Speedster Redux and the Zeva2 Fuel Gage

This week we go a little bit more in-depth with Ian Hooper's Zeva2 fuel gage driver.  I'm quite enamored of this little device which he apparently is still building by hand.  And improving.

In the latest incarnation, he's solved the fuel gage direction problem by using a 20 turn potentiometer.  The direction is indicated by which side of pot center you are on and the discharge rate, or really capacity of the cell, is indicated by how far from center you adjust it.

There are two other features I'm particularly excited about. First, it provides an instantaneous current indication as a series of pulses designed for a tachometer. A pair of little microswitches allows you to set your pulse count as in four cylinder, six cylinder, eight cylinder, etc. And the device automagically and with NO calibration puts out pulses to correlate 100 amps with 1000 rpm.

We set this up with a switch on the dash to switch either our normal RPM from the RECHARGECAR magnetic pickup on our motor, or this Zeva2 current output. I can switch between them at the press of a button.


I cannot express how much of a difference this makes.  Digital displays of rapidly changing current loads are just not very useful.  They update at the update rate of the display, and the result is just a dancing number, meaningless and without form.  When you put this on a BIG needle on a BIG gage, suddenly, it has all the missing rate and direction information - trend if you will.  We may only hit 1000 amps briefly.  But we'll get to see it.

Actually, as it stands, we won't.  We have a 6000 rpm tachometer and so our max display would be 600 amps.  I have a 10,000 rpm tach on order with Speedhut.

The fuel gage output is a rough AH count.   It uses a hall effect current device to measure current and integrates by summing instantaneous current outputs over time.  I've read endlessly how inaccurate and "drifty" this is.  Methinks they are letting the perfect delay and obscure the good.   Comparing hall effect output at different temperatures to the more conventional current shunt meters such as the TBS ExpertPro, I've found them within fractions of an AH after 100 amphours in and out.  Yes, it differs.  Yes, they drift.    But we're not building a piano here.  PLus and minus an amp hour would be plenty.

But charge and recharge errors can be cummulative over time.  We don't want to "get lost" on our pack SOC.  The answer is of course to reset the counter at the end of a charge cycle.  It would be nice if this were done automatically, but it is easy enough to do.

Unfortunately, the Zeva2 is a little crude in this respect.  You simply remove 12v power from the device and it resets. Also unfortunate, if you hit your maintenance switch to do some maintenannce, it also resets the device and you lose your AH count.

The solution was the addition of three components:  A LiPo battery backup, a diode, and a switch.  The LiPo powers the box when it loses system 12v.  The diode keeps the little LiPo battery from trying to power the car in this event.  And the switch lets us reset the device when we want to.

The Zeva2 is also a bare PCB with the hall effect mounted directly on it.  So we need to get it out of the weather.  That requires a plastic enclosure.  Since the conductor has to pass THROUGH the hall effect device, it also requires a couple of LARGE gland nuts or nylon domed cord grips to pass the cable through the box.

Finally, the Zeva2 provides an adjustable LED warning light driver you can use to turn on an LED when SOC reaches an adjustable level.  This is a five volt pullup with a 1k resistor.  We'll have to use a MOSFET or Darlington or some sort of thing to convert that into a 12v switch to actually do something useful, like disable our controller or switch a resistor across our throttle output to put it in limp mode.

I would prefer the LED circuit to actually provide a 3A relay.  But it is what it is.  At $179, that's pretty good.

In this episode, we also show some progress on Speedster Redux.  Our belly pack worked out pretty well, we have it installed beneath the frame quite securely.  This aluminum structure only projects 1.75 inches down, still leaving us with 7.5 inches of clearance, and provides room for 11 cells.  That's a 36 volt pack at 180AH for 6480 wH or an additional 28 mile range, more or less.

Matt Hauber engineered a front battery box allowing us 18 cells vice the 16 we had at that location in Speedster Part Duh.  And he built two rear battery boxes featuring 11 cells each instead of the 10 cells in each box we had in Duh.

We moved the Soliton1 to an aluminum shelf mounted to the motor and adapter plate.  This shows off the Soliton1 look very nicely.  It also made room for six more cells up on the shelf with a terminal strip and easy access to the cells and connections.  

We're also using EVWorks braided cell straps, with our own M8 bolts and NordLock washers to strap up.  These are working out marvelously.  The straps make a very flat, very strong connection between cells and the Nord Locks have just been a find.  They really grip it - forming a very tight connection that does not seem to loosen at all over the brief period of time we have been using them.  Infinitely superior to the lockwashers we once used.

For 180AH CALB cells, the 80mm straps seem to be the most useful.  Occasionally a 70mm strap is necessary to keep from humping up.

For cabling, we're using some yellow 2/0 from Genuindealz that is really quite good.  It's tinned and intended for marine use.  This makes it just about all you can squeeze into a 2/0 lug, but if we DO do 1000 amps briefly, I want the metal to handle it under that insulation.  It certainly was a pleasure to deal with the 1AWG we used a lot of in the much higher voltage, much lower current Mini Cooper.  But Speedster Redux probably needs the larger, albeit stiffer cable.  It's too bad I can't get this tinned marine cable in the proper International Orange color.

In this week's show, we also show my somewhat crude mounting of a J1772 inlet in the Mini Cooper.  I've grown to like the solid feel of the J1772 Yazaki plug.  We have it on the Texaco Fire Chief Plasma Ball Charge station now.  And we've added an interesting voltage/current/power meter from http://www.lightobject.com to that charge station.

Jack Rickard