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OT: Battery geeks ahoy.... I need some assistance

davisdavis Posts: 2,566
edited March 2013 in Commuting chat
I've kind of exhausted my ideas here:

I went skiing a couple of months back. I got agonizingly cold feet. I've always had fairly poor circulation, but this was enough that I simply didn't want to carry on skiing.

I've just ripped apart an old toaster, and embedded about 500mm of the heating element (nichrome wire) in a cheap insole with the help of a scalpel, and hooked it up to my Fenix bike light battery pack. It's got 2x 18650s in series, so 7.2v nominal. It gets to what I'd call "warm-ish" (pleasantly warm when held to your face), so hopefully it should help stave off the abject misery.

Approximate readings are 30 ohm resistance, and about 0.3 A flowing when run off the battery pack. Now, according to my maths, that means it should run for about 9 hours off the 2800 mAh battery pack.

Problems:

I'm worried that it's not going to put out enough power. That's only about 2W it's pushing, so I'd like to think about putting more volts in. Do I think about step-up transformers, or stacking more batteries?

Are 18650s the way to go? At this rate I'd need to charge all four batteries every night.

How well do these batteries cope with the cold? I suspect that skiing temperatures don't do any favours at all for batteries.

Now, I'm obviously going to need two battery packs (one per boot), so does anyone know where I can get really well made dual battery packs?

Any other ideas?

And, yes, I've tried warmer, better socks, and my boots are not too tight... I've apparently got awful circulation.
Sometimes parts break. Sometimes you crash. Sometimes it’s your fault.

Posts

  • thistle_thistle_ Posts: 5,157
    What kind of battery life do you want?

    9 hours is a long time for a bike ride and you could probably use 2 heating elements in series to get more heat for a lower runtime (as long as 2 elements isn't too hot, you don't want numb feet and melting boots)

    If you want a 9 hour run time, I'd double the batteries and double the elements. I think the Magicshine battery packs are 4x 18650 batteries, 2 in series in parallel giving 7.2v but they also have a voltage regulator circuit which I think is because you have batteries in parallel. If you've got 2 separate battery packs of 2x 18650 in series you might not need one.
  • wgwarburtonwgwarburton Posts: 1,863
    davis wrote:
    I've kind of exhausted my ideas here:

    I went skiing a couple of months back. I got agonizingly cold feet. I've always had fairly poor circulation, but this was enough that I simply didn't want to carry on skiing.

    I've just ripped apart an old toaster, and embedded about 500mm of the heating element (nichrome wire) in a cheap insole with the help of a scalpel, and hooked it up to my Fenix bike light battery pack. It's got 2x 18650s in series, so 7.2v nominal. It gets to what I'd call "warm-ish" (pleasantly warm when held to your face), so hopefully it should help stave off the abject misery.

    Approximate readings are 30 ohm resistance, and about 0.3 A flowing when run off the battery pack. Now, according to my maths, that means it should run for about 9 hours off the 2800 mAh battery pack.

    Problems:

    I'm worried that it's not going to put out enough power. That's only about 2W it's pushing, so I'd like to think about putting more volts in. Do I think about step-up transformers, or stacking more batteries?

    Are 18650s the way to go? At this rate I'd need to charge all four batteries every night.

    How well do these batteries cope with the cold? I suspect that skiing temperatures don't do any favours at all for batteries.

    Now, I'm obviously going to need two battery packs (one per boot), so does anyone know where I can get really well made dual battery packs?

    Any other ideas?

    And, yes, I've tried warmer, better socks, and my boots are not too tight... I've apparently got awful circulation.

    No experience with your specifics (I have good circulation, for which I'm thankful) but my thinking is that that's probably not enough power. Fischer, amongst others, make boots designed for their own heating system- it'd be worth trying to find the specs for those, as they'll presumably give you a fair idea of what'll work. I had a quick Google but didn't find any useful numbers- a bit more determined searching would probably get you a user-manual for their boot heaters that ought to have tech-specs.
    You're right that the batteries will suffer at low temperatures. The Fischer ones seem to be designed to mount above the back of the ski boot, presumably both to minimise the length of wire and to be kept warm by being close to your calf, inside your ski-gear. That seems a sound choice, if you can engineer it.

    Hope this is helpful.

    Cheers,
    W.
  • davisdavis Posts: 2,566
    No experience with your specifics (I have good circulation, for which I'm thankful) but my thinking is that that's probably not enough power. Fischer, amongst others, make boots designed for their own heating system- it'd be worth trying to find the specs for those, as they'll presumably give you a fair idea of what'll work. I had a quick Google but didn't find any useful numbers- a bit more determined searching would probably get you a user-manual for their boot heaters that ought to have tech-specs.
    You're right that the batteries will suffer at low temperatures. The Fischer ones seem to be designed to mount above the back of the ski boot, presumably both to minimise the length of wire and to be kept warm by being close to your calf, inside your ski-gear. That seems a sound choice, if you can engineer it.

    Hope this is helpful.

    Cheers,
    W.

    Cheers Buns, I think you've helped crystalize my thinking, and also led me to find these, which I hadn't seen before, despite Googling. They're £175, which leave some scope for engineering goodness.

    I can't find many technical specs on these, but they up the voltage to 8.4V. So, I suppose if I go for 3 x 3.7V protected Li-Ion 18650s, that should give me a reasonable amount of grunt (I think I need it; I have numb toes here in the office), and I can probably add some sort of voltage controller to drop the power if required. I suppose 6 hours would in fact be plenty of time. I can always switch it off when I'm having a Vin Chaud!

    cheers
    Sometimes parts break. Sometimes you crash. Sometimes it’s your fault.
  • tgotbtgotb Posts: 4,714
    I wouldn't worry too much about batteries suffering from the cold. Zinc-based and alkaline batteries suffer significantly, but Lithium-based batteries are much better at low temperatures. If you want to research it, it should be possible to find internal resistance/temperature curves (internal resistance is the root of the problem). From what I remember, Lithium-based batteries are pretty much unaffected down to around -30...
    Pannier, 120rpm.
  • kelsenkelsen Posts: 2,003
    How about these...

    http://www.amazon.co.uk/Warmthru-Rechar ... 376&sr=8-7

    £96 if ordered direct from seller according to one of the reviews
  • kelsenkelsen Posts: 2,003
    BTW, this wouldn't happen to be you would it davis?

    in-his-workshop-tony-stark-robert-downey-jr-works-on-a.jpg
  • davisdavis Posts: 2,566
    kelsen wrote:
    BTW, this wouldn't happen to be you would it davis?

    in-his-workshop-tony-stark-robert-downey-jr-works-on-a.jpg

    Actually, I did end up strapping the battery pack to my ankle when testing it. I looked like some kind of persistent offender....
    Sometimes parts break. Sometimes you crash. Sometimes it’s your fault.
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