Recently, I got a laptop battery from a Dell laptop about 5 years old. Battery life was down to about 1hr instead of about 8hrs when new. The battery in the laptop was replaced with with a new one and I took this one apart to test the cells and see what the cause of the low battery life reported on the computer was.
The pack is a dell YRDD6, rated at 3500mAh, 11.4V, so we’d expect to find 3x 3500mAh cells.
The cells all measured slightly above 4.2V straight from the pack. They were fully charged in the laptop and then were sitting for a few days before testing, so a failing cell with high self-discharge current was not the issue.
Testing was completed with a python program I wrote to control lab equipment for battery testing, and the test used 4-terminal sensing. For more details on the equipment setup and control, see an earlier blog post here (link).
1st cycle: discharge to 2.5V at 1A
2nd cycle: CCCV charge, 4.2V, 1A, end current 50mA. discharge to 2.5V at 1A
The discharge capacities are shown in the table below. All the cells still seem to be fairly well matched in capacity, so a single cell failing with low capacity does not seem to be the issue. The cells are pretty close to the typical end-of-life of 80% of original capacity, all just above that mark.
Next was an internal resistance test. If a cell or the pack had a high internal resistance, this could trigger the low voltage cutoff of the BMS when the pack is under load.
Cell 1 seems to have a slightly higher internal resistance than the other cells, but there is nothing to be concerned about here. The rise in internal resistance at low SoC is typical for any battery, new or old.
If the power draw of the laptop hasn’t changed significantly from when it was new, then there is no reason I see for this battery to only have a 1hr battery life now. 8hrs of battery life when new (100% SoH), should translate to over 6hrs of battery life now if we based the calculation only on capacity. Somewhere, we’re losing an extra 5hrs of life – extra power draw by the laptop, conservative battery management systems, or a higher low-voltage cutoff could all contribute. My own Dell laptop has started to give me battery warnings as well so I might replace that one soon and see what the issue with that one might be.