Charging a 12v battery bank w/alternator

[jonnpaull]

Hey,

First post on a forum ever.
I am starting a project to run our trailer off of a generator and a bank of batteries. I am not apposed to integrating some alternative energy sources in the future, but for now i just want clean consistent power but don't want to run the generator all day and night to run our fridges.

What i've planned so far is to have a bank of batteries feed an inverter and charge this battery bank on demand with a self starting motor with a shaft connected to a vehicle alternator.

I guess what i really need is some feedback on the layout of what i've got in my head and some ideas for circuits to take a set battery voltage to initiate a starter circuit for the generator and then when the bank is charged it needs to trigger the kill switch for the engine.

I'm concerned about what type of batteries to use, how many, and it definitely needs to be safe! so i am worried about the batteries being overcharged and spewing acid.

 

[tcmtech]

What level of abilities do you have with electronics or possibly basic programing or the willingness to learn application specific programing?
TO me this is a basic PLC or PLR type application with a few additional and very basic inputs signal conversion circuits. Well under $100 investment and few tools needed.

 

[tylernt]

Maybe it's just me but I like simple things. Most automotive alternators already have voltage regulators in them, so you should not have to worry about overcharging your batteries IF you select the right alternator. You should also consider a self-exciting alternator because a manually-controlled one will require extra effort (complicated circuitry) to deal with.

To detect end-of-charge, you could use some kind of current sensing device on the alternator output. As the batteries charge up the alternator's output should go down, so just pick a setpoint for the engine cutoff (will require experimentation). DigiKey, for example, lists many "Current Sensing Switch"es so this should be fairly straightforward.

As for auto-start, you can sense low battery voltage with a "voltage sensing switch" -- the second hit in Google for that phrase shows a device that looks perfect for this application.

One thing to consider is that when your fridge runs etc, it will draw the battery voltage down but this does not indicate the true state-of-charge of the battery. For example a fully charged lead-acid battery might read 12.8v with no load and 12.3v under load, so to avoid running your generator unnecessarily, you'll need to have the voltage setpoint lower than you might expect. Or, put another current-sensing switch on the battery output so that you have a better idea of what the battery's state-of-charge is between no-load and loaded conditions.

All that remains then is a starter circuit that is triggered by your low-voltage switch. Make sure that you design it so that the starter will not run for more than a few seconds, to avoid burning up the starter in case the engine does not start right away. If you wish to have multiple start attempts, you'll need a timer or a simple resistor-capacitor combo to delay the restart attempt for 30 seconds or so to allow the starter to cool.

One thing to consider for unattended operation is an oil pressure cutoff switch so the engine doesn't destroy itself should a lubrication failure occur. A over-temperature cutoff switch is another good idea in the event of a cooling system failure.

 

[jonnpaull]

I am pretty competent with basic circuitry but haven't tinkered in like 10 yrs. I would love to use a PLC just for the flexibility in programming and since it will be left unattended, i could run all sorts of inputs for safety. i love the safety suggestions of the oil pressure and temp cut offs and yeah i hadn't considered the output load idea for triggering the generator, that seems like the way to go.

Do you have suggestions for a PLC unit that is pre-made? How reliable are these, are they prone to glitches and malfunctions? i love to tinker but i also don't want this thing burning out all the time and causing headaches and money. Does this setup allow data logging so i can tell how often the generator cycles and what the average load is and stuff like that or am i getting too ambitious?

i would like to incorporate something into the starter circuit so that it knows when the engine has started but is also limited as to how long the starter will run for like you said and how many attempts it will make, sort of like a tach sense but it would have to run off of the magneto i guess.

 

[tcmtech]

I run the Teco PLR units in harcher environment applications that see -30 F to +140 F while running 24/7/365 and I have yet to have problems with one!

Here is there website. Industrial Communication Solutions - B&B Electronics The Teco PLR software is a free download to so you can see exactly what is involved in setting up and trouble shooting your software before you ever buy a PLR unit. All of what your monitoring can be done with either simple on/off inputs or by using a 0 - 10 volt analog input signals from sensors or what ever you want or need.

At worst being that the Teco PLR units are 24 VDC you would need a simple DC - DC converter to power them.

