# Help with battery math?



## Barry Allen (Apr 18, 2018)

Who here is good at battery capacity math?

I’m going on camping trips in the southwest next week and for warm weather sleeping I’ve got a fan to run in the tent. I’ll plug a 12v socket converter onto the battery terminals and use a small power inverter to run the fan at night. On low speed it has to be less than 10 watts draw, if that.

How long can I run the fan before the battery is too low to start the engine in the morning?


----------



## boraz (Aug 29, 2013)

a 12v fan will be more efficient 

you can buy an inline battery monitor/shutoff that you plug your 12v device into and it runs until battery is low, then shuts off to retain starting capability.

but whether you do it my way or your way (calculating and hoping) you should pack along a small jump box jic....


----------



## JLL (Sep 12, 2017)

In order to calculate that, you'll need to know the minimum amperage required to start the car.

A 10 watt device would use ~0.83 Amps on a 12V system.

But I agree with @boraz , a jump pack would eliminate the need to worry. At least for most people.


----------



## Valpo Cruze (Feb 23, 2014)

I know you are trying to run off the car but a typical RV dry camping setup would be twin 6V batteries in series as that will give more amp capacity. Twin 12s in parallel is a common setup as well.

These setups will last well past all night long pulling more power than just a fan. Your single battery should be fine. 

But as suggested already, bring a fully charged jump pack.


----------



## Valpo Cruze (Feb 23, 2014)

According to Google a 20 inch window box fan will draw 50 to 100 watts.


Solution B: For less than $30 you can get a small fan that runs on D cell batteries








RoadPro 10-Portable Fan with Dual Power Options of 12-Volt or D Cell Batteries RP8000 - The Home Depot
 

Keep your cool wherever your active life leads you with this great-looking portable fan. An easy and convenient way to enjoy a refreshing breeze in you truck, car, recreational vehicle or picnic bench,



www.homedepot.com


----------



## snowwy66 (Nov 5, 2017)

For $15 (price might be higher these days) you can buy a 12v fan from any truck stop. Plugs in to the cig socket.


----------



## Barry Allen (Apr 18, 2018)

I will check out the 12v fan. How long is the cord?

The clamp-on thing for the battery has maybe a 2 foot cord so I'll have to use the little plug-in inverter and then an extension cord to get the 120v output to the tent where the fan can be plugged in.


----------



## boraz (Aug 29, 2013)

Barry Allen said:


> I will check out the 12v fan. How long is the cord?
> 
> The clamp-on thing for the battery has maybe a 2 foot cord so I'll have to use the little plug-in inverter and then an extension cord to get the 120v output to the tent where the fan can be plugged in.


doesnt matter how long the cord is

they sell 12v extension cords


----------



## Valpo Cruze (Feb 23, 2014)

12V cords are limited to approximately 10 feet.

In concerns for 110V length limits and cable sizes needed depend on what load you are trying to power. 

The 14 and 12 gauge extension cords are among the most popular used types of cords. While there are different gauges, including the 16 and 10 gages, 12 and 14 are the most common. However, they carry different power amounts depending on the length of the cord in question. For instance, for a length of 25 - 50 feet, a 14 gauge cord will be great for 14-15 Amps, whereas the 12 gauge will work for 16-20 Amps. 
For 100 feet, the 14 gauge cord works for 11-13 Amps, while the 12 gauge cord is best suited for 14-15 amps. If your cord is 150 feet long, the 14 gauge is best suited for 1-7 Amps, whereas the 12 gauge will support 8-10 Amps. This shows that the extension cord's length also plays a part in the power it helps safely.


----------



## snowwy66 (Nov 5, 2017)

I wouldn't use an invertor.

That's one more to the equation of battery power useage.


----------



## boraz (Aug 29, 2013)

Valpo Cruze said:


> 12V cords are limited to approximately 10 feet.
> 
> In concerns for 110V length limits and cable sizes needed depend on what load you are trying to power.
> 
> ...


12v cords ARE NOT limited to 10'

rofl

its a 12v fan that draws half an amp


----------



## Valpo Cruze (Feb 23, 2014)

Please post a link to 1 to buy then


----------



## snowwy66 (Nov 5, 2017)

Google has them everywhere internet. 

Truckstops sell them. 
I think Walmart has them too.
Amazon.


----------



## boraz (Aug 29, 2013)

Valpo Cruze said:


> Please post a link to 1 to buy then





Amazon.com : 12v cigarette lighter power cord extension


----------



## Valpo Cruze (Feb 23, 2014)

Okay so the Amazon cords are 12 feet instead of what I said, approximately 10 feet. 

Just because you can buy a longer cord does not actually mean you should use a longer cord. For any electrical system you want no more than 3% power loss or 0.36V on a 12V system. It is recommended to keep a 2% loss for 12V automotive systems. Based on wire size and amp load the max distance can be calculated then there is the whole ampacity rating of wire to deal with.

