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-based 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 2016-01-29
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