House Battery Run Time

[Jon Wehrenberg]

OK, I understand. Different generation of inverters. Mine does not have the ability to operate Auto-start.

[BenC]

A lot of coaches have dual inverters connected to a common battery bank. A bank of 6 or 8 large house batteries (like 4D or 8D) need the added charge capacity of the second charger to minimize recharge time, even if you don't need the extra inverter capacity. Each inverter needs a remote panel for setup and configuring correctly to the battery type to which they are connected. The reason the batteries were not separated is for customer convenience. When at Vogue in the early 90's, we did about a 1 year worth of coaches with separate battery banks (one dedicated to running a refrigerator), and ended up connecting them together at a later date for longer "dry camping" time for the customers. With the batteries connected all together, creating a large single bank, the customer has the versatility and option to keep a refrigerator running a lot longer before needing to run the generator simply by turning loads off. With the bank split up into refrigerator-dedicated batteries and then "regular house batteries", the option of using all the batteries to run the fridge does not exist, even if the owner choses to reduce loads in an effort to reduce generator run time/increase the length of time between recharges. The fridge will run an average of 1/3 of each hour, regardless of if you watch television or not, use lights or not. In situations like that, the "house batteries" could be at 80% capacity, while the "refrigerator bank" is in need of a recharge. In addition, dual banks make alternator choice more restrictive (needing a regulator with "external voltage sense") since an isolator would be required. All this adds up to making the system more complex, with more parts to fail, and actually increases generator run time overall.

The Trace Engineering RV-series inverters had internal shunts between the battery negative and ground posts on the back of the inverter. When using two together, there was an External Shunt Kit available to wire up to one of the inverters. The inverter with the hardwiring of the exteral ground shunt was supposed to be set up as "External Shunt: this inverter" and would, in turn, show the "battery fuel gauge" as it was termed by Trace. The inverter without the shunt pigtail connected to it was to be set up as "External Shunt: Other Inverter" and would no longer show in the user menu the "battery fuel gauge" at all (it wouldn't be accurate anyway).
This series of inverters was unique with the shunt and "fuel gauge" feature, but it was never as accurate as hoped because to be a true indication of battery run time left, the load on the batteries and inverter would have to remain constant without any loads added or taken away. This is a difficult situation to obtain in an RV, with fridge compressor cycling on and off, water pump cycling on and off, etc, etc, etc. I had to tell a lot of customers to simply take the indication with a "grain of salt" and let your auto start feature take care of the batteries, instead of letting the gauge readings make you crazy, lol.

As far as charging and inverting, each Trace RV-series inverter operates independently. There really is no "primary" or "secondary" inverter. There are many factors that effect the dynamic in determining which one will finish a charge cycle first. The inverter that goes to float first does so because of what is "sees" as a result of battery cabling length/voltage drop, length of time at a certain "acceptance" current amount going to the batteries, and the 120vac loads it has for pass-thru (this effects if the inverter has cut back it's charge current in response to the input breaker size). All twin inverter systems will have one inverter drop to float first, then the other will finish up and eventually drop to float as well. If the inverters were installed to the optimum, the battery cable lengths, both Pos (+) cables are exactly the same length, and both Neg (-) cables are exactly the same, but all are as short as possible.

Hope this helps.

[charlesebrownjr]

Hey Ben, Thanks for the input. I understand the concepts now.

[travelite]

Whew... well that was easy, thanks Ben.

[JIM KELLER]

Thanks Ben, I enjoyed reading that.