 

[jonnpaull]

sounds like it's a good fit for this project thanks for the info! Now i can really get a plan in place

 

[mendoAu]

First off...hi all...new member with my first post. I'm just a simple gold miner living off the grid learning more and more about deep cycles and power inverters. Tired of cheap crap battery chargers with to many bells and whistles. Live in a 32 foot Hitchhiker RV. Use a generator for my power needs. I'm tired of spinning wheels and scratching my head. To keep this simple (SIMPLE) I use a four deep cycle array of 12v batteries with a 800 watt power inverter (cobra) and run the generater about three and a half hours and then, usually, get about three to four hours off the inverter. But every time I kick back and things are smoothly going something seems to pop up and go haywire. So here's my question....since someone is always around and paying attention couldn't I just go to the local car graveyard and pick up an alternater (with built in voltage regulator) and hook it up to a recycled electric motor (via pully and belt) to charge my batteries while running the gen and eliminate the battery charger. Gotta run the gen for shop tools and microwave and AC etc. anyhow and maybe it's just this cheap battery charger that does not charge those 12v's up over 12.3 anyway. I agree with the above poster "keep it simple" theory. I have no problem calculating motor rpm/pully sizes to max out the alternator. I even have some pieces of aluminum diamondplate to make a nice stand. Please make any comments and yes, I'd do understand that a couple of hundred bucks could get me one of those high tech 5 or 6 stage chargers to de-sulfide my batteries etc. but I'd rather put that money into new tires for the 4-runner before this winters snow fall, HA! (oh, I forgot to mention that this area seems to have some kinda aluminum foil dome over it and for some reason makes anything with high tech electrical circuitry fail. Last year we had a new Vizio Tv that was replaced three times because of failures of one sort or another and the list goes on and on...)

 

[MikeMl]

Running a gas engine to run an AC alternator to run an AC electric motor to turn another rotating machine (the DC alternator) isn't simple; its simple minded. The efficiency is like a few percent...

Connecting a gas engine to an alternator producing DC is slightly better efficiency-wise, but most automotive alternators (14V @ ~50A) still only produce 700W (~1hp), so you would need several alternators per engine...

A single 100AH battery would take one alternator for about three hours of running. Four of those batteries would take four alternators and about a 6hp engine....

 

[mendoAu]

Huh?... I don't think you understood. Re-read my post. I run the generator anyhow for general ac power to the RV. So...why not run an electric motor/ automotive 12v. alternator at the same time...plenty of generator power to power all. Never said anything about an AC alternator. I'm not simple minded but of course those who mis-read fall into that catagory. Geez.... and by the way alot of people run duel batteries in their trucks with no problems and ...believe it or not..with just one alternator. (and by the way..in my first post AC stands for Air Conditioner if any confusion)

 

[KeepItSimpleStupid]

FWIW: Please take it as a "comment only". One guy around here uses a 3 kW Honda inverter/generator to run a 120 VAC motor intermittently for a few minutes each customer. The generator automatically idles down when there isn't anything calling for power.
the noise isn't obnoxious at all. The generator hangs out in the bed of the pick-up which is towing the custom trailer.

 

[Les Jones]

I agree with the other comments that having a petrol/diesel engine driving a 120 volt alternator which then drives a 120 volt motor which drives a 12 volt car alternator is very inefficient. Would it not be possible to drive the car alternator directly from the engine that powers the 120 volt alternator. Some figures would be helpful. What is the horse power of the engine that drives the 120 volt alternator and what is the rating of the 120 volt alternator in KW or KVA. What is the amp hour rating of each of your batteries. What is the average current being drawn by the inverter when it is supplying your 120 volts. (And any other 12 volt load on the batteries.)

Les.

 

[alec_t]

If the recommended charging profile for your deep-cycle RV batteries is different from that of a standard car battery then a standard car alternator won't charge the RV batteries optimally, so may shorten their life.

 

[tcmtech]

If it was me I would just use a larger commercial application type alternator like a Delco 21si or 22si size that puts out 140+ amps continuous duty at 3600 RPM input as the charging unit and drive that directly off a second smaller engine.

I use those alternators for replacements on our bigger farm tractors that have heavy lighting loads and large batteries without problems. You can get them though most automotive parts stores or online for around $120 and the can take a pretty good beating without burning out.

 

[mendoAu]

Thanks for the input. I kinda think somewhere I didn't convey our "system" here in the backwoods well. Off the grid meaning no power lines, no cable tv, no natural gas, etc. Just big trees and a very nice creek. So along with the use of propane tanks for heat and the two refrigerators we have to run a 4000 watt generator daily 365 days a year for our 110v power needs. Ball park figure of $200 a month in fuel. The terrain here (tall mountains and 150 foot doug fir trees) pretty much make solar panels impracticable. We recently replaced all four of the deep cycle 12v batteries and keep up on the maintenence on them. Most of the time our power needs are fairly simple...a couple of lights, sat. TV reciever and when not switched over to the battery bank/inverter we have the RV on board inverter system plugged in which does a good job to charge the two deep cycles on board which supplies the 12v for lights and the water pump, etc.. I never use the inverter system for my workshop/outdoor needs, always generator power. So you see that the generator runs a good deal of the time. We had few problems until last week when the Cobra power inverter became faulty and just now sent it back to the factory and bought a (not my choice but ...) replacement Walmart unit. That's when things went downhill. Instead of the normal 3 to 4 hours of gen. on and switching over to the battery bank system for about the same 3 to 4 hours we are only getting about 45 minutes of inverter power. I'm returning that piece of junk Monday and have ordered another Cobra that will be in the middle of the week. So, after a couple of conversations with a friend he said that those batteries should take a charge of at least 13 plus volts and also said that he's not very pleased with the 100 dollar plus charger he resently purchased. Thats why I'm researching and leaning towards using the electric motor from an old retired air compressor and hooking it up to a car alternator w/regulator (thanks for the tip on the tractor units). Calculating rpm/pully sizes. Now this would illiminate the battery charger and would run while the generator is running anyway for several hours. This gen. has two 110 circuits and should easily cover the load. I'm trying to avoid NEW IMPROVED products with "take a dump" circuitry, HA! Living off the grid on a forty acre mining site has it's down side but it's also just a wonderful thing to look out the window at a picture perfect invironment with none of the big city problems.
PS. I did take heed about running a second gas engine for the alternator but that calculates into more gas usage and might not be needed if the 4000 gen would comfortably run an elect motor instead.

