Why the tension in a battery increases without a load?

Thread Starter

ajoaodetodos

Joined Jun 8, 2020
15
I have four 3.7V ion-lition batteries each connected in series (14.8V). The voltage measured in the batteries before starting the test was 13.5V. When connecting a load (DC motor) with consumption of 220mA, the voltage in the battery started to drop and I disconnected the circuit when the voltage measured in the batteries was 7v. Then without the load, I noticed that the voltage in the battery slowly increased again until it stabilized around 12V. Why did the tension increase without a load?
 

Alec_t

Joined Sep 17, 2013
12,064
Any battery has internal resistance, which drops the voltage at the battery terminals in proportion to the load current.
Unfortunately, if you discharged the battery down to 7V you have probably damaged at least one of the cells :eek:. Lithium-based batteries require proper charge and discharge profiles for safe operation and cell longevity. Read up about them at the Battery University site.
 

wayneh

Joined Sep 9, 2010
17,152
I have four 3.7V ion-lition batteries each connected in series (14.8V). The voltage measured in the batteries before starting the test was 13.5V. When connecting a load (DC motor) with consumption of 220mA, the voltage in the battery started to drop and I disconnected the circuit when the voltage measured in the batteries was 7v. Then without the load, I noticed that the voltage in the battery slowly increased again until it stabilized around 12V. Why did the tension increase without a load?
Part of the internal resistance in a battery is caused by diffusion gradients. Not all of the chemical energy in the battery is immediately available at the poles, the spent chemical must diffuse away and fresh chemical moves in. Any such gradient dissipates in the absence of a load, and the voltage recovers.
 

Marley

Joined Apr 4, 2016
411
Any real electrical generating device has internal series resistance. In the case of a battery, this resistance increases as it becomes discharged. The EMF also reduces as it discharges.
You can think of it as a generator of EMF (voltage) in series with a resistance. With no current drawn, there is no voltage drop so you measure the EMF. When current is drawn, the EMF remains the same but voltage is dropped across the resistance.
 

Audioguru again

Joined Oct 21, 2019
3,173
One Li-Ion battery cell is 3.2V when almost dead and is 4.2V when fully charged. Then your 4-cells battery is 12.8V when it is almost dead and is 16.8V when fully charged. Halfway in a discharge the total voltage is 14.8V (3.7V per cell).

If you discharge a lithium-ion cell below 3V then it becomes damaged. You discharged 4 cells to 7V which is 1.75V per cell so the battery is ruined and is dangerous (chance of a fire) if it is charged. All products powered with a Lithium battery have a low-voltage disconnect circuit except yours. What is the mAh rating of your battery or battery cells? Are they fake Chinese cells?

Read about a Lithium battery at www.batteryuniversity.com .
 

andrewmm

Joined Feb 25, 2011
1,465
I'd disagree slightly re all Li cells have an under voltage lock out
If you purchase a battery pack, as used in say a battery drill, they will undoubtedly have a under voltage lock out
If you purchase cells, then its very unlikely that they have UVLO,

The LiIon cells I know of, tend to have over current circuits , look at the test in the documents,

https://uk.rs-online.com/web/p/aa-b...jt-aUEd5fS28sZO9C2hoCUzIQAvD_BwE&gclsrc=aw.ds

But it does not affect the conclusion that you have most probably damaged some or all cells

NOTE Also,
These things can have a large amount of energy in them
treat with care,
 

Audioguru again

Joined Oct 21, 2019
3,173
His battery went down to only 7V so of course it has no low voltage disconnection circuit in it.
His battery cells are 4.2V and are rechargeable, not the Energizer 1.5V non-rechargeable batteries posted.
 

Phil-S

Joined Dec 4, 2015
205
A link to Energizer 1.5V AA Lithium batteries was in post #6. When they were introduced, I was given some by a pretty young lady in a park. They are very powerful, last for a long time and are very expensive. They are non-rechargeable.
I know people get given some strange things in parks, but some batteries?
Not a bad gift though.
The link to RS shows them as not available, but at £27 a pack of 10, plus tax, isn't bad.
The alternative Ansmann battery is almost twice the price.
Lithium batteries, decent ones, are getting a bit pricey now.
I use a lot of 1/2 AA 3.6V non rechargeable on wireless sensors, so am thinking about going over to rechargeable Lipo and use a phone power bank and Adafruit Powerboost to give the Lipo a top-up now and again. Expensive to set up, but I charge the power banks on off-peak power.
 
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