Hello SKYWLKR,
Li-Ion cells stop at 4.2 volts, regardless of capacity.
When you parallel Li-Ion cells, they equalize. NiMh and NiCd cells do not do this, or only do it in voltage, but not capacity. Li-Ion cells are different.
You can charge cells in series, but it is more dangerous, unless you have an active balancing circuit to insure that each cell does not go above 4.200 volts.
The first Li-Ion cells that came out were only charged to 4.1 volts. I don't believe any of these early cells are still in existence. All current Li-Ion cells can be charged to the same levels as Li-Ion Polymer cells. The older Li-Ion cells could handle charging to 4.200 volts, but cycle life would be reduced. There was not a safety issue with this level of charge.
It sounds like you need to take some time and think about, and figure out how parallel circuits work and what you can expect from parallel and serial battery packs. Once you have a working knowledge of that, charging currents and maximum current draw will become clear. Until then, I would recommend single cell charging.
Let me see if I can get you started...
When you parallel cells, the voltage stays the same and the capacity is added together. This means that if you were to parallel your 1800 and your 1300 mAh cells, you would end up with a 4.2 volt battery (a battery is made up of 1 or more cells) with a capacity of 3100 mAh. A 1C charge for this battery would be 3100 mA, with 1550 ma going to each cell. Since 1550 mA is greater than a 1C charge rate for the 1300 mAh cell, you should not charge at this high a rate. The highest rate you should use is 2600 mA. It is OK to go below that, so it looks like you would be charging at 1000 mA, or whatever the maximum charge rate is for your charger.
If you go back to a single cell, everything gets easy. 1C is the capacity of the cell.
Now, if you hook your 1800 mAh cells in series, you end up with a 8.4 volt battery that has a capacity of 1800 mAh. The maximum charge rate would be 1800 mA and your voltage would terminate at 8.4 volts.
Now, lets put 3 of your 1800 mAh cells in series. Let's also say that they are used cells and may not be well balanced. Now we have a situation where the maximum voltage we are charging to is going to be 12.6 volts, but this is going to be made up from 3 cells. If all 3 cells end up at 4.200 volts, we are good to go. However, if there is some build up of internal resistance of one cell, we could end up with the 3 cells at 4.2, 3.9, and 4.5 volts. This still totals up to 12.6 volts, but the cell at 4.5 volts is overcharged and could be close to rapidly venting with flame. This is the problem with charging Li-Ion cells in series. The advanced Li-Ion/Poly chargers have balancing taps that allow for cell balancing during the charge. If one cell gets above 4.2 volts, the charge is shut off from that cell to keep it from overcharging.
You can avoid all of this worry of overcharging by charging in parallel, or charging each cell individually.
Tom
