Battery Tender - negative terminal or chassis?

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I just got a Battery Tender Plus to go with the new battery for our truck which sits in the garage most of the time and is driven infrequently. The old battery went dead a few times from sitting and I'd been planning to get a maintenance charger once we had to replace it both to ensure the new battery lasts as long as it can and so we didn't have to worry about taking shorter trips in the vehicle from time to time.

The charger comes with a short cable with ring connectors and a rubber cap on the plug at the other end, and I'm planning to put the ring connectors on the [side post] battery and just leave it there tied up so it doesn't get into trouble. If that doesn't work out it also comes with an alligator clip cable but that'll be more of a pain on the side post battery I think.

At any rate the instructions say to hook up the positive end to the positive terminal on the battery and the negative end to a chassis ground. They specifically say NOT to plug the unit into the wall until all other connections have been made so I would not expect sparks. Why would you have to connect it to a chassis ground instead of the negative battery terminal? There is a chassis ground spot close enough I think, but the negative terminal would be more convenient. Is there an important reason for this?
 
It's simply a safety precaution on their (and subsequently your) part to help ensure there is no spark generation, as you noted, at the battery.

(I actually use one of their extension cables with the ring terminals attached to my side-poster and the SAE connector on the other end with another brand charger. It's a decent solution for a maintainer style device.)

By connecting the positive first and then the negative at a point somewhat distant from the battery... should a spark erupt it will have a very difficult time finding free hydrogen to ignite as it is away from the gaseous source. Doubly so as the charger is not yet energized by the AC line if you follow the directions properly.

Frankly it's an overengineered statement that loses significance when you are dealing with a low-current device - but you know, lawyers get involved...

Still a good idea to keep Safety first and foremost!
 
Because unsealed car batteries generate hydrogen gas and in the proper air to hydrogen gas ratio, it is a very good explosive.

When you make the second (last) connection to the battery, you complete a circuit. If you do ever generate a spark they want it to be far away from the explosive hydrogen gas that can be coming off of you battery. This is why you always make the last connection far from the battery. It minimizes the possibility of hydrogen gas explosion of the battery.

I actually once saw someone connect cables to a battery in a vehicle and have the battery explode when they made the second connection at the battery.

They were across the street from me at the time, and I was in a college dorm house sitting on the couch. It looked like someone threw a gallon of water into the air from where the battery was under the hood. The person this happened to ran into there dorm house and jumped into the shower. Later I questioned him about it and he said that he had made the connections with the correct polarity, but at the battery instead of making the last connection to the frame. He said it just blew up and when it did it threw acid all up into the air, and all over him.

They are also telling you not to plug the unit into the wall until after you have made the connections to the wall so that the unit is definitely off and therefore eliminating the possibility of a spark being powered by the unit.

Note, even with the charger unpluged, the battery could still power a spark (much more likley, if you had the polarity wrong), so you still want the last connection far from the battery.

BTW you should also turn off the unit (and or pull the plug) before removing the connections when you are done charging the battery. There is also hydrogen gas created when you charge the battery, and that could cause an explosion if you disconnect the leads when the charger is on.

And when you remove the leads, you always remove the lead that is farthest from the battery first. Because when you remove the first lead you are breaking the charge circuit, and there could be a spark. If there were a spark you want it to be far from the battery. That couple of feet of distance from the battery, can easily be the difference between a battery not exploding, or exploding. The hydrogen gas will be the strongest right adjacent to the battery. And just as a hydrogen or helium balloon will float upward, the hydrogen gas that is given off form your battery will quickly flow upward and not be anywhere near as strong of a concentration as you get a few feet away from the battery.

BTW, to really be safe, you should ALWAYS have a plan of where you could access either a hose that has water pressure to wash your face with, or a shower near by, when you are working with any liquid acid battery. In industrial settings OSHA requires an eye wash station, and for some larger applications an emergency shower.
 
Note there is no such thing as a 'sealed' automotive battery. Even the so-called 'sealed' ones can still vent if pressure builds.

I spent the afternoon in an emergency room when a battery explosion blew straight into my face! I was extremely lucky I came out of it with both eyes!
 
Beside the sparks, I've heard that its harder on the battery itself to charge through the negative post and easier on it if you use a chassis ground. Can't remember the reasoning on that one though.
 
I would do it on the basis of the less stuff hanging off the battery terminal, the more reliable it will be, especially for starting.

By the same token, most cars have an underhood fuse box with a 10 gauge positive wire off the battery ending in a lug. This lug makes a great tap point for medium to small current projects.

If one in 100,000 of those chargers leave the factory untested and with an internal short, it would be good for you to connect its ring far away from the battery. Call me crazy but I often "drag" a new connection to make sure there aren't huge sparks I wasn't planning for. (An exception is jumping a known dead car, then I make the last negative/block connection as fast and solid as possible to minimize the arcing)
 
No where in the installation instructions of a Battery Tender does it say to install the neg. small loop to the frame. It says to install it to the neg. terminal. Doesn't say you can't do it either. I use the small loop connection right to the battery terminals. Works great for me.
 
I use a 2 channel charger/tender for my rv with factory installed rings on the lines and its instructions say to go right to the battery (its a permy install)
 
I hook up my battery tenders to a positive and negative away from the battery terminals to avoid corrosion from the acid.
 
Thanks, this is about what I was expecting. This vehicle does have a lug in the fuse box but it's all the way on the other side of the engine compartment. There is a ground spot I could probably use near the battery on the body though.

It turns out the posts I have won't fit the rings anyway so I'm going to have to get new posts. If I can find both sizes (the positive is longer on this thing) with a good spot for the connectors I think I'll probably just do that.


Originally Posted By: Lakersguy
No where in the installation instructions of a Battery Tender does it say to install the neg. small loop to the frame. It says to install it to the neg. terminal. Doesn't say you can't do it either. I use the small loop connection right to the battery terminals. Works great for me.

Here (pdf link) it says to connect to the vehicle chassis but the diagrams here (pdf link) show the terminals so they're not really pressing the issue.
 
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