11-12Volt Low Battery Detector that cuts on a Switch or relay

Im Looking for a low 12volt car battery detector that cuts on a Switch or Relay.

Purpose: im trying to use car batteries for a motor but when the batteries get low, I want the circuit to cut on a Generator to Charge the Battery, later I will incorporate a timer for the circuit and a overcharge circuit, but for now just a low voltage circuit

Ideas: Transistor switch Circuit of some kind, Comparator? Schematics would be Great....thanks!

This is one circuit I saw, I was wondering can I remove the Red Led light and use a transistor as a switch, but I don't know which transistor to use that would work with that circuit and allow 12volts and enough current to start a Starter, Something like 40 Amps I believe.
http://www.electroschematics.com/6338/battery-level-monitor/

 

Looks like a good plan although others may have an better circuit.

When you decide what relay to use for starting the generator, an added transistor can be recommended.

You will want a lower value for ZD. Close to 10V or what ever cut-in point you want.

 

The starter is 12v ...50 amps max ,

 

What is your plan? Surely you won't be running the starter all the while battery voltage is low?

How do you plan to start and stop the generator?
Or is this "starter" the generator?

 

My plans was to make another circuit that senses current flow one starter wire after a 30 secs or so and switch of the negative wire on the starter(which will be otherwise also on(closed relay)

Whole circuit: Low battery causes transistor to allow power on positive cable of starter and then after timed run of 20-30 secs another relay shuts off power to the other starter wire (neg wire). Leaving the motor running. And starter off.

 

Maybe someone on here will design the whole circuit for you, now that there is more information.

If I was doing it, I would build the circuit as-drawn. Watch the leds, but operate the generator manually, to see if it works properly.

If it does, then a transistor drive can be added that drives a starter solenoid, which follows the led.

I see a small problem, in that solenoid will stay on for the whole charge cycle.
That is easily fixed by breaking solenoid circuit after delay, rather than "make another circuit that senses current flow one starter wire after a 30 secs or so and switch of the negative wire:.

Use an interval timer or timer controlled by oil pressure.

The first step would be getting the voltage sense circuit working.

I'm surprised there haven't been more circuits offered.

 

I would use comparators for this, and control a MOSFET with the comparator output. It's the same idea as the linked circuit - compare a reference voltage to the battery voltage - but I think having a quad comparator such as LM339 will be handy and for instance allow constructing a window comparator. Easier than discrete components. Using an LM3914 might be even more interesting.

You would need a big MOSFET to handle all the current, so it might make more sense to use a small MOSFET controlling a relay instead.

The problem here is trying to control by voltage. As soon as a load is removed or charging applied, battery voltage changes a LOT. Unless you can define precisely the voltage decision points and allow for hysteresis, the system may oscillate.

 

The problem here is trying to control by voltage. As soon as a load is removed or charging applied, battery voltage changes a LOT. Unless you can define precisely the voltage decision points and allow for hysteresis, the system may oscillate.

That would be my concern too. And since this battery is powering a motor there are also likely to be voltage transients to contend with.
What sort of generator do you have that needs 20 secs to start up?

 

Here is an idea that might work. When the battery voltage gets to a little below 12 volts it will turn on your starter relay. Once the generator starts and puts out some voltage higher than a little over 12 volts it will turn off the starter relay. I'm not sure what the starter will think about still being energized after the generator starts.

 

I would look for some other feed back from running motor. Tach signal or such.
I wonder if ripple is detectable?

 

Just one Question about that design...I like it, what is Vbat,- wire that connects to Battery...? and after motor is started I can find someway of cutting power off to starter later ...as of now im just trying to figure this part out....maybe pressure sensor like someone said before, when pressure in exhaust goes up starter circuit disconnects.

 

And Thx for Everyones Help...much appreciated.

 

Vbat is just a label so you can see the waveform above the circuit.

 

Thanks Ronv and others.

 

Ronv, do you have that file saved for the low battery relay with the tl431ed you sent me, I think you named it bat cutoff tl431? if so can you upload it so I can download it to mess around?...

 

Sure, Here it is with the TL431 model.

**********************************************************************************
* Model developed by Eugene Dvoskin "http://www.audio-perfection.com" 02/05/2012
* This TL431 model has been developed from schematic in the datasheet
* http://www.ti.com/lit/ds/symlink/tl431.pdf
* It matches most of DC, AC, Transient, Stability and Noise performance of TI TL431
* No attempts were made to cover Temperature dependences
*********************************************************************************
.SUBCKT TL431ED CATHODE ANODE REF
Q1 CATHODE REF N005 QN_ED
R4 N005 N009 3.28k
R2 N009 N012 2.4k
R3 N009 N010 7.2k
Q2 N012 N012 ANODE QN_ED area=1.2
Q3 N010 N012 N014 QN_ED area=2.2
R1 N014 ANODE 800
Q4 N003 N005 N006 QN_ED
R5 N006 N011 4k
Q5 N011 N010 ANODE QN_ED
Q6 N004 N013 ANODE QN_ED area=0.5
Q7 N003 N003 N001 QP_ED
Q8 N004 N003 N002 QP_ED
R7 CATHODE N001 800
R8 CATHODE N002 800
Q9 CATHODE N004 N007 QN_ED
R9 N008 N007 150
Q10 CATHODE N008 ANODE QN_ED area=5
R10 N008 ANODE 10k
Q11 N004 N004 REF QN_ED
D1 ANODE N004 D_ED
R6 N013 N012 1k
D2 ANODE CATHODE D_ED
C1 CATHODE N004 10p
C2 N010 N011 20p
.model QN_ED NPN(BF=140 Cje=1p Cjc=2p Rb=40 VAF=80 VAR=50 KF=3.2e-16 AF=1)
.model QP_ED PNP(BF=60 Cje=1p Cjc=3p Rb=80 VAF=70 VAR=40)
.MODEL D_ED D(Rs=5 CJ0=4.0p)
.ends TL431ED



Version 4 SymbolType CELL LINE Normal 0 20 -4 16 LINE Normal 32 20 36 24 LINE
Normal 0 20 32 20 LINE Normal 0 44 32 44 LINE Normal 32 44 16 20 LINE Normal 0
44 16 20 LINE Normal 16 64 16 44 LINE Normal 16 20 16 0 LINE Normal -16 32 8 32
WINDOW 0 32 0 Left 0 WINDOW 3 32 64 Left 0 SYMATTR Value TL431 SYMATTR
Description TL431A 2.5V shunt regulator SYMATTR Prefix X PIN 16 64 NONE 0
PINATTR PinName A PINATTR SpiceOrder 1 PIN 16 0 NONE 0 PINATTR PinName K
PINATTR SpiceOrder 2 PIN -16 32 NONE 0 PINATTR PinName R PINATTR SpiceOrder 3

 

I have my resistor at 23k before the tl431 so that my Low Voltage can be 11.9, and im getting 2.5v as reference, and only 1.20v as output... can 1.2v drive a irf1010n that's on this circuit? or is there another Mosfet you would recommend?

 

If you want to change the switch voltage you can vary R2, R3. R4 should remain about the same.
R6 determines the hysteresis - the difference between turn on and turn off. It is in parallel with R3 - it's not obvious in the schematic.
The TL431 performs like a comparator with a 2.5 volt reference in this configuration.