12V accessories tend to be small gauge wire so the ampacity is relatively small on them. Longer small gauge wire will heat up with amp load, any wire will heat with amp load.

The fan I linked to earlier has a 1 amp rating which means you can have about 35 feet of 18 gauge wire, only 15 feet total if its a 22 gauge wire. If the amp rating is lower then you can safely extend the wire but if its higher then it drops in length. 5A is 3 feet for 22 gauge, or 7.5 feet for 18 gauge.


You have to know your amperage load in order to determine how long the cord can be. And you cannot just take a larger cord and plug it on to the built in pig tail of the fan. You are limited to the smallest wire in the extension circuit.


----------



## boraz (Aug 29, 2013)

Valpo Cruze said:


> Okay so the Amazon cords are 12 feet instead of what I said, approximately 10 feet.
> 
> Just because you can buy a longer cord does not actually mean you should use a longer cord. For any electrical system you want no more than 3% power loss or 0.36V on a 12V system. It is recommended to keep a 2% loss for 12V automotive systems. Based on wire size and amp load the max distance can be calculated then there is the whole ampacity rating of wire to deal with.
> 
> ...


i literally linked a variety, you can choose 12-18awg, 3-25'


----------



## alleninsalem (Jul 20, 2021)

Barry Allen said:


> Who here is good at battery capacity math?
> 
> I’m going on camping trips in the southwest next week and for warm weather sleeping I’ve got a fan to run in the tent. I’ll plug a 12v socket converter onto the battery terminals and use a small power inverter to run the fan at night. On low speed it has to be less than 10 watts draw, if that.
> 
> How long can I run the fan before the battery is too low to start the engine in the morning?


I wouldn't chance it with an inverter and all that. Personally I use a fan that charges with a usb cable and has battery in it. I take several charged batteries and/or plug into my lithium jump box. Leave our car out of the equation.


----------



## Earthangel496 (Oct 17, 2017)

Valpo Cruze said:


> According to Google a 20 inch window box fan will draw 50 to 100 watts.
> 
> 
> Solution B: For less than $30 you can get a small fan that runs on D cell batteries
> ...


They sell these at Walmart in camping or fan section for around $20


----------



## 72chevman (Dec 28, 2018)

Search Amazon for Camping fan. They hang from top of tent, have huge battery packs, so they run 8-12 hrs and recharge via USB while you drive.








Amazon.com: Camping Fan with LED Lights 7-Inch, Rechargeable 4400mAh Desk USB Fan Battery Operated with 3 Level Air Flow, Portable Personal Fan for Camping Home Office Travel Indoor Outdoor… : Home & Kitchen


Amazon.com: Camping Fan with LED Lights 7-Inch, Rechargeable 4400mAh Desk USB Fan Battery Operated with 3 Level Air Flow, Portable Personal Fan for Camping Home Office Travel Indoor Outdoor… : Home & Kitchen



www.amazon.com


----------



## radio_davio (Dec 28, 2018)

Your battery's capacity is unknown. Cannot compute.


----------



## Cruzer4326 (Jan 22, 2017)

In the case that youb drain your car's battery and use a jump pack, you are leaving the alternator to charge a dead battery. This is a bad idea! Alternators are battery maintainers and you wil run the risk of overheating, and thus 'burning up' the alternator. I'm with the Use Another Energy Source group.


----------



## Blasirl (Mar 31, 2015)

Just for grins - don't forget to calculate voltage drop across any extension cord you use if you go that route.

Shorter cord and thicker copper - i.e lower gauge, is better. !2v DC is better than inverted AC voltage also.


----------



## Barry Allen (Apr 18, 2018)

Cruzer4326 said:


> In the case that youb drain your car's battery and use a jump pack, you are leaving the alternator to charge a dead battery. This is a bad idea! Alternators are battery maintainers and you wil run the risk of overheating, and thus 'burning up' the alternator.


As long as you are actually driving somewhere at speed and not just idling the engine to charge the battery, it's fine. The airflow through the engine compartment and through the alternator cools it sufficiently.


----------



## Cruzer4326 (Jan 22, 2017)

Barry Allen said:


> As long as you are actually driving somewhere at speed and not just idling the engine to charge the battery, it's fine. The airflow through the engine compartment and through the alternator cools it sufficiently.


Ok, You can tell yourself that to soothe your conscience. I rebuilt starters and alternators for many years. (Have you ever seen a burned up stator or rectifier?) They're not built to charge a dead battery. Sure it may survive that harsh treatment a given number of times, but I wouldn't recommend doing this regularly and I don't understand how anyone who calls himself a professional mechanic coud make such a statement as you have.


----------

