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I am connecting a load to a Li-ion battery (4.2V), but I don't know how much maximum current can pass through a Li-ion battery. When I know it, I will connect the load accordingly.

What is the maximum current which can pass in a Li_ion battery?

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    \$\begingroup\$ What does the battery data sheet say? Usually there will be specs for standard, rapid and maximum pulse discharge current. \$\endgroup\$ Nov 9, 2019 at 4:39

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As a rule of thumb small li-ion or li-poly batteries can be charged and discharged at around 1C. "C" is a unit of measure for current equal to the cell capacity divided by one hour; so for a 200mAh battery, 1C is 200mA.

Example: common 402025 150mAh battery from Adafruit: quick charge 1C, maximum continuous discharge 1C.

Slower charge and discharge eg 0.5C or 0.2C gives better capacity, close to the nominal for the battery, as well as longer life in cycles. Many battery datasheets only guarantee the number of cycles for 0.2C charge, even though they do allow up to 1C charge.

Individual batteries differ a lot; some I've seen for example only allow charging at 0.5C max in the spec sheet (which is pretty slow for a battery made nowadays), and others such as quadcopter batteries allow discharging at 10C or even 20C (which is impressive, but also potentially very unsafe...) So the rule of thumb is definitely not perfect; check the datasheet.

P.S. This is for small batteries under 2000mAh or so; large batteries are a different game, party because they are often packs made of small batteries in series and/or parallel, and with much more complicated management circuits.

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Do you have a part number?
What size is the battery (physical dimensions)?

Here is a method that will often give you a ROUGH indication for a wholly unknown cell:

  • Charge fully

  • Then discharge at a rate that doesn't greatly decrease the terminal voltage instantaneously until Vcell is about 3.8V.

  • NOW find the load current which will decrease the cell voltage instantaneously by about 0.2 Volt.

While this does not give bulletproof certainty it gives some measure of what the battery is "happy" with.

Method test:

Rather than using the 0 load to 0.2V drop load here I have used the available dischjarge lines of minimum discharge curve at 3.8V to discharge curve at 3.6V. The results are 'close enough' for this purpose.

  1. In this datasheet at 3.8V, loading to 3.6V takes discharge from 0.2C to 0.8C - thereby giving a fair indication of the battery C rating

  2. On the graph below - take the 3.8V / redline (1A) intersection. Draw a line vertically downrads to 3.6V. This about intersects the 5A yellow line. This suggests an "OK max" discharge rate is 5-1 = 4A. That's about 4A/3.5Ah or ~= 1.15C for this cell - ie, about right for an unknown capacity and capability cell.

enter image description here

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