How to make a cheap isolated dual-battery setup for $50

[DiploStrat]

Oh, and just to pile on, my automatic charge relay has a three position switch, complete with pretty dashboard lights:

-- Force combine (aka jump start),
-- Force separate (aka maintenance),
-- Old Timer (aka automatic).

 

[dlichterman]

But if forced combining for jumpstart is a stated goal.
Which battery will the coil power source ?

I know! I know !!j
Add yet another switch to make coil selectable...

Well, you can also quickly bypass the solenoid and join them together with something and then the solenoid would work.

 

[Hillbilly Heaven]

But if forced combining for jumpstart is a stated goal.
Which battery will the coil power source ?

I know! I know !!j
Add yet another switch to make coil selectable...

That is not a problem with the 3 position switch. Center pin goes to the coil of the contactor. First pin goes to the positive of the aux battery and the third pin goes to a wire that has 12 volts when the ignition on. This way you always jump start from your aux battery to your start battery.

 

[Hillbilly Heaven]

Added a picture to make it easy peasy

 

[dwh]

Exactly.

To add the force on/off and have ignition auto as well, you gotta feed both ignition hot and constant hot to the switch.

So you take the constant hot from the aux battery and you're covered either way.


Note: That diagram just posted does ignition auto and force on if the switch is DPDT. For it to do force off as well, the switch needs to be DPTT, or DPDT-Center Off.

 

[Hillbilly Heaven]

Sorry should have stated it was center off DPDT if using LED indicator or SPDT if not using LEDs.

 

[cgregs32]

Sounds good. I do have 4 guage wiring, a 70a alternator, and 70 amp fuses. I can switch my switch whenever I want!? I do have a solar setup for my AUX battery. I never see a charge from the solar panel higher than 6 amps. Is there a caveat to having 2 charging sources at one time? I'm about to hit the road and just want to make sure i'm not going to mess up my alternator/ voltage sensor or anything really.

 

[MichelleKiwi]

What would be the highest gauge wire for a simple setup? that is safe to use?

Note: the smaller the number the bigger the wire size.
8 is bigger than 12, 4 is bigger than 8.
It never hurts to go bigger except than the wallet

Depends if you want to self jump start using the aux battery.

If you do then I wouldn't go smaller than 4 AWG (25mm2).
If you don't then it depends how you have the setup.

If you connect the two batteries together using an isolator (relay) and fuses directly without powering other items on that cable:
I wouldn't go smaller than 10 AWG (6mm2)

Just make sure that the two batteries aren't linked together when you start the vehicle otherwise that starter motor could draw from the second battery and pop the fuses or worse
i.e. most vehicles stop power to the ACC items when starting the vehicle, so connecting the isolator to an ACC powered wire should fix that issue.


Checking that I'm correct?

 

[dwh]

I can switch my switch whenever I want!?

Yup.

I do have a solar setup for my AUX battery. I never see a charge from the solar panel higher than 6 amps. Is there a caveat to having 2 charging sources at one time?

Nope.

It's like having two air compressors to the same tank, each with its own pressure switch.

Of course, if they are set to different voltages, one will shut down before the other, but that doesn't matter.

I'm about to hit the road and just want to make sure i'm not going to mess up my alternator/ voltage sensor or anything really.

Have fun. No worries.

 

[dwh]

Just make sure that the two batteries aren't linked together when you start the vehicle otherwise that starter motor could draw from the second battery and pop the fuses or worse
i.e. most vehicles stop power to the ACC items when starting the vehicle, so connecting the isolator to an ACC powered wire should fix that issue.


Checking that I'm correct?

Well, since you asked...

If the batteries are a cranking type and a deep cycle type, the scenario you described is highly unlikely because a cranking battery has a lower resistance, so it would take something weird going on before the deep cycle was providing any significant power to the starter.

That effect becomes even more pronounced if the charge wire to the aux battery was something like #10, which adds even more resistance.

My truck has a dumb solenoid, ignition controlled. When I got it, it had #8 between the batteries and a 50a fuse. Starting the 460 engine with the solenoid engaged never popped the fuse.

It was like that for decades before I got the truck. I replaced it with a chunk of #4 battery jumper cable and got rid of the fuse, and then later replaced that with 1/0 welding cable. I upgraded the wire to max out the amp flow for bulk charging, not because the smaller wire was a problem - it wasn't.



Also, using ACC to control the solenoid instead of IGN is not recommended. The batteries should be isolated whenever there is no charge being supplied.

(Self jumping being an exception to that rule, but also an unusual occurrence.)

Having dissimilar batteries tied together during charging us fine, but keeping them tied when they are not being charged is unhealthy for the batteries, and using ACC would have them tied whenever doing something like listening to the radio with the engine off.

 

[MichelleKiwi]

Also, using ACC to control the solenoid instead of IGN is not recommended. The batteries should be isolated whenever there is no charge being supplied.

Having dissimilar batteries tied together during charging us fine, but keeping them tied when they are not being charged is unhealthy for the batteries, and using ACC would have them tied whenever doing something like listening to the radio with the engine off.

Thanks, dwh, I have learned a lot from your posts.
I would have thought that the starter motor would take power from everywhere it could. And didn't think about listening to the radio.
So in that case, it doesn't matter if the solenoid is engaged or not when starting the vehicle if the aux battery is a deep cycle.

My understanding is most vehicles keeps the IGN (ON position) powered while starting the vehicle so I used a 10 sec delay on relay before engaging the solenoid powered by the IGN (on my own vehicle).

Where I wanted to put my aux battery in my car, I couldn't find a deep cycle battery that would fit so I used a European style shorter starter type battery, as my aux battery

Question, What about using a normal starter type battery as the aux battery, it would have less resistance then, would it be possible to draw from the aux battery then (if the solenoid was engaged)?

 

[murrayargo]

If I wanted to be able to remove the aux battery. Would I just put a dash switch between the solenoid and wire that is hot when the key is on?


Sent from my iPhone using Tapatalk

 

[MichelleKiwi]

If I wanted to be able to remove the aux battery. Would I just put a dash switch between the solenoid and wire that is hot when the key is on?

Yes you can, if you remove the power source to the solenoid, then the wire after the solenoid to the aux battery will not be hot, if you remove the aux battery.

Make sure the positive wire is insulated after removing the battery just in case the solenoid is engaged, i.e. you accidently flip the switch back on, or the switch/wire shorts.

Also if you can put a small fuse like a 2A fuse going from the hot wire to the switch, so that way if there is a short and the fuse pops, you don't pop one of the factory fuses and maybe left stranded somewhere


Depends on the size of the wire, fuses, and solenoid between the two batteries, if you get a "SPDT center off switch", you can wire the other side the switch to a fused always hot wire to power the solenoid when ever you want, i.e. to self jump start, or to charge both batteries at the same time when you plug in a charger

 

[dwh]

Question, What about using a normal starter type battery as the aux battery, it would have less resistance then, would it be possible to draw from the aux battery then (if the solenoid was engaged)?

To be sure (just re-read a bunch of Asimov, he used that phrase a lot, "to be sure" - hehehe), it's possible, but still the battery closest to the starter will have the lower resistance. Especially if using smallish wire to the aux.



And a delay timer is cool. That's how Bluesea does it on their ACRs.

 

[dwh]

If I wanted to be able to remove the aux battery. Would I just put a dash switch between the solenoid and wire that is hot when the key is on?


Sent from my iPhone using Tapatalk

You can do that. The hot wire to engage the solenoid in my truck uses an insulated push on (friction) connector rather than a ring terminal, so I do the same thing by just pulling that wire off.