Minimalist Pit-Box

Here is the Pit-Box ready to use. I typically clip the bladder clamps onto the back wall of the box, and clip the glow-plug connector into the cap hole so it doesn't trail behind. Note the very heavy-guage floppy speaker wire leads for low voltage-drop (0.1 ohms total).

The front of the box shows a red button, the heavy-current leads connectors and the boost switch.

The red button is a glow-plug circuit tester. While the plug is connected, you can press the button and if there is a circuit, the light will light up brightly enough to see in strong sunlight (1.5 Volt bulb receiving 2 Volts).

The connectors are normal banana type rated at about 5 Amps so that there little chance of a bad connection and little voltage drop from them.

The white cable tie that can be seen holds the cell in place.

The boost switch bypasses the 0.1 ohm resistor in series with the battery which usually drops the voltage at the plug by about 0.3 Volts. With a typical glow-plug with the resistor switched in, you get about 1.4 Volts at the plug (much stronger than the 1.2 Volts from a NiCd "stick" starter). With the resistor bypassed (boost), you get about 1.7 Volts at the plug which really helps keep a bright plug when its getting drenched with fuel from a flooded crankcase. In "boost" mode you need to get the clip off the plug promptly so that combustion heating + high-current doesn't cook your plug.

The inside view (with plastic cover removed) shows (left to right) the bulb housing, the back of the "test" pushbutton, the 0.1 ohm series resistor, the banana plug sockets, the 2 Volt 2.5 Ah Hawker-Cyclon rechargeable lead-tin cell (http://www.batteryweb.com/pdf/Hawker_Cyclon_Manual.pdf - copy available below) and the boost switch. The 2.5 Ah cell should give about 1 hours plug connection time, enough for 25 starts or so. 2 Volts is about the minimum voltage you can use with "standing-start" length leads. So with a single cell, you aren't carrying around the weight of a 12 Volt battery with 5 wasted cells.

There is usually a plastic cover (from the side of a 2 litre milk bottle) over the electrics to stop anything from shorting out with stuff you carry in the pit-box.

Circuit diagram (freehand, sorry!) of the pit-box. Not much to go wrong! It will tolerate momentary dead-shorts at the plug clip without damage.

Circuit for charger - this is the difficult bit (haven't heard of any off-the-shelf chargers for these cells, so building your own seems like the only way to go . The link for the cell details above gives details on how these cells should be charged (using a constant voltage of 2.45-2.5V - here's a copy: Hawker_Cyclon_Manual.pdf). I've used a circuit based on a universal regular I.C. (http://www.national.com/ds/LM/LM117.pdf), but this is a bit complex to put together. But, ...

A simple circuit can be put together from a 3 Volt regulated "plug-pack" (e.g. from Tandy, for a game-boy advance) and a rectifier(diode) or two. The principle is to use the ~0.6 volt drop across the rectifier to reduce the plug-pack's 3.0 volts down to 2.3-2.5 volts. The circuit is simply putting the rectifer(s) in series with the positive lead of the charger with the polarity ring (marked on the rectifier) oriented towards the battery. For the common IN4001 series rectifiers (rated at 1 amp), it may be necessary to put two in parallel to handle the current rating of the 3V plug-pack (up to 1.5 amps). The figure to the side indicates the actual voltage drop across the rectifier (not a constant 0.6V as many would have you believe) and this indicates that, for a flat battery (1.9V) you should get a charge current of 4 amps (but your plug-pack will probably current limit to a lower current to protect itself) which will give maximum charge rate. At 2.2V the current will be 200mA, and at 2.45V the current reduces to a modest 10mA.

This seems like a good charging regime - I'll update this page when its built into my new pit-box. I'll put the rectifiers into the pit-box itself with a charging socket. This, in theory, would allow any 3V regulated supply to be plugged in (including a regulated supply from a car cigarette lighter, for example). Unless the 3V supply has a light indicating "charging", it might be worth putting an LED indicator in the pit-box to show a good connection.

Idea: connect an LED with a high value resistor to the 3V charge connector to show when charger is connected; and an LED with a low value resistor to the battery to show when the battery is charged (LEDs have a forward voltage drop at around the 2-2.4V of a mostly charged battery)). Alternatively, use a milliampmeter in two (switched) roles: indicating glow-plug amps (with a heavy shunt resistor), or battery voltage while charging or glow-driving - this can be done with a single-pole-double-throw switch. We wouldn't want to arrive at the competition armed with a flat battery (done that before ).