My below question is regarding batteries.

I have a USB charger and a mobile battery (Li-ion battery).

So, my mobile battery percentage has dropped to 0%.

So, what I do now - Take the USB Charger. Plug it in the wall socket and measured the output voltage of the USB Charged. I measured it to be 5V.

But once I connected it to my mobile battery, I measured the voltage in the USB Line and the current through it. (Ignored the D+ and D- lines)

I measured the voltage to be around 4.6V and current to be 0.92A.

My questions :

  1. When my mobile battery voltage is showing 0%, but the voltage between my mobile battery terminals in 4.6V. I have read that it stops around that voltage because some chemical reactions cannot occur. But My fundamental question is - Even while having 4.6V, why cannot my battery provide current to the mobile and keep it working? Like, the battery has potential difference but is not providing any current? Why is that?

(This is not specific to the only Li-ion batteries. I have seen 12V Lead acid batteries also not providing current even though the voltage across the terminals is around 11V? Why is this happening even though they have the potential difference? Like potential difference should cause current flow right? If potential difference is present, why is the current not flowing from the battery)

  1. My USB charger was showing 5V before connecting to the mobile battery. But once I connect the charger to the mobile battery, the voltage on the line is showing 4.6V and the current is around 0.92A? The output of the USB charger is supposed to be constant regardless of the load current, right? If not, then it is not a good regulator or charger, right? My question here - Why is the voltage dropping to 4.6V instead of being 5V, when connected to the mobile battery and what determines the current value to be 0.92A?

My USB Battery charger rating is 5V & 1A. So, shouldn't it output 5V & 1A instead of 4.6V and 0.92A?

Please help me to clear my misunderstanding the about two concepts of charging batteries


2 Answers 2

  1. 0% is an abstraction,

The voltage will never actually be 0V, but the batteries ability to provide current is greatly reduced and there is possibly risk to damage the battery permanently.

The battery controller says "no more " and shuts it down.

The open circuit (no load) voltage of a battery is dictated by chemistry and does not change much.

  1. Strict USB spec is 4.4- 5V is allowed range. So it is legal.

Everything in real life has a margin, that is there is always +/- around every specification. Some derating (reduced voltage , lifetime, current or other spec ) at 100% nominal load is typical.

These supplies are often designed to be inexpensive and do not have much "headroom" .

  • \$\begingroup\$ Do 12V Lead acid batteries also have the battery controller? \$\endgroup\$
    – user220456
    Commented Jun 15, 2020 at 5:15
  • 1
    \$\begingroup\$ No, they don't. \$\endgroup\$
    – Justme
    Commented Jun 15, 2020 at 5:25

The USB powerbank is more complex than you think. It contains a Lithium battery and the voltage of the battery must be withing range of approximately 2.7V to 4.2V. If the battery voltage goes below 2.7V or above 4.2V there can be permanent damage, and the battery might become dangerous. It also needs a safe limit for charging current.

So to connect it safely there are electronics, such as charging circuitry to allow it being charged from a 5V supply, and boost circuit to convert the battery voltage back to 5V for the mobile phone.

The charging circuit knows what is the safe charging current for the battery, and if it is set to 0.92A then it will draw only 0.92A from the mains adapter. The mains adapter may not be able to provide full 5V with full 1A load, and there is some loss due to wire resitance, so having sightly less voltage is normal, and some chargers can give voltages slightly more than 5V.

And when the battery is at 0%, the Lithium cell has about 2.8V voltage, so there is very little energy left in the battery, but as there is no load, the boost converter can still convert 2.8V to 5V. Since there is no load, the voltage may not be exactly 5V, the boost converter may be just probing with 5V pulses if there is a load connected or not, and it goes to sleep as there is no need to provide voltage. Predicting remaining energy purely from Lithium cell voltage is also not exact.

And yes, potential energy does exist without releasin it in the form of current flow. You lift a weight on the table, it accumulates potential energy, but the table prevents the release of potential energy. You store water into a dam and close the valves, you have potential energy but no current flowing. You have pressurized water in your garden hose, but the valve is closed so there is no current. Also, if you pinch the garden hose, the pressure still accumulates, but pinching the hose limits the current flow, and the water will only dribble out if you open the valve.

  • \$\begingroup\$ Two questions - 1. Why don't the battery work even if it has 4.6V instead of 5V? And the same for 12V Lead acid battery case (11V instead of 12V)? 2. And during 0% mobile battery condition, when I plug the charger, initially I see 4.6V? So, does it mean, the mobile battery at 0% will have voltage around 4.6V ? Or is the 4.6V because of the charger adapter since it cannot provide the full 1A at 5V? wanted to understand the 4.6V value whether it is due to the charger voltage drop due to high load current pull by the battery or is it because of the mobile battery voltage is stuck at 4.6V? \$\endgroup\$
    – user220456
    Commented Jun 15, 2020 at 5:35
  • \$\begingroup\$ Because when it's empty, it can't provide current, and trying to pull current makes the lithium ion cell voltage go below cutoff limit of 2.7-2.8V and the circuit turns off to protect battery from damage. Empty 12V battery can measure 11V under no load, but it still does not have the cemicals left to provide any current to a load without voltage dropping. And since we don't have the schematics for your powerbank, we don't know if the charging port is directly connected to the output port or not. The Lithium cell would not have 4.6V on it, it would be dangerous. \$\endgroup\$
    – Justme
    Commented Jun 15, 2020 at 5:43
  • \$\begingroup\$ I just looked upon the battery specification. It is a Li-ion polymer battery with a Nominal Voltage of 3.85V and a maximum voltage of 4.4V. But I used a USB charger to charge the mobile battery and it showed 4.6V? Shouldn't the battery get damaged if I apply 4.6V when it says its maximum voltage is 4.4V? \$\endgroup\$
    – user220456
    Commented Jun 15, 2020 at 8:09
  • \$\begingroup\$ I already explained that there is a charger circuit between 5V input and lithium battery. The 5V input is not directly applied to the battery. It would be damaged if it were applied directly. \$\endgroup\$
    – Justme
    Commented Jun 15, 2020 at 8:49
  • \$\begingroup\$ Could you tell me what the charger circuit does? Is it a buck converter which steps down the voltage to less than the maximum 4.4V? And one last question - Why do we say a battery is dead when it is below the 2.7V for the Li-ion case? How come it has voltage but not give current? I am very confused. I understood your analogy. But I am confused? How does the battery go dead at 2.7V? \$\endgroup\$
    – user220456
    Commented Jun 15, 2020 at 8:52

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