Mazda Miata batteries are pretty light and have 320-350 CCA. More than enough for a Europa. I don't know of a Car style battery smaller. There are lighter options though The Deka ETX 16 and 30 have good CCA, size, and are light. You can order auto style battery posts on Ebay for cheap.
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http://www.nsxprime.com/forum/showthread.php/138191-Lightweight-battery-odyssey-braille-etc-InformationBy Brand:
Braille**:
Braille B3121 ----------CCA: 550 ---- weight: 21lb ---------------- CA: 742
Braille B2015 ----------CCA: 425 ---- weight: 15lb ---------------- CA: 574
Braille B14115 ---------CCA: 360 ---- weight: 11lb,8oz ------------ CA: 486
Braille B106 -----------CCA: 210 ---- weight: 6lb,6oz ------------- CA: 283
Westco:
Westco Miata ---------CCA: 475 ---- weight: 25lb
Deka:
Deka ETX30 -----------CCA: 370 ---- weight: 21.7lb
Deka ETX20 -----------CCA: 270 ---- weight: 15.5lb
Deka ETX18 -----------CCA: 300 ---- weight: 18lb
Deka ETX16 -----------CCA: 275 ---- weight: 17lb
Deka ETX14 -----------CCA: 200 ---- weight: 12lb
Deka ETX12 -----------CCA: 180 ---- weight: 9.4lb
Deka ETX9 -----------CCA: 120 ---- weight: 6.3lb
Odyssey:
Odyssey PC680 --------CCA: 220 --- weight: 15lb / 16lb For MJT*
Odyssey PC925/MJT ---CCA: 380 --- weight: 24lb / 26lb For MJT*
Optima:
Optima 34&34R --------CCA 800 -----weight: 37.9lb --------------- CA: 1000
Optima 37/78 ----------CCA 800 ---- weight: 38.8lb --------------- CA: 1000
Optima 75/25 ----------CCA 720 ---- weight: 33.1lb --------------- CA: 910
Optima 25&35 ---------CCA 720 ----- weight: 31.7lb --------------- CA: 910
Duralast:
Duralast 8AMU1R ------CCA: 330 ---- weight: 25lb
Marathon:
Marathon MAR-8AMU1R-CCA: 320 ---- weight: 25lb