My wife’s 2003 Honda Odyssey needed a battery. Having changed the one in my car, I figured it was a no brainer. Looked it up online, found the right one and sent my wife to pick it up. Parts person confirmed it was the right one.
I removed the old one and put the new one in. Everything lined up great. Connected the negative terminal, and removed the little plastic cap from the positive. Hooked it up, and BAM! Only then did I realize the + and - were reversed on the new battery.
Now, yes, I should have confirmed the polarity. But it fit perfectly, and who the @#$#@$@# puts a protective cap on the NEGATIVE terminal!? Who in the world cares if you accidentally short - to ground?!?
My question - how much damage have I done?
Oh, and I made the last payment YESTERDAY! No kidding! 
A little 9V battery is designed with terminal that cant go on backwards, yet the terminals on a car battery are the same positive to negative? I guess this is the same reason why most cars don’t have a low oil level light. As far as damage, could be as little as a fuse, or as much as a blown computer control module. I would check fuses first hopefully there is built in protection for this sort of thing.
Honda learned along time ago that over protection was better. It took domestic car makers along time to figure that out. Your vehicle is equiped with a main battery fuse. It is like 80 amp and is screwed into the underhood fuse block. Its main goal in life is to protect the electrical system of the car from mistakes like this. Honda nor myself can guarantee that it will protect everything but it certainly helps.
Did the fuse burn out?
Does anything work?
Hopefully all is fine.
~Michael
Thank you Michael. I did find the main battery fuse. 60A I think? Anyway, it looks blown.
I did see the screws presumably holding it in, but wanted to check online first before I went at it with a screwdriver.
No, nothing is working. No lights, no nuthin.
I’m praying the fuse actually served it’s purpose or my wife is going to kill me! 
I have seen lots of these towed into my shop do to mistakes. I have only found one Honda that suffered any damage besides a blown fuse and that one had 150,000+ miles on it and may have needed the alternator to start with. Go ahead and change out the fuse and let us know how things are going.
~Michael
I’m still perplexed at why there was a plastic cap over the negative terminal? Is there any good explanation?
BTW - apparently I needed part number 24F-DL, and not 24-DL from Autozone.
Well, you can’t short out the battery if either terminal is protected so it doesn’t matter which one is protected.
When you install the battery, you’re supposed to hook up the positive lead first. There are a lot more chances of shorting the positive lead to a ground than the opposite situation. So, you connect the negative last. And to prevent shorting the battery while connecting the positive, there is a protective cap on the negative terminal.
On removal, remove the negative first for the same reason.
markts:
I can not explain why there is a cap on only the negative post. I looked on-line at both batteries and they both show a black cap over hat I can only assume is the negative post. All the Interstate batteries that I have in stock have a read cap over the positive post. The dual mount (top & side) have a red cap over the positive and a black cap over the negative.
Just as a humorous side note: About 3 months ago I had a full size Ford van drive into my shop. The customer had installed a new battery 3 days before. Every time he needed to start the van he had to jump start it. He indicated he thought he needed a new alternator. I opened the hood to check the alternator output and noticed when he installed the battery he forgot to take that little black cover off the negative post. Removed it and the van starts every time now.
~Michael
It’s something I’ve never really thought about, but what about the possibility of caps being used to shield the terminals a bit from air/moisture and prevent scale from forming?
If someone could install a battery without noticing a post cap then there is no doubt that they would also overlook a scaled over post.
A bit sheepish was he?
There are a lot more chances of shorting the positive lead to a ground than the opposite situation
I’m not sure I understand this. When you say “positive lead”, do you mean the terminal of the battery?
If I connect the positive lead to the battery first, the electrical system is “sort of” energized, isn’t it? If the ground of the battery and the chassis were the same, it certainly would be. But it seems a little ambiguous to me to have +12 to the car (or anything for that matter) and the negative “floating”.
If I connect the negative lead first, I know it means vitually nothing to the battery and the electrical system.
Just musing…
Replace the fuse. Then put in the battery like you are supposed to. Positive terminal first. It’s marked on the battery clearly (positive is a +, negative is a -) and usually the cables are color coded. Red is positive, black is negative. Every time. Positive is hot, negative is ground. Every time. Don’t guess. If you are prone to forgetting how things were set up, take a picture before you start. With digital cameras its easy and then you know how its supposed to be when you finish. Assuming a plastic cap is proof of anything is obviously not a good idea - anyone can take off or put on a cap.
I’m not sure I understand this. When you say “positive lead”, do you mean the terminal of the battery? [quote]
Yes.
[quote]
If I connect the positive lead to the battery first, the electrical system is “sort of” energized, isn’t it? [quote]
No. You could connect a power supply that is hundreds of volts on the positive wire as long as there is no return path to the power source, the whole circuit will float at that potential. No current will flow.
[quote]
If the ground of the battery and the chassis were the same, it certainly would be.
Of course, if you have a return path to the battery minus, when you connect the positive wire, there is a complete circuit and current can flow. This is exactly why you do not want to make the negative connection first.
But it seems a little ambiguous to me to have +12 to the car (or anything for that matter) and the negative “floating”. [quote]
But that is exactly what you want to do. It does not hurt electronic devices to have the positive connected without the negative. The devices do not have a “ground” reference so there is zero current flow.
[quote]If I connect the negative lead first, I know it means vitually nothing to the battery and the electrical system.
It’s the same with the positive lead.
Wow, quotes gone wild! I must have missed a closing quote at the beginning but without an edit post method, whatchaseeiswhatchaget.
If I connect the positive lead to the battery first, the electrical system is “sort of” energized, isn’t it?
This is incorrect, unless both terminals are connected you will get no current out of the battery.
You connect the positive first for a very good reason. When connecting the positive first, if you accidentally contact the wrench to the metal frame and the positive terminal, nothing will happen. Then you can connect the negative, now you only have to worry about touching the wrench to the positive terminal.
If you connect the negative terminal first the positive terminal is ‘energized’ so to speak when you work on it. By connecting positive first, neither terminal is ‘energized’ when you are wrenching on it.
Thank you. That makes sense!
Survey says!
Just the fuse was blown! The Odyssey lives to see another day.
It was a 120A “battery fuse” (!!) that blew and finding a new one took some chasing. But everything seems to be working fine after replacing it.
Thanks everyone!
If you are connecting the positive last and you touch the wrench to a ground, you will get some welding and heating action and you could burn yourself from the heat. Whenever you work near a positive terminal it is a good idea not to wear a ring or a watch. You could get seriously burned. The negative terminal can’t hurt you on a car with negative ground. Thinking and experience are both good, but safety helps too. If a procedure is recommended by a manual or a professional, there is a good reason. Don’t forget to wear some eye protection when working around a bad battery: they will sometimes explode for no reason. There are a lot of other things you could wear too.