Energy storage is the weakest element of any otherwise well-designed electric vehicle. The person who comes up with a much better rechargeable battery than what we have today will be very, very well rewarded. Compared to, say, a gas tank, batteries are very heavy, bulky, expensive, and slow to refill.

Battery technology is being driven not by large, high-energy applications like vehicles, but by portable electronics like cell phones, laptops, and toys hobbies like radio-controlled model aircraft. This means that if you want to use the latest technology for a vehicle, you tend to need to hook up a large number of small batteries intended for smaller devices. This introduces more labor costs, and every connection introduces electrical resistance and mechanical vulnerability. Charging and discharging large series of batteries in a safe, controlled way is then another problem. It’s hard to find stable, cost-effective sources for such technology, especially for small-fry startups like Cleverchimp.

tubepackStarting with Xtracycles as a platform has made more things easy than hard in designing Stokemonkey, relative to regular bikes, but coming up with good battery offerings has still been tough. After trying lots of different things, I’ve settled on nickel-metal hydride packs in two sizes: 324 and 468 watt-hours. These are good for about 40 and 70 minutes of high-rate discharge, which riders can portion out over the course of rides of any length. Customers can get singles or multiples of these packs to suit their needs best. The packs consist of thirty cells in series, spot-welded together and shrink wrapped.

I took it as a sign from the universe that when arranged in a near-cylinder, these packs fit just about perfectly in standard (cheap) 4″ ABS pipe for physical protection (several packs in less robust enclosures failed from physical shock in testing). I drill out the packs for ventilation and cooling, and cap them with standard rubber pipe caps for bumper cushioning. An adjustable strap makes carrying the packs for off-bike charging and theft prevention easy. The prototype shown in the photo below isn’t quite indicative of the production design, but it’s close. The packs fit in the Xtracycle’s inner velcro-flapped pouches, or right out in the main cargo slings. They can be lashed in any opportune place as well, including in the front triangle.

One of the sweeter touches are locking Neutrik Speakon connectors–far-and-away the nicest available for applications like this. One 4-pole connector serves for both charging and discharging.

I’m planning to import extra packs, and sell them independently of Stokemonkey to anybody with 36-volt needs. Ordering a surplus will help me get my sea-freight volumes up to the economical minimum, permitting smaller and more frequent shipments of other Stokemonkey components.

6 thoughts on “Batteries”

  • Mauricio Babilonia
    Mauricio Babilonia August 17, 2005 at 3:05 am

    Gotta wonder what kind of batteries a Prius or Insight would use…

  • Todd

    They use the same kind: Nickel-metal hydride (NiMH). About 10 times more of them (plus a gas motor, of course), reflecting the vastly higher energy requirements of multi-ton, high-speed vehicles.

  • Mauricio Babilonia
    Mauricio Babilonia August 18, 2005 at 3:54 am

    Okay, so I knew that they use a NiMH battery, but do you think they’re using something like 300 D-sized cells? Wouldn’t they use a smaller number of larger cells?

  • Mauricio Babilonia
    Mauricio Babilonia August 18, 2005 at 3:59 am

    Well, as it turns out, most everybody but Toyota is using D cells:

    Ford will use batteries from Sanyo. 250 in the pack. They are the “D” rechargeable NiMH type, literally the kind you can buy off-the-shelf (with high-cycle life). Those “D” batteries are shrink-wrapped together in series of 6, called a module. Honda uses the “D” type too, 120 of them from Panasonic shrink-wrapped the same way. Only Toyota uses a propriety design (supplied by Panasonic), which offers tremendous benefit; the energy-density is higher than the ordinary NiMH, the form-factor is smaller, and it has the ability to ignore a single cell if it fails. There are 28 rectangular modules internally arranged with 6 cells, each independently (non-series) providing the same voltage (but more amperage) as a “D” battery.

    Check out the photo


  • [...] Nearly a year ago, I discussed Cleverchimp’s battery packs. I say “Cleverchimp’s” instead of “Stokemonkey’s” because I conceive of the batteries as a commodity rather than part of Stokemonkey. Stokemonkey users shouldn’t have to use Cleverchimp’s batteries any more than drivers of Ford cars should have to buy Ford gasoline. We sell batteries solely because nobody else is offering them with suitable performance and physical characteristics at a reasonable price. That’s a little ridiculous, because there’s nothing exotic about these requirements. Anybody operating an electric bicycle, scooter, or similar has certain broadly similar desiderata. Meanwhile, we spend an inordinate amount of time sourcing, testing, rejecting, re-sourcing, and assembling batteries. [...]

  • Silvano Presciuttini
    Silvano Presciuttini April 30, 2007 at 8:01 pm


    Very nice and useful page. I am studying the idea of replacing the battery pack of my hybrid Mercedes bike (whose replacement part is probably not available anymore) with something like that depicted here. The original pack is rated 24V 5Ah.

    May I ask you for suggestions?

    Many thanks,


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