BU-910: How to Repair a Battery Pack

Learn to repair battery packs and open a business

Batteries for power tools and other commercial devices can often be repaired by replacing one or all cells. Finding a replacement cell in NiMH is relatively easy; locating an appropriate Li-ion cell to repair a pack is more difficult. Brand-name Li-ion cells are not readily available because a reputable battery manufacturer will only sell to certified pack assemblers in higher volume. (See BU-305: Building a Lithium-ion Pack)

When repairing a Li-ion pack, make certain that each cell is properly reconnected to a protection circuit. (See BU-304: Why are Protection Circuits Needed? and BU-304a: Safety Concerns with Li-ion) All Li-ion batteries must have a protection circuit. Some single-cell and power-tool packs may have simplified circuits; lead and nickel-based batteries are exempt.

While the classic lead- and nickel-based batteries share similarities in terms of cell voltage, the lithium-ion family diverges and some have different voltages. The typical Li-ion has a nominal cell voltage of 3.6V and charges to 4.2V, some specialty Li-ion charge to 4.1V and newer Energy Cells top at 4.35V/cell and higher. Li-phosphate is an exception with a nominal cell voltage of 3.2V and a charge limit of 3.65V/cell. Also unique is Li-titanate with a nominal cell voltage of 2.40V and a charge limit of 2.85V. (See BU-205: Types of Lithium-ion) These cells are definitely not interchangeable.

Besides diversity in chemistry, cobalt-blended Li-ion comes in the high-capacity Energy Cell for moderate loads and the Power Cell for high currents with less capacity. When replacing the cells of a power tool battery, use Power Cells with identical specification; laptops, e-bike, etc. use the Energy Cell. There are also hybrid cells that satisfy both applications.

Do not go cheap on the cells by sourcing them from an unrecognized manufacturer. Failing packs performances are often traced to inferior cells, and this also applies to the popular 18650 Li-ion format. One hears repeatedly of companies that took advantage of marked-down prices from a clearing house, only to have the packs fail after 2–3 years because of high cell failure.   

If a relatively new pack has only one defective cell and a suitable replacement cell is on hand, then exchanging the affected cell makes sense. With an aged battery, however, it’s best to replace all cells. Mixing new with old causes a cell mismatch that will lead to a short life. In a well-matched battery pack all cells have similar capacities and the weakest link in a chain determines the performance. (See BU-302: Serial and Parallel Battery Configurations)

Cells designed for a multi-cell pack require tighter tolerances than those for single-cell use, such as a mobile phone. Cell manufacturers cannot fully control the process and some cells will have higher capacities than others. Such a discrepancy won’t be noticed by the user in a single-cell device but uniformity is important in a multi-cell pack.

A battery shop may salvage good cells from a failed pack for reuse but the recovered cell must be checked for capacity, internal resistance and self-discharge; the three key health indicators of a battery. (See BU-901: Fundamentals in Battery Testing.) When checking a cell with a battery analyzer, mark the capacity so the value can be matched with a pack that might need a cell of similar capacity. Also make certain that the internal resistance is on par with a good cell and then verify self-discharge. Elevated self-discharge points to a cell with high electro-mechanical stress. To check self-discharge, the open circuit voltage of a fully charged Li-ion cells should be within +/-5mV after a 24-hour. 

Visitors of BatteryUniversity.com frequently ask: “Can NiCd can be replaced with NiMH?” In theory, this should be possible as both chemistries have the same cell voltage, but full-charge detection and trickle charge have different demands. NiMH uses a more refined charge algorithm than NiCd. (See BU-408: Charging Nickel-metal-hydride) A modern NiMH charger can charge both NiMH and NiCd; the old NiCd charger could overcharge NiMH by not properly detecting full-charge and applying a trickle charge that is too high. Unless otherwise marked, each chemistry requires its own charger.

Spot-welding a cell is the only reliable way to get dependable connection. Limit the heat transfer to the cells during welding to prevent damage. Insulate each cell electrically as the skin carries a voltage and is conductive. If the new cell is at a different charge level than the existing cell, apply a slow charge to bring them all to the same level. Observe the temperature during charging. Nickel-based cells will warm up towards the end of charge but must cool down after switching to ready; Li-ion should stay cool during charge. The rise in temperature should be equal for all cells; unevenness hints to an anomaly.

Measure the voltage of a repaired pack and check it again after 24 hours and then again after a few days, if possible. If an OCV drops lower than observed in another pack of same model, then there is fear of elevated self-discharge. (See BU-802b: Elevated Self-discharge).

