I have a couple of Gravely walk behind garden tractors that I use for brushcutting, etc. The problem with them is they don't have any charging system, so once the battery dies from cranking the engine, you're stuck. So I've been thinking about running a pair of wires to the back of my truck that I could hook up to the Gravely battery to charge while I'm driving it around between jobs, etc.

Assuming that my truck battery is normally basically fully charged, and the alternator is just keeping it topped off, is anything bad going to happen to the truck battery or alternator when I hook up a dead battery to be charged. Is there any danger of overcharging my truck batt. when the alternator output increases to charge the tractor batt? This wouldn't be something I would be doing all the time, so I'm not sure if I need to go all out and install an isolator to keep the batteries separate. I have located a "single" isolator (instead of the usual type I see where there's one input and two outputs, this one has just one in and one out) - would this work for my application, since it wouldn't be used constantly like a permanently-installed dual battery setup would be?

Any thoughts on this? And potential problems I need to watch out for? I would run pretty heavy wire to the back, and figure out some kind of "quick connect" to hook up to the tractor battery. If I wanted to get fancy I could also run a heavy duty relay, so the wires to the back wouldn't be live when I don't need them to be (which would be the case for most of the time I'm driving around). And in addition to a switch for me to turn the relay on/off, I could wire it so even if the switch was on it would shut off when the engine is off, so my truck battery doesn't get run down.

My truck is a '93 F150, with stock alternator (as far as I know, anyway). I don't want to hurt my alternator or electrical system, so if this ends up seeming like it's too risky I'll probably just forget about it. Seems to me like it's worth a try, but I'll wait to see what you all have to say. Thanks!

 

I believe a simple parallel system would work fine if you added a relay to turn the cables off when the keyswitch was off.

You will have some current flow and some increase demand from the dead Gravely batteries, but I think this can be controlled very simply by running regular size wiring instead of something heavy.

Run a 10 guage wire from the truck's battery, through a fuse or fusible link, and then to the relay. Then use the same 10 guage wire from the relay all the way back to the charge location, the resistance of the wire will will naturally control the rate of charge.

As the charging demand from the Gravely battery increases, the current flow increases. The resistance of the wire will stay the same, but as the current increases in the wire, the voltage dropped in the wire will increase. This will regulate the amount of current flowing to the Gravely batteries, keeping it from getting too high. As the voltage level(charge) increases in the Gravely battery, the current flow will decrease, and naturally the voltage drop in the wire will also decrease, letting it top of the battery without damaging it. Nothing bad will happen to the truck's system, because you are controlling the demand, which it should be able to handle fine.

I have to admit I am a Gravely fanatic. I think the old L's are one of the best designs ever made. I have one built about 1962. My Dad just helped me convert it over to a Li, and it's now even better than it was before. I felt a little rushed in low and almost never used high range. Now I mow in high and when the grass gets heavy I can switch to low. Believe it or not my 6.6hp L pulls a commercial 40" deck with me on a sulky up some very steep hills with ease. I could not use a regular mower on some of the slopes in my yard, which is about 3.5 acres. But the key was the dual wheels with ag tread tires.

 

Seems as I cannot find another picture some where else, this one is very simialar to the quick disconnects that I use for various projects.

 

I would definately head to your local parts store and pick up a battery isolator in the amperage range of your alternator output. This is basically a set of big diodes, which let your alternator charge each battery individually and proportionally. It also keeps the "dead" battery from trying to charge off of the "full" battery when the engine isn't running, which will discharge & eventually ruin both batteries. This is true even for good batteries of different brands, size, capacity, etc. They naturally want to reach equilibrium, and tend to destroy themselves in the process. The cabling is very simple, especially if your alternator has a single wire output. The isolator goes inline between your alternator positive output & the batteries/starter solonoid. It should come with several good diagrams. This way may be more expensive & complicated, but is good for your entire system. After all, you don't want to have to worry about disconnecting the back battery every time you park.

 

The relay I mentioned would keep this from happening. I didn't think he needed a full RV type setup with an isolator, since the batteries are on the tractors and will not be permanently connected.

