Not every frame has room for an internal Di2 battery inside the seatpost or downtube, and plenty of excellent bikes were built before internal battery routing became standard. The SM-BTR1 gives these frames a path to electronic shifting by mounting externally with a separate cradle that bolts to bottle cage bosses or tucks under the chainstay. If you're converting an older frame to Di2 or building up a bike without internal battery provisions, external mounting keeps the option open without requiring frame modifications.

The SM-BTR1 is a 7.4-volt lithium-ion cell with 530mAh capacity, which translates to roughly 1,000 to 2,000 kilometers of riding between charges depending on how often you shift and whether you're running a front derailleur. Cold weather reduces range somewhat, as it does with all lithium batteries. Shimano rates the battery for about 300 charge cycles before capacity starts declining noticeably, which means years of regular use before replacement becomes necessary. There's no charge memory effect, so you can top it off whenever convenient without worrying about degrading the cells.

When the battery runs low, the system gives you warning signs before it dies completely. The junction box indicator light signals low charge first, then the front derailleur stops responding while the rear continues to work. You'll still have 100 to 200 shifts left at that point—enough to finish a ride and get home. Charging takes about 90 minutes from empty using the SM-BCR1 external charger, which is sold separately along with its power cord. The battery slides out of its mount for charging, so you bring the battery to the charger rather than the other way around.... Read More