# li- ion battery discharge query

In a 3.7V 2000mAH Li-ion 18650 battery, does it mean that the battery can deliver upto 2A for 1hr depending on the load...Or it will deliver like a constant current source which will deliver 2A for 1hr.?

Can somebody clarify on this?

The battery will not act as a current source, but if you connect it to a constant current load that draws 1A, the battery will in theory be able to deliver current for 2 hours.

Chemical and resistive losses makes the real-world number lower than the theoretical one. And the more current you draw (the more amps), the higher the losses will generally be.

The voltage not coming into the equation makes Ah a less useful metric than Wh for many applications, but that's the convention. Especially when you're using the battery with constant power loads, you won't care as much about the current capacity as the power capacity.

If you know the nominal voltage of the battery, you can multiply the nominal voltage (V) by the current capacity (Ah) in order to get the power capacity (Wh). Using your example, 3.7 V * 2 Ah = 7.4 Wh.

• To be fair, many (most?) circuits are current "driven", as opposed to power "driven". I cant really think of many constant power loads. Cerainly most digital circuits will be current loads that dont care about the power. Commented Jun 25, 2018 at 15:54
• @BeB00, a battery usually drives a DC-DC converter, which usually outputs constant voltage/current and loaded with constant load. An example would be a well-designed flashlight. Therefore the draw from a battery is "constant power" in most cases. Commented Jun 25, 2018 at 20:11
• Many many circuits just use an LDO, although dc-dc converters are becoming more common Commented Jun 26, 2018 at 12:21

The Ampere-hour rating of a battery is a measure of the energy contained in the battery, and has little or no relation to the current that the battery can deliver.

A 2 Ah battery can theoretically deliver 2 Amps for 1 hour, or 1 Amp for 2 hours, or 0.5 Amp for 4 hours, and so on. In practice, you will get less total energy from the battery if you discharge it at a high rate. The Ah rating is gives the energy available when discharged over a "standard" time - for lead-acid batteries, this time is 20 hours. I think I've seen 5 hours mentioned for some lithium cells.

• "The Ampere-hour rating of a battery is a measure of the energy contained in the battery" I believe the Ah rating is a measure of the charge contained in the battery. The energy is measured in Wh. Commented Jun 26, 2018 at 9:21
• @Dampmaskin: would you accept "a partial measure of the energy...", as Ampere-Hours does not involve voltage, where Watt-Hours does. Commented Jun 26, 2018 at 15:16

In theory, the 2000mAh rating means the battery can provide 2A for the duration of 1 hour.

In practise, it won't be the case unfortunately. You have losses all over the place, heat from components, battery voltage dropping, power inefficiencies, and many more.

As a quick answer to your question, yes, your reasoning is correct that 2000mAh does mean 2A for 1 hour, however, don't expect it to be the case when putting it to the test!

A general equation for battery life would be (mAh/mA)*0.7 where the 0.7 is there to account for unexpected losses, and give you a slightly more useable figure to work with.

I disagree with a simple statement like: "2000mAh rating means the battery can provide 2A for the duration of 1 hour." It may be able to provide 2A or not. It's all up to the max discharge current. If that's above 2A, then the statement is true, otherwise it is false. It's like saying you can remove the content of a standard 0.5l beer bottle in under 0.1 seconds. Yes you can, but not without shattering it.

mAh is measurement of capacity and it stops there. Old 2000mAh cells have a discharge current in their specifications of 500mA or even lower, which means they will last 4h at that discharge rate rate. If you want to discharge them at 2A they will simply overheat and burn after a few seconds or minutes. That does not mean we don't have cells today supporting exactly 2A continuous discharge. Yes, we may find cells with .5A,1A,2A or even 30A discharge rate.

So to recap, mAh give you a batteries capacity while discharge current (in A) gives you the maximum current you are allowed to safely drain from it. Both are clearly stated in the official specifications of a 18650 cell or other li-ion battery.