Words and Images by Kraig Willett
In 2002 I bought a used SRM Pro powermeter. I’ve had the crank battery replaced once in the past 9 years (less than 20,000 km). Since day one, the powercontrol (the head unit) battery has been a bit problematic. First of all, the display that supposedly shows how much battery life was remaining was unreliable. For example, the PCV display might have said that I had 25 of 29 hours left on the charge, but inevitably, it would run out after only a few hours. Secondly, and probably related to what is mentioned above, is that the battery just wouldn’t hold a charge for very long.
As a result of this type of battery behavior in the powercontrol, I was trained to simply recharge the battery after every ride. No biggie from my perspective. A couple years ago, I sent in the PCV to have the battery replaced in hopes that I’d get a bit more reliability. At a cost of probably $75 plus shipping and a down-time of a couple weeks I was hopeful that my battery woes would be solved. Alas, the battery performance did not seem to improve. A bit disappointed, I had a nice long chat with Mike Hall of SRM during Interbike 2010 about a variety of topics and mentioned my battery issues. He kindly offered to make things right…and, well, one thing led to another and I never took him up on the offer.
Fast forward a year, and well, my PCV battery has recently taken a pretty serious turn for the worse. It hasn’t died during a ride in awhile (keep in mind that my long rides are in the 90 minute range), but if I leave it unplugged from the charger for more than a day or so the screen goes blank…a dead battery!
I’ve been working on my electronics skills lately, and so figured I’d give the PCV battery replacement a try while saving myself $75 (or more) in the process. The following words more or less describe how I did it.
DISCLAIMER: hey, if you try this based on my crib-notes and things go wrong – it’s your fault, not mine, OK? If you feel unsure about doing any of what is described in this article, spend the $$$ and send it to an authorized SRM service center. I was bored on a rainy So-Cal morning and had nothing better to do…plus, I’ve got serious soldering skillz, OK?!? Do not try this at home, kids!
Tools/supplies I used:
Phillips screwdriver, Dental pick, solder, soldering iron, electrical tape, replacement battery, wire cutters, wire stripper, razor blade.
Here’s the battery I used – it doesn’t have traditional leads like one might expect, but rather, it has connectors used for through hole soldering to a PCB (printed circuit board):
This particular battery is only an 80 mah NiMH battery, while the original SRM battery is 110 mah (which means it only will operate for ~75% as long before re-charging).
If you want your final product to be more pro than what I wound up doing, then you should pick up some silicone caulk, and some heat shrink tubing.
Unscrew the plastic powercontrol case using the Philips screwdriver, being careful not to dislodge the thin o-ring or rip the thin ribbon cable that connects the top of the case (and its buttons) from the bottom of the case. Open it up and lay it down on your workbench like it’s a book, with the guts of the powercontrol facing up. Did you know that the internal surfaces of the top part of the case are CNC machined plastic? Nope? Neither did I – no wonder they are so expensive! In a glancing light, one can see the flat end mill machining marks on the internal surfaces. I guess I would have thought that case was injection molded.
Remove the ribbon cable from the PCB connector by gentling wiggling and gently pulling. You don’t necessarily have to do this, but I didn’t really want to take the chance of melting that sucker accidentally (even though I’ve got mad soldering skillz!!!) during subsequent steps.
You should now have a top cover that looks something like this:
Pry out the old battery from its silicone prison. The original battery is held onto the bottom portion of the case with a dab of silicone. Gently pry it using your dental pick, or I guess a small std screwdriver might work. Be sure you pry from the lower thick walled portion of the case, the side walls of the case are thin-walled and I can see how one might break the case if using those as a lever point. You should now have something that looks like this:
Using a razor blade, trim off the outer plastic shrink wrap from the old battery exposing more of the red/black power leads and then cut the leads as close to the old battery terminals as possible.
As you can see, the new battery is roughly 30% smaller.
Bend the leads on the new battery to make installation into the case and the long/short leads from the PCB/PCV match up (the red/positive lead is longer, so place that end of the new battery towards the bottom of the case).
Strip the power/ground leads from the PCV and tin both ends, plus both ends of the new battery. Solder the connections up (and use heat shrink tubing for the pro version). The PCV display should come to life, if it doesn’t, check your soldering.
I wrapped the exterior of the battery up with some electrical tape to keep it tidy. You could use additional heat shrink tubing here, too, I reckon.
With the battery still out of the case (this gives you a bit of much needed thumb room), re-insert the ribbon cable from the top part of the case. This was tricky. I lined things up like it was just about to be all closed up (the pre-bend on the ribbon cable was left in a natural position that way) and then gently grabbed the end of the cable and wiggled it home. Be patient, it took me a few tries to get it seated.
Re-insert the battery (using additional silicone for the pro look) making sure all the wires are tucked in nicely and not interfering with the ribbon cable (the dental tool is nice for this).
Make sure the o-ring is sitting correctly on the base of the case, and then screw things back down. If you really want to be pro, you could probably find some di-electric grease to make things a bit more waterproof/sealed.
Charge up and go ride your bike!