# amp hours, watts, lights and battery life... help

Discussion in 'Electronic Basics' started by max slomoff, Feb 4, 2004.

1. ### max slomoffGuest

hi,
i'm trying to teach my self, but getting confused.

i'm checking out this big battery and i want to calculate how long i
will be able to run different combinations of lights with it.

the battery has a 60 amp hour rating.

how long can i run 200 watts worth of lights from that?

how about 1000 watts worth of lights?

2000 watts?

max

2. ### Glenn AshmoreGuest

Well, 60 amp hours is not all that big. but take the voltage, say 12V
and divide it into the wattage to get amps. i.e. 200 Watts/12 = 16.6
amps. Now divide that into the capacity of the battery to get the on
time 60 amp houurs/16.6 amps = 3.6 hours.

BUT the "60 amp hour" rating is at a steady draw of 3 ampsover 20 hours
.. Faster draw means less usable power so you will get a lot less than 3
hours at 16 amp draw. More like 2 hours. Also, a deep cycle battery
should not be discharged below 50% very often so you will not get more
than an hour or so at 200 watts or you will be buying a lot of batteries.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com

3. ### dBGuest

The Ah rating is the product of current and time.

Simply stated, a 60Ah battery will deliver 60A for 1 hour, or 6A for
10 hours, 3A for 20 hours etc.

Power is equal to voltage multiplied by current.

If your battery is 12V the current drawn by a 200W load will be 200
divided by 12 i.e. 16.7A.

Divide the capacity by the current, 60 divided by 16.7 = nearly 3.6
hours.

I'm sure that you can now figure things oiut if the battery voltage is

4. ### John FieldsGuest

---
The Ampere-Hour (AH) is the product of current (amperes) and time
(hours), so a 1 AH battery can deliver 1 ampere for one hour before it's
discharged, and a 60AH battery should be able to deliver 60 amps for one
hour before it's discharged.

The watt is a unit of work, and is the product of voltage, current, and
time, so if you had a 200 watt 12 volt lamp it would draw 200W/12V =
16.6A. 1 ampere is one coulomb per second and that's where the time
part comes in, but let's let that slide for now, since it'll just
clutter things up.

So, if you've got a 12V 60AH battery, and it'll let you draw 60 amps for
one hour before it's discharged, and your 12V 200 watt lamp only draws
16.6 amps, you can say this:

60AH / 16.6A ~ 3.6Hr

If you had a 1000W 12V lamp it would draw 1000W/12V = 83.3A and your
battery would only be able to keep it going for 60AH/83.3A = 0.72Hr.

But, of course, there's a catch. Most batteries will only be able to
deliver their rated capacity in AH at a rate of C/10 or less.

What that means is that if you want the battery to deliver what it's
rated for in terms of ampere-hours (ampere-hours being called the
capacity or the "C" of the battery) you can only get that much out of it
if you take one-tenth of the rated current out of it for ten times the
rated time. In your case, since you have a battery with a C of 60AH,
you'd get 60AH out of it if you took 6A from it for 10 hours. Looking
at the manufacturer's data, you might find that if you took 60A from it
it would be discharged in 45 to 50 minutes, YMMV depending on the
battery manufacturer.

5. ### max slomoffGuest

so i get the amps and the volts from the lightbulb i plan to use?

all of the bulbs are 120 v. say i use 3*65 w bulbs. 65/120 is .5 and
*3 is 1.5 - so if i ran all these lights it would draw 1.5 amps of
current? and the battery would last about 40 hours?

this seems wrong to me, but i'm totally assuming.

why was everyone using 12 volts for their calculations?

6. ### Keith R. WilliamsGuest

If your battery is 120V, yes. Though, as others have mentioned, you'll
kill the battery by fully discharging it like this.

A 60AH 120V battery would be quite the beast though.
60AH is a reasonable size for a 12V car battery. 120V batteries aren't
common, to say the least.

7. ### eromlignodGuest

A watt is not a unit of work, it is a unit of power. It is not the
product of voltage, current and time; it is the product of voltage and
current only. It is work (energy) per unit time. Multiplying it by
time (in seconds) would give units of energy, or joules. A volt is a
joule per coulomb. A more common unit of energy (or work) in
electricity would be the kilowatt-hour.

