Auxiliary 12 volt battery switch.

[Danie]

This is my first post to the Forum so please bare whit me if I’m doing things the wrong way.

I want to switch a 70 Amp automotive relay ON to charge a second battery in my truck when the engine is running and the voltage on the first battery raised to ± 13 volt. I did build a circuit with a LM741 OpAmp but the relay oscillate like crazy when the OpAmp output goes high.

I looked at the topic “12v Battery Low Voltage Monitor” then realized the LM431 circuit could do the job for me if inverted. If I’m correct the relay will drop out as soon as the voltage drops below the preset value.

I’m a rooky in the electronics world and hereby request your help.

 

[mbarazeen]

your circuit may be correct, but the problem i feel is the hysterisis setting of the OPAMP, when you switch the 2nd battery obviously the voltage of the running battery comes high again, so the circuit again reconnect it to 1st battery then imediately to other...it continues and you get such osscillation.

modify your circuit only to trigger one time, or to give you an indication so that you will manually change it from one to another.

Edit: i understood your post wrong, since you need change over when the 1st battery is charged. so check wheather your input to the realy is stable or not, also make sure you use a freewheeling doide across the relay, drive the relay through a transistor after OPAMP.

 

[shortbus=]

A voltage comparator would be better for this application than a op-amp, as that is what they are for. A lm139 is a automotive use comparator. But hysteresis is still needed. Data sheet; **broken link removed**

 

[crutschow]

The problem is that when the aux battery is connected, the first battery voltage may drop well below the comparator threshold, especially if the aux battery is significantly discharged, causing oscillations.

To cure this you could add a latch circuit to the relay so that once the relay is energized it won't open until the motor stops (infinite hysteresis). If the relay coil and latch are powered by the ignition switch, then the relay will open when you turn off the motor.

 

[MikeMl]

Danie, here is a circuit using an LM431/TL431 that should work. It can drive a contactor with a 350mA coil. The plot shows current through the coil vs rising battery voltage (X-axis) for different pot positions. The circuit has hysteresis based on the 2.2Meg resistor R6. Reduce R6 it if you need more. Wire it downstream of the ignition/accy switch so it doesn't kill your truck battery when it is parked.

I would adjust it to allow the truck battery to come up to ~14.0V before you parallel your auxiliary battery. Then adjust the hysteresis R6 so that the circuit doesn't drop out when the aux battery is switched in...

 

[Danie]

Thanks for all the replies.

The relay in the LM 741 circuit was oscillating on it's own or with only a 12 volt flicker globe as load on the bench and in the truck. The oscillation starts when voltages on pin 2 and 3 are equal (6.6 volt) and stop when the supply voltage rises by ± 0.5 volt. The relay then stay on. The same happens when the voltage goes down after the engine stopped.

Mike I will buy the parts and build the LM431 circuit over the weekend, I will reply when done. What software are you using to make the drawings?

 

[bryan1]

Hi Danie,
First welcome to the forum . Generally when a second battery is used on a vehicle it is for use when the engine is off to power DC equipment like a fridge etc. There are many commercial products that all basically do the same job which is :- when the vehicle is running the primary battery just does it's job of running the electrics in the vehicle, the second battery is charged and monitored for full charge then it is trickle charged to keep it topped up. As soon as the ignition key is turned off the primary battery is cut from any circuit and the second battery is the power supply. The circuit will monitor the battery and cut out at a pre set voltage.

Now when the vehicle is started the primary battery is still in full charge and the circuit will look at the second batteries charge state then determine the charge needed.

The siliconchip magazine has done several projects devoted this and are well worth checking out.

Regards Bryan

 

[Danie]

Bryan

Thanks for your reply, do you have a link or web page for siliconchip magazine?

 

[bryan1]

Hi Danie,
The article I was thinking of is here Silicon Chip Online - 12/24V AuxiliaryBattery Controller and it featured this charge controller circuit Oatley Electronics

Don't click on the link at the bottom of the siliconchip page to read the rest of the article or it will take to a site where they want $$$ to read the rest. I reckon that Oatley kit will do just what you need and they can ship overseas too..

Regards Bryan

 

[Danie]

Bryan

I live in Pretoria South Africa but I driving a Chevy Blazer 6.2 Diesel. On this truck I retrofitted a 120 Amp Bosh alternator an used the exiting signal from D+ to trigger a latching relay. That mean the engine must run to connect the Aux. battery to the alternator, this is a simple but bulletproof circuit.

The circuit I’m working on now is for a Nissan Patrol 4.2 diesel (61Y) and believe it I can’t get a signal from the alternator. I’m too scared to fiddle to much and stuff up the electronics.

Thanks I found the article you send the link for.

 

[MikeMl]

The circuit I posted was drawn and simulated using LTSpice (free download here)

 

[Danie]

Thanks Mike

 

[MikeMl]

On further simulation, a better starting value for R6 is 330K. With the pot adjusted to cut-in the relay at 13.9V, the relay will release when the battery voltage sags below 12.9V

 

[Danie]

Mike

I will first build the circuit on the bread-bord and will make R6 a trimpot to get the best value. I can’t wait to get going.

 

[MikeMl]

It is very easy to add a cut-in time delay to my circuit, so we can do that if you would like a smaller hysteresis. The problem might be the momentary sag in main battery voltage when the discharged aux. battery is switched in parallel with it. It will take the alternator ~1/4sec to increase its output to increase the voltage again, so adding a small delay to the TL431 is easy.