Math check on battery usage

solomon_says

12V x 100Ah = 1200W

I want to find out total watts for a single 12V 100Ah battery assuming 100% efficiency. Measuring a 60 W bulb for one hour, I can run the bulb for 20 hours?

Do I measure bulb usage in watts or watt hours? I don't need a sizing, I just need to know the math.

mvas

Your formula is OK, except it has the wrong units.
12 Volts x 100 Amp-Hours = 1200 Watt-Hours
You must use the correct "units".

Also, Watt-Hours = Watts x Hours

Yes, you can run the bulb for 20 hours, but it will be very dim.
Typically, we run for 10 hours or less, then we stop.
Typically, we run until a specific "Low Voltage" is hit, then we stop.

EDIT: fixed typo!

solomon_says

My battery is 12V and 100Ah. If V x Ah = Wh I will only have 100 watt hours?

myocardia

solomon_says said:

My battery is 12V and 100Ah. If V x Ah = Wh I will only have 100 watt hours?

You find watt hours by multiplying nominal battery voltage times the battery's amp hour rating. 12x100 is not 100, it is 1200. BTW, unless you plan on buying yourself a new battery every few weeks, you'll want to use no more than 50% of it's amp hours, and 20-25%, if you're wanting it to last as long as possible.

solomon_says

I was confused cause mvas said 12V x 1200Ah = 1200 Wh.

If I know 1 battery has 1200 watt-hours, is it correct to calculate run time of a 60W light bulb if 1200Wh ÷ 60W? I read somewhere that watt-hours are found by V x Ah ÷ 100 so now I'm very confused.

myocardia

solomon_says said:

I was confused cause mvas said 12V x 1200Ah = 1200 Wh.

If I know 1 battery has 1200 watt-hours, is it correct to calculate run time of a 60W light bulb if 1200Wh ÷ 60W? I read somewhere that watt-hours are found by V x Ah ÷ 100 so now I'm very confused.

Yes and no. Yes because that's how you figure out what you're trying to figure out, and no because a brand new battery will only survive a few weeks worth of you treating it that way. That would get really expensive, really fast. You should never plan on discharging any rechargeable lead acid battery more than 50%, and if you want the batteries to last a lot longer, you only want to discharge them 20-25%, meaning you'd need to divide 100aH by either 4 or 5, to find out how many ampere hours you can use from them, before you'll need to fire up your generator to recharge them, instead of replacing them..

mcgivor

solomon_says said:

I was confused cause mvas said 12V x 1200Ah = 1200 Wh.

If I know 1 battery has 1200 watt-hours, is it correct to calculate run time of a 60W light bulb if 1200Wh ÷ 60W? I read somewhere that watt-hours are found by V x Ah ÷ 100 so now I'm very confused.

You are getting confused by mixing a time based figure with a value that has no time base.

Example
A  12V, 50 W lamp is required to operate for 10 hours from a 12v battery,  what would the recommended battery capacity be, for a maximum 30% depth-of-discharge ?

Current is power divided by voltage, so 50/12 =  4.167 amps
Ampere hours would be current multiplied by time in hours, 4.167 × 10 = 41.67 AH
Battery size for 30% depth-of-discharge 100/30 × 41.67 = 138.9 Ah

Personally I prefer to do the electrical calculation first then involve the time, working with Ah and Wh together is confusing, batteries are sized in Ah. so easier to transpose.

So the 60W, 100Ah battery question.
60W/12V = 5 A
The 100 Ah battery is really 30Ah, for a 30% depth-of-discharge 
Therefore 30Ah/5A = 6 hours running time to 30% DOD.

Of course this is a rough calculation, assuming full capacity etcetera, voltages would be slightly higher, therefore power higher, it's always better to be conservative, by increasing battery capacity, or reducing the run time.

Hope this helps.

solomon_says

@mcgivor ok I finally see how the numbers work. Thanks for the help everyone, I think I'm getting smarter now

mvas

Fixed typo in message #2 ...