I’ve now got enough data on Alfine Di2 to jump to some conclusions about Di2 battery life in general. Some of what follows is still conjecture, hence the jump, but seems reasonable based on the data I’ve got so far. Update: Click here to cut to the chase; a running report of my real-world battery life since January 2015.
I’ve made it through one full discharge cycle and am currently one bar into the second cycle. It took 35 days to completely drain the battery, 25 days until I was down to 1 bar, which seems pretty decent to me. However the amount of time moving during that cycle was a paltry 17 hours. That represents only about 3 days touring at my typical 5 hours per day. Pretty disappointing.
Separating Di2 from the Alfine hub itself, performance has been exceptionally good the entire time. Fast, consistent, silent shifts. Though not my ideal style of shifting, the pushbutton shifter is solid and works well with or without gloves. A huge improvement over the crappy SRAM twist shifters I’ve been using and until a Di2 twist shifter comes along, hands down the best shifter I’ve used on a trike. All of the components including the exposed battery, wiring, and junction box have endured without complaint the rain, snow, mud, and indescribable road sludge that coats everything in wintertime. The display looks and works great attached to my side mirror. The installation shown on the right looks pretty clean even though it was quick and temporary. Even the behavior with the battery completely discharged was well designed. The display went to zero bars and shifting stopped working with the gear stuck in its last position. But after a few seconds a single bar appeared and I was able to shift through all 11 gears before shifting died again. I repeated this about 15 times before losing interest. Granted I might not get as many chances in warmer weather but it looks like even when the battery dies you’ll get at least one chance to jam it into a gear that will get you home. Di2 is definitely a keeper if I can make peace with battery life.
Considering impact on battery life, the display is the most obvious difference between my setup and Di2 configurations boasting phenomenal battery life. I needed to find out how much that pretty display was draining my battery. So I screwed up my courage, squinted my eyes, and took a knife to an incredibly expensive 200mm length of E-Tube wire so I could break out one of the leads and insert an ammeter to measure how much current the system is using. Cutting tiny pieces of wire with a sharp razor while squinting isn’t always the best policy, good that my calling wasn’t a surgeon, but I came out of it without any bloodstains and all fingers intact. In the process I learned that an E-Tube wire is nothing more than two very thin, stranded, individually insulated wires, one red and one black, inside the exposed thick black outer sheath. The outer sheath is easy to cut with a knife but seems resistant to abrasion; combined with the stranded conductors it makes a particularly strong and flexible wire. Add to that the thin, waterproof, click-to-lock connectors and you have a high quality cable, designed specifically for the environment in which Di2 is used, that probably justifies its high price. You could use any 2-conductor cable for Di2 but I’m certain nothing else would work as well.
Now for a little math. I chose not to post WARNING: Math Ahead at the top of the page for fear that readers might just click away to the latest cat video. Consider yourself warned.
The relevant formula for battery life is C = I * T, where C = charge capacity in milliamp-hours, I is current in millamps, and T is time in hours. I can be divided between Idisplay, Ishift, and Ioff, where Idisplay is the current used to drive the display, Ishift is the current used to shift gears, and Ioff is the current used while the system is “off.” Note that the system is never really off, but is in a low power mode while not in use. So the formula becomes:
C = (Idisplay + Ishift + Ioff) * T
Ishift is a short burst of current that is difficult to measure with just the multimeter that I have at my disposal, but measuring Idisplay and Ioff is easy enough. I inserted my handy-dandy breakout cable between the motor and the first junction box in the system and measured current draw with my multimeter. I measured the current while the display was on and off. Then I replaced the display with the cockpit junction box and measured the current in it’s “off” state, which I’ll call Ijunction. Here’s what I found:
|Idisplay = 4.97 mA
Ioff = .015 mA (15 microAmps)
Ijunction = .014 mA (14 microAmps)
A few things become immediately obvious with these measurements. One, in their “off” states, the display and SM-EW90-A cockpit junction box (shown right) are essentially identical. So from the perspective of battery life, the display is the only difference between the two. Two, the current draw while off is small enough relative to the other loads that it can be ignored. With these numbers the system could sit for over three years before the battery is discharged. I’m sure there are other factors that will kill the battery long before that, but you get the idea. Three, the display is drawing a relatively large amount of current. If the system sat idle but with the display on, the 5mA of continuous load would drain the 500 mAH Di2 battery in 100 hours. I’m currently running the display with the default 300 second timeout which means that it is on constantly whenever my trike is moving. In this configuration, 100 hours becomes the absolute maximum moving time I could get, assuming a full 500 mAH of capacity, perfectly efficient discharge, and shifting only enough to keep the display on, once every 300 seconds. So reality will be far less than 100 hours. Take the display out of the equation by either turning it off or using the cockpit junction box instead, and the run time is limited only by Ishift. I don’t have a measurement of Ishift and it is directly proportional to the number of shifts made (averaged over time), but it’s safe to say that battery life will be significantly better with that display turned off as much as possible.
