BU-802a: Rising Internal Resistance

Understanding the importance of low conductivity

High battery capacity is of limited use if the pack cannot deliver the stored energy effectively. To supply power, the battery needs low internal resistance. Measured in milliohms (mΩ), resistance is the gatekeeper of the battery; the lower the resistance, the less restriction the pack encounters. This is especially important on heavy loads such as power tools and electric powertrains. High resistance causes the voltage to collapse on a load, triggering an early shutdown. Figure 1 illustrates a battery with low internal resistance in the form of a free-flowing tap and a battery with elevated resistance with a restricted tap.

Effects of internal battery resistance

High resistance

 

Figure 1: Effects of internal battery resistance

A battery with low internal resistance delivers high current on demand. High resistance causes the battery voltage to collapse. The equipment cuts off, leaving energy behind.

Courtesy of Cadex

Lead acid has a very low internal resistance, and the battery responds well to high current bursts that last for a few seconds. Due to inherent sluggishness, however, lead acid does not perform well on a sustained high current discharge and the battery needs a rest to recover. Some sluggishness is apparent on all batteries at different degrees. This hints that power delivery is not based in internal resistance alone but also on the responsiveness of the chemistry. Nickel- and lithium-based technologies are more responsive than lead acid.

Sulfation and grid corrosion is the main contributor to the rise of the internal resistance with lead acid. Temperature also affects the resistance; heat lowers it and cold raises it. Heating the battery will momentarily lower the internal resistance to provide extra run time. This, however, does not restore the battery but will add stress.

Crystalline formation, also known as “Memory”, contributes to the internal resistance on nickel-based batteries. This can often be reversed with deep-cycling. (See BU-807: How to Restore Nickel-based Batteries) The internal resistance Li-ion also increases with use and aging, but much improvement have been made with electrolyte additives to keep the buildup of films on the electrodes under check. (See BU-808b: What causes Li-ion to Die?)

Alkaline, carbon-zinc and most primary batteries have a relatively high internal resistance, and this limits its use to low-current applications such as flashlights, remote controls, portable entertainment devices and kitchen clocks. As these batteries discharge and deplete, the resistance increases further. This explains the relative short runtime when using ordinary alkaline cells in digital cameras.

Two methods are used to read the internal resistance of a battery: Direct current (DC) by measuring the voltage drop at a given current, and alternating current (AC) that takes reactance into account. Measuring a reactive device such as a battery, DC and AC resistance results can vary greatly and neither one is right or wrong. The DC method looks at pure resistance (R) while the AC method includes reactive components and provides an impedance (Z) reading. (See BU-902: How to Measure Internal Resistance)

Resistance values lend themselves well to predicting power delivery to a DC load, whereas Z provides more accurate results when working with a digital load. The battery has reactive qualities in form of capacitance (alike a giant capacitor) and performs well with digital loads. In comparison, the electric grid is resistive and helps to cook our meals at the end of a long day.

A battery pack has resistive and reactive values that are connected in series. Below is a DC resistance summary of a single-cell mobile phone battery and a power pack in which the cells are connected in parallel and series.


Internal Resistance of a Mobile Phone Battery

Cell, single, high capacity prismatic 50mΩ subject to increase with age
Connection, welded 1m  
PTC, welded to cable, cell 25mΩ 18–30 mΩ according to spec
Protection circuit, PCB 50mΩ  
Total internal resistance ca. 130mΩ  



Internal Resistance of a Power Pack for Power Tools

Cells 2P4S at 2Ah/cell, 18mΩ subject to increase with age
Connection, welded, each 0.1mΩ  
Protection circuit, PCB 10mΩ  
Total internal resistance ca. 80mΩ  

Courtesy: Siemens AG (2015, München)


Figures 2, 3 and 4 reflect the runtime of three batteries with similar Ah and capacities but different internal resistance when discharged at 1C, 2C and 3C. The graphs demonstrate the importance of maintaining low internal resistance, especially at higher discharge currents. The NiCd test battery comes in at 155mΩ, NiMH has 778mΩ and Li-ion has 320mΩ. These are typical resistive readings on aged but still functional batteries reflecting the chemistry. (See BU-208: Cycling Performance that demonstrates the relationship of capacity, internal resistance and self-discharge.)

