I had started to build my own smart charger but made one major mistake with the MOSFETs so I threw out that design and went simpler. I converted the chargers that came with the batteries to trickle chargers based on some advice from a coworker who pointed out that more often than not, I just want a charged drill when I need it, and rarely will need to charge the battery quickly.

Rechargable batteries are rated in terms of Amp-Hours (AH) or milliamp Hours (mAh). Theoretically, a battery rated at 1AH can deliver 1A of current for 1 hour, or 2A for 1/2 hour, or 1/2A for 2 hours, and so forth.

The product of Amps and time is the Coulomb, so the capacity of the battery is sometimes written as C. To charge the battery one must put back the number of coulombs that were taken out (plus some for transfer loss). Charge rates are usually expressed as a rate of C. Most cheap drills I’ve run into have battery packs rated at 1200mAh. To charge the drill I could do 1.2A for 1 hr (which would be a rate of C), or .6A for 2 hrs (C/2), or 2.4A for 1/2 hr (2C). Practically speaking though, the cheap batteries need a lower charge rate for a longer time, such as C/2 or C/4.

The chargers that come with the drills are actually just a transformer to limit the current. The only electronics are a transistor, a resistor, and an LED to indicate the current is flowing. The transformer limits the current, so if you have a 300mA transformer you need 4-5 hours of charging at C/4. Leave it any longer at that rate, and you’ll be hurting your batteries.

Drop the rate really low, to C/20 (~60mA) and you can keep the current on indefinately. It will take a long time to charge (20-24 hours), but you never have to worry about pulling the drill off. This is how most smart chargers work, they give the battery a high charge until it’s done (ie at C, or C/2), and then drop to a low charge like C/20 to keep the battery topped off.

Converting an existing charger to a trickle charger is actually very simple. There is a part called the LM117 that’s an ajustable 3 terminal regulator. Normally the LM117 is used to provide a constant voltage given a variable input, but it can do other things. The circuit I used is even in the datasheet, it makes the device a 50mA constant current source. That is, no matter what voltage you put in, the LM117 will pump out that voltage at 50mA. You have to apply 24 ohms across terminals 1 and 2 (read the datasheet for which pin is which), for which I used two 47ohm resistors in parallel soldered directly to the lm117. Throw that in between the battery and the source, and you’ve got a constant current source.

The parts are easily available and should cost no more than a few dollars. You might even be able to order the LM117s as samples, or go to a local hobby shop to get it. Glitchbuster is a great place to order stuff from too.

Be careful when working with batteries. This only works for NiCad and possibly NiMH, and not LiON, and certainly not alkeline.

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I’ll be the first to admit I don’t know enough about the protocol, but what always struck me as odd was that there’s no multihoming for enterprise customers. Only carriers get to multihome (advertise their prefixes out multiple providers), and by strict interpretation anything longer than a /46 is to be dropped (the default allocation for companies is /64).

The talk went over some of the barriers to IPv6 adoption, and one of the reoccuring themes was lack of multihoming. For some companies, this is essential since the Internet is their only revenue stream. So maybe it’s not that multihoming isn’t available, it’s just that there is no good high availability solution for enterprises, and in our IPv4 mindset we think of multihoming.

The argument against this is that it goes against aggreagation. If a network is to be multihomed, it can’t be aggregated, and there’s one more prefix out on the Internet. Already on the Internet we have almost 180,000 prefixes on the Internet. Increase the address space by a factor of 2^96 and routers will be overwhelmed.

The counter argument to that is largely that it’s possible to build routers that can outpace the growth of the Internet routing tables.

Regardless of the arguments, I’m happy to see that groups are taking a pragmatic view of IPv6 adoption, and looking to overcome the hurdles.

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I’ve always thought along these lines, but my recent trip to see a large call centre in action reinforced it.

It’s difficult to gather my thoughts on this issue and explain, so start here:

The End of The Middle – a condensed version of the famous Rise of the Stupid Network

Tim Denton wrote some papers on the same subject.

Traditionally, the voice world has been dominated by big iron, running on leased lines, and making the network the intelligent part. Think of how dumb your phone is… You pick up the handset, the other end sends you a dialtone. You press buttons that make music, the other end decyphers it into a number. The other end figures out where to send the call. The other end rings the other person’s phone.

The old world is only good at building channels from person A to person B. Anything else.. Call waiting, voice mail, and call hold, is done on the PBX end.

Compare this to the Internet. Your web browser speaks HTTP. The web server speaks HTTP. You talk. The network doesn’t know what you’re doing, nor does (or should) it care. It’s just a packet.

Bring this into the voice world.

An organization needs flexibility and to be cost effective. TDM lines take weeks to be installed. PBX programming is complex. Getting into the SS7 integration is expensive and even more complex.

Data connections are built and rebuilt thousands of times a second. All you need are two endpoints, and it’s there. An organization that needs to connect two voice endpoints over its WAN just needs IP addresses.

Moving the PBX functionality to the network, and onto a piece of open hardware allows tighter integration of existing software into the system. Suddenly you have access to everything you need. You have the flexibility to easily monitor the system in real time. You can move agents between queues or change queues in a flash.

While I am rather PBX naive, I know that a lot of these things can still be done on big iron. But I know enough that the stupid network, the one that just flips packets, is a better bet than one that has all the smarts in the centre.

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Hopefully, the third time is the charm. I’ve put up the EngineeringWiki, built on WikiTikiTavi.

If you’re not familiar with a Wiki, it is a web site that can be updated by anyone. Each time you type a term with WordsSmashedTogether (like that), it makes a link to that page/node. The first time you type in that term, it creates the page. It also has its own markup syntax, which is designed to make it easier for people to add content.

I’ve gone through and added a bit, with the intention of adding more. If you’re an Engineer, are thinking of becoming one, or are in the midst of your studies, go ahead and add stuff. The emphasis is on content, not looking good (which is par for the course in Engineering).

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