蓄 means 'to store / accumulate',
电 'electricity',
池 'pool/tank' - literally 'electricity-storage tank'. So
蓄电池 = rechargeable battery (one you can refill), as opposed to a disposable
电池 like a household AA. Compare
充电池 (also 'rechargeable battery', emphasizing the
充 'charge') - interchangeable in everyday speech.