I knew this was bound to happen. One day, it simply didn’t wake up. After months of trusty service my SRM Dura Ace Powermeter did not connect to my bike computer.
I bought the SRM second hand, it was fully functional, however, I can tell it has been used a lot. Maintenance history unknown, I was anticipating how on that really important time trial I would lose power data.
Well it was less dramatic than that, one autumn day it simply took 15min till the SRM paired up and the next day I got no response at all. Couple of rides before that I would notice the 3s average power acting weird ( dropping to 100-150W when I was doing a steady 220W average) on some occasions. That should have rang a bell that I need to look into changing the batteries.
Being the DIY type and training with power junkie I set up on replacing the batteries ASAP!
SRMs come with two types of batteries – a round lithium cell (mostly older wired units) or 1-2 square 3.6V lithium thionyl batteries. These batteries have extremely low rate of discharge so can be stored for 10 or so years. On one of those lazy weekends I should have opened up my powermeter to see which type it required so when the time came I wouldn’t have to lose a week of having no power data. I always learn the hard way…
My wireless SRM Dura Ace requires 2xLTC-7PN-S4 batteries. The LTC-7PN-S2 is the exactly same unit, however, the wiring terminals are longer and not straight. You can cut and bend them yourself so either type works fine.
In Europe you can find them here under Lithium Primary cells.
In the US here (or Google: LTC-7PN-S4)
DIY SRM Battery Change
Remove the crankset from the bike; unbolt the chainnrings (the small ring holds the plastic cover down)
Carefully lift the plastic lid (some SRM units have wires connected to the lid, DON’T break them!). In my case the lid is not connected to anything so it comes right off. Vóila you see the two yellow batteries. Notice how the wires are pressed against the walls of the SRM. That is why they need the black plastic sleeves.
Slide off the plastic sleeves of the wires; Using a soldering iron disconnect and remove the old units
Re-solder (RED is “+”; BLACK is ground) and slide the plastic sleeves back over the wires. This one was actually tricky since the terminals need to bent towards each other so that they are not touching the walls of the SRM. I couldn’t get the plastic sleeves back on, however, there is about 1-2mm gap between the solder joint and the metal frame. The black plastic cap holds the batteries down so they shouldn’t be moving back and forth. If it ain’t broken don’t fix it.
If you have a small magnet at hand you can “wake-up” the powermeter and see if it can be paired to your computer. Simply move the magnet back and forth the two long blue components.
Follow the first steps in reverse order to re-assemble the crankset. Wipe off any dust/road grit from the rubber O-rings (and add some silicon grease) to ensure that no water can get inside your precious powermeter.
Re-check if the slope is what it used to be (in my case 18.0Hz/Nm). The zero offset value was around 630-650 before the battery change. It was 555 immediately afterwards. I check it before every ride and will report if anything changes over time. UPDATE: After my first ride I checked the zero offset a couple of times (Garmin Edge 500 updates it as well). It started at 559 and finished at 628, so overall the SRM is back to it’s pre-battery change state.
Re-install the crankset and go ride!
Total time was about 1h, since it has been a while that I have used a soldering iron and also getting the battery terminals bent the right sway took some fiddling. It is a tight space packed with (sensitive) electronics so I was overly careful not to break anything or tear any wires.
It was quite easy to do, just remember to take your time or you risk damaging a very expensive unit.
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