urgent battery detail help

[aruna1]

Hi guys;
I'm working on a small robot.I saw this battery(MN27A) in local shop which is 12V and have 21mAh capacity.

battery details
http://www.dhgate.com/5pcs-gp-27a-mn27-gp27a-12v-alkaline-battery/p-ff80808129b0c4df0129d4e159113eba.html

my question is if I use a regulator and lower the voltage to 6V will capacity become double?

what i mean is that I can use 12V 21mA for one hour with this batery.
but if i use 6V with 21mA then time will be two hours?

coz
P=VI

12*21*1=6*21*2

thank you

[waltr]

Unfortunately no.
If you draw 21mA from the 6V regulator output the regulator will dissipate 6V * 21mA = 126mW as heat.
Ptotal = Iload *Vload + Ireg*Vreg

The battery will still only run for 1hour theoretically. I say theoretically because the capacity is usually rate at a lower discharge rate.

[aruna1]

oh,too bad
thanks anyway

[dunk]

Unfortunately no.
If you draw 21mA from the 6V regulator output the regulator will dissipate 6V * 21mA = 126mW as heat.
Ptotal = Iload *Vload + Ireg*Vreg

The battery will still only run for 1hour theoretically. I say theoretically because the capacity is usually rate at a lower discharge rate.

that is correct for many regulators oftain referred to as "buck" regulators.

a "switching" regulator on the other hand pulses the power (kind of like PWM) and then uses capacitors to give an average voltage.
this kind of regulator does attempt to give twice the battery life at half the voltage
but they are never 100% efficient.

80-90% efficiency is typical with switching regulators.
they usually require a few more components and a little more expensive but not much.

alternatively, Dimension Engineering (one of SORs advertisers) produce a switching regulator module that has all the extra components included in a 3 pin package.


dunk.

[Soeren]

Hi,

Unfortunately no.
If you draw 21mA from the 6V regulator output the regulator will dissipate 6V * 21mA = 126mW as heat.
Ptotal = Iload *Vload + Ireg*Vreg

The battery will still only run for 1hour theoretically. I say theoretically because the capacity is usually rate at a lower discharge rate.

that is correct for many regulators oftain referred to as "buck" regulators.

a "switching" regulator on the other hand pulses the power (kind of like PWM) and then uses capacitors to give an average voltage.

Err?
A buck regulator/converter is a switch mode converter (a step down switcher, the opposite of a boost converter) - I guess you meant a linear regulator?.

The common topologies of switchers are: Buck (down), boost (up), buck/boost (up and down), SEPIC  (up and down) and CUK (up and down).

The 3 latter types are particular useable when the needed voltage is somewhere between the min. and max. voltage of a battery, eg. for getting a stable 6V from 6 cells of NiMH going from 8.4V down to 5.4V during a discharge period.