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#### howmanyds

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Does anybody know how many Watt/hours are held in a fully charged pair of dual batteries?
I want to calculate how long I can run a pair of 18W lights without turning on the engine. Or my 300W lightbar.

#### Airedrifter

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Does anybody know how many Watt/hours are held in a fully charged pair of dual batteries?
I want to calculate how long I can run a pair of 18W lights without turning on the engine. Or my 300W lightbar.
Mine came with 2 -- 75ah batteries.

You're question is easy but hard. With 150 ah you have 1875 watt hours. (vlts x amps = watts)

So theoretically you would have 1875/36 or 52 hours of lights

and

1875/300 or 6.25 hours of the light bar.

But that means total drainage. Not a good thing for a battery. So 1/2 of 1875 is really usable and then you still need a reserve for starting the vehicle.

Using the SWAG method that gives you about 20 hours of the little lights or 2 hours of the light bar.

#### morey

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Using the SWAG method that gives you about 20 hours of the little lights or 2 hours of the light bar.
What does SWAG stand for?

#### howmanyds

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Awesome. Thanks for calculating for me!

#### howmanyds

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Does anybody have the wattage of the interior lights that stay on when the slider is open? I'm thinking they could stay on an entire weekend without much affect on the battery.

#### Chance

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Awesome. Thanks for calculating for me!
I'll add a tip so you can do it easier yourself in the future.

For some reason I don't know, many sites use units of Watts/hour when doing these kinds of estimates. That is an incorrect unit of measure and leads to confusion. It is not watts "per" hour.

The correct unit of measure in this case is watt multiplied by hour, not divided. Obviously if you divide instead of multiply everything will be wrong, or the numbers won't make sense. Or the units won't make sense.

As far as energy goes, 1000 watts for 2 hours is the same amount of energy as 2000 watts for 1 hour, or 1 watt for 2000 hours. They are each 2000 watt-hours. Or 2 kilowatt-hours.

I'll also add that the three examples above, while equal in amount of energy, do not have the same effect on batteries. That's because the faster batteries are discharged, the lower amount of energy we can get out of them.

The standard battery rating is based on discharging over a 20 hour period. So the 75 Amp-hour capacity only applies at that 20-hour rate. And that's around 4 Amps, or roughly 50 watts for these particular batteries (I'm rounding off numbers). When we start pulling higher loads like 300 watts we will get less out of the same battery.

#### lkruper

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I'll add a tip so you can do it easier yourself in the future.

For some reason I don't know, many sites use units of Watts/hour when doing these kinds of estimates. That is an incorrect unit of measure and leads to confusion. It is not watts "per" hour.

The correct unit of measure in this case is watt multiplied by hour, not divided. Obviously if you divide instead of multiply everything will be wrong, or the numbers won't make sense. Or the units won't make sense.

As far as energy goes, 1000 watts for 2 hours is the same amount of energy as 2000 watts for 1 hour, or 1 watt for 2000 hours. They are each 2000 watt-hours. Or 2 kilowatt-hours.

I'll also add that the three examples above, while equal in amount of energy, do not have the same effect on batteries. That's because the faster batteries are discharged, the lower amount of energy we can get out of them.

The standard battery rating is based on discharging over a 20 hour period. So the 75 Amp-hour capacity only applies at that 20-hour rate. And that's around 4 Amps, or roughly 50 watts for these particular batteries (I'm rounding off numbers). When we start pulling higher loads like 300 watts we will get less out of the same battery.

Chance, good explanation. If you want to take it further and know the Peukert's constant (k) for your battery, you can use the formula to calculate the actual time a particular load can last (t) and the AH rating at that load (It). That might be good for the final analysis before setting a system in stone.

First calculate
t = H (C/IH)^k

then

It = C (C/IH)^k-1

H is the rated discharge time, in (hours).
C is the rated capacity at that discharge rate, in (Ampere-hours).
I is the actual discharge current, in (Amps).
k is the Peukert constant, (dimensionless).
t is the actual time to discharge the battery, in (hours).

