[Ran D. St. Clair]

Warning – Discussion of electrical systems can be dangerous (To your wallet)

Lets talk about the electrical systems. This is theory and philosophy mind you, since I haven’t actually built it yet. I am an electronics engineer, so I at least know ohms law, but I am not an electrician, though I do at least know the difference between ground, neutral, and hot. I will try not to lead anyone astray with mis-information. I have read “Managing 12 Volts” by Harold Barre and I recommend the book.

My project requires that I be able to operate independent of the grid (shore power) indefinitely, though I may have access to the grid at times. I realize that is very different from what many of you are doing, so it drives a different approach. I also have a bent towards efficiency. It’s not some green save the whales thing, it’s just my nature.

It seems to me that there are four kinds of electrical power that I need to handle and distribute:

1. 12V DC (Direct Current) related to the truck starting battery
2. 12V DC related to the house batteries.
3. 110V AC (Alternating Current) from the on board generator, or shore power if available.
4. 110V AC derived from the house batteries via an inverter.

I realize that many of you will have a shorter list. You may not feel the need for an inverter, or even batteries beyond the starting battery. You may only have 110V AC (AKA 115V or 120V AC) and not even use 12V DC beyond the cigarette lighter in the cab. Certainly you probably only have AC wiring in your fixed base home.

You might wonder why I make a distinction between the 12V starting battery and the 12V house batteries. Some really simple systems just use the starting battery for everything, or use diodes, relays, solenoids, or manual switches to connect them together at one time or another. All of those might be a reasonable option depending on the situation, but I think it is often a bad idea, and I don’t plan to do it.

The 12V starter battery and the 12V deep cycle house batteries are fundamentally two different types of batteries. They may both be 12V lead acid batteries and they may look similar, but they are not. The starter battery has porous lead plates for maximum surface area and maximum starting current. It should never be deeply discharged, and while one or two short term deep discharges might not kill it outright, each one takes a toll and it will soon be useless. A deep cycle battery has thicker stronger solid lead plates. Since the plates have less surface area it won’t be able to deliver as much short term current for the equivalent physical size. It can be discharged and charged many more times, but it should still not be discharged below 50% of capacity and each deep discharge cycle still takes a toll that will eventually kill it.

I have to say that lead acid batteries in general are a very poor technology for house batteries. There are many far better technologies out there, and new technologies are being developed every day. For the time being though, lead acid batteries are the only cost effective solution. I expect that will change dramatically in the next 10 to 20 years as electric cars become more popular.

Every method for connecting the starter battery to the house batteries has potential problems.

Manual switches can be great, if you know when to turn them on and can remember to turn them off. I might consider one as an emergency option, but I know that when it comes to remembering things I am an idiot, so I wouldn’t want to rely on it all the time.

Diodes have a voltage drop, dissipate a lot of power and get hot, and send all of the energy from the alternator to the weaker (more discharged) battery. If the house batteries are weaker, they get all the juice, and the starter battery never gets charged. Keep doing this long enough and eventually the starter battery is just as discharged as the house batteries, which isn’t good for it. Of course, if you run the alternator long enough, eventually all the batteries will get fully charged and everything will be OK, assuming you have somehow compensated for the voltage drop of the diode. That might take a long time though because regular alternators are designed to charge very slowly and conservatively, especially as the battery gets closer to being fully charged. You could use an expensive high current alternator and a matching expensive 3 stage voltage regulator, and then adjust it to compensate for the diode drop, but even after all that, you better run the engine long enough to fully charge all the batteries. You could even hook the house batteries to an AC charger driven by shore power and never let them get discharged, but if you are going to do that you really don’t need house batteries at all.

You could use a continuous duty solenoid between the house battery and the starting battery, to connect them only when the engine/alternator is running. The problem here is that a discharged house battery will immediately suck up all the current from the alternator, and will even start pulling current out of the starting battery, drawing it down even faster than in the diode example above. The current surge when the solenoid turns on will be tremendous, so the wiring had better be able to take it. It’s very much like giving another car with a dead battery a jump. Do it once and it’s not a problem. Do it over and over again without giving your battery a chance to recover and you will need a jump yourself. Considering that your house batteries might have much more capacity than your starting battery, they can suck it dry and still be hungry for more.

One of the most sophisticated options for connecting the house batteries to the starter battery is a Battery Combiner or Electronic Feedback Relay. It uses solid state circuitry and the intelligence of a small computer to attempt to get around the problems with diodes, solenoids and manual switches. The general idea is that it monitors the voltage on all the batteries, which could include multiple banks of house batteries. If any one of them has a high voltage, indicating that it is being charged, then it connects all of them together so they all get charged. Unlike a diode, the voltage drops are minimal, but like a solenoid, the current surge can be very large when the batteries are first connected. They can also discharge the starting battery into the house batteries, just like with the solenoid. Battery combiners are also quite expensive.

All of these methods are being used, and there are probably many people out there that are happy with every one of them. For my purposes they are all not good enough. I might consider a manual switch just so I can use the house batteries to jump a dead starter battery though. In general I want my house batteries and starting batteries to be completely separate systems. I don’t want to compromise the reliability of the truck starting system. Besides, running my truck engine all day to keep my batteries charged would be terribly inefficient. It might be reasonable to charge the house batteries from the alternator on a long drive, which I can still do with a manual switch, if I can just remember to turn it off when I get there.

I could put a 2nd high current alternator on the truck, with a 3 stage voltage regulator, and charge the house batteries completely unrelated to the starter battery. I am sure it would work, but it would be expensive, and require mechanical modifications to the truck. I am too cheap for all that. Better to use a high efficiency generator, and a battery charger to keep the house batteries up to snuff.

Speaking of generators, I am told that the little Honda generators (or similar) are up to 3x more efficient than some of the larger gas or diesel gensets that are often used on luxury RV’s. Those big generators have many other nice features and may be the only good option for you, but not for me. I don’t need to run 2 air conditioning units and all the electrical what not’s for a family of 5. That brings up the question of what do I, or you, need?

I’ve done the math and it is amazing how cheap and convenient electricity is from the grid. You don’t fully realize that until you try and make your own, or worse yet, store enough to meet your needs in batteries. You have to pound yourself down to what you really really need if you want to live off of batteries for more than a day or two.

To be continued...