Avoid the discharge of the car battery when not used using a simple relay and interruptor

Relay connection scheme

An old Fiat Bravo was draining battery when not used for a couple of days, and the battery had to be recharged every time: The electronics had an excess of consumption, far surpassed the admissible 0.3 Amps.

After reviewing the possible cause (alternator regulator in poor condition, or something that was connected) by removing fuses and checking consumption every time, I did not found the source of the problem, so we decided to place a switch controlling a relay to disconnect the battery when the car was not in use.

Thus the battery, although the car is not used for 1 full year, only 30-40% is discharged (typical self-discharge at 1 year for batteries Pb / Calcium). Furthermore we lengthen the life of the Central Computer by completely disconnecting it. 

I also take the opportunity to see how these relays are for my next project, which would be programmatically control the battery charging to save fuel by turning off the alternator.

NOTE : Keep in mind that if the battery gets disconnected, you lose the date and time of the control panel (if any).

Using a relay just have to carry three thin wires into the cabin to turn the switch and voltage indicator, because power is handled as close to the battery as possible.

I opted for a solid state relay for its compact size and long life ; Today the Chinese are manufactured by high amperage and very well priced, even more than the mechanical cheap.

These entirely electronic relays are very convenient when many connections / disconnections in a short time are made; no sparks occur in the connection , lasts longer in these cases, with the only drawback of offering some resistance to current flow, so they need cooling if high amperages are controlled (with tie it to a plate of the car, it will act as heat-sink).

Data Relay:

Load current 120A

Load voltage 5-60 VDC

Control voltage 4-32VDC 

2-15mA DC control current

In voltage ≤ 1.5 V

Leakage current ≤ 4mA

ON and OFF time ≤ 10 ms

Dielectric strength 1500VAC

Insulation resistance 500mω / 500VDC

Ambient temperature -20 ~ +70 ° C

Mounting: fixing bolt

LED activation

Note : You will never Trust the technical characteristics of electronic Made in China, rarely are real, they allways exaggerate capabilities to avoid compettition, it appears to be a standard for those parts. For example, I use thhis relay with the announcement "120A" but it is the same with "80A" marks, so I do not recommend using it to greater consumption of 50A at 50V; one of those who acquired indicates that goes well with a 28A 50V without generating almost heat; for now we give a vote of confidence to the brand "Wenzhou Jomall Electrics".

Necessary material

• Amperage DC relay 30% higher maximum instantaneous consumption. It is important to note DC, because if it is not used for alternating current, the circuit would remain closed at all times.
• Small switch to turn; You have to handle milliamps, so worth whatever we have at hand.
• LED voltmeter , voltage indicator, convenient to know the status of the battery.
• Small material : Plugs for cable connections according to the thickness (watts), duct tape, heat shrink tubing of various diameters, cable thick enough (to switch it to us the finest available).
• And the usual tools ; soldering iron, digital multimeter, scissors, etc.

Knowledge required : Welder Management, basics of electricity, electricity of the automobile.

Checking consumption

Placing the tester in series with the positive wiring, and set to DC amps, testing the derivations, I found the terminals with high consumption standing, waiting a bit until it stabilized, that we had to cut the supply to them with the relay:

While indicating 5,6A, then drop to 0,7Ah, still excessive

They proved these two, so we need to check the maximum consumption of the wiring ;after finding that going to the switchboard, feeding as much 60Ah fans and other electronics (we'll know by amp fuses protecting each device), I looked for a relay to admit 80A (although the Chinese indicate 120A think are Ah peak .) If you do not have it clear, it's best to get a cable from the sum of the diameters (copper conductor) hose, for connection from the battery to the relay:

These two outputs will control the positive terminal to the relay

Cables going to the dashboard

The following is to find the optimal position of the relay to prepare the passage of cables from the cockpit control to the same. 
In this case the sheet covering the wheel had a sufficient space to place the relay tied to it, so take the three cables (negative and positive switching) to that area. Beware than an area with cables close mass or plates that may accidentally touch the relay:

What took me more time, not knowing the electrical configuration of the car, was to seek the possible passage of the cable to the cockpit;aAfter much rummaging under the dashboard, I found a rubber already passed by a cable , which only had to drill a bit for the cable to pass just to ensure a seal, as the water in that area of the windshield falls. 

