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  1. #1
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    At home SRM battery replacment.

    Hi everyone , this is my first post but need your help!

    i currently am running a wired dura-ace 7800 srm with PCV head unit. now...of course..two weeks from
    nationals it decided it was going to run out of battery power and is currently a very expensive heartrate
    speed and time computer. so in the quest to get back into a working state i decided to go down the route
    of replacing the batteries myself well i opended it up this evening and found 2 square yellow batteries
    made by keeper ( 3.5v 750 mAH LTC-7PN-S4 ) < thats the battery code i belive.

    so so far seems pretty easy came apart easy enough (one stripped chainring bolt ) ,
    lets just order two new ones! well not that simple... theres place in my city that carrys
    all kinds of batteries and they do have this batterie....i belive? well this where i need your help.
    the ones they have are identical ( same company and demensions) but they are 3.6v instead of 3.5v . would this be a big diffrence? or any diffrence?.
    if so i need to order some new ones asap ( if you know a link ..that would be epic )

    any one with some adive that would be great! help me please i need this done as soon as possiable as i leave
    in a week and half!

    thanks all!

    will

    (im from canada keep in mind)
    heres a link to what the battery looks like

    http://www.bing.com/images/search?q=srm ... tedIndex=0

    http://www.bing.com/images/search?q=srm ... tedIndex=1

  2. #2
    Super Moderator
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    I've replace my own batteries, both in the crank and the PCV. I have a Cannondale SI spider so different from yours, just one battery.

    Get your battery specs and Google it. I found this, first hit:
    http://www.batterybob.com/product.asp?intProdID=166006

    You need two. You also need a soldering iron (I use a butane one). I would recommend a voltmeter thing to verify that the battery is making the right voltage AND that once you solder it in that the SRM spider is receiving the same voltage, meaning you touch the voltmeter leads to the leads going to the circuit board and make sure the battery's full power is going through to the board.

    Finally I'd recommend using a brush on electrical tape. I used it to seal the solder joints. I have heat shrink tubing also but there's no room to work with that stuff. The brush on electrical tape is great.

    My own procedure detailed below. Now it's a 30-40 minute process, but the first time I was really hesitant and it took maybe 2 hours. I did both cranks I think and one, which I think I did 3 years ago? needs to be done again.
    http://sprinterdellacasa.blogspot.co...r-battery.html

    I also replaced the PCV battery. Much more straightforward:
    http://sprinterdellacasa.blogspot.co...placement.html

    Remember that the volts number is absolutely critical, at least I treated it that way. I don't understand the implications of changing the voltage but my understanding is that it screws everything up.

    The mAh is a measure of how long it lasts, sort of. With the PCV I used a lower capacity battery because the full capacity one is not readily available, at least I couldn't find one. Since I plug in the PCV after every ride the lower capacity has not been an issue.

    As far as measuring the slope (the number you program into the PCV head, like 20.9 or something) I found that the slope didn't change when I changed the battery. If you want to measure it there's a process for doing so. I haven't detailed it in the blog but it's here:
    http://weightweenies.starbike.com/fo...hp?f=8&t=77383

    I used two 25 pound weights (weighed each one separately on a digital scale allegedly good to 1g), some chain (ditto on the weight), hooks, etc. The idea was to have a non-stretching set up to hang the weights. It takes some doing to balance everything with just one person but I got my bike on my workbench, hung the weights so they hung below the workbench, and the numbers on the PCV head magically stabilized. So it works.

    A week and a half is not a lot of time to get everything into the house and soldered up. Depending on your commitment to Nationals you may want to try and source another PCV head.

    Hope this helps and good luck,
    cdr

  3. #3
    Junior Member
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    Thanks for the reply , glad to speak with someone whos done it before ! So in your opinion will the
    3 .6v work ? Is it just like a newer model ? Also i will be having the local computer store
    Install the new batteries! To much risk and if they break it there responsiable thats also a bonus.

  4. #4
    Senior Member
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    3.6 or 3.5 is not going to be an issue. The battery will start it's life at 3.6v but will gradually go down over the life of the battery and the voltage will also change based on temperature. The electronics within the SRM won't have any problem dealing with a slight change in voltage.

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