Secondary battery and isolator installation

2013-11-26 08:29 by Ian

I bought a 250A battery isolator on ebay for $70, but when the thing arrived, I realized that I had a serious problem on my hands. The thing was too long to fit in any convenient mounting location. So I had to get creative.

Before pictures....

I used a Smittybuilt dual battery tray. The bottom half of it is already installed and holding what will be the auxiliary battery. Installation of the top tray would mandate moving the fuse box forward by about an inch. This wouldn't have been a big deal had the aftermarket intake box not been in the way. But the added complexity is why the crank battery isn't installed yet.

About the intake.... The previous owner of the Jeep installed a cold-air intake and washable filter that obstructs a good deal of space in the engine compartment. The scope of this problem is well-represented in the first image. It was the single biggest factor influencing the mounting location that I eventually chose. My only other good options were alongside the battery stack, and on the driver-side of the engine compartment. The battery stack option wasn't good, because it would have obstructed access to the oil filter. The driver-side option wasn't good because it would have involved at least two meters of additional 4ga wire. I2R is not your friend in this case. Chrysler used very high-quality cable throughout the electrical system. Fortunately, I was able to re-use all of it. Insulation is high-temperature and hydrocarbon resistant, and has very fine strands.

The completed isolator assembly

The isolator is basically just two monstrous-big diodes tied together at the anodes with each cathode bound to a given battery. The anode in this case is the center terminal of the isolator. The round object on the bracket is the jump-start switch....

After pictures....

As can be seen, I fabricated two brackets to hold the isolator to the passenger-side radiator retention bar. I left enough space on the brackets to install the jump-switch (the little black cylinder protruding through the rear bracket). The brackets are held to the bar using 4 5/16" rope clamps (which work beautifully).

To prevent rotation of the assembly, I also installed two blind nuts into the intake box (which I was pleased to discover was made of steel). This gave me two orthogonal mounting points and adequately prevents any rotation of the isolator bracket.

The A/C compressor pipe was routed in a really dumb way. The high-reaching loop near the firewall was at serious risk of being damaged by collision with the battery tray when being subjected to vibration. So I used a section of vinyl hose to buffer the two parts from any undesirable contact. I will have to keep tabs on this hack for a few months to ensure that the vinyl is not being chafed away by contact with the tray.

Post-installation tests went smoothly. There was no alternator adjustment required to account for the voltage-drop across the isolator, as the alternator's output voltage is under computer control based on the voltage arriving at the battery.

As a matter of course, I improved on Chrysler's electrical layout with respect to the alternator/battery/fusebox connection. The stock configuration had the feed cable from the alternator not only necked-down to a thinner gauge (why?!?), but also directly wired to the fusebox. The stock configuration didn't take advantage of the battery's large electrical "mass" to buffer alternator-induced noise, instead feeding it directly to the broader electrical system. My changes now interpose the battery between the alternator and the rest of the electrical system.