Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uwm.edu!zaphod.mps.ohio-state.edu!brutus.cs.uiuc.edu!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!phil From: phil@ux1.cso.uiuc.edu Newsgroups: sci.electronics Subject: Re: NiCad "Deep Discharger" Message-ID: <34700007@ux1.cso.uiuc.edu> Date: 28 Jun 90 04:18:00 GMT References: <154259@<1990Jun15> Lines: 77 Nf-ID: #R:<1990Jun15:154259:ux1.cso.uiuc.edu:34700007:000:4366 Nf-From: ux1.cso.uiuc.edu!phil Jun 27 23:18:00 1990 The reversal of a cell in a battery (battery = many cells) of NiCd cells happens when 1 of the cells is weaker than the others. The high forward current causes a voltage drop across that cell that, if higher than the charged level of the cell if it has any charge left, will appear to be in the opposite direction of ordinary charging. The stated figures for the lowest voltage to discharge a battery to are some attempts to prevent a cell reversal. If your battery is SUPPOSED to have N*1.2 volts, then discharging it NOT LOWER than (N-1)*1.2 volts will assure you of not actually reversing a cell since the weak cell will have to pass 0 before becoming reversed, and at that point the voltage of the bettery must be (N-1)*1.2 volts or less. I have a radio that can operate on voltages from 6 to 15 volts. I have a bettery pack that is 11 cells and rated at 13.2 volts. I can run this pack until the radio dies at about 6 volts. By this time, if I am going to be reversing any cells, I may have ALREADY done so. Many electrical devices manage to function over a significant range of voltage. Alkaline batteries have a gradual voltage drop, so many devices are designed intentionally to work under these conditions. The normal capacity of alkaline cells is rated in terms of how long for their voltage to drop to 1.0 volts (from 1.5). So it may be moot to even attempt to discharge your NiCd battery DOWN to (N-1)*1.2 volts since it may in fact already be BELOW that level. So what can you do? I have for a long time suspected that the "memory effect" is caused by the internal discharging of a cell. This is a chemical reaction that is very different than the reactions to chanrge or normally discharge a cell. This reaction apparently increases the internal resistance. Think of a single cell as an array of cells in PARALLEL. The chemical surfaces of the cell are electrically in parallel, so this is valid. When the cell is PARTIALLY discharged in normal use, half of the cell is used up, and half (the weaker half) remains charged. The first time around this is not a distinct boundary. The cell is then recharged. As this cycling is repeated, part of the cell is constantly discharged and recharged while the rest of the cell is NOT. When this abused cell is used for longer than the cycles it is used to, it is forced to start discharging from the lesser used portion since the rest of the cell is discharged. This portion, having a higher resistance, conducts less current and appears to have a significant voltage drop. This may even be so severe as to prevent the device to function, and the capacity of the cell will apparently be diminished. I also believe that higher discharge current bring this condition on faster. Also, cells are not manufactured equal in capacity. Typically you might see as much as a 10% variation in capacity and/or internal resistance. So your battery may already have a weak cell fresh out of the box before any abuse gets to happen. Given that it is not really practical to discharge battery packs down to the (N-1)*1.2 volt level, and that cells can get weaker if they are allowed to only be partially discharged, and that weak cells can become reversed in discharging UNDER NORMAL USE, my practice is to TOTALLY discharge my battery packs to ZERO volts EACH AND EVERY cycle they are used. After I am done with a battery pack, I set it up to be fully discharged AT A LOW CURRENT for 24 hours or so, then put it on charge. The idea is to prevent a cell from becoming noticeably weaker than the other cells in the pack. What I am doing is keeping the battery pack BALANCED. I have been doing this for over 120 cycles so far on 3 packs, and a 4th was added a few months ago which has had about 40 such cycles. Ordinarily NiCd's last much longer than 120 cycles, so my testing period is not over yet, but neither has this caused an untimely death. One pack was inadvertenly subject to sitting in a very hot car for a day a couple of weeks ago, but it seems to have survived the ordeal just fine. I also have carried and used the packs in deeply freezing (below -15 C or +5 F) weather many times this past winter, without any apparent problems. --Phil Howard, KA9WGN-- | Individual CHOICE is fundamental to a free society | no matter what the particular issue is all about.