Unfortunately the smallest display time the E-Tube software lets you set is 5 seconds, so I can’t completely turn off the display. I’m sure from Shimano’s perspective this makes complete sense—if a customer doesn’t want the display they should just use the cockpit junction box instead. But in my humble opinion, Shimano needs to broaden their collective mind a bit. I use my trike in two very different modes: Daily commuting and recreational riding where a display is useful and battery life is not an issue, and touring where a display is optional and battery life is everything. In either mode the display unit is mechanically better than the cockpit junction box, so I’d prefer to use it even if the display is blank. It’s better because it doesn’t have the third, unused port and it allows a cleaner install on my handlebar. I’m sure one of the arguments for a non-zero minimum display time is that the display is the only way of learning the battery’s state of charge. Fair enough, that is especially critical information on tour. But I don’t need 5 seconds to read the battery gauge. A half second would do just fine and I’d see it whenever I shift. Still, 5 seconds might also work. Only more time on the system will tell.
Conjecture time. Stay with me.
The bounds of moving time for my current Alfine configuration, display always on while moving, is between the (hopefully worst-case) 17 hours I measured in my first discharge cycle and the (completely unrealistic) 100 hours due to the load of the display. Let’s say that once the outside air temperature increases and the battery breaks in to it’s optimal charge capacity, I start seeing 25 hours of moving time between charges, a number that is roughly supported by results so far in my second discharge cycle. With these assumptions I can calculate the average Ishift with the following formula:
Ishift = 500 / 25 – 5 = 15mA (averaged over the time moving)
Now I have all I need to estimate battery life in various configurations. I’ll use 75% of the rated battery capacity to account for inefficiencies and my desire to recharge once the battery gauge reaches one bar instead of complete discharge, to provide some buffer. So C = 375 instead of 500. With these assumptions I’d expect to get 375 / 155 = 25 hours of moving time between full charge and one bar, if I could turn the display completely off. That represents 5 days of my style of touring with a roughly 20% buffer. Not great but it does meet my initial requirement that I consistently get a full work week of touring between charges. Even with 5 seconds of display time each shift I think I could make this work.
Now for a really pessimistic view. Let’s say that instead of 25 hours moving in my always-on configuration, I never see better than the 17 hours I’ve already seen. That puts Ishift at 500 / 17 – 5 = 24mA. I’d be getting around 375 / 24 = 16 hours before the battery gauge gets to one bar (with still maybe 20% capacity left). Ouch. If this is the best I can expect I’ll have some serious reservations about Di2. But I think this number is excessively pessimistic.
For example, even my most optimistic calculation of 25 hours of run time (display off) is dramatically lower than the lowest numbers I’ve seen reported about Ultegra Di2 (I haven’t seen any numbers on Alfine Di2). This makes me wonder if I’m not accounting for the difference between the two. After the display, the next obvious difference between Alfine and Ultegra Di2 is the motor and shift mechanism. Up until now I’ve assumed the energy required to shift the Alfine IGH is no more than what’s required to move a chain around an external set of gears with Ultegra. However I don’t have much basis for this assumption and currently have neither an Ultegra derailleur to test nor an instrument capable of testing it. If I’m wrong about this, the Alfine itself could be another reason I’m not getting the fabulous battery life that others report. With my Alfine experience so far I may not need it, but this may be another reason to switch from Alfine to Ultegra.
Update: I have now seen a report for Alfine Di2 battery life. Roughly 360 miles in 6 weeks on a charge. The mileage is better than I’ve seen so far.
Update 4/25/2016: With 3 charge cycles under my belt in the Ultegra Di2 configuration, I’m averaging 48 hours moving, 100 elapsed days, and 527 miles from full charge to one bar. Now we’re talking.
All right, so what can I currently say about battery life with any degree of certainty? Really just two things: 1) the battery life I’ve experienced so far does not meet my minimum requirements, and 2) turning off or eliminating the display should significantly improve battery life. Time will tell, but I think battery life for Alfine Di2 will still work out, though probably at the low end of my acceptable range. Given that reports on Ultegra Di2 battery life are dramatically better than what I’m seeing with Alfine, and in the absence of evidence to the contrary, I’m willing to believe that Ultegra Di2 might be significantly better than Alfine. This would put it well over my minimum requirements. Stay tuned as I compile more data, the most recent found here.