# how to charge a li-ion battery with a small amperage?

I have created a power supply which provides 5V and 15mA. My battery (3.7V,520mAh) doesn't charge because this current isn't strong enough,it starts charging from ~150mA - 200mA. But my application consumes 2mA in average. For my battery to accept the charge, I plan to use a shunt battery charger. Is it the good thing to do?

• A lithium-ion battery will still charge (slowly) at very low current. To avoid overcharge you must keep the voltage below 4.23V. Normally this is done by reducing charge current when it gets to 4.2V. I don't know what a 'shunt' battery charger is, but proper Li-ion charger IC's and modules are cheap and readily available. – Bruce Abbott May 17 '17 at 18:23
• If I decrease my voltage, I could charge my battery ? Do you think it's sufficient ? – Dipo May 18 '17 at 7:58
• If you limit your charging voltage to 4.20V then yes, it should be sufficient to charge the battery (very slowly). You should also ensure that if the charger is not powered it won't discharge the battery (eg. disconnect the charger when it is not powered, or put a reverse current blocking diode in the charging circuit). – Bruce Abbott May 18 '17 at 8:11
• For now, I tried to charge with 5V voltage and 500mA. My battery answers positively so i don't understand why my battery would charge if i decreased the voltage? – Dipo May 18 '17 at 11:10

Use a dedicated charging chip like MCP73831. Maximum current is set with a single resistor.

• Please elaborate, maybe you could explain how a charging circuit would be better than a shunt. – laptop2d May 17 '17 at 17:46
• If i use this type of circuit, i resolve my problem? – Dipo May 18 '17 at 8:07
• Yes, everything is automated. The chip looks at battery voltage, does constant-current charging, stops charging when current is low (battery full), when battery self-discharges it will top up the battery. – filo May 18 '17 at 9:06
• Good. Even if i've small currents (30mA), i could charge my battery? – Dipo May 19 '17 at 12:42
• Yes - read the datasheet at least once, it says: "Programmable Charge Current: 15 mA to 500 mA". – filo May 19 '17 at 12:44

Firstly you should not be charging with such a high voltage. Your charger should only supply a maximum of 4.2V to 4.3V. Secondly the charge current available is far too low and at that rate (15mA) it will take over 34 hours to charge.

Aby

• Ok but you don't answer my question so could you do it? I agree with you, the voltage is too high. i can increase my current. As i said, my application consumes 2mA so it's not a problem because charging the battery more slowly (in theory) also results in higher efficiency as you loose less energy to ohmic losses and the ESR of the battery. – Dipo May 17 '17 at 14:50

First, if someone is concerned about efficiency, a shunt regulator is not the most efficient dc to dc regulator.

Secondly, LI-ion batteries are not forgiving of design flaws in their chargers. To avoid major problems (fire and explosion) their charging profiles need to be carefully observed - Voltage, current, time, battery temperature...

Here's a reference to app notes and data sheets to get the full story. http://www.linear.com/search/search.php?q=li-ion+charger