On Tue, Feb 5, 2013 at 5:06 PM, RussellMc wrote: > 1. What they all said. Noted! > 2. If you want efficient use of energy and you use linear regulation (whi= ch > you said is your preference) then NimH allows you to increment so Vmin is > just above Vout . Also (Vmax-Vmin)/Vmin for NimH is better than most > chemistries (except perhaps LiFePO4). Vmin for NimH is about 1V so 5cell= s > give you 5V+ at end of life and Vmax of about 1.3 x 5 =3D 6.5V. So loss o= f > energy due to excess voltage is about 0% at end of life and ~~ 1.5/5 =3D = 30% > at start of life. At current drains of C/5 or less Vout avg is about 1.2V > so mean efficiency is 1/12. ~=3D 83% which is as good as many boost buck > converters. > > Using a buck converter designed to ruijn in continuous mode will give low > noise and should be as quiet as most things need with > enough care. Efficiencies can be >=3D95% with great care and >=3D90% with= some > care. There are some very nice synchronous conversion ICs available (boos= t > or buck or a few for buck/boost which are highish efficiency and generall= y > lower noise. That seems to be where I'm heading. I'm starting to realize I have greater noise problems than just the PSU, so it's dropping lower on my list as a major concern. I'm trying to stick with a 4 cell solution for a few reasons, and this seems to be a good way to get a nice output given what I'm constraining the input to. > NimH is about as energy dense as LiIon in many cases these days - it has > somewhat crept up over recent years. eg an AA NimH at 2500 mAh x 1.2V =3D > 3000 mWh. An AA Liion at 3.6V nominal and 1000 mAh =3D 3600 mWh. With a > linear regulator you need 2 x LiIon for 5V out and the 7.2V mean is very > energy wasteful with linear regulation. The NimH wins nicely there by ma= ny > measures. > IF you use a buck converter the LiIon will be competitive and you can > decide on other issues such as ability to drop in std cells if needed. > > LiFePO4 x 2 is a good choice by most measures except use of std AA cells > when needed. Also energy density is lower than NimH or LiIon. The main mark against LiIon is that I can't swap in regular cells easily, and there isn't an easy way to hold the cells, short of making my own replaceable pack of some kind. I've considered it (and still am), but for the moment I'm trying to stick to AA cells. I thought about doing some kind of switchable Alkaline/NiMH AA/ 14500 LiIon, but the lithium cells I've found just don't have the same capacity, so they wouldn't be very useful, especially given that it'll be harder to keep track of them and keep them charged. Josh --=20 A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. -Douglas Adams --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .