Advise on building an auto start for my generator.

[mc5a Jon]

I have: 7kw gen set, 3600 watt inverter, 50 amp 24v battery charger, and 4 8d sized gel 12v batteries hooked up to make 24 volts. It works well, but I would like my gen set to start up when the battery pack reaches 22 volts. And shut off when they're charged. I know it can be done, I just wonder if I could build it myself? At this point when it comes to my bus "on a budget" has a whole new meaning. Cost is my main concern.
Thanks Jon.

[Jeremy]

If you are proposing to build the electronics yourself I'm sure Google will find some circuits for you. The first result I got was for this home-built board:



from this discussion:

http://www.edaboard.com/ftopic307379.html

Jeremy

[luvrbus]

Jon, about all custom vans in the 1990's era had auto start check your local wrecking yard you could save a lot of time and just adapt it to your generator FWIW mine was a Starcraft Van with the Clifford system remote and auto start. 


good luck

[rv_safetyman]

Jon.  What inverter do you have?  Many of the inverters with the capacity you have already have the auto start for the generator.  There are also aftermarket units.  

I am not an expert on these units, but one issue is how they sense the low voltage.  22 volts "at rest" is pretty low state of charge as I recall.  My guess is that the commercial units are designed to catch the 22 volts at the very first reading.  That would be when something is drawing big current.  I suspect that there is a somewhat sophisticated timing circuit for the voltage reading.  

The other issue is that you should have a clock function to make sure that the generator is not turned on during quiet hours.

I suspect that Sean will jump in here and give you a "real" answer .

Jim

[JackConrad]

If your generator requires holding the switch in the preheat position for a few seconds prior to going to the start position, this will have to be incorporated into the auto switch. Just something to consider. Jack

[James77MCI8]

You could accomplish this task with a Pico relay. It is basically a programable relay. They have 4 inputs and 6 outputs(I think).  Programing is typical ladder logic and the unit can be ordered with analog an digital inputs.

[rv_safetyman]

It looks like both Xantrex and Magnum make self-standing auto-gen starts.  Onan also makes one, but I think it is only for Onan.

Given that they are pretty sophisticated, I would think it might not be worth re-inventing the wheel.  

Here are a couple of links:

http://www.altestore.com/store/Enclosures-Electrical-and-Safety/Miscellaneous-Electrical-Parts/Generator-Starters/Xantrex-Automatic-Generator-Start/p5649/

http://www.ecodirect.com/Magnum-ME-AGS-N-p/magnum-me-ags-n.htm

I debated long and hard about saying this, but I have to add that one of my biggest fears is a coach with auto-gen start and a generator fire when you are not there.  My Trace has the auto-gen start feature and I would not activate it till I had my system in place.  Indeed, I still do not have it activated.


Jim

[luvrbus]

Auto starts are really advanced now some will start the generator and turn the AC'S on all done by temperature, can be programed to start and stop at different times, remote starts don't you love this hi tech stuff at least till it quits.
I would find me a old Murphy systems from the dark ages they worked good with out all the problems with todays electronics  


good luck

[Dreamscape]

Not to put down your idea at all, but I prefer the way I do it. I use my finger.

I like things simple, easier to troubleshoot, at least for me anyway. I've been around electronics some, they do have their limitations at times.

Paul

[Sean]

Lots of good comments here already, including the one about 22 volts being too low.  FWIW, my 15-minute start voltage is set at 23, and my LBCO is set at 22.9, which overrides the quiet time.

Before I can give you any concrete advice, I need to know the brand and model of your battery charger.

Starting the generator, of course, is the easy part -- whatever voltage you pick, you simply monitor the battery voltage, and when it dips below your set threshold for some set period of time, you start the genny.  Really good autostart systems have several thresholds and time periods; the Xantrex SW4024 that I use has different programmable thresholds for 24 hours, 2 hours, 15 minutes, and absolute (LBCO), which is a 30-second reading.  It also has the ability to start and stop the set based on load.

The real trick is figuring out when to stop the generator.  It's not as simple as measuring battery voltage, because the charger will be running, and so what you will be reading is actually the charge voltage.  If you look at a typical voltage curve for a charger, you will see that the initial voltage will be higher than the final voltage, but neither of those will be as high as the peak voltage.  Moreover, the approach to final, or "float," voltage, can either be an abrupt step or a gradual slope, depending on the charger.

Chargers with integral generator control, of course, know when they are done charging and can stop the genny at an appropriate time (and it is the charger portion of inverter-chargers that handles this aspect of the auto-start system).  To build your own system, you will need to know how to tell when the charger is finished; alternatively, you can estimate the average time it takes to complete charging, and stop the genny on a simple timer.

In most cases, the amount of time the charger takes to finish will be variable, even starting from the same Depth of Discharge (DoD), because you will likely have a variable amount of DC load connected to the system during charging.  During the charging process, the charger will be powering the loads at the same time it is charging the batteries, and the amount of current left over for charging will thus vary with the load.  This is also the reason it is difficult to determine generator shut-down based on charge current; with no load connected, the current will taper off to a floor value (whatever the batteries need to float), but with loads connected, the charger will usually continue to supply whatever current is demanded by the loads.

FWIW, commercial add-on auto-start units stop the generator strictly based on peak voltage.  This means that they generally will stop the set at a fixed amount of time after the "bulk" phase of charging has been completed.  That's probably acceptable for your purposes as well.

Lastly, I need to point out that monitoring voltage over a period of time (which needs to be done for both starting and stopping functions) is not something that can be achieved with strictly electromechanical means -- it requires electronics.  Nowadays, there are tons of Integrated Circuits (ICs) that take a lot of the work out of building these circuits, but there is still no way around having to design and build a circuit for it.  As Jeremy pointed out, there are tons of circuit diagrams available for free on the 'net.  However, if you are not experienced or comfortable with building printed circuit or breadboards and soldering components on to them, then testing and troubleshooting the result, you might want to go the ready-made route, or else find someone with the test equipment and experience to build the circuit for you.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[mc5a Jon]

This forum is great. I hadn't considered alot of your points and I think I will keep my eyes open for one of the old school conversion van auto start systems. The inverter and charger are Vanners and the charger is set up to not over charge.(don't have the part# on me, but they are as

old as the hills.1980's) You know something electrical is old as hell when it's made in the USA.lol  I also have a new 24volt 20amp battery tender that I can leave pluged in all the time but it is 20amps and the Vanner is a 50amp. I had no plans of leaving this system unattended I

would just like to be able to go somewhere and use the bus the same way I use my house. Until this system falls into my lap, my finger does work very well.
Thanks Jon

[Jeremy]

Regarding the 'conversion van auto start systems':- I'm probably misunderstanding what people mean by these, but aren't these systems simply for starting the your car or van's main engine from the remote key fob? If so I'm not clear on how such a system will help with a generator.

Jeremy