Help me understand this battery charger

[Caio Negri]

I've taken apart an old NiCd/NiMH 4xAA charger to figure out how it works. I was expecting the usual transformer + full brigde rectifier + smoothing capacitor but the thing actually has no capacitors and only 2 diodes. I traced everything and drawn the schematics (see attachment) and I can see it sends half the pulses to each pair of batteries, but what I dont understand is:

• Why the need for R1 and R2?
• Would a circuit like this be able to fully charge batteries without damaging them?

[Audioguru]

You show R1 and R2 shorted so they do absolutely nothing.
I think the circuit is too simple to charge without damaging the battery cells so maybe it undercharges them. A charger is supposed to detect a full charge then shut off.

I have one charger that continuously charges forever but the cells it charges overnight are fine I guess due to the low charging current. I have another charger that is simply a stupid timer. It resets and overcharges if the power fails when it is charging. My third charger is smart and fast but it shows some battery cells with an error and refuses to charge them. The refused cells charge perfectly in my other chargers.

[Caio Negri]

You show R1 and R2 shorted so they do absolutely nothing.

Yea that's what I thought, makes no sense  . I'll attach some photos of the actual circuit board in case someone wants to double check. The metal fingers are connected to the positive terminals of the batteries (2 for each) and the negative terminals are labelled GND1 to GND4.

[MrAl]

I've taken apart an old NiCd/NiMH 4xAA charger to figure out how it works. I was expecting the usual transformer + full brigde rectifier + smoothing capacitor but the thing actually has no capacitors and only 2 diodes. I traced everything and drawn the schematics (see attachment) and I can see it sends half the pulses to each pair of batteries, but what I dont understand is:

• Why the need for R1 and R2?
• Would a circuit like this be able to fully charge batteries without damaging them?

Hi,

As others have mentioned, the circuit is not drawn correctly yet.

Chargers like this are really mostly for NiCd cells not NiMH cells.  They will keep charging indefinitely until you take the batteries out of the charger.  They rely solely on the robustness of the cell(s) to take an over charge, which NiCd cells can do better that NiMH cells.  Yes it will work for NiMH cells but there is more risk of damage if you leave them on charge for too long.

The basic design criteria is not that accurate for this kind of circuit either.  They also rely on the transformer output voltage with series resistor versus the cell(s) voltage(s) such that the current tapers off as the cells charge and thus do less damage to the cells than a constant current if the cells are left on too long.  This causes problems right away with line voltage changes. 

The charger works just so-so for NiCd cells but for NiMH cells you have to be careful not to overcharge, and that is not that easy to do unless you are sure the NiMH cells have been completely run down before charging, and you time the charge process yourself.

The internal mechanism that comes into play when NiCd cells are being overcharged is more robust than with NiMH cells, so the bottom line is they work for NiCd cells but for NiMH cells you have to be more careful and time the charge more carefully.
There are many new modern chargers that work much better however such as the Panasonic BQ-CC55 which is a medium speed charger and also does ONE cell at a time up to FOUR cells at a time, so you can do just three cells too if you like.  The kind of charger shown in the pseudo schematic in this thread can only two two cells at a time or four cells at a time, you cant do just one or three.  If you short out one cell bay in the charger, then you can charge just one cell, but then the current level is much higher and so there is more risk of damage to that one cell.

BTW chargers like this do not need smoothing capacitors because the full wave or half wave rectified AC current is believed to charge the cells better than a smooth DC level, and so the cells are actually better off with this kind of pulsing charge.  How much it matters though is hard to measure.  In any case the charge is accumulative, so that if you charge for 1/60 second and then charge for another 1/60 second the equivalent total charge time is approximately 1/60+1/60=2/60 seconds.