# Measure charge during charge??

Discussion in 'Electronic Basics' started by steamer, Feb 10, 2005.

1. ### steamerGuest

--I've been having trouble figuring out how to know when a battery
is fully charged. That is, sure, I can measure its voltage after
disconnecting the charger, but how could I do it in series, so to speak,
*during* the charging process? Dumb guy here..

2. ### Guest

Ed,
What kind of batteries and charger are we talking about here?
Richard

3. ### ILYAGuest

Just wait some time and it will be fully charged.

4. ### JeffMGuest

how to know when a battery is fully charged
Temperature.
When input current no longer adds to the *charge* of the battery,
that current starts making the batteries warm.
Modern battery packs / chargers sense this.

5. ### John FieldsGuest

---
That's not necessarily true. For instance, when a lead-acid battery
is being charged with a constant voltage current-limited charger (as
it should be), the amount of current the charger will force into the
battery will decrease as the battery's voltage increases toward its
fully charged value, with the result that it will run cooler as it
gets more and more fully charged.

I don't think it's true, either, for batteries which are charged with
a constant current, since what heats a battery up is the power its
internal resistance is dissipating with current passing through the
battery. Since the internal resistance of a battery _decreases_ as it
charges, and since the power the battery dissipates is:

P = I²R

Then, for a constant I, as R descreases P (and temperature) must also
decrease.
---

6. ### Jim MeyerGuest

Connect the voltmeter in series with the batteries and charger.

why ?!

10. ### Jack// aniGuest

This is something really funny!

11. ### tlbsGuest

Some commercial battery chargers have a switch that "samples" the
battery voltage by momentarily disconnecting the charging circuit and
connecting an instrumentation circuit (to measure the voltage).

To do it manually without disconnecting wires, you could use a DPDT
switch or a relay to switch between the charger and a voltmeter.

Depending on the type of battery, voltage is usually the best indicator
of full charge, or as others have mentioned, temperature can also be
used.

12. ### peterkenGuest

Tried it by measuring the weight before and after charging ?
Since charge is put in, the weight will increase, thus the amount of weight
is a measure for charge...
Might even build a clever circuit that disconnects leads when the weight is
enough (eg using a spiral spring or so...)
;-)