No, I am not that smart The full explanation is here -> http://www.batterystuff.com/kb/tools/peukert-s-law-a-nerds-attempt-to-explain-battery-capacity.html

#### howmanyds

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#### Airedrifter

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Chance, good explanation. If you want to take it further and know the Peukert's constant (k) for your battery, you can use the formula to calculate the actual time a particular load can last (t) and the AH rating at that load (It). That might be good for the final analysis before setting a system in stone.

First calculate
t = H (C/IH)^k

then

It = C (C/IH)^k-1

H is the rated discharge time, in (hours).
C is the rated capacity at that discharge rate, in (Ampere-hours).
I is the actual discharge current, in (Amps).
k is the Peukert constant, (dimensionless).
t is the actual time to discharge the battery, in (hours).

No, I am not that smart The full explanation is here -> http://www.batterystuff.com/kb/tools/peukert-s-law-a-nerds-attempt-to-explain-battery-capacity.html

SWAG method is simplier. #### lkruper

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SWAG method is simplier. Or do what engineers do.... calculate to 0.0001% and then multiply by 2X just to make sure #### Chance

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SWAG method is simplier. That works pretty well unless one starts to discharge battery very fast. For example, if a person tries to run a microwave or air conditioner off an inverter that is powered from a single 75 Ah battery, the battery will likely drain down way faster than expected.

A roof AC that pulls 150 Amps from battery will certainly run far less than 1/2-hour before battery is drained. It's kind of hard to even guess. I know this happens too often to some running microwaves that may pull around 100 Amps from battery.

This relationship also confirms that doubling battery capacity will like more than double run time. It's an interesting balancing act.

#### Chance

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Chance, good explanation. If you want to take it further and know the Peukert's constant (k) for your battery, you can use the formula to calculate the actual time a particular load can last (t) and the AH rating at that load (It). That might be good for the final analysis before setting a system in stone.

First calculate
t = H (C/IH)^k

then

It = C (C/IH)^k-1

H is the rated discharge time, in (hours).
C is the rated capacity at that discharge rate, in (Ampere-hours).
I is the actual discharge current, in (Amps).
k is the Peukert constant, (dimensionless).
t is the actual time to discharge the battery, in (hours).

No, I am not that smart The full explanation is here -> http://www.batterystuff.com/kb/tools/peukert-s-law-a-nerds-attempt-to-explain-battery-capacity.html

How often do you find the "k" value listed for individual battery models? Or is it just about lumping all AGMs as one, for example? Perhaps the article covers this. A briefly saw a range for different types.

#### lkruper

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How often do you find the "k" value listed for individual battery models? Or is it just about lumping all AGMs as one, for example? Perhaps the article covers this. A briefly saw a range for different types.

I have not done it. Some manufactures have this in their technical information. But there is calculation application on that site that allows someone to generate the factor, I believe.

#### Airedrifter

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That works pretty well unless one starts to discharge battery very fast. For example, if a person tries to run a microwave or air conditioner off an inverter that is powered from a single 75 Ah battery, the battery will likely drain down way faster than expected.

A roof AC that pulls 150 Amps from battery will certainly run far less than 1/2-hour before battery is drained. It's kind of hard to even guess. I know this happens too often to some running microwaves that may pull around 100 Amps from battery.

This relationship also confirms that doubling battery capacity will like more than double run time. It's an interesting balancing act.

I understand, but those factors are in the SWAG system. #### Chance

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I have not done it. Some manufactures have this in their technical information. But there is calculation application on that site that allows someone to generate the factor, I believe.
I've seen the data but in the form of a chart, so if you don't have an engineering or math background it's easy to follow. I just can't find the chart. It's similar to the chart for battery life as a function of discharge depth, except it shows energy as a function of discharge rate.

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#### lkruper

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I've seen the data but in the form of a chart, so if you don't have an engineering or math background it's easy to follow. I just can't find the chart. It's similar to the chart for battery life as a function of discharge depth, except it shows energy as a function of discharge rate.

The Concorde/Lifeline manual has a bunch of charts including a Peukert plot at http://www.lifelinebatteries.com/manual.pdf

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