# Using existing Fuel Guage



## madderscience (Jun 28, 2008)

I believe several of the modern BMS (battery management system) offerings output an analog, 0-5V SOC signal, with 5v == full and 0v == empty. Many newer (but not too new) cars' fuel gauge is actually a 0-5v volt meter, so it can be driven directly from that signal. Elithion's BMS I know does this for sure (it is the one I have). Pretty sure the orion does it too. I think there are add-on modules to the basic manzanita BMS that will do it as well.

If your car is new enough that everything is CANbus, including fuel gauge, and the actual gauge is digital, then you would need to have something that spits out the CAN signal to emulate the fuel level sender, or if you can get at the sender itself and figure out how to hack it to replace its mechanical float arm variable resistor with the BMS voltage input signal that might work. I don't know off the top of my head if any BMSes output CANbus signals in for fuel level designed to replace those coming from a sender unit but it is worth asking around.

If your car is old enough its gas gauge will probably be an analog volt meter using a 0-12V signal. In that case you would need to make an adapter circuit (probably using an op amp or similar) to scale the 0-5V from the BMS to 0-12V. Such a circuit isn't too hard to do: the correct op amp and two resistors with a 12:5 overall ratio will do it.

BH


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## rtz (Jul 3, 2013)

2 or 3 options to drive the stock dash gauges presented in the Jan 30 episode: http://evtv.me/


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## rtz (Jul 3, 2013)

RX8 with all the gauges/lights working:

http://forums.evtv.me/post/instrument-cluster-6780278?pid=1281740292#post1281740292


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## skeeterb (Feb 6, 2009)

The RX-7 I'm wanting to convert will be a mid to late 80s model. I have no idea if it will be a normal electrical or be CAN Bus type gauge. How can you tell if it is a CAN bus type or one that can understand regular electrical signals?


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## frodus (Apr 12, 2008)

What about this:
http://www.evworks.com.au/zeva-fuel-gauge-driver-plus-soc-plus-tacho-ammeter

Hook up volts and run the power cable through it and it'll drive a gauge.


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## mizlplix (May 1, 2011)

I have NEVER gotten mine to work in 2 1/2 years.

I know of two other people who have also had probs. either in not working or not accurately.

To calibrate it you must run your pack down to 3.10 volts/cell (Then tow the car home where your charger is) then set it for empty. Sometime during charging it will go to full and any pack charged after that is above full.

The actual fuel gauge movement is fast and to me not enough warning of emminent fuep depletion.

"............The FGD+ uses multi-turn pots to allow precise adjustment of three parameters:








The position of the needle when the pack is Full







The expected battery capacity (i.e rate and direction of amp-hour counting)







The state of charge level where the low battery warning LED comes on".........

Mine is an early model and NOT adjustable for battery hours or anything else.

I cannot recommend this to anyone. I hate wasting money.


Others exp may vary. (And Yes, I have gotten lots of help trying to fix it)

Miz


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## sirwattsalot (Aug 27, 2012)

skeeterb said:


> I am wanting to convert a Mazda RX-7 and want to use the existing fuel gauge for the SOC of the battery pack. What do I need to get to measure the SOC and can output something the fuel gauge can use to show how the battery pack stands?


This is tricky because the battery voltage drops as you press the accelerator. Even a dead battery may read Full on the gauge and then empty as soon as you start to drive. You need to account for current (I) and voltage (V) and calculate power (P). P=V x I, and compare p to total Ah of the battery to determine the percentage of remaining power by first calculating watt hours used verses total watt hours held by the battery. There is no quick and easy way to accurately determine the amount of charge left in a battery and the total Ah value of the battery will change with age. To be perfectly accurate, you would need to determine the amount of energy that was stored in the battery during each charge and use that value for comparison.

Tricky but, it can be done with some head scratching, a micro processor, some C++, etc. Time is a variable most people ignore in the equation.


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## Thaniel (May 25, 2008)

skeeterb said:


> The RX-7 I'm wanting to convert will be a mid to late 80s model. I have no idea if it will be a normal electrical or be CAN Bus type gauge. How can you tell if it is a CAN bus type or one that can understand regular electrical signals?


80's Rx'7's didn't have can bus or any other types of fancy gauges. The Speedo is still driven by a cable in that car  I've owned a couple and did a conversion (not electric) on one. The fuel gauge should be just a varying voltage. Check the wiring diagram for the car and you should be able to see.


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## jwiger (Oct 18, 2014)

I just bought a full ZEVA EVMS with their current sensor. I will let you know how it works for me. Mine will be a bit different than what you're planning though. 

If you want to use the stock fuel gauge then you need the FGD+ with its adjustable limits. The idea is you run a traction pack cable through the current sensor. With the sensor between your battery pack and your charger/controller/DC-DC converter/(whatever else draws from your battery pack). The FGD+ tracks all of the current going in and out of your battery during charging or use. Supposedly you can hit a reset when you finish charging to reset your gauge to full (when needed for re-calibration).

I'm using an aftermarket gauge and the whole EVMS system, I will share how it goes in my build thread.


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## mizlplix (May 1, 2011)

Then there is this one. I have not used it though, I do run one of their BMS systems. The board is available separately and will drive your gauge.

http://minibms.mybigcommerce.com/products/EV-Display.html


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## Nabla_Operator (Aug 5, 2011)

For me, using the existing fuel gauge as a BMS (SOC) indicator was an unexpected long adventure that started with the question: is this gauge a voltage or current dependant system? What is inside that gauge? Without finding that answer I was able to connect the BMS output with a dashboard from the sixties this way:






(top row = conventional fuel tank connection, with a heat-up bimetal fuel gauge needle. Bottom row = mine, a freaking needle)
(left colom = original i.c.e. car. Right colom = connection with a BMS in a B.E.V. )


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## alvin (Jul 26, 2008)

This might be a little bit over kill, but I use this LCD display that does control my fuel gauge. It counts amp seconds out while driving and then amp seconds back in when you charge. You have to put a DIY kit together.
Here is the link to the thread about it.


http://ecomodder.com/forum/showthread.php/lcd-instrumentation-cougar-controller-16132.html

Alvin


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