Anecdotal Evidence - M1 Cells (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.

This article relates some unscientific experiences with the DeWalt 36V (a.k.a. M1) cells from A123 Systems. These experiences have highlighted areas where they seem to be superior to LiPo cells. 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. I titled this article Anecdotal Evidence because it falls in the area of knowledge that includes user observations, hearsay, and even old wives' tales. Anecdote #1 This anecdote consists of several incidents. Each incident was due to stupidity on my part, and in my flustered state I failed to properly document which cell(s) had what done to them. The subject of the anecdotal evidence is the pack shown to the right. I built the pack to illustrate the ability to stack cells. I never properly insulated it and subsequently attached large leads to each junction so I could charge/discharge individual cells and hook up different cell counts for testing LiPoDapter+. My first mishap was to short the entire pack on the case of one of the cells as shown. We have already learned that doing this to a LiPo pack will frequently result in ruined cells and sometimes fire. The M1 pack seemed to charge and discharge fine. My next mishap was to leave the pack sit for several days after discharging it to 2.1 volts per cell. I found that one cell (the burnt one) measured 0.0 volts, one measured 1.1 volt, and one was 2.4 volts. With LiPo, these cells would be dead. Anything below two volts is almost certain death for LiPo cells. Anyone who has left a pack plugged into an ESC for several days has learned this the hard way. I charged the M1 cells to 3 volts at a C/10 rate, then cycled them at 5 amps. They seemed OK. What more could these cells suffer through!? It didn't take long to find out. I needed a 12S pack for testing so I connected a new 10S DeWalt pack in series with two of the cells from my 3-cell test pack. Being in a hurry, I didn't balance the cells. I thought the 10-cell pack had somewhat more charge than the two added cells and everything would shut off before any cells got overcharged. WRONG! The two cells were each reading 5.0 volts after charge cutoff! This kind of overcharging causes explosions and fires with Lithium. At a minimum you have a puffed up, ruined cell or pack. I slowly bled off the excess charge on the cells and cycled each one. Oh, did I mention that I dropped the pack four feet to the floor hard enough to break solder joints? A LiPo pack would have been smooshed. The next picture shows the end result of all this abuse. The curves show each cell discharged at 33 amps. One of the cells (the burnt one) appears to have a higher internal resistance (more voltage drop) and about a 25% reduction in capacity. The others appear comparable to new cells. I'm sorry I can't associate each mishap with specific cells. Perhaps the burn caused a small breech in the case. Bottom line is that this pack suffered most of the things that destroy LiPo packs yet I'm only out 16 bucks for one bad cell. These mishaps would have probably caused the loss of three or four LiPo packs at hundreds of dollars. We won't even speculate on possible fire damage.

Anecdote #2 This anecdote speaks to the relative safety of M1 cells and also to the issue of balance. One of my original M1 packs consists of two end-to-end-soldered 3S sticks to make a 6S pack with a center-tap for use with one of my MaxCim motors. I've never flown this pack, but have been using it for M1 testing and for testing Dapters. It has between 50 and 100 full and partial discharge cycles. Normally 5 amps charge and 5-10 amps discharge. I was charging 1/2 of the pack (3S) and had a "problem". The pack had charged to 15.4V or 5.1V per cell. This is almost a 50% overcharge! It would have destroyed a LiPo pack. I discharged each side of the pack separately on my CBA II. I then charged the pack as a 6S pack and discharged each half separately at 10 amps. The graph above compares the overcharged half with a previous test of the same pack half. The red line is after the overcharge and the black is an earlier test. The results imply perhaps a higher internal resistance, but not much else different. These cells might not be good as new but they did not fail or puff up or burst into flame. I then put about 1 amp-hour into the 6S pack and checked cell balance. All six cells measured within one millivolt of each other!