The high energy density of new technology batteries, for example lithium-ion, comes with a downside; potentially extreme short-circuit current. This paper will describe the capability of a demonstrator for a new main battery for a submarine, the consequences of connecting it to the existing DC distribution system, and how the release of stored energy is to be controlled. A complication is that proprietary battery management systems will shut down on excessive current demand, but that behaviour is not consistent with the UPS support function of the battery or with making conventional fault clearance discrimination work in the extended network it serves. An innovative technique to meet the real-life use requirement will be described, giving an energy source with both power and responsibility.