GSM Discharge NiCd
Figure 2: GSM discharge pulses at 1, 2, and 3C with resulting talk-time
The capacity of the NiCd battery is 113%; the internal resistance is 155mΩ.

 

GSM Discharges NiMH
Figure 3: GSM discharge pulses at 1, 2, and 3C with resulting talk-time
The capacity of the NiMH battery is 94%, the internal resistance is 320mΩ.

 

GSM Discharges Li-ion
Figure 4: GSM discharge pulses at 1, 2, and 3C with resulting talk-time
The capacity of the Li-ion battery is 107%; the internal resistance is 778mΩ.

All three figures courtesy of Cadex

Notes:   The tests were done when early mobile phones came with NiCd, NiMH and Li-ion. The performance of Li-ion and NiMH has since improved. The maximum GSM draw is 2.5A, representing 3C from an 800mAh pack, or three times the rated current.

Last updated 4/22/2015

 


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Comments

On December 12, 2011 at 10:16am
Guillaume wrote:

If anyone could provide me with a link of a lead-acid battery internal resistance vs temperature, it would be awesome.

On December 13, 2011 at 6:37am
girish yadav wrote:

lead acid battery performs better at low intl resistance at low temperrature intl resistance rises while at higher temperature vice-versa.

On December 13, 2011 at 12:40pm
Guillaume wrote:

@girish

I should have been more explicite in my question. I already knew that a colder temperature meant a higher internal resistance (yeah, I know I should have mentionned it).

What I would like to know is by how much the internal resistance is affected by a change of temperature. For instance, take the internal resistance of a battery at 30°C (say 4mH).

What will it be at -40°C ? 2 times 4mH ? more than that ?

On November 28, 2012 at 11:48am
danwat1234 wrote:

Why is is that cheap aftermarket lithium ion polymer camcorder battery packs often, over time, get high internal resistance to the point where you can’t record that long in 1080p before the voltage lowers too much and the camcorder shuts down. If I wait a few minutes I can then do some more video.
Eventually though, internal resistance gets so bad that the battery just can’t deliver a large load for any amount of time.

This has happened to me with many aftermarket DB-L80 batteries. The charge cycles is something less than 100, probably around 75, and they aren’t exposed to high heat all that much except for summer.

What changes in a li-ion battery to cause it to wear out in the form of rising internal resistance instead of just reduced capacity?

On December 5, 2012 at 11:37pm
Tysen wrote:

danwat1234, the Batteries in question were most likely rated at a high Capacity but have very poor Discharge Rates. for instance the OEM batteries in a Li-ion drill are rated at only 1300-1500 mAH of storage, but are Capable of Delivering 10-15x their capacity (18amps) for a few mins. this is Just Like a starter Battery. Most Laptop batteries are rated around 2400-2600mAH and can Deliver usually 2x their total Capacity in amps which results in a 30 min Battery life when playing games (5200/0.5 =2600mah). now the aftermarket maker prob tried to sell you on a Larger cell. you did indeed get a larger cell but the Battery is unable too keep up with the demands of your camcorder.

On December 5, 2012 at 11:47pm
Tysen wrote:

Sorry to further Add onto this. Generally with the 18650 cells (most laptops use) the larger the Mah rating the lower the rate of Discharge. one of the things ive found from these cells is
1300 +/- 18 amps discharge rate
2000 +/- 10A
2200 +/- 6.5A
2400-2600 5A
2700+ 0.5-2A in a perfect world. and I have seen a Samsung 2900 mah cell with a 3A discharge rate. but it required a hobby charger to take it to the rated 4.35 volts and in standard systems it only ran 2640mah

On December 6, 2012 at 12:07am
danwat1234 wrote:

No they are regular sized DB-L80 polymer single cell batteries. They have just about the same capacity as OEM but internal resistance is higher. It’s just 1 of those things you have to look at when you buy $3 aftermarket 2.5 watt-hour batteries.
Thanks it’s interesting how higher density batteries can’t put out as much energy at once.

On April 14, 2014 at 3:28am
MUZAMMIL SALEEM wrote:

I want to deseal/open a lead acid battery without damaging. Is it possible? Is there any machine or specific tool available for this purpose.