Mine came with 2 -- 75ah batteries.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.
What does SWAG stand for?Using the SWAG method that gives you about 20 hours of the little lights or 2 hours of the light bar.
I'll add a tip so you can do it easier yourself in the future.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.
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.
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.SWAG method is simplier.
Thanks, great link. Will certainly bookmark to read and study later.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
Thanks, great link. Will certainly bookmark to read and study later.
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.
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'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.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.