So basically how can I charge both the cells without adding any more major components?
You severely underestimate the complexity of two-cell LiPo charging. A specialised IC designed for two-cell Lipo charging is recommended, as there is a complex sensing and 'balancing' process that needs to happen during charging to ensure both cells are changed at the same rate/voltage, else problems can occur.
A possible IC for charging 1,2, or 3 Cell lipo batteries is the Texas Instruments BQ24133 and similar devices. They have a large range of battery managment ICs, check it out.
arduino needs 7volts to operate.
Well that is not entirely true - it needs 7 volts because it has a crappy cheap linear regulator on the Vin input jack. It has a large drop-out margin, so needs the 2V gap between the 5V output and the 7V+ recommended input. You can bypass this with your single cell lipo by putting it directly into the VCC pin (V5+ pin). PLEASE NOTE YOU MIGHT DAMAGE YOUR ARDUINO, AND YOU CANNOT RUN THE ATMEGA328P AT 16MHZ ANYMORE!!
Basically, you will need to somehow get a 3.3V, 8Mhz version of an arduino (Like the arduino Pro [mini] with 3.3V 8Mhz setup on the board) and do what I mentioned above. Please be careful, there is no reverse protection or over-voltage protection on this pin, as it's assumed to be nicely regulated and protected by then. Don't say I didn't warn you!
(Or how can I power this setup?)
I suggest you just get a much larger capacity single cell Lipo pack and let the bluetooth module's integrated charger do it's thing. Nothing wrong with 3.7V. Also your 2-cell Lipo will need regulating down to 3.3V somewhere, so there is losses involved with this as well. You must check that if you will use a single cell lipo charged fully to 4.2V, if the system will not be damaged by this - check max ratings for all devices in their datasheets. Maybe you should look at getting a 3.3V LDO (low dropout regulator) and plug the single cell lipo into this, which will nicely drop out the system at 3.4~3.5V and remain very efficient the whole time.
I suggest a Micrel MIC5205 for that purpose I mentioned above.
There are benefits to doing high voltage (like lower input currents), for motor driving etc and avoiding regulation of those high current driven devices, but for a low-power embedded system I suggest just staying with a single cell lipo with high capacity.
good luck!
There are very expensive $250- $300 chargers that do higher than 6s, probably closer to 10s, but not a lot to choose from and right now my memory is drawing a blank on their names, but they do exist.
I have no idea why there are not more, but I suspect that the demand is simply not there yet. Lithum batteries at those higher voltages are not as common and can be very expensive as are they chargers.
The charger you link to balances its battery by having a charging connection and balancing connection at the same time. The charging connection is directly connected to the + and - of the battery and supplies the main charge. The other connections are more complicated and, for example, in a 6s battery there would be 7 connections, one at the "-", on at the "+", and a connection or wire coming from every single cell connection. So each time another cell is added to make it a 2s or 3s, a wire comes out between the "+" and "-" of each cell added. So a connection between all 6 cells and one at the botom or "-" and one at the top or "+" and you have 7 wires coming out that will then plug into the side of the charger.
The charger then monitors each individual cell's voltage as it is charging the battery as a whole, but most chargers don't seem to start balancing until the battery is essentially full, or at least one cell is at 4.2 volts. Then it uses the seven wire connection to balance the battery, usually by discharging the higher voltage cells a little via a small current, and then charging the whole battery again slowly. Then repeat until all balanced.
It looks like what you linked to would work, except that they are for smaller number of cells in series than what you want to do.
Another option that would work for you is to do what you are doing - split the 10s into 2 5s and charge them independently, but parallel charge them using a parallel charging board and then you could charge them at the same time.
Check this out: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14856
Best Answer
It looks like this setting is OK.
One LiPo cell will have a (maximum) voltage of around 4.2 V.
2S means that there are 2 cells in series. In series means that the voltages add up so for a 2S battery you get 4.2 V + 4.2 V = 8.4 V.
Now things get confusing because on your battery is says 7.4 V!
When a LiPo cell is fully charged its voltage is around 4.2 V
When a LiPo cell is almost empty its voltage is around 3.7 V
What they list on batteries is the "almost empty" voltage which is around 3.7 V per cell, of which there are 2 in series so: 3.7 V + 3.7 V = 7.4 V.
Your charger also says "2S (7.4 V)" so that's OK.
The Current setting might be a bit high. I would take that down to 2 or 3 A especially if you can accept the longer charging time.
Charging / discharging cells with high currents decreases their lifetime so if you can, take it slow, use a lower current.
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