Auxiliary Battery in Truck Bed - How to keep charged

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Auxiliary Battery in Truck Bed - How to keep charged

Postby Rusty99 » Mon Jul 29, 2019 9:37 pm

Hi folks,

I am scrambling to prep for an upcoming 1 week long road trip and need some help and advice. My electrical knowledge is low but I am willing to try anything with basic skills and your suggestions.

I have a 12 volt cooler that I want have always running in the bed of the truck. Cooler consumes 48 watts / draws 4 amps according to specs on Koolatron website. My idea was to purchase an deep cycle battery to have in the truck bed and power the cooler.

Advice - Help #1 Charging the Deep Cycle Battery

A) Use my 2000 Watt Honda Generator and Automotive battery charger to top up the battery each night. Also how do I calculate how long I can run the cooler on the battery? What is the calculation?

B) Wire up something easy in the truck so that the trucks electrical system will trickle charge the auxiliary battery when driving. We will be on the hwy every second day for about 3 hours. With limited time how would I go about doing this? I want to ensure that I do not drain my trucks starting battery.

Advice - Help #2 How do I connect power to cooler

The cooler has a 12 volt connection that would plug into a 12 volt Socket. If I buy a 12 volt socket off amazon do I just connect it directly to the 12 volt deep cycle battery? Would there need to be a fuse? or shut off switch?

I hope this makes sense in what I am trying to achieve. Is a last minute add on and hope to get it done. Thanks in advance for your help. :D
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Re: Auxiliary Battery in Truck Bed - How to keep charged

Postby Pinstriper » Tue Jul 30, 2019 12:45 am

I can answer a few of these.

How long you can run depends on the battery size, which will be rated in Amp Hours. So, a 100Ah battery would run your 4 amp draw for about 25 hours. But not really, as you will damage the battery by drawing it down past 50%. So call it 12 hours.

Now, can you recharge that ? It isnt as simple as saying a 4 Amp charger for 12 hours, as the battery will absorbe charge slower asit approaches 100%. You will want to spec a battery charger that can top off that battery in the time available.

If it were me, I would not mess with charging the battery from your truck on your three hour drives. I would run the cooler itself off your car during that time. It is 3 hours less draw on your battery, and when you are stopped, you can run both the cooler and the battery charger off your generator.

The real question is how hot the weather will be. No cooler will keep up with being in direct sun and hot temperatures. Keep the cooler in a shaded area will be key. Remember also that high heat kills batteries.

A visit to your local batteries+ store should set you up with the right sized battery and charger combination, provided you give them the draw/load and the expected run time, along with how much time you can make available on the generator.

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Re: Auxiliary Battery in Truck Bed - How to keep charged

Postby RJ Howell » Tue Jul 30, 2019 6:48 am

First and foremost: Can your alternator handle all this extra work? Not all can.

I do know of a few charging their batteries from the truck and using a Solar battery charge controller. Cheap enough on Ebay or Amazon.

You could also go with a small charging station that has a 12v and USB sockets.

How long will it last? As another stated depends on your battery setup.
Amp hour rating is key! Battery type is also.

For example I'll use a 100ah battery:
12v lead acid battery should only be drawn down 75% of rating. 25amps available
12v Gel/AGM 65%. 35amps available
12v Lithium 95% 95amps available
Pair of 6v lead acid 50% 50amps available.

I have dual 6v's. I consider those to be the only 'true' deep cycle batteries out there.
I'll go lithium once the dollar/cost gets better.
All these %'s are based upon a good charge/dis-charge cycle and not damaging the batteries.

Hope some of this helps!
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Re: Auxiliary Battery in Truck Bed - How to keep charged

Postby John61CT » Tue Jul 30, 2019 8:13 pm

very inefficient to use that 12V cooler rather than a 12V compressor fridge

also how cold will it get when ambients hit 100° ?

Many Engels for example will freeze ice cream in that, long as you shade and vent them well.

As a cooler can be as low as 15A / day.

50A in 24 hours is considered a lot even for food-safe use.

And they last for years of rough usage.
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