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In another question, I was trying to figure out what kind of battery I could use to provide enough current to power my circuit. Someone pointed out that batteries often have a maximum discharge rate. In looking at data sheets, I found that these rates are often noted in units of "C."

At first I assumed it was related to coulombs and 1C was 1 amp for 1 second, but that doesn't appear to be the case (except, I guess, coincidentally for a 60,000 mAh battery maybe?).

Instead it appears that 1C is the nominal rating of the battery? So, a battery rated at 3500 mAh was tested and from fully charged it will discharge 3.5 amps continuously for 1 hour before it is fully discharged?

So, if I have a circuit that requires .5 amps and a 840 mAh battery, I only need a 1C maximum discharge rating to be able to power my circuit? Will it power my circuit for roughly 1.68 hours (potentially more because of the reduced stress)?

If, on the other hand, I switch to a battery that's rated at 2700 mAh, then my current draw is closer to .2C or .54A/hour? Would that power my .5A circuit for roughly 5 and a half hours?

Finally, why do we rate batteries this way? And why isn't the maximum discharge current rated in amps instead of ratios of discharge rates?

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    \$\begingroup\$ Your understanding seems to be generally correct. As for why... I wish I knew. \$\endgroup\$ – Hearth Jan 30 at 15:29
  • \$\begingroup\$ Why would the current draw change when you choose a different battery (".54 A/hour")? Also, A/hour would be a rate of change of current. Ah and Coulombs are both units of charge, A is a unit of current. \$\endgroup\$ – pbfy0 Jan 30 at 16:44
  • \$\begingroup\$ @pbfy0, the current draw doesn't change; it's about .5A in both cases. My understanding is that based on the rated capacity of the battery, the value of 1C changes proportionately. I agree regarding amps being a rating of current. I guess I'm asking why they give me maximum discharge rate in C instead of amps since what I care about is amps no matter what the capacity is. I guess I just have a hard time imagining saying, "hmm, I wonder if this battery will let me discharge its entire capacity in 5 minutes," vs. "I wonder if this battery will explode if I put it in this flashlight." \$\endgroup\$ – D. Patrick Jan 30 at 18:28
  • \$\begingroup\$ @D.Patrick It's useful for charging, I guess... because it tells you how long it will take to fully charge. I'm as lost as everyone else for why that is the quoted number for discharge. \$\endgroup\$ – pbfy0 Jan 30 at 18:34
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The way batteries are specified has a very long history. I don't like it either but I've learned to live with it.

An important thing you should know is that the capacity (C) is specified at a specific discharge rate and you need to know what that rate is. Sometimes it is specified like "C5" or C\$_5\$", which would mean that the rated capacity assumes that the discharge current is 1/5 of C.

If you discharge at a higher rate then the actual, available capacity will generally be less. Discharging at a lower rate generally provides a higher actual capacity.

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    \$\begingroup\$ I wonder if it's possible for any battery chemistry to have a non-zero optimal discharge rate, where discharging either faster or slower would give a lower total capacity. \$\endgroup\$ – Hearth Jan 30 at 15:33
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    \$\begingroup\$ I think if you wanted to use a discharge rate that was lower than a battery's self-discharge rate then the actual capacity would be lower than expected. As an extreme example, if you used the C value of a AA alkaline battery and expected to discharge it over 50 years you would be sorely disappointed. \$\endgroup\$ – Elliot Alderson Jan 30 at 15:37
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    \$\begingroup\$ Okay, good point. Didn't think of that, but you're right. \$\endgroup\$ – Hearth Jan 30 at 15:39
  • \$\begingroup\$ Most battery data sheets these days will have a family of discharge curves, showing the volts per charge used at various discharge rates. It can be very useful when choosing what cell to specify for a product. \$\endgroup\$ – TimWescott Jan 31 at 4:24
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C is a current equal to capacity of the battery in one hour.

  • for a 2200mAh battery that would be 2200mA
  • for a 7Ah battery 7A

It's used algebraically like any other constant.

  • C/5 ( or 0.2C) is one fifth that current
  • 2C is twice that current

C is not necessarily a suitable discharge current.EG lithium button cells with 360mAh capacity struggle tp produce more than 100mA

So, if I have a circuit that requires .5 amps and a 840 mAh battery, I only >need a 1C maximum discharge rating to be able to power my circuit?

if the maximum discharge rat is given as 1C then yes, C is 840mA, and your requirement is less than that so the battery is suitable.

Will it power my circuit for roughly 1.68 hours (potentially more because of the reduced stress)?

Batteriy capacity is often specified at a C/20 discharge current, (the current that depletes the battery in 20 hours is C/20). Discharging at a higher rate may reduce the avaialble energy.

So C may have been measured at a lower discharge rate. I woiuld expect at-least 1.5 hours, but that's just a guess. the datasheet for the battery may give discharge curves.

If, on the other hand, I switch to a battery that's rated at 2700 mAh, then my current draw is closer to .2C or .54A/hour? Would that power my .5A circuit for roughly 5 and a half hours?

Yes something like that.

Finally, why do we rate batteries this way? And why isn't the maximum discharge current rated in amps instead of ratios of discharge rates?

becasue it gives a number that scales with the size of the battery, which is very useful when generalising about similar batteries that ony differ in capacity.

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