How many batteries does it take to light this bulb up?

[physicsboi12]

Hey guys
So lately, I've been interested in circuits. I am good at maths, but my physics isn't as good (hence why i signed up to ask questions). So I have a question. Imagine you have a simple circuit, like below:

Simple circuit, 1 bulb and X amount of batteries. So my question is, how many batteries would it take to light up the bulb, given the following specification?

BULB
1.5V
0.3 amp

Each battery has a specification of:
1.5V
1200 mAh

----------------------------
So here's my working out, I am not sure if I have done it correctly.

for bulb:
P = VI
P = 1.5 X 0.3
P = 0.45W

So the power required to light up the bulb is 0.45W.

for battery:
1200 mAh = 12Ah
12Ah/60 = 0.2A

So we have the amp (0.2) and the voltage (1.5), we can work out the power:
P = VI
P = 1.5 X 0.2
P = 0.3W

So each battery produces a power of 0.3W, but we need at least 0.45W.

So two batteries is 0.6W.

Number of batteries needed = 2.

Is this correct?

 

[mjc123]

for battery:
1200 mAh = 12Ah
12Ah/60 = 0.2A

What are you trying to do (wrongly) here? Do you know what a mAh is?

 

[physicsboi12]

What are you trying to do (wrongly) here? Do you know what a mAh is?

Yeah I think that bit is wrong, I don't think you can turn mAh into A. How would you find the A?

 

[mjc123]

You would need to know the time over which the battery is discharged. You don't. Assume that the battery can deliver the required 0.3A current.

 

[russ_watters]

Yeah I think that bit is wrong, I don't think you can turn mAh into A. How would you find the A?

A battery will supply the required amperage or die trying.

mah is capacity, analogous to the gas tank in your car. it has no direct bearing on how fast the car can go (how much power the battery can put out).

 

[physicsboi12]

You would need to know the time over which the battery is discharged. You don't. Assume that the battery can deliver the required 0.3A current.

How do I find this out, without actually discharging a battery myself?

For example, if you had this specification sheet for the battery: https://d2ei442zrkqy2u.cloudfront.net/wp-content/uploads/2016/03/QU1500_US_UL1.pdf

How do you find the A from this?

 

[russ_watters]

How do I find this out, without actually discharging a battery myself?

The way you worded the question, we don't know if you are specifying or asking how long it can stay lit, or if you want to calculate it.

You have to tell us what it is you want to know!

 

[physicsboi12]

The way you worded the question, we don't know if you are specifying or asking how long it can stay lit, or if you want to calculate it.

You have to tell us what it is you want to know!

Basically I don't know the specifications of the battery. I guess I have to calculate the A? I don't know how.

 

[Merlin3189]

Welcome to PF. Thanks for a clear question and statement of your ideas. That enables people to understand your problem and offer pertinent advice.

So here's my working out, I am not sure if I have done it correctly.

for bulb:
P = VI
P = 1.5 X 0.3
P = 0.45W Correct.

So the power required to light up the bulb is 0.45W.

for battery:
1200 mAh = 12Ah should be 1 point 2 amp hours
12Ah/60 = 0.2A but what is this? 1.2 Ah means it can supply 1.2 A for 1 hour. If you discharged the battery in 1 minute, you could draw (in theory) 60x that current, 72 A. I can't see any reason to divide by 60

So we have the amp (0.2) and the voltage (1.5), we can work out the power: But this is not a relevant calculation
P = VI
P = 1.5 X 0.2
P = 0.3W

So each battery produces a power of 0.3W, and this is not a valid conclusion, but we need at least 0.45W.

So two batteries is 0.6W.

Number of batteries needed = 2.

Is this correct?

First, power is not a consideration in the way you asked the question.
Power is the rate of using or supplying energy - how much energy in a given time. But your question does not specify a time.

You have a battery that holds a certain amount of energy 1.5 V x 1.2 Ah = 1.5 V x 1.2 A x 3600 sec = 6480 J
That energy can come out slowly or quickly, like 1.2 A for 1 hour, or 12 A for 6 min or 120 mA for 10 hour.

In your case you simply look at the current you need, 0.3 A, the 1.2 Ah / 0.3A = 4 h
Your battery can supply 0.3 A for 4 h. Since the voltage is just what is required, you need just this 1 battery to light your bulb for 4 hours.
If you need longer, then 2 batteries will give 8 h, 3 batteries 12 h, etc.

