This is about something people are ignoring when selecting a charger especially new comers to Electrics.Generally people take into account is the no.of cells the charger can charge and the maximum charge current it can handle.And completely ignoring to know  how many watts the charger can handle, which is also a very important factor.
Like in the case of BC6 charger which is a very good charger at a great price and works very well in its limits. When selecting a BC6 or any charger we usually see how many cells it can charge( here 6S) and how many amps it can handle (here 5 amps) and generally assume that it can charge 6S Li-Po battery pack at  5 amp current rate. But it is not correct. BC6 is a 50 Watt charger.Which means it can handle only 50 watts. Wattage is the product of charge Current and Battery Voltage.
Suppose we want to charge a 6 S battery i., V= 6x 4.2 = 25.2 volts.@ 5 amps , we would need a charger which can handle  25.2 x 5 =  126 watts. which BC6 can not handle. because W = V x I,
we can see that the maximum voltage V = 50/5 = 10 volts. which is more than 2S but less than 3S.So BC 6 charger can safely charge a 2S (8.4 volts) at 5 amps.Similarly BC 6 can charge 6S(25.2) at 1.9 amps only.
By using the formula W= V x I or I = W/V we can see that BC6 charger can charge different cell count batteries as below.
2S battery (8.4 v)  , Maximum charge current = 5 Amps
3S battery (12.6 v),                                   = 3.96 Amps.
4S battery (16.8 v)                                    = 2.96 Amps
5S battery (21 v )                                      = 2.38 Amps.
6S battery (25.2 v)                                    = 1.98 Amps.

The above given figures are calculated for BC6 charger. But one could calculate for any charger using the formula  Maximum Charge Current = Max Watts the charger can handle/Full charge Battery Voltage.

Good peice of information. Thanks Rao!

Yeah really useful.  I never knew this.  I assumed that the BC-6 is a charger capable of 126W because it is rated for 6S and 5A!! And now I also know never to charge my 4s pack at 3.3Amps!! 

One more doubt though.. Even if I charge my 4s at 3.3 Amps, I dont think I would be damaging the charger or battery... Only thing is, as the battery voltage increases (when charge is pumped in), the rate of charge (Amps) might drop!!  So probably when the battery voltage reaches 15.15V, then after that the charge rate would drop from 3.3A to a lesser value! Would do some monitoring when I charge it this weekend.

Also, high time I invest in a bigger charger!!

- Chan

Quote from: chanvivek on June 25, 2009, 11:04:45 AM
Only thing is, as the battery voltage increases (when charge is pumped in), the rate of charge (Amps) might drop!!  So probably when the battery voltage reaches 15.15V, then after that the charge rate would drop from 3.3A to a lesser value!

Lithium batteries require constant current to charge. So, even if the battery voltage increases the charger will pump up its charging voltage so that a constant current is maintained.

If you talk about the Lead acid batteries, used in automobiles they are constant voltage charge. The charger will maintain the same voltage no matter what the cell voltage is.

This is what I infered after playing around with batteries sometime in the past.

-Ismail

Can someone recommend a good charger with links?

nice tip never payed attention to wattage/cell numbers

thanks rao 

Quote from: suniljoseph on September 12, 2009, 05:02:16 AM
Can someone recommend a good charger with links?

We have  just received the  following GT chargers in addition to RCPower BC 6

1) GT Power B 6    : Rs 3150/-
2) GT Power A6D   : Rs 4300/- ( including built in 5 A Power supply )
3) GT Power A6 10 ( 10 amp 200 watt charger ) : Rs 5700/-

Rgds
Sai

Rgds
Sai

If anyone is looking out to buy a new charger...

I would strongly recommend the G.T. Power chargers as i use them myself on the field and found them to be superb..

All the best !

atul g.

please post some link to buy 3) GT Power A6 10 ( 10 amp 200 watt charger ) : Rs 5700/
thank you