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5 volts out of 4 rechargeable batteries

Hi folks. I'm pretty new tor robotics, and I am hoping to solve the  issue of providing 5volts to my cpu and digital circuits.   To start with I use 4 1.2volt rechargeable batteries, this provides 4.8 @ 1.2 volts per battery... unless they are freshly charged in which case each battery is now at 1.4volts and 4 of them bump the voltage up to 5.6volts which is a tad too high for my gear.   

One idea is to use a low drop 5volt voltage regulator, but what happens when the voltage hits 5.0v, I take it that regulators and low dropout ones need a little bit more voltage than the target voltage to work.  Most of the gear I use will work down to 4.5 volts, so having a battery pack with a voltage regulator not working once it hits 5volts would really limit the amount of time my robot will run for.

Another thought was use a diode to drop 0.7 of a  volt so instead of 5.6  I get 4.9volts, I am thinking this is a cheap and easy solution.

I am wondering how much power would this diode be consuming.




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You can use a boost regulator too, within current limitations. I love these things!

http://www.pololu.com/catalog/product/798  (outputs 5V with a 0.8V-5V input, you could use this with one to three series batteries, and suppose you could also use it with two pairs of series batteries in parallel)

http://www.pololu.com/catalog/product/791  (this adjustable one is what I'm using on my current project)

I also really like these 3.6V rechargeable lithium ion coin cell batteries ($2 each!), which would limit current output and require a regulator, but would minimize size and weight:


Thanks guys. I think I'll stick with the diode as I like the idea of reverse polarity protection.  I already fried one pic for by getting the + and - wrong. :).

I'm using 4x AA rechargeable batteries very often to power Atmel AVR micros, without any problem. The Atmel AVR mega328 f.e is specified from 1,8..5,5V power range (6V absolute maximum). So there is no need to put a diode between. For others micros check the datasheet first.

On the other side if you plan to switch between normal batteries and rechargeables a diode would make sense. Use a jumper to shortcut the diode, when using recharchables. The diode must match the maximum current frowing through the circuit. Advantage of the diode is the protection of the micro from reverse powering.

What microprocessor and circuits are you using? Some of them will tolerate over 5V. For example, I recently discovered that several LMR members have used the Picaxe at 6V with no ill effects.

One of the lesser used battery chemistries: Nickel-zinc. Nominal voltage of 1.6V, safe to Ms. Gaia.

I haven't done it personally, but you can also use a zener to turn a voltage regulator on and off (I'm specifically thinking of the old LM2575/2576's) at a specific voltage with its on/off pin. Most of the datasheets for the regulator have the circuit example included.

Just a couple options.