My vehicle calls for a Standard R121 group battery. Posting to ask if I can replace it with a Standard R124 group battery?
Tester
Probably not, the 124 is 2" longer
It is a bigger battery, and you would need to measure to see if it fits.
21R -8 3/16 x 6 13/16 x 8 3/4
Group 24 Batteries- 10.25 x 6.8125 x 8.875 inches
Why do you want to use a different battery ?
One other consideration, even if you can shoe-horn a bigger battery into the engine compartment, one of the connection leads may not reach the posts. I’ve never encountered a 121 before. Is that a common size in cars these days?
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To answer Volvo V-70, I can get an Interstate brand group 124R battery from my local Costco for anywhere from $50 to $75 less than a group 121R store brand from my local Auto Zone, O’Reilly’s, NAPA and even WalMart. And while not true in every instance, Interstate has been known for supplying quality batteries for many years. As for if the 121R group is common, it is widely available high would lead me to believe it must have a fairly broad application, perhaps across Korean and other Asian imports at least.
To quote Ray, or maybe Tom, it is the stingy person who spends the most.
@Wpollock:
I’ve upgraded battery sizes, like you’re trying to do, many times over the years, on cars, motorcycles, and tractors. It worked successfully every time.
However, it’s rarely straightforward, and is almost never a simple “drop in”. The burden is on you to verify beforehand that it will work. It often requires improvising or modifying something in or around the battery mounting area to make it work.
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The last time I replaced my battery I bought the largest that would fit. It’s worked.
Size that fits . That is the key point . Plus are the post in the right place ? A tape measure and noting the location of the + and - posts with a trip to the store will answer those questions .
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Agreed. Whether it fits is the question. The bigger the better and the more CCA the better but that is not always possible. With many cars the cable ends barely reach as it is so adding a inch or so might involve cable work.
If the battery cables and starter motor are in good shape you should not need anything larger than what the car was born with.
Assuming the engine is not full of 60 weight motor oil and it’s not 5 below outside… 
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I would do this swap in a heartbeat if there is room for the longer battery. A tape measure is your friend. In th´old days I replaced many group 24 batteries with 27s.
I replaced a group 35 with a group 24f battery in my 2006 Honda Accord V6. It took maybe a hour to lengthen the plastic base and case 1 3/4" with scrap plastic material. In a few years when the group 51 battery in my Honda HRV is due for retirement, I plan to replace it with a group 56 battery. Yeah, I hate getting stranded!
I generally agree, but I’ll only do it if the larger battery fits WITHOUT modifications
65 instead of the smaller 58
49 instead of the smaller 48
78 instead of the smaller 75
and so on