Ideally electrical cells would be able to hold their electrical charge without any losses for all time. However, all electrical cells have some amount of self-discharge. Self-discharge is the loss of charge over time while sitting unused and not connected to anything. Self-discharge is a normal attribute of all cells. It results from chemical reactions taking place within a cell. Primary (non-rechargeable) cells have extremely low self-discharge which can give them a shelf life of more than a decade. Secondary (rechargeable) cells have greater self-discharge. Rechargeable lithium ion cells, after a larger initial loss in the first month after being charged, typically lose around 1% of their charge per month thereafter.
Additional self-discharge can result from leakage current paths existing within the cell. Particulate contaminates and dendrite growths produce internal “micro-shorts”, creating such leakage current paths. These are not normal attributes of cells and as they can lead to catastrophic failure of the cell detecting them early on is a top concern to lithium ion cell designers and manufacturers.
To detect higher-than-normal self-discharge in lithium-ion cells, developers and manufacturers have traditionally relied on measuring the drop of a cell’s open-circuit voltage (OCV) over a period of several weeks or longer to get good validation. Having to wait this long during development results in lost opportunities by being late to the market with new designs. This problem is further compounded if self-discharge testing must be repeated. In manufacturing, having to store large quantities of cells for a long time to screen them for self-discharge presents major expense, logistics, and safety problems to contend with. Clearly, ways of reducing the time to evaluate the self-discharge of cells is highly valued.
Measuring a cell’s self-discharge current provides an alternate means to directly determine a cell’s self-discharge rate. Cells exhibiting excessively high self-discharge can be identified and isolated in a small portion of the time required by the traditional OCV approach, greatly reducing the associated expenses, difficulties and potential hazard. The challenge of performing direct self-discharge current measurement is suitable equipment possessing required stability and resolution has not existed to make it practical.
Keysight has recently introduced two new solutions that directly measure the self-discharge current on cells, greatly reducing the time required to evaluate cells for self-discharge, compared to the traditional approach of measuring a cell’s OCV loss over time. The first of these solutions is the BT2191A Self-Discharge Measurement (SDM) System, shown in Figure 1. The BT2191A allows engineers to reduce design cycle time and optimize self-discharge performance of new cell designs.
Figure 1: Keysight BT2191A SDM System
The second new solution is the BT2152A Self-Discharge Analyzer, shown in Figure 2. The BT2152A measures self-discharge current on many cells at the same time, greatly reducing the time required to discern bad cells from good cells for self-discharge in manufacturing. An example of this is shown in Figure 3. The BT2152A can provide significant reductions in work-in-progress (WIP) in manufacturing, along with associated reductions in expenses, logistics issues, and safety problems incurred with carrying the WIP for that time.
Figure 2: Keysight BT2152A Self-Discharge Analyzer
Figure 3: BT2152A measurement discerning a bad cell from good cells
So, if you are involved with the design or manufacturing of Lithium Ion cells then cell self-discharge should be a top concern for you. To learn more about BT2191A Self-Discharge Measurement System the following link will take you to its home page: <BT2191A home page> To learn more about BT2152A Self-Discharge Measurement Analyzer the following link will take you to its home page: <BT2152A home page> Here you will find additional information providing greater details about self-discharge measurements on cells and about these two new solutions!
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