 

That would work, for sure, but he will be using this over and over for, potentially, years. I'd rather suggest the safer more durable route. I would also never suggest downsizing wire to regulate voltage. How many car fires/re-calls have happened due to undersized wiring? Does anyone remember the Pontiac Fiero? A dead battery is going to draw more current -period. Resistance to this draw will cause heat, and will slow the charging process. It will not, however, lower the draw on the charging system. That power is now being lost as heat in the undersized wire. This will not "protect" the Graveley battery from the effects of rapid charging, because it is not limiting Voltage nearly as much as it is limiting Amperage (you still have ~12V at the 18AWG wire at your trailer's taillights, right?), and is causing hazards and undue stress to the rest of the charging system in the process. If it does not habitually blow the fuse, it may cook the insulation off of the wire. Have you ever tried to jump start a car with those $8 Auto-Zone 10-guage jumper cables before? They're only about 6 feet long and you can burn your hands on them when you finish.

Please don't get me wrong, I'm not taking a hard line just to be a jerk, and I don't intend to cause a flaming thread. I'm an electrician and a former mechanic, and I think you suggested something that is functional, but potentially impractical or unsafe (and it got me a little worked up - sorry).

Now I don't know, but am assuming that these batteries are or are similar to deep cycle batteries (I have seen tractor batteries before). Deep cycles are made to drain down low and then be charged back up (at least they're more capable at it than standard batteries), so I'm hoping that they can handle the charge/drain cycle rather well. All-in-all, the isolator, battery tray, connectors, and some 6 or 4 gauge wire (positive) from a stereo or welding supply shop, and a ground strap (negative) should run under $100. If that's a little steep, Franklin2's suggestion will work for close to half that, but I'd heavily reccomend larger gauge wire.

 

"will slow the charging process" is correct. So if you are charging the batteries more slowly, then there will be less demand on the charging system.



This is correct. But given the length of the wire, and the environment it is in(not enclosed in conduit, and probably not bundled tightly with other wires), it should work fine without overheating the wire.

As you know, voltage, current, and resistance are inheritantly tied together with the formula E=IxR. And the amperage is what we want to control, since we want to limit the load on the charging circuit, and also keep from charging the batteries too aggressively. So you are correct, if E stays the same, and we increase R, then I (amperage) will have to go down.

Don't get to worried about starting a war on here about how to properly do something or fix a problem. There are sometimes several ways to accomplish something, and I like little debates on technical stuff.

Your charging idea would definitely work, and is the recommended method for someone who has serious power demands. The gravely uses a standard car battery which usually lasts probably up to 10 starts before it needs charging(he will have to help me out here, mine has a pull rope). So I am thinking he will just be "topping off" his batteries everytime he loads them back on the truck after mowing a lawn.

 

I feel a lot better about that. I had visions of the 2-foot long tractor batteries I've seen before. I was envisioning a lot more draw on the system. I suppose if I'd looked a little more carefully the first time the term "walk behind" tractor would have rang a bell for me.

 

What about getting a small "one wire" alternator and running it on the tractor?

 

It has been done. But there is not much room to do it. The whole electric starting system on the Studebaker motors looks like an after thought. Later on they went to Kohlers with the built-in alternators.

 

I think you are going to end up cooking the tractor batteries. I'd put an inverter in with a GFI plug the back of the pick-up and plug a regulated battery charger into that. You'll also have the inverter to charge ni-cad tool batteries, and run small power tools off of.

 

Yeah, but do we really have to go up to 120v, just so we can "safely" drop down to 12v? Seems pretty inefficient.

 

My suggestion for the connector is the type they use to charge electric forklifts. You can order them from grainger. You order the male and female end separately. I do not remember the cost for these but the cheapest ones will be more than sufficient because they are made for big forklift batteries.

 

The newer inverters are very efficient. This was the best Mod I ever made, could charge the Racing Jet ski battery up without worrying about cooking it. No battery, no race because there's no charging circuit. Also it was great for small power tools, chargers, and drop lights. The only mistake I made was the inverter was getting beat up do to its location, but I put a HD electrical box over it later on.