Vehicle battery charger using alternator signal

Thread Starter

pavelvoivoda

Joined Dec 23, 2021
3
Hi everyone

First to let you know, I read thru a few articles about powering a relay with a capacitor, but did not find them helpful in regards to installations in vehicles. Also Im hoping this thread is in the right category on this forum.
Any advice to narrow down my search, or further investigate into these ideas, would be greatly appreciated.

Here is an electrical diagram that I made the other day. The cap does work when I am testing it on the workbench to energize a relay for 2-3 seconds, but not when in the vehicle.

1640301557003.png

The two ideas are:
1 - to have a relay(2) switch ON to charge the AUX battery from the alternator, a few seconds after the alternator has started producing energy (when the car is OFF, it stays normally closed to allow current from the solar system);
2 - to prevent any current from the AUX battery going to the starter when cranking.

The capacitor provides enough charge to energize relay 1 for a few seconds (enough for me to let go of the ignition key after engine starts).

The only signal that is available to be used as an indication that the alternator is producing energy, is a disconnecting ground inside the alternator's voltage regulator assembly. When the ignition key is rotated to ON, the ground is connected and the control indicator lamps on the combination meter turn ON. The moment you start the motor and the alternator starts producing the 14.3volts, the control indicator lamps on the combination meter turn OFF - a.k.a. their ground thru the alternator is cut. I translate this signal to control an SPDT (low current relay 1) which controls the ON/OFF switching of the high current relay 2 (also SPDT) shown above, which it in turn, switches between the sources for charging the AUX battery - Solar or Alternator.

When tested on the workbench, the capacitor does power relay 1 for a few seconds, which would be enough to keep relay 2 without power for the same amount of time, which in turn would be enough to prevent the 15amp fuse on the aux battery to burn from the starter drawing way too much current (i will switch the fuse with a higher amp rating, the minute this starts working).

But it doesn't work when in the vehicle - i see on the volt meter on the AUX that the voltage drops from 12.6 to 11.8 for a mere second and then it doesn't get up to 14.3, just stays at the AUX current state of charge.

I added the two diodes as to prevent current being drawn back in the reverse direction and ultimately emptying the capacitors (thinking that it's because of the starter, which starves off every other component for a second, until it stops cranking). But that didn't work either.

The relays have a current draw of no more than 1.7 micro amps per relay (shown on the DMM), and the capacitor used is 10000 microfarads which does provide close to 3 seconds of charge for relay 1, and the diodes are 1N5819 which are 1 amp thru them at max 40 volts, (im writing from memory).

Relays are pretty strong and cheap, so no need to worry if the contacts burn out at a time 1/3 of their life.

So far the fuse burns, as the relay 1 doesn't stay energized for the 3 second duration.

Im stuck. It's basically a dual charging system but cheaper than the T-max and equivalents.

Ask me anything else you need to know
 

crutschow

Joined Mar 14, 2008
29,466
Why not just connect the starter control voltage to the Relay 1 coil?
That way you don't have to add any delay time, and it will work no matter how long you crank the engine.
 

LowQCab

Joined Nov 6, 2012
1,913
If the "Aux-Battery" is the usual Car-Battery type Lead-Acid-Battery, or a large Deep-Cycle-Battery,
there's really no need to switch anything,
unless, of course, if You plan to routinely run the Aux-Battery dead.

If You want to get fancy,
just add a Circuit like this one to allow the Alternator to
charge the Aux-Battery when the Engine is running.
.
.
.
Dual-Aux Battery 1 FLAT .png
 

crutschow

Joined Mar 14, 2008
29,466
just add a Circuit like this one to allow the Alternator to
charge the Aux-Battery when the Engine is running.
It would appear the source and drain of the MOSFET should be interchanged, so the the Aux-Battery is not discharged during starting, but is charged when the engine is running.

That solution also uses an expensive MOSFET.
It should be feasible to use a smaller, cheaper MOSFET with a small-value power resistor in series to limit the charge current.
 

Thread Starter

pavelvoivoda

Joined Dec 23, 2021
3
Guys,
thanks a lot! It worked when I connected it as @crutschow mentioned and it did the job! I also connected it to the preheating timer relay which has a prolonged signal sent to the glow plug relay, depending on how hot the engine is. The hotter, the shorter the time to keep the glow plug relay ON. And this does managed to save the fuse from burning when cranking!

As a side effect, the preheating timer relay allows for a little bit of time for the alternator to charge the starter battery before it connects the aux to be charge.

@LowQCab - the battery is just an ordinary lead acid deep cycle battery.

Only thing that I did was to connect two diodes to each of the OEM relays, so that current doesn't activate the other...ask me how I found out!

Here is a revised diagram of all the components, and this thread is open for discussion until the admins see appropriate, otherwise I got what I wanted and will keep you posted how it's going!


1640360393361.png

Please guys have a happy holiday and let me buy you a beer! Send me a paypal email!!
 

crutschow

Joined Mar 14, 2008
29,466
It would seem the diodes are shown backwards.

Isn't the glow plug voltage on when the starter is engaged, or is it shut off during that interval?
 
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