Charging DeWalt 36V Packs (A123 Systems)

DISCLAIMER: These articles are the findings and opinions of one person. Long-term durability of these cells is not known for our application/construction techniques. Any cell technology that we use is potentially dangerous. Do not work with these cells if you do not have a healthy respect for them, or if you lack the requisite skills.

Some of this article is a repeat of the introductory article. It won't hurt to read it again. LiPo cells have caused significant property damage due primarily to their tendency to burst into flame if overcharged. They are also subject to catastrophic failure if over-discharged or called upon to provide high current, e.g. momentary shorts. These failure modes are exacerbated by any unbalance between cells in a pack. The M1 cells behave differently from other Lithium technologies. The specified end of charge voltage is 3.6 volts rather than 4.2 volts. If the charging source for a conventional Li cell is greater than 4.2V, the cell will continue to accept energy, sometimes with disasterous results. The M1 cell, on the other hand, accepts very little additional energy above 3.6V. From the A123 site: "A123 materials are designed to ensure all the Lithium is fully extracted from the cathode when the battery is fully charged. As a result safety issues relating to overcharge are eliminated because there is no Lithium available to plate on the anode in an overcharged state. This is in contrast to conventional Li Ion cells, which only extract half their Lithium content when they reach their upper cut-off voltage. Conventional Li Ion cells are easy to overcharge and once in this state they can continue to extract Lithium putting the cell in a dangerous mode and making it prone to fires and explosions." The Figure below shows the cell voltage of an M1 cell charged with a LiPoDapter and an Astro 112D NiCad charger. The M1 cell has a nominal 2.3 ampere-hour capacity. Normal Li technology dictates a 1C (2.3A) charge. The curve below was from a 6A charge. A123 Systems claims their cells can be charged in as little as 5 minutes without damage. Realistically, our chargers can't do better than a 15-30 minute charge. A123 Systems recommends a maximum of 10A charge. This is just short of 15 minutes. The LiPoDapter cuts off charge at a bit less than 4.2V per cell. As can be seen on the graph, once the 3.6V point is reached, the cell voltage rises rapidly to cutoff (less than two minutes). After cutoff, the voltage quickly "sags" back below 3.6V. The purpose of this test was not to endorse the charging of M1 cells on LiPo chargers. This will cause long-term harm. On the other hand, their were no fires or explosions or self-destructive behavior. Even though cells were charged to 4.2V, the LiPoDapter cut off the charging before significant energy could be pumped into the pack. Conventional Lithium chargers, on the other hand, continue to put energy into the pack until it essentially accepts no more energy. It appears that you could charge M1 packs on lithium chargers if you removed the pack before it reached 4.2V/cell. Not a good idea. The Astro 109 provides an audible tone at this point. This is better, but A123 Systems maintains that this will cause long-term harm. Based on A123 recommendations, the new LiPoDapter+ will cut off at 3.7V and will handle up to 8A. This results in essentially a full charge in less than twenty minutes. The LiPoDapter+ will handle up to 10 M1 cells or 8 LiPo cells depending on the charger capacity. Even though these cells can be charged at up to 10 amps, most popular chargers are limited to lower currents and output current goes down for higher cell counts. The chart below shows estimated charge time in minutes for NiCad chargers using LiPoDapter+:

If you have an Astro charger, you're in good shape for charging M1 packs with the LiPoDapter+. Most LiPo chargers simply don't have the horsepower to charge M1 packs very quickly. The Astro 109 has the horsepower. It's not clear yet that the 109 will come up with a cell count that will properly charge all cell counts. It does appear that the 109 does not like the rapid voltage rise at end of charge and shuts off. That's a good thing.