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Help with charging 2 18650 in series

Lameass

New Member
Hi, this is a part of a project I am working at right now. I have a solar panel (12V/20W) and I want to use it to recharge 2 18650 batteries in series and these batteries will be simultaneously powering Arduino nano. My question is: Will the solar panel be able to keep these batteries charged (arduino with all the components won't take more than 50mA) and is this chip MCP73213T that is used to charge the batteries connected right? (don’t want my **** catching on fire) Thank you
 

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gophert

Well-Known Member
Most Helpful Member
You'd be better off operating it with one battery so you don't have to worry about balancing the two lithium cells. Adafruit.com makes a small circuitboard that can charge a lithium battery safely and then boost the lithium battery voltage to operate USB (5v) devices. Super simple if you can regulate the voltage from your solar charger to 5v (input voltage for the LiPo charging circuit.

 

rjenkinsgb

Well-Known Member
Most Helpful Member
Battery negative should connect to VSS and charge negative; you do not seem to have them connected together?

Note that you also need a "balance" circuit and over-discharge protection, to avoid damage to the batteries.
Look for a two cell BMS board on ebay, that also has balance included.

Without that, if the cells end up with slightly different charge levels, the one that reaches full charge first can be over-voltaged and possibly burst, before the charger cuts out.
 

Lameass

New Member
Alright thank you guys so much and yes the battery terminal were wrong.
Just to be sure the BMS board should be connected to the battery terminals of the MCP73213T and then on the BMS board I will connect the batteries right?
And can I power the Arduino while the batteries are charging ?
 

gophert

Well-Known Member
Most Helpful Member
And can I power the Arduino while the batteries are charging ?
yes, the Adafruit device is intended as an uninterruptible power supply so it can be changed and power an external device at the same time. See last part of below description of the adafruit part. Also, for projects needing less than 1 amp, there is a 500mA version for $5 as well.

Since the built-in battery charger has load-sharing, it will automatically switch over to the USB power when available, instead of continuously charging/draining the battery. This is more efficient, and lets you charge-and-boost at the same time without any interruption on the output so its fine for use as a "UPS" (un-interruptable power supply). Please note that this board is designed to be used only with a LiPoly attached, it won't work without a battery.
 

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