If you want to think about power, the bulb is using 0.45 W or 0.45 J/sec, so your 6480 J battery lasts $\frac {6480 \ J} {0.45 \ J/sec} = 14400 \ sec = 4\ h$
But the reason batteries are described by their Ah capacity (I guess) is that it is easier to calculate the current rather than the energy.

 

[JoeDeschesnes]

To my understanding you have:
- 1 bulb requiring 0.3 Amp at 1.5V
You want to figure how many batteries to power it.
- Batteries giving 1200 mAh at 1.5V

Since
1200 mAh = 1.2 Amps/hour
Then with only 1 battery you could power your bulb for 4 hours:
4h × 0.3amps = 1.2 amps/hour (1200 mAh)

 

[physicsboi12]

To my understanding you have:
- 1 bulb requiring 0.3 Amp at 1.5V
You want to figure how many batteries to power it.
- Batteries giving 1200 mAh at 1.5V

Since
1200 mAh = 1.2 Amps/hour
Then with only 1 battery you could power your bulb for 4 hours:
4h × 0.3amps = 1.2 amps/hour (1200 mAh)

Thanks, this has helped me. So this is similar problem, this time its 1 battery and 3 bulbs.

Each bulb has:
0.3A
1.5V

And the battery (just 1) has:
2000mAh
1.5V

So for the bulb, the total A is:
A = 0.3 X 3 = 0.9A

And the total V is:
V = 1.5 X 3 = 4.5V (but I don't think this is relevant?) 2000mAh = 2Ah

h = Ah/A = 2Ah/0.9A = 2.22 hours

So 1 battery, of 2000mAh, will power the three bulbs for 2.22 hours.

Is this right?

 

[JoeDeschesnes]

Thanks, this has helped me. So this is similar problem, this time its 1 battery and 3 bulbs.

Each bulb has:
0.3A
1.5V

And the battery (just 1) has:
2000mAh
1.5V

So for the bulb, the total A is:
A = 0.3 X 3 = 0.9A

And the total V is:
V = 1.5 X 3 = 4.5V (but I don't think this is relevant?)2000mAh = 2Ah

h = Ah/A = 2Ah/0.9A = 2.22 hours

So 1 battery, of 2000mAh, will power the three bulbs for 2.22 hours.

Is this right?

Yes! That would be right! Well... I'm just the average Joe and I may be wrong but that's what I believe would be right! Haha

 

[davenn]

And the total V is:
V = 1.5 X 3 = 4.5V (but I don't think this is relevant?)

yes it is relevant, because you cannot supply 3 bulbs in series from a single 1.5V battery. 1.5 does not equal 4.5
so you would need 3 x 1.5V batteries in series

 

[Mister T]

for bulb:
P = VI
P = 1.5 X 0.3
P = 0.45W

So the power required to light up the bulb is 0.45W.

That's not the correct conclusion to be drawn from that calculation. What the calculation tells you is 0.45 W is the power consumed by the bulb when the voltage is 1.5 V and the current is 0.3 A. You would be able to get the bulb to light up while consuming far less power, but of course not as brightly.

 

[physicsboi12]

yes it is relevant, because you cannot supply 3 bulbs in series from a single 1.5V battery. 1.5 does not equal 4.5
so you would need 3 x 1.5V batteries in series

Ah okay, got it! How about if the bulbs were in parallel - would you still need 3 batteries or would 1 battery be ok for them to light up with equal brightness?

 

[davenn]

Ah okay, got it! How about if the bulbs were in parallel - would you still need 3 batteries or would 1 battery be ok for them to light up with equal brightness?

no, in that case you only need one battery
and for the bulbs to light up to their normal/full brightness, the battery needs to be capable of supplying enough current

Dave

 

[physicsboi12]

no, in that case you only need one battery
and for the bulbs to light up to their normal/full brightness, the battery needs to be capable of supplying enough current

Dave

So in the example in post #11, the bulbs would light up to their normal/full brightness with 1 battery, right?

 

[davenn]

So in the example in post #11, the bulbs would light up to their normal/full brightness with 1 battery, right?

yes, as said before, if x 1.5V globes in parallel then a 1.5V battery is all that is needed and it needs to be able to supply the current required



Dave