Well, I guess it was bound to happen at some point... drained battery .
This got me thinking, researching, planning, etc. Browsing these forums, there are a couple threads I read through, but nothing too definitive to completely make up my mind.
Basically, I'm wanting to add a second battery for the electronics (laptop/amp/sub/etc.). I read about wiring up through an isolator or a relay, and I guess isolator is the way to go for electronical components.
2nd battery will run all the electronics, so main battery will act normal, as if no extra electronical components were added (this should prevent main battery from draining again). Since all the power is hooked straight to the battery and not the IGN or ACC lines, I control the power on/off via dash switch. I should've known better, but the girlfriend forgot to flip the switches to power off the electronics and drained the battery overnight .
Now, since adding the 2nd battery I want to get wiring setup correct before I try anything. If the 2nd battery is wired through an isolator, I am led to believe the batteries won't see each other which is good, but will the 2nd battery charge when the car is on? Will the alternator charge the 2nd battery as well as the main battery? If the 2nd battery discharges/drains and the main does charge the 2nd battery, is there any manual operation I would need to 'start' the 2nd battery again or would it be 'act' just like normal?
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If you have a charge lamp on the dash (on when not charging), all you need is a relay.
That's what is now being called "The Ultimate Intelligence Battery Isolator" in response to "Smart Isolators".
It is fully automatic, simple & cheap.
The only catch is sometimes if the relay's coil requires too much current, but that is rare in older alternators without EMS interaction. Even then, a MOSFET can be used in place of the relay, else to drive the relay (or one relay per extra battery).
The relay should be sized for your load - ie ~$3 for 30A or ~$25 for 200A-400A. (And of course (self-resetting) circuit breakers or fuses at each battery end!)
The only other recommendation is a low voltage cutout (LVCO) to protect the 2nd battery.
I often recommend the MW728 (~$20) with its 10A capacity & 11.2V shutdown.
In other posts I explain both above and the various tricks (voltage increase; latching output; manual on & off; complete LVCO isolation after dropping out; higher LVCO currents).
PS - I recommend against diode isolators. IMO they are the worst choice except where small currents are involved etc.