Last updated 2017-05-09


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Comments (17)

On April 17, 2013 at 7:21pm
vikash wrote:

i have dell inspiron n5010 laptop battery and it is dead. i want to re-built it,but i confuse with volt it’s volt is 11.1v but i do not not the single cell voltage. how i found that the voltage of a single cell of my laptop battery.it has 6-cell battery,4080 mAh,capacity 48Wh, rechargeable li-ion Battery,type J1KND,

On November 27, 2013 at 12:45am
jørgen eriksen wrote:

kan ikke lsde mit nyeop det siger st lav Batterithilsen jørgen eriksen.emp.

On February 4, 2014 at 12:47pm
s cohen wrote:

can batteries exploded at certain temperatures

On March 5, 2014 at 3:30pm
bryan wrote:

I bought a new charger for my my phone the exactly like the last one but it said that the battery is not supported with the phone what can i do.

On March 13, 2014 at 12:07am
Ed wrote:

Is it the same identical battery (including model number as the old one?
Is it the same identical charger as the old one—including model number?

If THAT, my guess is that they just de-listed the old battery or old phone.  IF it is the same battery and same charger, it still is supported but they don’t list it as that anymore.  Assuming, of course, it is EXACTLY LIKE—including same model number of same manufacturer.

On March 13, 2014 at 12:10am
Ed wrote:

Batteries not only can but DO explode—about 20 years ago now, there was a rash of Ericsson Cell Phones exploding.  Also about a decade ago, Toshiba laptop batteries catching fire, which they “fixed” by reducing the capacity of the battery by half.

On April 2, 2014 at 3:59am
Chipo wrote:

how can i repair my li-ion battery for my laptop without replacing it? the battery nolonger store power

On July 1, 2014 at 5:18am
Chris wrote:

I ordered a new battery for my mid 2010 13” macbook pro from China.  It arrived DOA.  I know it isn’t the computer as it was charging the old battery before the replacement. Do you have any pointers to make a fix to this battery or test it?

On May 8, 2016 at 5:25pm
jay wrote:

my phone have 3 virus and i want to clean or repair?

On July 4, 2016 at 12:28am
russ wrote:

Jay, if you know you have three virus, not two, nor four, you must already know the answer to your query. Its simple jay. 

Go to an appropriate forum, batteries don’t get viruses.

......  Or do they? Hmmm I guess with smart batteries…... Lol ed¿

On April 8, 2017 at 1:37pm
John wrote:

My mechanic discharged my battery & wants me to buy a new one. Somewhere along the line I remember adding Magnesium sulphate.. I don’t remember how. Appreciate any assistance…..J

On August 20, 2017 at 12:21am
jon wrote:

I’m thinking that most of the brands of LIthium Ion 18V drill batteries would be exchangeable,just requiring putting in the battery from one battery housing to another so housing fits the drill.So,if I can find a same voltage,amperage new battery from any other drill manufacturer,shouldn’t I be able to sub the guts of that battery into my housing?

On November 29, 2017 at 12:34am
kingmaker wrote:

My lithium ion battery indicates in portable charger that the battery is fully charged and in multimeter its shows no voltage in battery ,so how to i repair my battery..

On January 23, 2018 at 2:32am

there is no power in my phone battery,how do i repair it.

On April 13, 2018 at 8:04am
Ken wrote:

On August 20, 2017 at 12:21am
jon wrote:
I’m thinking that most of the brands of LIthium Ion 18V drill batteries would be exchangeable,just requiring putting in the battery from one battery housing to another so housing fits the drill.So,if I can find a same voltage,amperage new battery from any other drill manufacturer,shouldn’t I be able to sub the guts of that battery into my housing?

I don’t see an answer to this question, and I have the same question?

On July 13, 2018 at 10:03pm
Brad wrote:

I have a Mavic Pro drone battery that has only been cycled 10 times and is being reported defective by the drone because the voltage of the middle of the three cells is lower than the other two. 4.26 / 4.11 / 4.27 volts respectedly.

I’ve ran it down to zero and charged it to full with no improvement. What’s happening to the middle cell not to charge all the way?

Is there any way I can reset this battery or target specially the middle cell with a bit extra charge?

On September 3, 2018 at 12:32am
Gabriel Mohan wrote:

I have this question.
I have this 2 lithium ion battery packs connected in series to an electric wheelchair.
Using 18650(3.3v 2600mA) in 7S4P configuration. (24V 10AH). They are charged by a charger of class 2. - model no. HP0060W(L2). They are charged together at the same time as (packs) are connected in parallel.
As one of the packs had failed. I can only replace the batteries but the new setup is now at either 11.2AH or 12AH.
Can the battery pack one at 10AH and the other at 11.2AH or 12AH be charged by the charger.?
Is this mixing of different capacity incorrect.?
Would there be any issue. ?
You advise would be most appreciated.