"Accumulator" is an alternate term for "battery", but uncommon in American English usage. In British English usage, it means a "storage battery".
In other technologies, such as hydraulics, the term is used for analogous equipment that stores energy.
(Going off at a tangent...)
To me (native British English speaker, whose travelled a bit) "Accu" is a European/continental way of saying "battery". "Accu" either means battery (e.g. Dutch/scandinavia) or has been used in trademarks so that it has become analogous with whatever a battery is called elsewhere in Europe.
The use of "accumulator" for a rechargeable (usually wet cell) battery is archaic British English, For me, it comes under the heading of "I understand what you are saying, but I would never say it".
When dabbling with battery operated valve radios, it's a useful way of differentiating between the low voltage/high current battery that feeds the filaments, from the higher voltage/low current battery which provides the plate voltage, which was usually a dry non-rechargeable battery.