Battery Usage Calculation

[casejeep]

I know this will seem pretty basic to a lot for you people. But I can not find the answer in terms I can understand.

How do I figure out how long I can run said electronic on my dual battery set up?

Also, let me run this by you guys. Run a live wire from the vehicle battery to a cut off switch. When the vehicle is on turn the switch on so the 2nd battery charges. Turn switch off when vehicle is off? Is this too simple to work? Am I missing some very fundamental electrical engineering?

Thanks

 

[mm58]

Without considering any variables, basically an 80 amp hour (A/hr) battery, fully charged and in good health, will power a one amp load for eighty hours. Or, a .5 amp load for 160 hrs; a 2 amp load for 40 hours, 8 amps for 10 hours, etc etc

 

[DaveNay]

Without considering any variables, basically an 80 amp hour (A/hr) battery will power a one amp load for eighty hours. Or, a .5 amp load for 160 hrs; a 2 amp load for 40 hours, 8 amps for 10 hours, etc etc

Also, for a deep-cycle battery, you should not use greater than 50% of the capacity. A standard "starter" battery on a vehicle isn't really suitable at all for a lengthy discharge (5%-10% of capacity?)

 

[casejeep]

How do watts factor into this?

Without considering any variables, basically an 80 amp hour (A/hr) battery, fully charged and in good health, will power a one amp load for eighty hours. Or, a .5 amp load for 160 hrs; a 2 amp load for 40 hours, 8 amps for 10 hours, etc etc

 

[mm58]

Volts x Amps = watts.
Say you're running a 12v appliance that draws 5 amps. That's 60 watts of 'power'

 

[DaveNay]

How do watts factor into this?

Watts divided by volts = amps.

A 120 watt device at 12V will draw 10 amps per hour.

An 80 A/hr battery would run that device for 4 hours (@ 50% reserve capacity)

 

[casejeep]

Like always I have been lead out of the dark into the light!

Thanks much!

 

[dwh]

Also, let me run this by you guys. Run a live wire from the vehicle battery to a cut off switch. When the vehicle is on turn the switch on so the 2nd battery charges. Turn switch off when vehicle is off? Is this too simple to work? Am I missing some very fundamental electrical engineering?

Thanks

That would work. Keep in mind that it might take anywhere from 8 hours to 24 hours of drive time to fully recharge a 100 amp*hour battery that's halfway drained depending on your wire size, and how your truck's voltage regulator behaves.

 

[croix]

Watts divided by volts = amps.

A 120 watt device at 12V will draw 10 amps per hour.

An 80 A/hr battery would run that device for 4 hours (@ 50% reserve capacity)

Please keep in mind that this is a good starting point, but results may vary. Things like line loss, temperature, battery age, etc. will reduce the number of hours you actually get from a battery. If the loads you plan to use for the number of hours you plan to use them push the limits of the battery, you may consider stepping up to a battery with more amp-hours. Just my 2 cents.

-croix

 

[croix]

Also, let me run this by you guys. Run a live wire from the vehicle battery to a cut off switch. When the vehicle is on turn the switch on so the 2nd battery charges. Turn switch off when vehicle is off? Is this too simple to work? Am I missing some very fundamental electrical engineering?

Thanks

Although this would technically work it could create significant problems. For example, batteries of different age will try to equalize when connected. This can degrade one or both batteries. Also - depending on which loads are run each battery you could end up draining both batteries if the switch is left in the wrong position. Personal opinion here... but a dual battery setup works better with a proper isolator to control when each is charged and prevents primary battery from being drained below starting voltage.

 

[Hilldweller]

Please keep in mind that this is a good starting point, but results may vary. Things like line loss, temperature, battery age, etc. will reduce the number of hours you actually get from a battery. If the loads you plan to use for the number of hours you plan to use them push the limits of the battery, you may consider stepping up to a battery with more amp-hours. Just my 2 cents.

-croix

Plus this.
You've got a theoretical 100 ah battery in pretty good health with 95 actual ah available. A 40 watt appliance. The battery is charged to a nominal 13v to start.
Appliance, a fridge, duty cycles according to its thermostat ---- 40 watts divided by 13 volts. But the volts begin to drop and the amps rise. It's a logarithmic loss...

 

[DaveNay]

Plus this.
You've got a theoretical 100 ah battery in pretty good health with 95 actual ah available. A 40 watt appliance. The battery is charged to a nominal 13v to start.
Appliance, a fridge, duty cycles according to its thermostat ---- 40 watts divided by 13 volts. But the volts begin to drop and the amps rise. It's a logarithmic loss...

The OP is a beginner....throwing all these finer details at him is not going to help him grasp the fundamentals.

 

[Hilldweller]

The OP is a beginner....throwing all these finer details at him is not going to help him grasp the fundamentals.

Okay.

In other words, aim high. Have more amps than you think you'll need, drain your batteries as little as you can, & fill them back up as soon as you can. Formula for happy batteries.

 

[DaveNay]

Okay.

In other words, aim high. Have more amps than you think you'll need, drain your batteries as little as you can, & fill them back up as soon as you can. Formula for happy batteries.

You did notice that I suggested he de-rate his battery by 50%? :iagree:

 

[Hilldweller]

You did notice that I suggested he de-rate his battery by 50%? :iagree:

Yeah. I see so many people using the water analogy too seriously. Thinking they can drink the well dry and just fill it up again.
Poor batteries...