While my 04 HCH was in for transmission judder/burnishing (again), they recommended replacing my 12v battery. After 6 years it's probably time to replace it but I declined to get the Honda battery installed for $125. I know I can get a Costco/Kirkland 51R battery for only $60, but that replacement has 500 CCA (cold cranking amps) while the OEM Honda battery has only 332 CCA. Aside from carry extra weight from the heavier battery, will a higher capacity battery reduce gas mileage since it is charged from the IMA battery? Anyone with an non-OEM replacement battery notice a drop in mpg?
I can't speak from actual experience but I seriously doubt the larger size 51R battery will cause any negative impact on mpg. In fact, a fresh 12V battery may help by supporting all the electronic controls better and recharge more efficiently.
From my research it appears that the OEM battery is a standard size battery used in several small Japanese cars, like the Honda Fit. Unfortunately, the North American battery manufacturers have not come up with an exact match as yet, but this may change. The 51R is a popular size used in 4 cylinder Honda Accords, CR-Vs, and standard Civics, so this size is easy to find. It is the same height and width as the OEM battery but about 1.5" longer. The plastic battery cover/tray will not fit but there is enough space for the actual battery. The "R" refers to "reverse" post placement. You want the positive post to be in the left rear corner in order to attach the cable properly.
I did replace it with a non-Honda battery which seems to be working fine. Fortunately the tray and cover both fit the larger 51R battery and after about 2 days the IMA charging improved and leveled off (had been very erratic with the old battery). I vaguely recall reading some discussion and opinion that a larger capacity battery puts a bigger load on the IMA charging. I also checked the owners manual which had no cautions or warnings about 12v battery replacement.
I've been having issues with my 2005 Honda Civic Hybrid. The car won't reliably start (as if the battery is dead). Lights come on in the dashboard, but it sometimes takes a few tries before the car starts.
I figure this is an issue with battery in the engine compartment not the IMA batteries. The IMA light has not come on. It's the original battery up front. Sounds like I just need to take the car into the dealer and buy a new 12V battery.
If your 12V battery is still the original one then it is very likely failing. As explained above the least expensive replacement is a size 51R like is used in regular Civics. The smaller OEM Hybrid 12V battery is not manufactured in North America as far as I know. Because the 51R is popular, you should find it easy to locate one at Costco, WalMart, or auto parts stores in your area.
If you have never replaced a battery before, find a friend or neighbor to help you. It's quite simple but you need to be careful like with jumper cables, and remember a battery is full of acid. (I have ruined more pairs of jeans that I care to admit)!! Also be sure to have any radio security code available before you disconnect the old battery.