How to: Split-Charging

If this post has already been made please forgive me, but I searched and could not find it.

I find this set-up easy, cheap, realible and convenient as you can run 2 batteries( one for the vehicle and one for the audio) to play your system when the vehicle is not running and still be able to drive home once you kill the extra battery (which should be a DEEP CYCLE battery!) but still recharge the extra battery. Oh, and make sure your charging system AND alternator are up for the task!

U will need to use a 30a bosch relay and attach it as follows: 30-to the main battery's positive terminal; 85-to ground; 86- to the IND(indicator light terminal wire) on the alternalor; 87-to the extra battery's positive terminal. Of  course your positive terminal from your extra battery should be connected to your equipment. If I missed anything let me know. One Love!

Inexspensive, No Voltage Drop, and Compact. Great qualities.

That system work but it does have some serious drawbacks too that we think should be addressed.

First is that the charging setup is not prioritized and can actually impair the vehicle's primary battery. In some cases, the battery was no longer recoverable (even on deep cycle bats.) Basic Kirchoff's Law

Second, this method cannot be used on vehicles that have alternators without D+ terminals. Using it risks a backlash in the charging system (rare but it has been known to happen)

Third, after some time there is a risk of damaging the relay since the energy going through the relay is unregulated, and during startup due to the current flow in SOME batteries (700+ CCA in most cases) and HO Alternators may cause 87/30 fusing. (Also rare, but more common than 2)

Fourth, this is limited to low current. If you plan on running for extended periods and draw more than 40 amps on your system, failure and 3 become plausible.

This is not an attack on you djdowdell, but we feel that advantages and risks should be put on the table as fully as possible.

For those who want to hit a system hard for extended periods, we recommend getting a system that can handle 100/120+ amp flows through them. They will most likely have a regulator with priority and isolator built in. Many are hybrid systems so those vehicles without D+ on the alternator will also be able to use it safely. Most systems will fall into the $175 - $500 range. The only drawback we have experienced is that in RARE cases, the system will isolate the secondary system and may not let the audio system hit. A strong majority of the time that this happens (80% or so) it is because the audio system, battery, and alternator is not planned properly and helps to prevent deep cycling the primary battery and browning the electronics.

Right idea, use a high current solenoid like the Scosche 200 amp model (and the same one used by all other companies as well).

Use a bigger relay (go to an RV trailer store.. they have them), fuse the wire tying the two batteries together through the relay (fuse at both ends) and activate the relay off an ign wire.

This setup has been used for decades in motorhomes..

Jim