You say 6V 1500 mAh Nimh - which implies 5 cells rated at 1.2V nominal. You have a 5V load. A Nimh will be reasonably well exhausted at 1V and if you want to treat them well you gain substantial lifetime by early discharge cutoff. Actual terminal voltage depends on C discharge rate. At say C/5 - C/10 or less you are liable to use most of the capacity at 5V in a 5 cell pack, and so an LDO is a very attractive choice. Using a P Channel MOSFET as pass element you can get dropout down to don't-care levels with relative ease - usually better dropout than most regulators give you. If you care greatly about current limiting and over temperature cutout etc then an LDO may be a better choice. A linear regulator gives you better than 5V/6V =3D 83%+ across essentially the whole range - and it approaches 100% as battery nears 5V - you'd be vvvhard pressed to match that with a buck converter, and less likely again with a buck boost. BB usually are disappointing in efficiency and often have a nasty spot efficiency wise around the buck to boost changeover point - where they SHOULD be able to approach 100%. In your case Vin has a limited range and if you used a BB you would be near the un-sweet spot the whole time. I you use a converter, if it is not synchronous you will have one of two diode drops and can lose around 5% in the diode when conducting, even with a Schottky diode. _______ If any of Dave's work saving help is public domain (or CCSA :-) ) I'd be pleased to hear. I'd be happy enough to not have to deal with NimH again in energy poor situations. Where you have consistent copious charging energy available from mains or other sources then they may be bearable. Dave mentions a 1% or so fatal early failure rate with Chinese cells. Not all Chinese cells are equal and some are very unequal. Buying from the more highly competent manufacturers should help, and if you buy far cheaper than they are selling you are very likely doing yourself a disservice. I'd need significant persuasion to buy Chinese NimH that were not made by one of BYD= *, GP or BPI (in no relevant order). There are no doubt others who do as well but who are they, how do you know, who says so and how much cheaper are they. Annoyingly high MOQs may be a reason to buy elsewhere. May. I thought that LiFePO4 were going to be the answer to a maiden's, or engineers, dream, but while they are good, they too need a smidgen of care in their feeding. DO NOT FLOAT LiFePO4 at charge cut off voltage, that their days may be long on the face of the land. In your application, 2 x LiFePO4 cells would alow linear regulation, less efficiency than NimH at high battery levels, much easier care and feeding overall and a lower whole of life cost (but greater purchase price. Russell McMahon * I have no shares or vested interest in any of these. Warren Buffet does (or did). --=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 .