I just replaced my battery. It was still good but I wanted something with more reserve, cold cranking amps, quality, etc... Something more drive in worthy, so the drain on the battery from running the stereo through 2 movies doesn't hurt it as much.
The OEM civic battery is size 51R. The R means reversed polarity. The battery terminals are on one side of the battery. Put that side facing you and the positive (red) should be on the right hand side. A plain old 51 will have the positive on the left. Optima makes red tops and yellows in that size now so it's easy to get a good AGM battery replacement for our cars.
51 is basically the smallest size of common battery for cars. I wanted more reserve capacity for the stereo without the car running, so I went to the next step up. Size 35. Same height, same length, but about 1 1/2 inches wider. Size 25 is the same as 35 but the polarity is reversed. I'm not sure why they don't have a 25R or 35R like the 51. I don't know the meaning behind the size designations. 35 is a very common size.
I went for an Exide Edge battery. Exide is a huge battery manufacturer and many house brands of AGM batteries are just rebadged Exides. They are very well priced as well. Cheaper than an Optima or Stinger, and availability was much more common in my area. Even Costco sells them. The Exide Edge uses a graphite technology which gives it the same benefits as the spiral technology in the Optimas. It's pretty much the same quality as a Yellow Top, being a starting battery with drain characteristics for stereos and electronics. I haven't seen any negative reviews for the Exide, but lately Optima has been getting some negative publicity regarding their quality and reliability. Many "top 5 batteries on the market" lists include the Exide Edge, so it's a quality battery.
Just to compare, the original Honda 51R battery had 450 cold cranking amps. The Exide Edge 35 has 650. The reserve capacity isn't marked on the Honda battery but comparing Exide Edge, the reserve capacity at 25A for a 51R is 75min while it's 110min for the 35. Optima has pretty much identical specs. So there is a significant advantage to the larger battery.
Now for the installation. I removed the old battery, the plastic housing, and the plastic bottom tray. You can't reuse the plastic tray or the housing. The housing isn't required since the new battery is a high end AGM and doesn't require it. The original battery is just a normal flooded lead acid and has a chance to leake. The new battery just fits in between the 2 holes for the J bolts for the hold down. There is a little rubber mat that was originally on the bottom plastic tray and I put that under the new battery to give it some cushion from the steel of the battery shelf since you no longer use the plastic tray. If you try to use the plastic tray, the larger battery bulges it out against the mounting holes and the J bolts will no longer go in. I also flipped the orientation of the J bolts so they are on the outside of the mounting holes instead of the inside to accommodate the wider battery. I also purchased an adjustable plastic battery hold down. Just adjust it to the width of the new 35 battery and bolt it down using the original J bolts and nuts. It's a very tight fit but it does fit. I believe it would be the largest battery that is easily installed in the civic without any mods aside from not using the tray and getting a new wider hold down.
The only difficulty was trying to hold the J bolts and get the hold down mounted and tightened. The larger size of the battery makes for less room to get your fingers in. I ended up pre-installing the tie down to the J bolts with the nuts tightened just enough to hold it all together. That way I wouldn't drop the bolts into the engine compartment. Fit it into place and tighten it down. The first time I didn't do this and lost the J bolt in the engine compartment
Overall an easy upgrade and the best that can be done aside from installing auxiliary batteries in the trunk. I just have the Honda Premium 360W stereo so it's nothing huge. If I had bigger aftermarket amps I'd go the trunk route and just keep a 51R under the hood.
