"蓄电池容量" can be translated into English as "battery capacity" or, more specifically for rechargeable batteries, "rechargeable battery capacity" or "accumulator capacity" (though "accumulator" is less commonly used in everyday English compared to "battery").
In most contexts, especially when referring to the capacity of batteries used in vehicles, electronics, or energy storage systems, "battery capacity" is the most straightforward and commonly used term. For example:
The battery capacity of this car is 60 kWh. （这辆车的蓄电池容量是60千瓦时。）