 

[tcmtech]

If I was you I would be looking for a old commercial or industrial type battery charger. They are not that hard to come by and many of the larger ones can put out a huge amount of current continuous duty. I have rebuilt several over the years that were for large 12 volt based standby power systems and could put out around 200 amps continuous duty without overheating.

Ideally when charging your 12 volt deep cycle batteries you want something that will put out around 14.3 - 14.5 volts until they are fully charged ( current draw drops below~ 5 - 10% of their total AH rating) then drops back to around 13.8 volts to float charge them from there while power is available.

Given your load cycles the floating function would likely not really be necessary though so direct charge off of a alternator would work just fine for what you are doing.

As for using a regular heavy duty high current transformer based automotive battery charger/booster unit it should work just fine to provide your ~14.4 volt charging voltage provided your generator is putting out the correct 120VAC 60Hz. If the voltage is too low then the charger's DC output will also be too low and it won't do any good.

At this point if you have four deep cycle batteries and a 800 watt inverter is draining them down to the point it shuts off in 45 minutes they are severely undercharged. Given a decent 100 ah rate 12 volt deep cycle battery should be able to supply ~1000 watt hours of usable power your set of four should easily carry a 800 watt inverter maxed out for at least four hours even after factoring in conversion losses.

As for the charging side of things obviously you would need more than the 1000 watt hours per battery to recharge them so if they are drained down to the point of the inverter shutting down a set of four 100 Ah deep cycle batteries would need a power source capable of supplying at least 130 amps at 14.4 volts for four hours or more to get them back up to full or near full charge.

 

[tcmtech]

Another thought on where to get a high powered battery charging systems is to repurpose an old MIG welder power supply.

Wire feed welder power supplies are constant voltage units like normal battery chargers are and most decent ones rated for 150 - 200 amps have a lower end working voltage down in the 14 - 15 volt range usually with 100% duty cycles which makes them pretty easy to adapt to work as high current battery charger units.

 

[mendoAu]

Thanks for the info. I'll be making some phone calls and maybe post a "wanted" add on craigs list. You have helped me believe that the charger we have now is just not cutting it. No matter what setting the most it seems to pump out is 12.3v which would probably be just fine in an auto-recharging situation. I also double checked the 4000 and yes it is 120/240 60Hz. and fairly new. It's a Honda clone (Harber Freight Predator) that just hummms right along. Beats the Briggs and Stratton at twice the price hands down.

 

[MikeMl]

... Never said anything about an AC alternator. ...

What do you think your "4kW generator is"?

 

[tcmtech]

Interesting being my brother has that exact same generator.

I have a slight suspicion that your genset might be putting out too low of voltage which is why your charger only puts out 12.3 volts instead of the 14+ it should.

My brothers genset would only put out around 105 volts when he got it from a buddy of his cheap who didn't like it for that very reason that with the too low of output voltage it would not power things properly.

I think the simple fix for his was to adjust the voltage regulator module in the end of the generator up to the normal 120 - 125 VAC that it should be at with no load. I think that generator head uses the common 4 - 6 KW type crescent shaped automatic voltage regulator with has a small brass screw on the back side that is for setting the voltage.

BTW if yours ever goes bad that voltage regulator is easy and cheap to change out. It's a universal type that can be found on line for around $15. I buy them to use as refit replacements for older gensets that have expensive or obsolete AVRs in them.

Also check your engine RPM's as well they should be around 3750 at no load giving you a ~63 Hz AC frequency as well.
If not you will have to turn your engines governor up a bit too.

 

[dr pepper]

Take some care in choosing an alty, vehicle alty's are designed for a certain capacity battery, if you have 100's of amp hours on a vehicle alty it might overheat.
You can get wround this by under running the alty maybe 50%, by sensing o/p current and controlling rotor current.
The most sensitive things seems to be the rectifier, ok its harsher application however I fried the diode pack in an alty that was converted as a mig welder.