Dual battery - question on operation (1 Viewer)

[t00manyuserz]

Hi all,

I got a dual battery setup going using a dual battery isolator (the usual Amazon one, that will charge the secondary battery once primary is over 13V or whatnot), that I use for accessory stuff. Nothing too fancy, but it does what I need. I have a couple of questions on the electric setup, though.

1. what happens when the secondary battery is completely dead? The other day, the ground cable from the battery isolator fell off, and the secondary battery did not charge at all, i.e. it fully drained to 9V. When I hooked it up again, the isolator kept flicking to secondary and back to primary (the LED kept going on and off), and after a couple of minutes, I decided to stop the car and use a battery charger to charge the secondary to avoid this. However, if I were out in the bush, what would've happened? Would it have _eventually_ charged, or would it have killed my alternator, or anythig else?

2. If I need to take power from the secondary battery for a boost, do I just disconnect the + from it and is that sufficient, or will the isolator now assume that the battery has 0V and start trying to charge it?

3. If I need to work on the car and want to make sure there is no power to it, do I remove the negative from the main, the secondary, or both? I feel like with the engine off, only main is sufficient because by default, the isolator should have the secondary removed from the system, but this is for my sanity.

Btw, the isolator is kind of like this one: https://www.amazon.com/Battery-Isolator-Sensitive-Compatible-Vehicles/dp/B0CG174DMR

Thanks for any help.

 

[e9999]

well, not exactly a question you asked, and maybe not what you want to hear, sorry, but personally I would be quite hesitant to put in a generic no-name chinese (probably) gizmo switching high currents in my prized vehicle, especially if it does odd things and if the manufacturer feels compelled to print "High quality" on it...

 

[t00manyuserz]

well, not exactly a question you asked, and maybe not what you want to hear, sorry, but personally I would be quite hesitant to put in a generic no-name chinese (probably) gizmo switching high currents in my prized vehicle, especially if it does odd things and if the manufacturer feels compelled to print "High quality" on it...

lol. I knew this kind of response would come and I should’ve prefaced by saying I’ve done a modicum of research on this and it appears to be a somewhat common setup when you’re not trying to do extreme things like using the second battery for starting the car, for example - and therefore, no high currents are switched because the battery is never called to deliver a high current via this device. Only charging current.

So was your post because of any particular bad experience with a similar setup, or was it simply because it _appears_ to be a cheap Amazon product? Because if the latter then I’m not sure it’s a justified position.

 

[e9999]

no, I don't know anything about this particular device, so yes I may be completely off. I just have a bit of experience with chinese inexpensive no-name electrical devices that makes me wonder. There are many places where I'm willing to take chances and experiment. Power electronics in my vehicle are not high on that list. But that is just my personal take, FWlittleIW.
I don't want to sidetrack your thread any further, but since you mentioned something about it, I would just add that charging currents are not always small and harmless, I've melted some stuff doing that before.

 

[mobi-arc]

With your discharged second battery, the relay is responding to system voltage as the relay connects and disconnects the discharged battery into the system. When closed, the discharged battery causes voltage to sag below the low voltage disconnect threshold, relay opens, voltage pops up above 13 volts, relay closes and attached discharged battery, voltage sags again with the load applied, relay responds and opens. Relay will keep cycling until the discharged battery is in a better state of charge. This scenario is why our solid state battery isolators are attractive.

 

[t00manyuserz]

With your discharged second battery, the relay is responding to system voltage as the relay connects and disconnects the discharged battery into the system. When closed, the discharged battery causes voltage to sag below the low voltage disconnect threshold, relay opens, voltage pops up above 13 volts, relay closes and attached discharged battery, voltage sags again with the load applied, relay responds and opens. Relay will keep cycling until the discharged battery is in a better state of charge. This scenario is why our solid state battery isolators are attractive.

Ahh I see. That does make sense, thanks. I’ll probably consider a solid state isolator sometime, but this explains the behaviour.

 

[LandLocked93]

1 - You need a "smart" 2-way relay if you want the relay to decide on your behalf when to bind the batts and when not to. This allows a bind when either battery is low, helping to charge the lower batt when running or equalize the two batts when parked up. This does present a possible drain from both batts tho should the smarts of a "smart" relay leave the chat. If YOU want to decide when to bind the batts and when not, a 2-way smart relay with a dash-switched manual override can facilitate that. Or just a dumb relay that simply binds the batts when you flip a switch from inside the truck. (my solution)

2 - Again, the "smart" relay w/a manual override switch can accomplish this. So too the dumb relay. Yet both solutions are assuming that at least one of the batts is charged enough to actually start the rig. And neither solution requires "removing a wire" to make that happen.

At this point let me mention as I've seen it countless times, for a dumb relay the trigger power for the dash switch must come from each battery. This is so that regardless which batt is dead, you still have power to run the relay. Wiring just a single trigger wire from a single battery, and that battery goes dead, the relay is now inop. "Smart" relays can pull from either source to which it is connected, but that too is up to the relay to allow. And if the smart relay has gone dumb for some reason, you're screwed w/o some trail fixin'.

3 - Well, if the second batt is not connected in any way to the OEM wire harness and the bind device is not powered, then just the main disconnect is required. I personally go overboard and disconnect both just for the heck of it even tho I know there is nothing being powered by the second batt as it is NOT tied into/picked off of the factory harness.
Just for sanity's sake, as you say.

 

[t00manyuserz]

1 - You need a "smart" 2-way relay if you want the relay to decide on your behalf when to bind the batts and when not to. This allows a bind when either battery is low, helping to charge the lower batt when running or equalize the two batts when parked up. This does present a possible drain from both batts tho should the smarts of a "smart" relay leave the chat. If YOU want to decide when to bind the batts and when not, a 2-way smart relay with a dash-switched manual override can facilitate that. Or just a dumb relay that simply binds the batts when you flip a switch from inside the truck. (my solution)

2 - Again, the "smart" relay w/a manual override switch can accomplish this. So too the dumb relay. Yet both solutions are assuming that at least one of the batts is charged enough to actually start the rig. And neither solution requires "removing a wire" to make that happen.

At this point let me mention as I've seen it countless times, for a dumb relay the trigger power for the dash switch must come from each battery. This is so that regardless which batt is dead, you still have power to run the relay. Wiring just a single trigger wire from a single battery, and that battery goes dead, the relay is now inop. "Smart" relays can pull from either source to which it is connected, but that too is up to the relay to allow. And if the smart relay has gone dumb for some reason, you're screwed w/o some trail fixin'.

3 - Well, if the second batt is not connected in any way to the OEM wire harness and the bind device is not powered, then just the main disconnect is required. I personally go overboard and disconnect both just for the heck of it even tho I know there is nothing being powered by the second batt as it is NOT tied into/picked off of the factory harness.
Just for sanity's sake, as you say.

Great info. Thanks very much - gives me something to think about!