I look for a trickle charge circuit operating from the vehicle electrical system, that would allow me to keep my jump starter battery continuously charged, but without the risk of overcharging. Can anyone suggest?

 

What type of battery? Most battery chemistries don't tolerate constant charging well.

 

The battery is Lead-Acid, gel type, like those used in UPS, but larger capacity (18AH). Thanks for your response.

 

I look for a trickle charge circuit operating from the vehicle electrical system, that would allow me to keep my jump starter battery continuously charged, but without the risk of overcharging.

he battery is Lead-Acid, gel type, like those used in UPS, but larger capacity (18AH).

A Schottky diode, to prevent back-charging, in series with a small value resistor, to limit the charging current, should work for that purpose.
The diode and resistor can be connected directly to the vehicle battery with an appropriate fuse in series to protect the circuit (10-15A depending upon the wire size).
A resistor value of a half ohm should work to limit the maximum charge to about 2 amps.

 

I have already connected a common diode (not Schottky) to avoid loss of jump starter charge in cases of main battery failure. Would that be OK? What if I connect the jump starter via a cigarette lighter plug (outlet is available in the trunk)?

 

I have already connected a common diode (not Schottky) to avoid loss of jump starter charge in cases of main battery failure. Would that be OK? What if I connect the jump starter via a cigarette lighter plug (outlet is available in the trunk)?

A Schottky diode has a slightly lower voltage drop, so allows the jumper battery to be charged closer to the main battery voltage.
A regular diode is okay, as long as it keeps the starter battery charged.
The normal resting voltage (after 12 hours) of a fully charged 12V lead-acid battery is about 12.6V.

Yes, the cigarette lighter plug should be fine for charging the battery.
But you should add a small resistor in series with the diode, as I suggested, to limit the charge current after the battery has been discharged after being used to start a vehicle.

 

I will keep the regular diode at the time, as I installed in the inside of the jump starter and opening/closing the case is a dreadful operation (about 20 different screws to remove and then to guess where each one goes...). I should imagine that the 0.3V (0.7-0.4) voltage drop difference will not have a huge effect on the charge time. Of course, I will add the resistor too.
Thank you so much, my friend, for your valuable advice.
Vasilis