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ArcticMyst Security by Avery

Wiring a battery charger to a circuit

Joined
Feb 19, 2009
Messages
879
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18
Hello everyone,

Can anyone explain to me where I would add a battery charger to a circuit where a battery is already wired into? If the battery is already wired into a circuit, should I just connect the charger directly to the positive and negative of the battery (extended wires from the battery), or do I connect the charger to another end of the circuit?

I want to use this to connect to the batteries...
https://www.sparkfun.com/products/9614

And put 2 of these batteries in series...
https://www.sparkfun.com/products/731

My phone charger cable will work with the breakout board. It has the micro B connector on one end and a USB on the other. It plugs into my phone charger. Would my phone charger work with the two batteries above?

I hope some of this makes sense. I'm having a hard time explaining it because I myself am confused about how to do this.

Thank you for the help!
Matt
 





Joined
Sep 12, 2007
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No, your phone charger will not work. The main reason is it's not actually designed to charge batteries - it needs additional circuitry that's in the phone. It just feeds the phone 5V to do with what it pleases. It's a bad idea to charge two lithiums in series unless you monitor the voltages or have a circuit that automatically balances them. You have lots more reading to do on battery charging.
 

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Yes, but not if they're connected in series. What is your application here? Why do you need 2 packs in series?

If you could modify your project to only need 1 LiPo pack, possibly by using a boost converter like I just linked you, charging and protection becomes a lot simpler. As soon as you go above a single cell, you have to start worrying about cell balancing, monitoring etc.
 
Joined
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I need them to run a 3.3V Arduino Pro Mini which is going to control a tiny pager motor and another micro dc motor which i think is rated at 3V, an LED switch, and a separate led. Im afraid one of those cells wouldnt be enough so i wanted to wire two in series. The arduino wont be running continuously though, just for a moment as i hold down the switch... Which will be no more than 5 seconds.
p
 

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2 cells in series would give you 7.4V, which would just be turned into waste heat by the Arduino's onboard regulator. You can run the Arduino directly from a single LiPo, by feeding it into the Vcc pin (not the RAW pin). This is the pin directly connected to the power input of the ATMega, which will cope with the 3-4.2V range of a normal LiPo.

If the runtime is not enough, you can put more cells in parallel, which is a lot easier to charge than in series, just treat them as if they were a single cell. ie, 2x110mAh cells in parallel, charge them as if it was a single 220mAh cell.

Alternatively, use that board I linked before, then you can just treat it as a regular power input, and the cell charging is taken care of. LiPo cells don't like going below about 3-3.2V per cell, and below about 2.8V they start taking damage.

The regulator on the Pro Mini has a dropout voltage of 0.05V, which means you can use any voltage above 3.35V. So you could even feed the single LiPo into the RAW pin, you'd just have 0.05V less of runtime.
 
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Joined
Feb 19, 2009
Messages
879
Points
18
That was incredibly helpful! I really appreciate it :)
So when wiring it to charge, do I extend wires from the battery from the beginning of the circuit or do I just choose any point of the circuit to run the positive and negative from the battery to the 5V and GND on the charger/booster?

EDIT: Nevermind! There is a tutorial for the board! Thank you very much for your help!
 
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