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I'm currently building a portable USB charger. It uses 2 Li-ion batteries in series connected to a USB port via a 5V regulated buck converter. While a single Li-ion battery would suffice for most applications, this USB charger needs to be able to output 1A at 5V; a single Li-ion battery of the type I have is only able to output 1A at 3-4V, or a max of 3-4W - therefore stepping up the voltage would reduce the maximum current output. The converter itself is not an issue other than requiring more than 5V to operate (hence wiring the batteries in series to produce an input voltage of 6-8V), since it has a maximum current output of 3A.

For the purposes of charging, I have a relatively cheap TP4056 charging micro-board specifically designed for charging 3.6V nominal batteries such as the ones I have. It's capable of CC-CV charging and automatic shutdown when current drops too low (indicating saturation). However, it's only designed for operating with one battery - as such, it can only detect whenever the battery it is connected to is full. Is there anything I could set up that would allow the charger to switch between batteries whenever one is full, and cease to charge either whenever both are full?

I imagine a relatively simple solution of a manual switch for operation after one battery is full might work, but this introduces micromanagement that I believe can be fairly easily avoided.

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  • \$\begingroup\$ Are you trying to charge 2 li ion batteries (as per your question title) or trying to charge something else from two li ion batteries in series? \$\endgroup\$ – Andy aka Apr 3 '14 at 13:53
  • \$\begingroup\$ Both - the Li-ion batteries are first charged, then used to charge a portable device - but the only issue is the charging of the internal Li-ion cells. \$\endgroup\$ – Alex Freeman Apr 3 '14 at 13:59
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    \$\begingroup\$ are you taking them out of the device for charging purposes? or you want fully built-in two-cell charging? \$\endgroup\$ – KyranF Apr 3 '14 at 14:15
  • \$\begingroup\$ The latter. The system is entirely enclosed within the casing. \$\endgroup\$ – Alex Freeman Apr 3 '14 at 19:19
  • \$\begingroup\$ @AlexFreeman , man, unless you do similar to what i said in the comments of my answer, it will be very hard without a huge and complex balance charge circuit. Those designs are hard to come by, i've been looking all week, for a guy at work. The only thing I can see is you have a carefully set up two seperate single cell batteries, charge them seperately and the at the end join them in series to give the 7.4V Nominal. This only works if you disconnect the system load and charge everything (fast charge style) and then reconnect the load. doing the swapping logic like we said before. \$\endgroup\$ – KyranF Apr 4 '14 at 0:28
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When charging lithium batteries in series, you need what's known as a balance charger to keep the cells at similar voltage. One way that some chargers do this is by stopping the charge of both when one is full, discharging the full one until they are even, and then charge them both up again. There are fully featured chargers meant for the remote control hobby, but they require setting the charge options on the screen before every charge. I cannot seem to find any "plug and play" balance charger boards similar to the TP4056. I am fairly sure that ICs exist that will handle the buck/boost power supply and the balancing, but I can't seem to find one and you would need to manufacture your own circuit board, and the chip is almost certainly surface mount.

That being said, there are a few workarounds. One is to have the batteries in series for operation and parallel for charging, and use either one or two single cell chargers. This can be accomplished through a 3PDT switch, and a good explanation is here. Note that you don't need to use the same charger, your TP4056 should be able to be substituted.

A simpler and more elegant way to do it would be to use a boost mode power supply, and don't put the cells in series. You can attach multiple of your current cells in parallel, so that if you have two cells and are drawing 5W, it would be 2.5W per cell or ~0.67A. There are many chargers that are designed for this application, handling both the CC-CV charging and the boost regulator for the output, such as the Lipo Rider Pro from Seeed Studio which will do 1A output. This is what I would recommend.

Alternatively, you can buy more powerful batteries that will put out more than 1A instead of putting multiple in parallel.

Or if you want to take all the fun out of this project and just buy something that sort of works, there are USB battery banks available from China that will take care of all the charging and discharging of 18650 LiPo batteries. I have seen banks that will take from one to six 18650 cells in parallel, and some of the larger ones can do as much as 3A output. Here are a few examples, and a video review. Their build qualities and user interfaces look pretty bad, but apparently they work.

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A very simple digital switch could be made using an Arduino or other readily available controller for sensing voltage over each (with ADC), and when the first one reaches 4V or whatever your limit is, switch the charging load over to the other one. When both are full, have the controller light up some LEDs to let you know the charging is done.

Is that what you were sort of looking for? That is more automated than just a physical switch (which would just be a "double pole single throw" or whatever)

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  • \$\begingroup\$ I'm afraid it isn't as simple as that. Li-ion batteries are only 70-80% charged when they hit max voltage - after that, they go into constant-voltage charging, where the current is slowly reduced until it becomes so low ~3% of nominal current) that it is considered fully saturated. The TP4056 is capable of detecting this, but a simple voltage detector wouldn't be. In addition, I need charging to actually cease whenever both are charged, rather than simply an indicator activating. Perhaps a hysteresis effect whereby whenever both cells are charged, the circuit switches off until reset manually? \$\endgroup\$ – Alex Freeman Apr 3 '14 at 14:04
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    \$\begingroup\$ yes indeed. If you want fast charge to 80% capacity, it's a useful solution. If you wanted, you could spend $1 for a current shunt monitor (TI has one, INA138 looks good) and use a second ADC input to monitor that too. you can have a system load switch, and a path-switcher for swapping which battery is connected with the output \$\endgroup\$ – KyranF Apr 3 '14 at 14:08
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    \$\begingroup\$ you could indeed have a SR Latch/Flip flop for the condition of "its all done!" and have a little push button which resets the latch condition \$\endgroup\$ – KyranF Apr 3 '14 at 14:09
  • \$\begingroup\$ I see. That helps. I'm not entirely sure how to implement some of these though, especially ADCs. \$\endgroup\$ – Alex Freeman Apr 3 '14 at 20:08
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    \$\begingroup\$ That assumes that both batteries will remain at equal charge the whole time. Slight inconsistencies in discharge may lead to discrepancies in the individual battery charges - and if the charging circuit is only designed to charge one battery at a time, it could prove fatal for the battery that has higher charge. \$\endgroup\$ – Alex Freeman Apr 3 '14 at 21:09
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You can use 0505 DC-DC converter and two TP4056, one of them floating, to charge batteries independently. In this case, second TP4056 is powered from DC-DC, ground of this TP4056 connected to common wire of two batteries connected in series.

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The li-ion normally need to have a connection in between the first and the second one to monitor the voltage, and the picture you show may cause fire, because the after the first battery reach 4.1 or 4.2 voltage it will continue charge and will not stop charging, it need to be connect separate circuit or protection circuit, so the first one battery will stop charging after reach the 4.1 or 4.2 volt and continue charge the second one. It does have the protection PCB for Li-ion battery of 8.4 v. Go look it up on Google search.

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