How To - Wired SRM Powermeter Battery Replacement

So...

I have an SRM.

(waits for applause to quiet)

SRM calls the cranks and the computers two different but similar things. The cranks are the Power Meters (because I guess they measure power) and the computer on the bar is the Power Control. I don't know why, but it's the only place you can click a button or something. So it's the Control.

For short, the cranks will hereforetowith (that's lawyer speak according to a non-lawyer) be called the cranks or the SRM. The computer will be called the Power Control V or PCV or head unit.

I have a wired SRM. That means I don't have the "wireless SRM" (cranks) or the Power Control VI or VII (PCVI, PCVII). I have the regular wired cranks and the PCV.

Okay, SRMs have a bunch of things going for them and one thing not. The good things are all the stuff that you never want to deal with, the stuff that you don't realize you want until you don't have it. Some things you want in a power meter include things like accuracy, ease of use, reliability, ease of data download, data analysis, and durability.

There's one bad part: batteries.

The PCV head unit takes a fancy rechargeable battery pack that is no longer manufactured. Seriously. Someone is getting them made kind of on the side, but you seriously cannot buy them anymore. The battery pack, from the guy who does it on the side, costs $10 per battery pack plus $10 shipping for however many batteries you get.

The cranks take a non-rechargeable battery. Depending on the crank, you may find a round battery, a rectangular one, or two rectangular ones. There may be other variations, but the moral of the story here is simple: before you order a battery, figure out what's in your crank. Then order that one.

Luckily someone still makes the crank batteries.

As anyone with an SRM will tell you, when you start riding, you need to reset some variable thing. I have no idea what it is, but the crank tells the PowerControl some number and when it's stable you hit Set. The full version (it takes 5 seconds) is as follows:

1. You wake up the crank (spin the crank back a revolution or two) and wake up the head unit (spin the back wheel or hit a button on the it).
2. You hit Mode and Set at the same time. You get smaller font number down low - that's the head unit's value for this variable. And you have a bigger font number in the middle - that's the crank's opinion on what the variable should be in the computer.
3. When the crank number (the larger font) stays still, you hit Set. The lower number changes to whatever the upper number displayed.
4. Hit Mode to exit. You're done.

Now, a little while ago, I was getting ready for a race. I woke up the crank and head unit, went and hit the Mode + Set, and watched the crank number.

It kept moving. Like a lot. Like up another 100-200-300. Normally it'd be in the 250 range. This time it was bouncing up to and over 800.

On my rides I'd get some crazy power numbers. 3000w max. 1200 average. Etc.

A good way to tell if your crank batteries are dying is see if the crank number keeps moving in one direction. That's my experience. If it does, you need to replace the battery.

Waitaminute.

You'll point out that I said up there that the one bad thing about SRM are the batteries. And you're right, I said that. The SRM units are not necessarily user friendly when it comes to battery replacement. So, at least in my mind, they decided to do the work themselves. If you want to replace the battery, you need to send the unit back to SRM.

However, if you have relatively still hands, can work with small parts, know how to solder, and have the guts to put a solder near your $2000 cranks (that's a guess on current eBay prices on the crank), then you can do it yourself.

This covers only the crank. I'll do a head unit later.

First order of business - find out what battery you need for your crank.

My initial foray into my SRM spider.
I realized those holes are for lightening. Cool.

Open the case (use a thin screwdriver to just pop off the white casing). Carefully examine the guts. You should see a battery like the one above, maybe two, or a round battery that resembles a stack of quarters inside a cannister.

Now Google the battery specs. You'll see some sites come up. I chose one. There may be others.

I opened it up more. Cool.
Note large o-ring on big part of crank, small o-ring hanging off of white cover.

Now wait for the battery to arrive. You have to wait because the batteries cannot be air shipped. It's got to pound pavement all the way to your shipping address.

I ordered a couple since I have a primary and backup SRM system. Both, mind you, have dead or dying crank batteries.

Soldering at work.

Note my various tools, from left:
1. Soldering torch. Radio Shack, if you must know.
2. Black oven pan, so I don't drip hot solder onto desk pad.
3. Moving up, some electric solder from Radio Shack
4. Box above that batteries came in.
5. Little batteries (two of them) in a plastic bag. The sense of scale here is important. These suckers are little.
6. Clear tube is the cap to the soldering torch. Important because it holds the flint to light the soldering torch.
7. The crank arm.

I use a butane torch because no need for electric outlets. No chance of touching soldering iron to wire. No electric burning smell by default. Plus refilling them with butane is cool.

I had to put the pen there to hold the battery in place.

There are these tiny brass looking wire things that loop around the spider. Don't mess with them.

Basically you heat up the wire. The solder melts and the wire pops off.

Then the hard part. You need to heat the wire again for the new battery, touch the solder to the joint, let it flow, and you're done.

Don't dwaddle with the soldering iron else you'll keep it hot, the solder won't set, and the wire just pops off.

Next time I'll use some kind of clip. I got stressed with the balancing act I did to hold the wire, battery, and everything else in place.

Although SRM checks the battery to make sure it's up to snuff when they install a new battery, I don't. I don't have the tools to check the charge/discharge and other stuff they do when they install a battery. I guess I can always redo it if I do it myself. But to pay someone else to do it, well, they have to get it right on the first try.

I started thinking of ways to mod this, so I could either install a rechargeable battery (and a recharge port) or adapt the connectors so there'd be no soldering.

I didn't think of anything so I didn't do anything. But I'll keep thinking.

Someone else, either SRM or someone, is trying to make a transmitter that would pick up off this wired arm and transmit Ant+ protocol up to the head. That would be totally awesome.

Make sure you remember which battery is dead and which one is not.

Luckily the new battery has some extra mounting pins on it, else I might have soldered in the old battery. That would really suck.

Getting closer. Magnifying glass so I can see minute details.

It got tedious, especially with the hot torch. I ran out of time at work and had to head home, the crank carefully transported with a half connected battery.

Once home I redid both connections and popped the cover on. The white cover is a bit stubborn so it takes a bit to pop on. The o-ring in the center doesn't help any. Make sure both the outer and inner o-rings are seated properly. Press firmly around the perimeter of the white cover.

Ultimately the small ring holds it in place, but until you get the rings mounted you need to get it at least close to in place.

And... well, I could use the same picture for before and after.

So... now for a month or two of testing. I hope this works. First indications look good, but I need to wait for some bumping around and stuff before I call it a success.

I actually did all this work in mid-September. About 3 weeks later, so far so good. Wattage is back to mortal numbers. I'll see how it continues. If things go well, I'll do the other spider during the winter sometime.