Battery Charging Question

[kamon]

Hey guys!

I'm working on a project that I need a 7.4v-8.4v output(essentially two 3.7v LIPOs in series).

I'm looking to power the project with 2 18650 batteries, but I am curious on how I should handle them, and handle the charging.
Note: I'm looking to charge with a 5v, 2a USB charger.

I'm thinking of two different options:

1. Put the batteries in parallel, then use a buck-boost converter to up the voltage to 7.4v. Only question is charging here, would I be able to use a tp4056 charger(e.g: http://www.ebay.com/itm/221756164059), since they are the same voltage, just double the mAH? I'd have two identical 18650's, so don't think this is a problem, but I'm still a bit of a noob, haha.

2. Put the batteries in series and use a buck-boost to up the USB input charge to 8.4v, but that doesn't seem very safe at all...Plus, I know LiPos can't be charged directly like that, it's a multi-step process. Would something like this: http://www.banggood.com/2S-7_4V-2A-Li-i ... 44249.html allow me connect the two in series, then just connect a 8.4v power connection to the board to charge them?

I know these are probably idiotic questions, but I'm hoping you guys can look past that and give me some input on how I should handle this .

Thanks in advance!

[moosepr]

That board you have linked from banggood is designed as a charge/project board. It will allow you to throw 8.4v at the battery, and it will (allegedly) ensure that the batteries share that voltage correctly to charge. In my experience, they will even disconnect the battery from the input/output when it is full/empty.

Having the batteries in series simplifies the input, you can basically just throw in a voltage regulator and drop it down to 5v and your golden.

Having the batteries in parallel will allow you to use your tp4056 boards for charging. As far as that board is concerned, it will just act like a bigger capacity 3.7v battery. Now there are dangers to this of course. Batteries from the same manufacturer, and even the same batch will vary. Slight differences in capacity will cause issues if you charge/use then together. When you charge them, one will become full first, meaning this battery will become over charged while the other cell keeps charging. The same is true in reverse when you are discharging them, one will empty first. Your better off buying a larger capacity cell to begin with, or one that has been factory matched and balanced. You can get away with using multiple cells together, but you need the be aware of the risk

I'm by no means an expert, just read a lot!

[kamon]

That board you have linked from banggood is designed as a charge/project board. It will allow you to throw 8.4v at the battery, and it will (allegedly) ensure that the batteries share that voltage correctly to charge. In my experience, they will even disconnect the battery from the input/output when it is full/empty.

Having the batteries in series simplifies the input, you can basically just throw in a voltage regulator and drop it down to 5v and your golden.

Having the batteries in parallel will allow you to use your tp4056 boards for charging. As far as that board is concerned, it will just act like a bigger capacity 3.7v battery. Now there are dangers to this of course. Batteries from the same manufacturer, and even the same batch will vary. Slight differences in capacity will cause issues if you charge/use then together. When you charge them, one will become full first, meaning this battery will become over charged while the other cell keeps charging. The same is true in reverse when you are discharging them, one will empty first. Your better off buying a larger capacity cell to begin with, or one that has been factory matched and balanced. You can get away with using multiple cells together, but you need the be aware of the risk

I'm by no means an expert, just read a lot!

Thank you!

Still a bit confused, though. If I connect the batteries to the protection circuit like this:

battery_circuit.png (16.82 KiB) Viewed 8754 times

I want the output voltage of the batteries to be 7.4v, but I want the batteries to charge with 5v.
Also, am a bit confused on how I connect both the charger and the circuit I want to power with 7.4v at the same time.
Do I need to put in a switch or am I completely missing a piece?

[moosepr]

How strict is the 7.4v need? The output will range from 8.4v when full, to about 6.6v when empty (depending how low the protection allows on that board). Now you could use one of the jazzy adjustable buck/boost boards that keep your output steady, but you need to watch the current loading, most boards only like a maximum of 2 amps.

With the charging you could use another boost board to get the 5v up to the required 8.4v, but it might be safer to get a proper charger and have a different connector for charging.

There is another, slightly less convenient option. You could get some holders for the 18650 cells, then just pop then into a regular charger when they need juice. It's by far the safest option, but no good if you're sealing your batteries inside a case somewhere

[kamon]

Yeah, I was just saying 7.4v to simplify the normal output of two 18650's in series, since they're considering "3.7v" by most descriptions/standards. My project is fine with the 6.6v-8.4v range.

Okay, so let's say I went with the boosting the 5v from the usb to 8.4v for charging, how would I go about connecting that?
Just connect the output of my buck-boost converter to the P+ and P- on the protection board?

And, then, where do I actually connect the power rails for my project? P+ and P- as well?

[moosepr]

Those protection boards tend to have just the one connection for charging and discharging, so that will be your P+/P-. you could connect your boost board up to there (might be safer to add in a switch so it cant become a drain on your batteries), maybe something like this

http://www.ebay.co.uk/itm/201730643792

you will need to adjust the little blue variable resistor to set the output voltage to the required 8.4v (might be worth setting it lower so you never overcharge the cells) then connect that to the P+/P- and it will charge the cells. The only thing you may struggle with, is knowing when they are charged. You could manually measure the voltage (after removing the power in) and see if they have reached 8.4v yet. a proper charger would be able to tell you that it has reached the end by changing the colour of an LED, something like this

http://www.ebay.co.uk/itm/181936173887

sure its not as nice as USB charging, but it would be lots easier!!!

[tiyaray]

Having the batteries in parallel will enable you to utilize your tp4056 sheets for charging. To the extent that board is concerned, it will simply act like a greater limit 3.7v battery. Presently there are perils to this obviously. Batteries from a similar producer and even a similar bunch will fluctuate. Slight contrasts in limit will cause issues on the off chance that you charge/utilize them together. When you charge them, one will turn out to be full first, which means this battery will end up overcharged while the other cell continues charging. The equivalent is valid in turn around when you are releasing them, one will exhaust first. Your happier purchasing a bigger limit cell regardless, or one that has been manufacturing plant coordinated and adjusted. You can pull off utilizing different cells together, however, you need the know about the hazard