I have found a battery pack with 74Wh capacity and it supports Qualcomm Quick Charge 3.0 on its output (up to 18W). I wonder is there anything like a safety limit of battery discharge? Is it even possible to increase the discharge rate above 18W and ensure that the wattage remains stable as the battery packs drains down to zero? I tried to google it quickly without much success but I must admit I have a very limited understanding of this field.

PS: if there is any info or variable missing from my description please let me know, I will be most happy to specify to make this case more useful to everyone

  • \$\begingroup\$ The discharge rate is limited by your load. If the load consumes N Amps then your only choice is a) Reduce the load current b) drop the voltage. \$\endgroup\$ Mar 2 '17 at 15:02

You did not mention the voltage. What you need is the battery's discharge rate. How many amps per hour. Lithium ion usually charge at 0.8 of discharge rate.

Charge and discharge rates of a battery are governed by C-rates. The capacity of a battery is commonly rated at 1C, meaning that a fully charged battery rated at 1Ah should provide 1A for one hour. The same battery discharging at 0.5C should provide 500mA for two hours, and at 2C it delivers 2A for 30 minutes. Theoretically. Faster discharge rates will diminish the rated capacity.

If the battery gets too warm then you need to reduce the charge rate.

The advised charge rate of a Lithium Energy Cell is between 0.5C and 1C; the complete charge time is about 2–3 hours. Manufacturers of these cells recommend charging at 0.8C or less to prolong battery life; however, most Power Cells can take a higher charge C-rate with little stress. Charge efficiency is about 99 percent and the cell remains cool during charge.

Some battery chemistries can take more than a 1C charge.

Lithium Manganese Oxide 3C max
Lithium Titanate: 5C max

These should be charged a .7C - 1C, may be able to go over 1C, check temperature.
Lithium Iron Phosphate
Lithium Nickel Manganese Cobalt Oxide 

These do not exceed 1C.  .7C-1C recommended.
Lithium Cobalt Oxide above 1C shortens battery life.
Lithium Nickel Cobalt Aluminum Oxide, above 1C shortens battery life.

If you drain the battery to zero you will be significantly reducing the number of charge cycles.

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QuickCharge will itself be limited to 18W (9v, 2A?) By virtue of the circuitry being able to cap at 2A, it will prevent you from drawing more.

You're only going to be able to exceed 18W electrically if you bypass the charger itself and tap off the lithium cells. You may (will) damage a lot of things doing this -- the cells themselves may be rated for 18W continuous discharge only, since it doesn't make sense to buy higher-rated cells from an engineering perspective.

  • \$\begingroup\$ Not so. The charger limits only 'charging'. It does not limit discharging rates. \$\endgroup\$ Mar 2 '17 at 16:33
  • \$\begingroup\$ Are you sure? I've read the question again, and it sure sounds like he's trying to increase the output wattage of the powerbank. In this case the battery IS the charger for a secondary device. \$\endgroup\$ Mar 2 '17 at 17:01
  • \$\begingroup\$ If what he has is a powerbank for charging downstream product, then the protocol and driver for the downstream product being charged limits the current to that appliance. In all probability the powerbank is at a much higher voltage and uses a buck convertor (current limited) to supply the downstream controlled current (although it could also be a boost convertor of course). The current from the batteries is NOT the same as the charger controls and as the battery voltage drops the current increases. Hence My comment the charger only limits charging current not the discharge current. \$\endgroup\$ Mar 2 '17 at 21:20

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