Dual Battery Question

[EricG]

I'm looking at the Costway 55 Quart fridg/freezer and I'm looking at having to add an additional battery, I have an 03 Discovery and they run a group size 24 battery. I think I went up a size when I replaced it. The fridge says it has a rated power of 50 watts and I'm not sure what that means. What I'm looking for is a 2nd battery to run the fridge overnight or longer with no issues. If I'm at a camp ground with power I'll plug it in and the set up will be mainly used for weekend trips and maybe a few longer trips.
What should I look for in a battery? I'm guessing a deep cycle and should it have a minimum number of amp hours?
Thanks,
Eric

 

[Buddha.]

If the battery is mounted under the hood I'd just use the biggest conventional flooded lead acid that would fit the space. If you're mounting it inside the vehicle then I'd do a sealed AGM. Either way I'd use some kind of an isolator to disconnect the auxiliary battery when the truck isn't running.

 

[EricG]

I'll be using an isolator for charging.

 

[pugslyyy]

The Costway 55 is rated at 50Watts. 50W / 12V = 4.2 Amps. Let's do some math!

For a 48 hour weekend, maybe the fridge runs half that time? So I'll use 24 hours as the amount of time the fridge is actually running.

4.2 Amps x 24 hours = 101 Amp-hours (Ah).

For an AGM battery (deep cycle, not start), usable power is half rated, so for 100 Ah usable you would need a 200 Ah AGM battery.

For a LiFePO4 battery, usable power = rated power, so for 100 Ah usable you would need a 100 Ah LiFePO4 battery.

 

[Florida Native]

My Costway 30L runs less than 15% of the time in the house.

Sent from my SM-G950U using Tapatalk

 

[Martinjmpr]

As far as battery choices go, start off with what will fit because the best battery in the world is no use to you if it won't fit where you need it.

IOW the first choice you have to make is where you will put it. That will dictate which options you can choose.

 

[pugslyyy]

You've neglected duty cycle. A fridge running is consuming 50 watts or 4.2 A in your calculation but only while it's running. If it ran continuously then that's 100% duty cycle and if you let it run for an hour that would be 4.2 A-hr.

But they don't do that. Say it runs 2 minutes and stops running for 2 minutes, which isn't unusual in my experience with my Engel at least. That means it's a 50% duty cycle and that means instead of 4.2 A-hr it's actually only 2.1 A-hr.

So in a 24 hour period a fridge that consumes 4.2 A running at 50% duty cycle consumes 2.1 A-hr x 24 = 50.4 A-hr. The actual number depends on environment, what you set it to, how full you keep it, how much you open it, ambient temp, if you loaded and cooled it at home first.

I actually assumed a 50% duty cycle. "For a 48 hour weekend, maybe the fridge runs half that time? So I'll use 24 hours as the amount of time the fridge is actually running."

 

[EricG]

It'll be under the hood and a group 24 or 27 is all that I think I can fit.

As far as battery choices go, start off with what will fit because the best battery in the world is no use to you if it won't fit where you need it.

IOW the first choice you have to make is where you will put it. That will dictate which options you can choose.

 

[DaveInDenver]

I actually assumed a 50% duty cycle. "For a 48 hour weekend, maybe the fridge runs half that time? So I'll use 24 hours as the amount of time the fridge is actually running."

Didn't see you extrapolated. My mistake.

 

[pugslyyy]

Didn't see you extrapolated. My mistake.

no worries. Duty cycle is always a tough guesstimate anyways - you know it is either too high or too low.

 

[Martinjmpr]

It'll be under the hood and a group 24 or 27 is all that I think I can fit.

As someone who's done the dual battery dance before, I'd highly recommend you figure out your physical placement FIRST. If you can truly fit a 24 or 27 battery, you're in luck because there are many, many batteries in that size to choose from.

Once you have a rugged and secure place for the battery, doing the wiring is comparatively easy.

When I did the dual battery on my Suburban I was limited to a 34 or 70, and there aren't nearly as many options in that size.

 

[EricG]

The wiring is not an issue for me. Just making sure I have enough power to run the fridge is my main concern. I'll either need to buy a battery tray of build one. Once I pull the plastic tray and housing I should have plenty of room.

As someone who's done the dual battery dance before, I'd highly recommend you figure out your physical placement FIRST. If you can truly fit a 24 or 27 battery, you're in luck because there are many, many batteries in that size to choose from.

Once you have a rugged and secure place for the battery, doing the wiring is comparatively easy.

When I did the dual battery on my Suburban I was limited to a 34 or 70, and there aren't nearly as many options in that size.

 

[RoyJ]

If space is a concern, it may be worthwhile investing in pure lead AGMs, like the Odyssey type. They are the only starting / deep cycle hybrid rated to 80% DOD with a reasonable service life (400 cycles IIRC).

The only other type that can stand this deep of discharge would be golf cart and industrial flooded batts, but they are much too large / heavy.

A group 27 pure lead is rated to 90 Ah I believe, or 72 Ah @ 80% DOD. This is enough for 2 nights in my experience, where the fridge runs ~25% duty cycle.

 

[rayra]

Consider a roof or ground-deployed solar panel kit as an adjunct to extend your power capacity. You'll get ~~5A out of a panel and could combine with a max-fit flooded battery for about the price of an Odyssey AGM alone. And it will be power supplied when the fridge is using the most.

 

[EricG]

I have a group 65. I think I went up from a 27. I'm actually looking at an AGM that will fit and has 95 amp hours.
Just have to decide what I want to do.

If space is a concern, it may be worthwhile investing in pure lead AGMs, like the Odyssey type. They are the only starting / deep cycle hybrid rated to 80% DOD with a reasonable service life (400 cycles IIRC).

The only other type that can stand this deep of discharge would be golf cart and industrial flooded batts, but they are much too large / heavy.

A group 27 pure lead is rated to 90 Ah I believe, or 72 Ah @ 80% DOD. This is enough for 2 nights in my experience, where the fridge runs ~25% duty cycle.