Not trying to nitpick, but I didn't want to confuse the poster any

Don

9. ### Greg NeillGuest

Ten 60AH 12V car batteries in series. That would take
up a lot of room, and you'd have to be careful about
venting evolved gasses to avoid explosion hazards.

You can see why using a wall plug is so convenient...

10. ### Don BruderGuest

You need to turn "Watts" into "Amps" and "Time".

Let's assume you're working with a 12 volt battery - You don't say what
voltage you've actually got, so I need a starting place. Change "12" to
whatever voltage rating your battery actually is and recalculate
accordingly.

You now need to know what the resistance of the light bulb is. Start
with your first one - 200 watts - You know the wattage of the load, and
you know the voltage of the supply. You want the resistance of the load.

Ohm's law says that the bulb's resistance can be calculated from:

Watts = volts^2/ohms.

So we solve for ohms:

200 = 12^2/ohms

200 = 144/ohms

144 / 200 = 0.72 - lets check that - 144/0.72 = 200, which is correct.

So, assuming your battery is 12 volts, your bulb must have a resistance
of 0.72 ohms to be able to dissipate 200 watts.

Now you can figure out the amperage it burns, using another facet of
Ohm's law:

Amps = volts/ohms

So Amps = 12/0.72, or approximately 16.6 amps.

Now you've got the critical piece of the puzzle that you need to answer

You've got a 60 amp-hour battery - which means it can supply 60 amps for
one hour, or 10 amps for 6 hours, or 1 amp for 60 hours, or 0.1 amps for
600 hours, or... (If you haven't caught the pattern, the equation is

So to get your "run-time" for a 200 watt bulb on this 12 volt, 60
amp-hour battery, divide battery capacity (60) by amps (16.6) to get
hours of run-time. In this case, that would be a bit more than 3.5 hours.

Don't expect to get that exact amount of run-time, though - resistance
from less-than-ideal real-world wiring is going to burn up some of the
battery's capacity.

The other examples are left for you to play with. The exact same math
applies to all of them. Don't forget to adjust for your actual battery
voltage - if you've got a 6 volt battery, replace all uses of 12 in my
examples with 6. If you've got a 9 volt battery, use 9 instead of 12,
and so on.

11. ### Don BruderGuest

Because it's a convenient place to start from since you didn't specify
battery voltage, and it wouldn't be at all unusual to see a 60 amp-hour
12 volt car battery.

12. ### David WoodGuest

This discussion is academic since the OP is trying to light multiple
120V rated bulbs with a 12V source. A 65W 120V (from message 5) bulb
has a resistance while incandescently hot of ~222 Ohms and just a tiny
resistance when cold. 12V from his battery is 1/10 the voltage needed
to get the rated wattage and luminosity from that bulb. It will get
warm but not light up.

This sounds like one of those emergency/back-up/power failure lighting
scenarios. Dare I suggest a 12V light source be acquired to go with
the battery.

13. ### Keith R. WilliamsGuest

I wouldn't want to get across such a beastly lashup. 12V is
pretty safe. 10X that is rather lethal!
Indeedy!

14. ### max slomoffGuest

david,
the adds / reviews for this battery say that people run lights and
refridgerators etc from them. it runs 115 v ac. light bulbs are ac
right?
heres the stats http://www.macyummies.com/xaxp15.html

and one related question:

if i shine 2 250 watt lights onto the same subject, will the
illumination be as bright if i use 1 500 watt light?

15. ### Peter BennettGuest

That thing includes a 1500 watt inverter ("state-of-the-art
electronics that convert 12 volts from the battery pack to household
power")

Because power is constant through the system (to a first
approximation), if you plug a 60 watt 120V light into the outlet, it
will draw 0.5 amps at 120 volts from the inverter. To supply that
power, the inverter will draw 5 amps at 12 volts from the battery.

I think the marketing department had too much say in the design of the
Powerpack 1500 - with only a 60 AH battery, you can't draw the
advertised 1500 watts from it for more than 10 - 15 minutes.
I would expect so.

--
Peter Bennett, VE7CEI
peterbb (at) interchange.ubc.ca
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