The significant disadvantage of NiMH batteries is the high rate of self-discharge; NiMH batteries lose up to 20% of their charge on the first day and up to 4% per day of storage after that. In 2005, a low self-discharge (LSD) variant was developed. LSD NiMH batteries significantly lower self-discharge, but at the cost of lowering capacity by about 20%.
Those eneloop batteries (NiMH) are "on sale" today [only] at woot.com8 AAs and a charger (it goes to a "trickle mode" after re-charging), $18They hooked me for 2x.
LiPoly and LiIon batteries can have very big charges - it all depends on which batteries you have. After all, they fly planes on them. Real planes. Like the Dreamliner Why do you have 2 relays? You can do it just as well (better in fact) with one (if you really *must* use relays). Just a single SPDT relay will switch between the two + connections, and you have a common ground. With the power running the relay is energised, so pulls the contacts to connect the incoming power to the arduino, with it off they spring back to connect the battery to the arduino. A massive capacitor should cover the time the relay is not connected to either source.With diodes, if the main power is higher than the battery power the diode will be reverse biased. You will get a small amount of leakage current flowing backwards into the batteries from the main power - we're talking micro amps here, so nothing to even consider worrying about. I don't see where the discharge of the batteries would happen - other than the self discharge that happens over time anyway.For MOSFETs, you might like to take a look at the arduino's schematics - it uses a small op-amp and a MOSFET to seamlessly switch between the main power and the USB power.
Those eneloop batteries (NiMH) are "on sale" today [only] at woot.com8 AAs and a charger (it goes to a "trickle mode" after re-charging), $18They hooked me for 2x.> > > I'm not stumping. I saw this subject with "Eneloops" in it and thought I'd mention it, as I did in fact just buy a couple of packages featuring "same" (coincidental). I got two of 'em, there's just one shipping charge, $5 (in toto - including anything else from any of the other sections), and I paid some sales tax, too, and very convenient.Anybody can pick (or cherry-pick) anything apart, but I think it's nice as a package.I think they're part of amazon, but I'm not certain.
Most eneloop batteries has lower voltage, I think fully charged one measure around 1.35V and it can go down to 1.2V but still have plenty of power. Most photographers use it for their flash guns. I have 32 of it for my flash guns (but I ain't a photographyer).