 

[RustyKnee]

Now, if I took one nimh battery, and applied 1.2 volts (from a constant source) across it. Once the battery is fully charged, would the current flow not stop?

You shouldn't just wack a discharged bttery on a constant voltage alone. say the battery eventually reeaches 1.2V when charged, it may drop 0.9 or 1 when discharged. Just wacking a 1.2V on it may result too much current if the supply is meaty. you need to limit the current from the supply, which means the voltage applied is only a bit higher than the battery voltage at that moment. once the voltage applied reachs the "1.2" it stays at 1.2 and current drops off to near zero (depends on what current the charger is set to cut off at).

A capcitor you can generally just applie the voltage at the "1.2" it will have asurge current and will charge considerably quicker than abattery...so the high current only lasts a short time, things shouldn't get very hot.

Stu