Will this charge larger lithium batteries?

[BatteryCharger]

I have a "desktop" charger for my cell phone that I bought on ebay for cheap. It charges the phone and then there's a place for a second battery. There are only 2 contacts for the battery, so it must sense that the battery is full by the voltage. If I get some larger lithium batteries, say camcorder batteries, to use in flashlights, will this thing charge them ok? They would be the same voltage, just higher capacity...

 

[Doug S]

You need more infomation. Many chargers are *really* dumb and rely on the battery pack circuitry to terminate the charge. If your camcorder battery does not have such circuitry, then this charger may be inappropriate.

 

[BatteryCharger]

How can I find out? I opened up the charger, and there is quite a bit going on in the circuit board. Doesn't look very dumb to me, but I don't know that much about electronics...

 

[Doug S]

[ QUOTE ]
BatteryCharger said:
How can I find out? I opened up the charger, and there is quite a bit going on in the circuit board. Doesn't look very dumb to me, but I don't know that much about electronics...

[/ QUOTE ]
Your could put a bare li-ion cell on to charge and monitor the voltage over time. If it quits rising below 4.25V then it is at least minimally OK. You will have to decide for yourself if using a cell/charger combo where the degree of failsafing is unknown is an acceptable risk. Generally cell protection and charger protection are coordinated in the design phase, hence all that fine print about using battery packs / chargers that are specified for each other.

 

[PeLu]

The only problem/danger with chargers made for smaller cells is, that their shutoff current (end of charge) may be too low.
That means that a charger for a 700mAh cell may have some 50mA end of charge current and a 2.2Ah cell will need more like 150-200mA. This may even more be important when the cell is not charged with the full initial current.
But I do not have any experiences about how much this will influence lifetime (and it it is worth taking care about it therefore).
Vcal charges his/her cells with a lower start current and they tend to be full when reaching the voltage limit.

 

[Doug Owen]

Peter has put his finger right on it. Due to the capacity differences you're going to overcharge the larger cell every time. Whether this just eventually wrecks the cell or *blows it up* is something I can't guess.

My advice is 'don't mess about with these rascals', they're known to be hazardous if abused (why do you think they don't sell us the bits? It's not because they don't want our money.....).

Doug Owen

 

[Doug S]

[ QUOTE ]
PeLu said:
The only problem/danger with chargers made for smaller cells is, that their shutoff current (end of charge) may be too low.
That means that a charger for a 700mAh cell may have some 50mA end of charge current and a 2.2Ah cell will need more like 150-200mA. This may even more be important when the cell is not charged with the full initial current.
But I do not have any experiences about how much this will influence lifetime (and it it is worth taking care about it therefore).
Vcal charges his/her cells with a lower start current and they tend to be full when reaching the voltage limit.

[/ QUOTE ]
Peter is quite right that most Li-ion manufacturers recommend charge termination at a C range of 1/10 to 1/20C. In actual practice it doesn't seem to be too important to cycle life longivity. For years I charged packs that I constructed using no charge termination of any sort. I simply used current limited constant voltage chargers with Vout in the range of 4.21 to 4.23V. Packs were generally charged once a week and I would get 100-150 cycles before noting significant capacity loss. I think that the relatively low number of cycles was likely due to general aging [150 cycles = 3 years at once per week] and due to my 4.21-4.23 charge voltage. Now that I am a bit smarter, I have reduced the charging voltage to the 4.15-4.20V range for my more recently constructed chargers.
These observations were made on Sony and Sanyo graphite cells. Extrapolate to other cell types as you wish.

EDIT: FWIW, at a charging voltage of 4.20V, the residual current into a 18650 size cell in good condition is under 1mA so even chargers with very low charge current termination thresholds should terminate charge on even larger cells. This residual current does increase with use. Probably with enough data one could estimate the general condition of a cell by measuring it's residual charging current.