Dual Battery Question: Mixing Battery Types and Sizes

[tsherrygeo]

Hi All, with all the downtime at home I'm finally getting around to building out the back of my 99 Ford Explorer with drawers and a sleeping platform for camping. Now looking at electrical.

Background:
I'd like to run some LED lights and other accessories off of a second battery. Most of the info on dual batteries I can find is for adding a second full size battery to run MEGAWINCH7000 or fridges. I'm not doing that. Just LED lights, PC fan, phone charging, bike computer charging, camera battery charging, etc. The occasional laptop might be the biggest device.

I know I would probably be fine just running off my 750 CCA starter battery, but I like the peace of mind of having the accessory battery for trips where I'm parked a few days in a row.

I've read through much of Expedition Portal's famous cheap dual battery setup: https://expeditionportal.com/forum/threads/how-to-make-a-cheap-isolated-dual-battery-setup-for-50.77503/

As for location, second gen Explorers have a cramped engine, so I am thinking of locating it inside the back of the vehicle, either on the left or right side. I'm undecided as of now, if anyone has suggestions, I'm all ears.

Question: Is it safe to set up a small sealed lead acid battery like: https://www.amazon.com/dp/B00K8V2VD0/ as a dual battery with a solenoid as suggested in the $50 Expedition Portal thread? I am worried about overcharging a small battery.
Would I be better off getting one of those "smart battery isolators" https://www.amazon.com/dp/B00WTAFR5W
With the smart style isolator, do I still have to run a wire between negative terminals on both batteries?

The "Easy Button" option might be to just use a portable power source like this: https://www.amazon.com/dp/B07FYJVFNK. I want to price out my various options before whipping out my wallet.

Thanks for any advice.
-Tim

 

[dreadlocks]

Yeh doing it backwards, Go get a Deka (and not some MightyMax ********) ETX20L and wire it up to your starter directly, put an isolation relay in em and use the big car battery for your loads and the little battery for starting.

Remember with lead based chemistry your usable energy is 50% of spec, if you discharge em below 50% you kill em.. so a 30AH battery is really a 15AH battery, and your 80AH starter battery is really 40AH.. Your starter only needs a ton of amps for like a second or less, a battery used only for starting is always >98% charged so it can be the smaller battery just fine and dandy as long as it can output enough amps to fire up the motor.. and the Deka's output more than advertised, that ETX20XL puts at least 300CCA out of it, many test em and find closer to 500CCA.. more than enough to start your engine unless its an 11L Diesel.

 

[tsherrygeo]

Nothing ”wrong” with that sealed battery shown. But depending on voltage your alternator outputs and wiring distance to that battery you might have problem charging at its specified voltage.
Also, have you determined your average Ah needs between charging ?
To maximise its lifespan, that battery 35Ah, should not be discharged below about 50% of its capacity...
Btw, Being a sealed battery, its reasonably safe inside your passenger area. But all batteries should be shielded/enclosed in some fashion.

That automatic charging relay shown,
As a kit of parts, I would be suspicious of it. Aside from its not a well recognised brand, No idea of the relay itself. Its supplied wiring shown as 7AWG. Which exists, but its an oddball size. Leading to further suspect its inferior such as non-listed/recognised component, low temperature rating, aluminum, etc. Same thing with the ring terminals. The battery terminal clamps, you wont need with the proposed system anyway. But you will need two OCPDs for the battery connections and probably more for whatever appliances you connect.

Regarding the automatic charging relay... I just picked one at random from amazon for example. And yes, fuses are good
Specifically, I'm asking if I need to run a negative cable to connect both of the batteries. I know I would need to do this with a "dumb" solenoid...