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I just recently purchased a 'Smart' battery charger ( https://www.ebay.com/itm/37526391005...Bk9SR57U1M71Yw ) and was/is concerned with the 'repair' mode and the 'smart' charging sequence of operation (up to 9 stages). I decided to 'service' the 3+ year old battery but before doing this, I set up a slave power system used to keep the car powered up ( NOT RUNNING & KEY REMOVED ) while using the 'Smart' charger. I recently saw (and I thought commented) where someone wanted to isolate the battery and while doing that to keep the car powered. The following picture/narrative is what I did.







I used two 12v 9ah UPS/alarm system batteries in parallel and clipped it to the POS cable along with the POS battery charger cable (with two separate voltmeters monitoring the two 'systems' hooked up after the 'isolation'). I then clipped the slave battery to the NEG cable, unscrewed the terminal bolt, and slipped the clamp off of, and held it back away from, the battery NEG terminal with a piece of cardboard (isolation). Finally, I connected the NEG clamp of the charger to the battery NEG terminal. Simple to then clip the meter's NEG's to the two systems. This allowed the charger to do whatever it wanted to do without having any input to the isolated car electronics & at the same time keeping all the car's circuits active during it's sleep state. The picture doesn't show the exact wiring but isolating the battery is pretty simple and as long as you keep the polarities right nothing will cross-circuit. IF your not sure of what your doing, I suggest you not do this because if you hook it up wrong you'll possibly do damage either the car or yourself.


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