Battery Questions

[Foxx373]

Ohm's Law - How to calculate Power, Current, and Voltage.

Power (P) is measured in Watts (W)
Current (I) is measured in Amps (A)
Voltage (V or E) is measured in Volts (V)

Follow me so far?

P = E*I
E = P/I
A = P/E

With that said, calculating how much Power (in Watts) anything should consume.

My question is, how can we calculate how long a device will last using a batterys rated mAH (Milli-Amp Hour)? Lets use this for an example:

Screen - 7.5V @ 750mA = 10W (unmodded Verge Gamecube Screen)
Battery - 14.8V @ 6300mAH (an unmodded laptop batt.)

I have read about putting rechargeables in parallel to up the current is a bad thing, why?

Is there a way of putting them in parallel safely?

How can we calculate the total Voltage and Current output of regaregables?

Perhaps ther can be a Sticky of common used formulas for refrence..?

[nos_slived]

1. My question is, how can we calculate how long a device will last using a batterys rated mAH (Milli-Amp Hour)? Lets use this for an example:

Screen - 7.5V @ 750mA = 10W (unmodded Verge Gamecube Screen)
Battery - 14.8V @ 6300mAH (an unmodded laptop batt.)

2. I have read about putting rechargeables in parallel to up the current is a bad thing, why?

3. Is there a way of putting them in parallel safely?

4. How can we calculate the total Voltage and Current output of regaregables?

5. Perhaps ther can be a Sticky of common used formulas for refrence..?

1. Same voltage is Time=Battery/(Screen+Console) where B,S, and C are current consumed, and T is in hours. I can't remember how to calculate if it is different voltages though. However, if you are just using that battery for everything, then you just measure how much current is used from the start of the circuit to the end, and divide Battery by that ammount.

2. They tend to start charging themselves because the current is not distributed equally amoung the batteries. You then end up with one battery at twice it's max current, one dead battery, some odd voltages, a lot of flames, and a broken console. Kind of fun really.

3. Not really. You can buy battery packs that have them in parallel, like 9V(4 AAAA in parallel), but make sure that if you do this, you buy from a good quality company. Just safer that way.

4. Same as alkalines. Series=Voltage1+V2+V3 and the lowest battery's current. Parallel(dangerous as you know)=Current1+C2+C3 voltage should be the same for all.

5. You cant call this a formula sheet without the ohms formula(resistance):

Resistance(R)=V/I or
Resistance(R)=P/I(squared) or
Resistance(R)=V(squared)/P

I=V/R
V=IR

These are all just pyramid formulas.

--/ \----- -----/ \
-/ V \--------/ P \
/ I|R \------/ I|E \

All you do is cover the letter you want to find, and do the remaining operation. If you cover V, you are left with I|R or I*R, so V=I*R. If you cover I, you are left with V/R so divide, and you have I=V/R.

[Foxx373]

... how to calculate an LEDs needed resistor:

R = (VS - VL) / I

VS = supply voltage
VL = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 20mA), this must be less than the maximum permitted

Ohms law; easy as pie! ;}


../ \
./ P \
/ I|E \

and I figured we could all contrubute usefull formulas and a sticky could be made out of the results.