atatistcheff
Solar Enthusiast
- Joined
- Sep 20, 2019
- Messages
- 176
I'm looking for some thoughts or experience on lithium power tool battery packs. I've been repairing some of the Ryobi 18V packs which are dead or just below the BMS charge voltage. It's pretty easy and normally just involves opening the pack and injecting some volts to get the pack up to a point that the regular charger will work. Of course, sometimes there are dead cells in the pack which can be replaced to make it usable again.
I'm finding quite a bit of variation however between the watt hours available in various packs. What I'm seeing is that when I replace all the cells (5) in a pack with known, tested values the total watt hours does not add up to what it should be. For example, 5 cells tested at 1100 mAh should provide about 84% of the rated value (new cells are 1300 mAh). However, this 1100mAh pack is coming in at about 60% of the rated value instead. I suspect this is due to the cells being out of balance. One cell gets low and the BMS cuts off the pack before the other 4 have discharged fully.
I think my mistake was not to ensure the cells were all perfectly matched - at least in their voltage - before putting them in the pack. This brings me to my question. Does anyone know if these power tool pack BMS boards actually do any cell balancing? The BMS looks pretty complex so I would assume there is some balancing going on. It seems that the pack capacity would prematurely degrade over time as the balance drifts. However, if they do balance then my oversight of not matching the voltages well should eventually be corrected.
I can't seem to find any information on how the Ryobi BMS works (not that this surprises me). Does anyone have any knowledge or experience with these BMS boards?
I'm finding quite a bit of variation however between the watt hours available in various packs. What I'm seeing is that when I replace all the cells (5) in a pack with known, tested values the total watt hours does not add up to what it should be. For example, 5 cells tested at 1100 mAh should provide about 84% of the rated value (new cells are 1300 mAh). However, this 1100mAh pack is coming in at about 60% of the rated value instead. I suspect this is due to the cells being out of balance. One cell gets low and the BMS cuts off the pack before the other 4 have discharged fully.
I think my mistake was not to ensure the cells were all perfectly matched - at least in their voltage - before putting them in the pack. This brings me to my question. Does anyone know if these power tool pack BMS boards actually do any cell balancing? The BMS looks pretty complex so I would assume there is some balancing going on. It seems that the pack capacity would prematurely degrade over time as the balance drifts. However, if they do balance then my oversight of not matching the voltages well should eventually be corrected.
I can't seem to find any information on how the Ryobi BMS works (not that this surprises me). Does anyone have any knowledge or experience with these BMS boards?