Dual battery W/h - Ford Transit USA Forum
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post #1 of 16 (permalink) Old 06-18-2015, 12:55:AM Thread Starter
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Dual battery W/h

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.
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post #2 of 16 (permalink) Old 06-18-2015, 12:58:PM
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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.

2015 Transit Wagon 350 XLT, 3.5 Ecoboost, 3.31 limited slip, Medium height, Medium length, Tow package, dual batteries, 110 vlt, SYNC with CD, ...
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post #3 of 16 (permalink) Old 06-18-2015, 02:07:PM
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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?
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post #4 of 16 (permalink) Old 06-18-2015, 02:25:PM Thread Starter
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Awesome. Thanks for calculating for me!
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post #5 of 16 (permalink) Old 06-18-2015, 02:28:PM Thread Starter
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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.
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post #6 of 16 (permalink) Old 06-19-2015, 08:09:AM
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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.



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post #7 of 16 (permalink) Old 06-19-2015, 10:22:AM
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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...-capacity.html

Last edited by lkruper; 06-19-2015 at 11:58:AM.
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post #8 of 16 (permalink) Old 06-19-2015, 12:27:PM Thread Starter
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Nice. This is helpful.
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post #9 of 16 (permalink) Old 06-19-2015, 12:53:PM
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Originally Posted by lkruper View Post
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...-capacity.html

SWAG method is simplier.

2015 Transit Wagon 350 XLT, 3.5 Ecoboost, 3.31 limited slip, Medium height, Medium length, Tow package, dual batteries, 110 vlt, SYNC with CD, ...
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post #10 of 16 (permalink) Old 06-19-2015, 01:02:PM
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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
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