Extended to general comment on NimH use based on other comments as well as John's > The flashlights I use start erratically around 1 volt, > & will shut off at about .9V. > > Whether this is deliberate I don't know, but the cells > seem to tolerate it. The following figures are "out of my head". I have various notes etc but these are probably close enough. Check any which matter. 0.9V NimH end point is acceptable but there are few disadvantages in cutting off at say 1.0 V. Energy is close to exhausted by 1V so a 1V termination costs you little and helps the cell life somewhat. At any significant current (say C/10 up) the internal impedance will be high enough to be dropping noticeable voltage so the loaded voltage will be lower again than what you see just after cutoff. You get substantial "bounce back" in the short period after cutoff at low voltage. Say 0.1V+ and maybe 0.2 depending on prior load, cell state etc. The OLPC project claims 2000 cycles on NimH by limiting capacity to 80% of nominal with restrictions at the top and bottom end. Their end of life capacity is 60% or 70% of new, so the 80% range would need to be within the current capacity. This is more easily managed by watching voltage under known conditions than by coulomb counting. Somewhat surprisingly OLPC claims equal cycle lives for NimH rated at 80% of capacity and LiFePO4 rated at 100% capacity - 2000 cycles in each case. Their LiFePO4 figure is the same for cells from two major Chinese manufacturers. NimH working voltage depends significantly on load. At 0.1 - 0.2 C the majority of energy is delivered in the 1.15 - 1.25 Volt. As someone noted - fully charged Voltage can be 1.3V + but as also noted, this drops very rapidly. A logical conclusion is that with NimH, if you have control over battery capacity, then by specifying substantially higher capacity than needed initially and then using it very conservatively you get lower whole lifetime costs AND better performance - you can get better output energy efficiency (lower Vdroop at given load), better charge efficiency, far longer cycle lifetime, far lower capacity loss per cycle / lower capacity droop with time / far better capacity with age (3 views of the same thing). By eg doubling the initial capacity and using it at 50% of rated value you get probably 4 to 6 times the lifetime so an overall cycle cost reduction of 2 to 3 times plus benefits from longer replacement times, more capacity with age etc. The potential disadvantages of eg doubling capacity is that higher energy density NimH tend to have higher % self discharge rates - addressed by using low self discharge cells, and the tolerance to trickle charging drops to about zero at higher energy densities. eg a 2400 mAh NimH AA is rated by reputable manufacturers as having NO long term trickle charge tolerance and only modest short term overcharge balancing capacity. A low energy density cell (eg 800 mAh AA tends to allow continuous trickle charge at C/10 or higher (although limiting this doesn't hurt). FWIW NimH end point voltage under "typical" charging regimes is about 1.45V but this varies with cell temperature, rate of charge, state of battery and to some extent by manufacturer. Voltage can be used for end point termination if more conventional means are not possible (as in eg small solar charged equipment (think about how standard methods are affected)) but delta temperature or negative delta V are preferred when possible. As also mentioned - Alkaline cells have a higher open circuit new Voltage than "Leclanche" or Zinc Chloride. Alkaline is typically just over 1.6V o/c new. This can be reliably' used as a check on whether a cell is Alkaline ornot - sometimes suspect with el cheapo cells. > I'm a fan of the Maha C9000 charger - Following their recommendations > for cell maintenance seems to work. > www.mahaenergy.com/download/mhc9000.pdf I'm very pleased with the facilities the MHC90000 offers. Remote programming and logging would have been easy enough to add and would greatly enhance the value of the charger. Russell McMahon --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .