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!

SRM Batteries

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)

With a small screwdriver pry the plastic lid open (there are small indentations at each corner.) start with one side and gently go around.

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

Adjust (cut/bend) the terminals on your new batteries to match the old ones.

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.

Good luck!

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While experimenting you will inevitably notice a couple of things. First, your floor pump dial is usually reaaaaly far away for you to see small increments and second, the gauge has no small increments; 5psi and 0.5 bar is the finest you can go…again if you can see it. Of course you can get a nice tyre pressure gauge and pump-check-pump-check again, but I mean, pumping tyres shouldn’t be a multi-step activity.

I was watching the “8 Ball Diaries” cyclocross movie and I saw one of the mechanics there using something that looked like a cordless drill, it had a hose attached to it and he was pumping tyres. The unit had a nice backlit display. Where do I get one of these?!?!?!

What I have seen was the green Bosch PAG unit, long discontinued (as of end 2012) and not appearing on any sites like eBay. Tough luck… But no! Apparently there is an alternative – the Craftsman Cordless Inflator available from Sears in the US or eBay for the rest of the world (price ~EU35/$45). Similar to the Bosch in function with a 200psi (13.7bar) max pressure. The unit’s main purpose is inflating car tyres and various things like beach balls, air mattresses so it came with a hose for a Schrader (car) valve. All accessories attach to the Schrader head, so not ideal for bicycle use.

You can use the provided screw on Presta adaptor, however, neither convenient, nor elegant. I have heard of people simply cutting off the Schrader head off and attaching a bicycle pump head of their choice such as the Silca. Again, not optimal. So with some modification I put a Lezyne pump hose to the Craftsman Cordless Inflator. I have been impressed with the durability and the usability of all Lezyne pump accessories so it is no wonder it was my first choice for my conversion. For this project the Lezyne hose is a perfect fit into the head of the inflator. You can use the standard included Flip Thread Chuck for either Presta or Schrader valves, by just…unscrewing and flipping it. Car or bicycle tyres, no problem. In addition the Lezyne Slip Chuck is super fast and compatible with disc wheels as well! So read on…

The Conversion

• Craftsman Cordless Inflator
• Hose nipple – 4xM5 or similar that you can find in a hardware/air compressor store, go small rather than big sizewise.
• Lezyne floor pump replacement hose with a Flip-thread chuck
• M5/6 steel washer+a similarly (over)sized rubber ring
• Lezyne Slip Chuck (Optional for convenience and for disc wheels)

The Steps

• Unscrew the red cap and remove the provided hose.
• Remove the rubber bushing.
• Cut the Lezyne hose to the desired length; 30cm (1ft) is plenty.
• Slide the red cap onto the hose, than the rubber washer onto the chuck; than the steel washer; insert the hose into the chuck assembly
• The rubber washer should be able to cover the whole front of the inflator
• Slide the red cap over the chuck assembly, it is a tight-ish fit; make sure the rubber washer fits square against the inner surface of the red cap.
• Screw the cap back on; finger tight is enough
• Go pump your tyres and ride your bike! (the gauge reads 2psi higher accounting for the pressure in the hose)

You can use the Speed Chuck so you never have to unscrew the Flip thread chuck should you need to pump up a disc wheel.

So far I pumped 4x car tyres form 22 to 32psi, 4x2in wide bike tyres from nothing to 60psi and I have been using the inflator daily on my bike. Battery is still going strong.

NOTE: The Craftsman Cordless Inflator does NOT come with a battery or a charger so you need to order that separately, it costs 2x the price of the tool. I recommend the Li-ion battery. Also the charger is 120V ONLY!!! Get a converter before you connect it to a 220-230V socket!

P.S. I wondered what is the difference between the Red and Yellow Flip-thread Chucks. All I could see is that the internal parts of the Red one are metal rather than plastic so in theory they should be more durable.

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