I fitted a 2,000 mAh battery into my Sena SRL2. The longer pack wouldn't fit in the original case, so this became more than a battery swap: I trimmed the housing and mounting plastic, made a new covering, and removed some foam inside the helmet to get it all back in.
At your own risk: modifying the helmet's foam liner is not recommended by SHOEI. I accepted that risk on my own helmet because I was removing only a small amount of foam.
The replacement battery
The replacement was a BGB DTP103450, 3.7 V, 2,000 mAh pouch battery. Side by side, the extra space it needed was obvious. I had to make room for the pack along with the circuit board and wires.
The replacement pack beside the battery I removed.
An edge view of the two packs and the board beside them.
Making room in the housing
I started by prying off the plastic support that holds the SRL2 in place. With that out, I trimmed the supporting case to accommodate the longer battery.
I kept trying the assembly in the helmet recess as I worked. Each fit check was with the battery and board together, rather than just the loose battery. I also trimmed the mounting plastic to give the assembly more clearance.
Removing parts of the case left the battery and circuit board exposed, so I wrapped them with amber battery tape to recreate a covering around them.
The mounting bracket after trimming, with the cut edges visible.
Trimming the helmet liner
There was still interference inside the helmet. I used a Dremel to take a little off the white foam in the recess—roughly 3 mm.
The white foam in the recess where I fitted the assembly.
Reusing the connector across two helmets
I reused the old battery's connector, cutting it off and soldering it onto the new battery. I worked on two helmets, and the battery connections were different: one had three wires and the other had two.
On the two-wire installation, I connected the red and black wires and cut and insulated the yellow lead. On the three-wire installation, I connected all three.
Covering the exposed assembly
After the amber wrapping, I used black silicone to form a softer covering around the assembly, followed by black cloth wiring tape. After another fit and power check, I reglued the mounting bracket with T7000.
The circuit-board side after wrapping.
The idea was to cover the electronics again, especially with water exposure in mind.
Back together
It powered on. I'll come back to battery life after using it.