On Sat, 12 Mar 2011 18:10 -0800, "Andre Abelian" wrote:

> What's the best way to calculate battery life if boost converter is
> evolved.=20

wattage out / efficiency of conversion =3D wattage in

>=20
> 1. I have boost converter that starts at 0.9v and batteries I used are
> 2xAA=20
>     at this moment I have 2 rechargeable batteries each one is 2000 mah
>     total=20
> voltage battery side
>     is 2.4v and output of boost converter is 3.3v
>    standby current boost converter takes 6ma assuming every thing else is=
=20
> disabled or in sleep.
>    I made a mistake in schematic by not enabling burst mode. it suppose
>    to be=20
> 38uA on stand by mode not 6ma. LTC3402
>    mod can be done but any time I do the mod the chip burns out.=20
> 2. I use PIC24FJ256GB110 according to datasheet it takes 1ma run time
> assuming=20
> 2v power but the clock is not mentioned so I do not know.=20
>     I am using 4mhz 3.3v and run time takes 5ma. I changed the crystal to
>     1mhz I=20
> see very small difference in load. I used 4mhz
>     only because of USB requirement. I am able to switch internal clock
>     to=20
> 4.000.000 when usb is not used and 48.000.000
>    when usb is plugged in.

The USB peripheral probably is to blame for the higher-than-predicted
current. But other peripherals you have enabled may also be having some
effect.


>=20
> my question is how should I calculate battery life based on 8ma load
> since there=20
> is a boost converter that runs at 0.9v and=20
> I know from passed that 80% of battery should be used. I know its rated
> as=20
> current per hour lets say
>=20
> 80% of 2000ma =3D 1600ma/0.008ma=3D200.000 hours
>=20
> I am just adding extra 2 ma incase of enabling other IC chips.
> does this make sense? 200.000 hours

Small correction, I think you mean 8 mA not .008 mA

I'd calculate the current required first by the wattage into the boost
converter.

If you need 3.3V at .005A that is .0165 W. If your boost converter is
80% then you need .020625 W in. At 2.4 V in that is .0086 A

So your .008 A estimate is pretty close only if you need .005 A out. But
your battery voltage will not always be 2.4 so you can figure it better
if you know the average.

But basically I would say 200 hours is not a bad estimate. But not if
you need another 2 mA for other chips.

Friendly regards,

Bob

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