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I'm doing a hobby project where I'm designing a smart RC car around the size of a mini sim card. The BOM has been narrowed down, and I reckon it's doable with a 4-layer board.

enter image description here Image source: https://www.gsmarena.com/glossary.php3?term=sim

I'm now doing research on possible batteries to use. I'm eyeing the Panasonic CG-320 (15 mAh rated) or this 70mAh LiPo battery. Both are well within my size targets, but I'm unable to find any datasheets that specify a maximum discharge current.

For reference, my power budget is less than 5mA when sitting idly, less than 100mA when running around, and 300mA worst case (e.g. in case of motor stall).

Are there any rules of thumb regarding discharge? Based on what I've read, a maximum current of 0.75C for charging is ideal, but I'm unable to find any figures for the other way around.

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    \$\begingroup\$ Generally: No datasheet, no sale. It's really as simple. But, really, usually lithium batteries have their voltage, their nominal capacity and the maximum sustained discharge current printed right on them or very prominently mentioned in a datasheet. \$\endgroup\$ Aug 20 '20 at 10:19
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    \$\begingroup\$ Also, you mixed up "Ah" and "A" in your link description. If the difference isn't clear to you: you need to read up on this, and it will show you whether the 15 mAh battery is even a candidate for you. \$\endgroup\$ Aug 20 '20 at 10:19
  • \$\begingroup\$ (when you draw 100 mA, your 15 mAh battery will last 15 mAh / 100 mA = 0.15 h = 9 minutes; don't know whether that's acceptable to you.) \$\endgroup\$ Aug 20 '20 at 10:21
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    \$\begingroup\$ I think you might be happy about this datasheet, especially! \$\endgroup\$ Aug 20 '20 at 10:33
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    \$\begingroup\$ no, but batteries are really among the things that are still "if in doubt, measure yourself" \$\endgroup\$ Aug 20 '20 at 11:19
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Discharge is rated in "C" for example if your selected battery states 20C the maximum discharge is 20 * Battery capacity.

One of the reasons LiPo batteries are used in RC projects is the fact they can normally handle a high C rate (They can deliver a punch to the high-power motors).

If we look at the two options, you provided

Panasonic CG-320 - I've found a more detail breakdown of its specs here - LINK

The highest diagram they shown details a max discharge of 1C, they also mention in the handling guidelines to not let the battery voltage drop below 2.7V so you may need to do some voltage monitoring to be safe. Annoyingly thats the only mention I can find of discharging, so from that you could possibly assume the max discharge is 1C.

enter image description here

The 70mA Battery you linked has basically no technical documentation I'd recommend looking for a better documented version. Without a datasheet its impossible to know whats safe.

A better documented alternative would look like this Farnell LiPo this part has a datasheet with all the info you need here - Datasheet. I would recommend looking around the high end suppliers, Digikey, Farnell, RS & Mouser they generally require suppliers to provide proper technical documentation.

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To choose a minimum capacity battery use the 1C rate on small cells ie. Ah capacity remaining/ 1h and worst case impedance e.g. Vmin/IMax=3.2V/0.3A=≈10ohm load and let that be 50x higher for 2% loss in battery voltage.

Thus ESRmax≈200 mOhms Max say at 10%SoC. ESR can be derived from the discharge curves between 0.2C and 1C whereas best ESR is generally highest where the curve spread is least.

The spec examples are slightly below this (for best case SoC Max.)

Thus my rule of thumb battery bank ESR<2%Load surge and average load for 1 hr , Ravg>Vavg/Ah

Your requirements may be different.

E.g. instead of 500 charge cycles you can use 2 cells alternating down to 50% SOC and 4.0 CV max and get only 40%Ah yet get 2500 charge cycles each or so or over charge to 4.35 and undercharge to get only 100 ish charge cycles.

The physics are based on Peukert’s Law.

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