Using a useful "son" to avoid disarming all the trim

I passed the yellow plastic guide into the interior:

And I realized that I was leaving the area that interested me:

While moored cables we use to pass to the other side, in my case at hand only 1.5 to 2.5mm cable , but it perfectly up to 0.5 mm. or less. The brown cable to the negative, and two blue for cutting the positive:

We pull the guide wire and leaving plenty of inside, let tying with flanges. 

NOTE: Ideally, for safety, you first place a corrugated tube through which pass , or use an existing one be aware that if you play with the mass of the car one of the blue (friction, breakage, etc.), a fire may be caused by sparks from short , careful to protect well.

On the inside we do the same, passing it where it is not rubbing against anything, and holding it well to leave fixed:

Protect where necessary:

Until arriving at the chosen site, passing behind covers and trims:

Excuse dirt, it is not mine;)

Then we connect the hoses to the driven side of the relay, the negative, leaving the final connection to the battery, and seek the optimal position to force us cabling:

I have a little sanding sheet and the contact side of the relay epoxy paste, once made ​​the holes for sheet metal screws, so that the epoxy is securely fastened and ensure proper heat transfer. Then I realized that silicone had stayed the same, besides being more flexible, cheaper and better withstand high temperatures:

The paste and dry moor before the Araldit 5 minutes . The blue cable is loose go to the positive side of the relay (top right), is the power switching power MOSFET driver relay:

We have almost all the work done, with the control cable supplied with the negative side and the positive to the switch . The negative control also connect the cable to the voltmeter:

Placing the voltmeter and switch on the dashboard

Open holes with the dremel at the most appropriate place where the switch can not be activated by mistake. 

Cutting the plastic to fit the voltmeter

The voltmeter and placed

Remember that if you want to activate the voltmeter only to operate the switch, connect to the positive pole on the switch is activated .

Be sure to protect the pins with shrink:

It is important to locate it in a place that can not be actuated by accident (the motor would stop inmediatelly; imagine going at 120 kms/hour on the highway). In this case it is covered by the handbrake when releasing it, greatly limiting its possible activation.

To activate the switch, also we activate the voltmeter, which also serves to check if the relay is activated.

12,7V, after a couple of days show 12.9V, paying off arrangement

Connecting the positive battery

Finally, after checking that it is properly connected to the positive . I used 3-wire 2.5 mm welded together at the ends as the equivalent of going to the relay,

to finally protect black heat-shrink tube and flat terminals to the ends welded with tin, to ensure firmness and correct connection:

And so is the set:

Finally, to prevent any shock, I have covered the positive terminal with silicon, which if necessary is easy to remove, this way we prevent accidental short circuit with a ground wire of the vehicle (I couldn't put back the protection cover for lacking space under the air filter).

In a couple of days of use, disabling the battery at night, the battery is fully recharged , saving the user more trouble with this house-hacking.

We can improve this by conecting the relay to the ignition key, so it is automatically disconnected/connected.

I hope you liked it, waiting for your comments and suggestions !!

Links and references: 
Here even they offer "circuit breakers to turn the switch off" to do the same; an antitheft good;) 
green digital voltmeter DC 2,7-30V in Aliexpress 
Running a SSR SSR-100DD-DC-control-DC-SSR-Single-phase-Solid-state-relay SSR relays Advantages 100A-SSR- Control-3-32V AC-DC-output-24-380V-AC  solid state relay control to 380V ac 
Solid State Relay in Wikipedia Spanish 
http://www.physics.unlv.edu/~bill/PHYS483 /ssr.pdf
http://www.ixysic.com/home/pdfs.nsf/www/AN-145ES.pdf/$file/AN-145ES.pdf