Adding Additional AGM Batteries

Overview

The Allegro Bus 42QRP on a Spartan chassis comes with a total of eight 12 volt AGM batteries to supply the coach with adequate power. Four of these batteries are located in the rearmost curbside basement compartment. Freightliner typically used the compartment to the right of this for their engine starting chassis batteries. As such they were covered with a sheet metal cover. Spartan installs their chassis batteries in the rear engine area so this area has now been utilized as storage space for four additional AGM coach batteries, bringing the total to eight. These batteries are the Interstate Batteries DCS-88BT battery, which is rated at 88 amp-hrs. This brings the total battery bank capacity to 704 amp-hrs. I've found that it's possible to get at least 8 hours of use from this configuration when dry camping overnight. However, if conditions are colder and the HydroHot boiler runs this could be marginal. Therefore I decided to add four more batteries to my system to increase the runtime by 50%. This was easy to achieve in my particular coach because there was room available in the small curbside compartment that gave access to the HydroHot and was nearby the inverter with it's massive #4/0 battery cables. The following images and text will show how this was done in the event that others want to add more capacity to their coach.

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Mark Quasius - "Cruzer"

The Installation

These are the existing battery compartments. The compartment on the left has the four primary batteries. The coach power goes to a manual disconnect switch, which then passes through a 400 amp fuse that supplies power to the entire coach as well as the 3,000 watt inverter/charger. The basic output connections are found in the large terminal junction panel above. The compartment on the right houses the second bank of four batteries. The steel cover over the batteries really isn't necessary and exists because that's the way that they always covered the Freightliner chassis batteries and they just haven't figured out they don't need it. The compartment is sealed from road dirt and the AGMs don't outgas hydrogen so feel free to remove it if you wish. Above this is a white board connection panel with the charge solenoid and slideout interlock solenoid as well as few circuit breakers.

On the left side battery bank you'll notice the large fuse mounted horizontally. This 400 amp fuse powers the whole coach so it wouldn't hurt to carry a spare one of these along with you when traveling. On the right side of the fuse is a very larger #4/0 battery cable that runs to the middle of the coach and supplies the inverter/charger. This is of great benefit when adding extra batteries because it saves anyone from having to fish heavy cables to the new location.

This image shows the original inverter setup. This was the Xantrex RV3012 modified sine wave inverter/charger. I've replaced that with the Xantrex RS3000 true sine wave inverter/charger but it's the same basic size and has a similar battery cable requirement so for all practical purposes, it's the same for our battery project. As you can see to the right, there's a large open area next to the frame rail that is available for any connections that need to be made. This area will be modified to take advantage of that.

This is the curbside compartment. Previously I'd been using it as a storage locker for my blocking and tow bar accessories. It had pretty much been a wasted space though so I dropped the extra batteries into this area. By stacking them alongside the wall I still had plenty of room to store my blocking and tow bar accessories. I then made some angle iron rails to hold them in place as well as a hold-down strap (not yet shown in this image) to keep them from jumping out should I drive through the world's largest pothole or whatever. The nice thing is that this compartment is immediately next to the inverter so cable runs are short and easy. I made up 3 pairs of short jumpers to tie the four batteries together then ran a pair of #4/0 cables to the inverter area, although #2/0 would probably have been adequate for this auxiliary bank. Since this picture was taken I also fabricated a steel enclosure to cover these batteries. This enclosure is lined with the same rubberized 3M Safety Walk that I put on the RV's roof and protects the battery terminals from any accidental shorts should I drop something conductive on them. It also allows me to utilize the space above the batteries for additional storage.

This image shows the new RS3000 inverter. The only difference is that the battery cables connect to the right side of the inverter rather than the front corner, as in the RV3012. But the existing cables have plenty of slack in them so they can be easily moved. I made a heavy steel plate in a U shape. I slid this plate up to straddle the frame rail then placed two long bolts through it to hold it in place. This gave me a panel to mount things to without having to weld, drill, or cut the frame rail itself. I then added an additional 400 amp fuse to this panel as well as a ground stud. The OEM battery cables from the rear now connect to the ground stud (negative) and the fuse (positive). I probably could have just went with two studs but I wanted to utilize the existing fuse by the batteries to feed the coach, as it now does, but relocate the long battery cable to the hot side of the fuse. That way all 3 battery banks are on the same side of the fuse. The hot lead from the OEM battery banks will meet up with the hot lead from the new battery bank at the hot side of the second fuse. The inverter will then be connected to the other side of the fuse, giving it it's own fuse to give it adequate overcurrent protection. In the image below, the inverter cables are not yet connected. If you're wondering what the two small 12 volt circuit breakers are for in the top right corner of the picture, they're for the 12 volt feeds to the two basement freezers I have. I also added a disconnect switch for this third battery bank should I ever have the need to disconnect it from the system.

Summary

This effectively increases my battery runtime by 50%. With the residential refrigerator this gives me the ability to not have to worry about running out of juice in any reasonable conditions. It also gives me the flexibility to run the genset to recharge the batteries at my convenience and removes the "emergency" issue from the equation. It also lets me straddle any quiet time hours easily without having to be present at the coach. I highly recommend that you visit the RV Tech Library articles on Inverters. This will give you a better understanding of how inverters work as well the differences between true sine wave power and modified sine wave.

This review written 12/1/06