Current limiting while charging a remote 12 volt battery

Thread Starter

DavidLaidig

Joined Apr 19, 2019
6
Hi ya'll
I am installing a power jack and a group 24 battery on the tongue of my boat trailer. I intend to keep the battery charged with the aux pin from the trailer cable. The aux connection is on a 20 amp fuse. How do I limit the current when the battery is low?
 

crutschow

Joined Mar 14, 2008
23,335
What's the purpose of the battery?
What's the maximum current you will draw from it?
 

crutschow

Joined Mar 14, 2008
23,335
Offhand, one simple way to limit the charge current it to just add a series resistor to the aux pin connection.
For example, if the battery is 12V and the charging voltage is 14.5V, a 0.5Ω resistor would limit the charge current to 5A.
The resistor would be dissipating 12.5W (it can get very hot) so it should be a 15-20W resistor.
That should be more than sufficient to keep the battery charged if it's just used occasionally to operate the jack.

The jack switch is connected directly to the battery, of course.
 

Thread Starter

DavidLaidig

Joined Apr 19, 2019
6
Thanks, yeah I considered that. I am going to mount it outside of the battery box which is plastic. It will be mounted on the aluminum trailer frame and I will encapsulate the connections with epoxy.
 

BillB3857

Joined Feb 28, 2009
2,497
Make sure the tab is isolated electrically from the resistance element if you are going to mount to the frame.
 

Thread Starter

DavidLaidig

Joined Apr 19, 2019
6
LOL I'll check it but that would be a first for me! If it's not Amazon will get it back. Nothing like a "hot" heat sink!
 

marcf

Joined Dec 29, 2014
256
You did not mention it but I assume that you are going to use a split charging method for the two batteries.
 

mvas

Joined Jun 19, 2017
538
IF NEEDED - an "Ideal Diode" may be a good choice here, since it would not get hot like a resistor.
Is the Pin at 1:00 o'clock in the 7 Way Connector a "Battery Charge" Pin?
If true, then you may not need to limit the amps externally.

Is the Group 24 Battery an AGM SLA type battery?

Typically, you do not connect the Power Cable until after the Trailer Hitch is safely on the ball of the truck.
 
Last edited:

crutschow

Joined Mar 14, 2008
23,335
an "Ideal Diode" may be a good choice here, since it would not get hot like a resistor.
How would an ideal diode help? :confused:
The purpose of the resistor is to limit the charging current below his fuse rating.
 

mvas

Joined Jun 19, 2017
538
How would an ideal diode help? :confused:
The purpose of the resistor is to limit the charging current below his fuse rating.
I don't know anybody, that "needs" a resistor, for their battery on their trailer.
You've invented a solution for an imaginary problem.
I have a 12 volt 12 AH battery, actually my third SLA battery in 10 years, on my trailer.
I have never blown my fuse on my 7-Way Connector.
I don't know anybody that has blown their fuse on their 7-Way connector due to a "low" battery.

The ideal diode is to prevent the trailer from "reverse-powering" the vehicle if the engine dies.
 

crutschow

Joined Mar 14, 2008
23,335
I don't know anybody, that "needs" a resistor, for their battery on their trailer.
Maybe he doesn't "need" it, but now you know someone that's requested a solution for a (possible) problem that can be solved with such a resistor. :rolleyes:
You've invented a solution for an imaginary problem.
Perhaps.
he ideal diode is to prevent the trailer from "reverse-powering" the vehicle if the engine dies.
The resistor should also minimize any "reverse-power" if the engine dies.
 

mvas

Joined Jun 19, 2017
538
Just a very small charging current of 1 amp x the 1/2 Ohm resistor will cause a 1/2 Volt drop.
A 1/2 Volt drop may be too much, such that the Trailer Battery never fully charges to 100%.
If the Lead Acid battery never fully charges to 100%, then it will sulfate and it will die an early death.

The resistor solution is worse than the imagined potential problem.
If a 1/2 ohm resistor is added, then it must be bypassed, after some small delay.

The Ideal Diode will stop the Trailer Battery from ever being discharged by the vehicle - when the Key is in the ON position but engine is OFF ( ie someone playing radio or using Power Ports ). The Trailer Battery must be at 100% Charge when the trailer is disconnected for an extended period, otherwise the battery will sulfate and die.

A 10 Amp 12V-to-12V Battery Charger is an option.
I think, Power Stream and other mfrs, carry such a product.

A small 4 Amp model + an hour or two of travel time, could recharge the trailer battery to 100% ...
https://www.etrailer.com/p-2024-07.html

A Push-Button Resettable Circuit Breaker, that is exact replacement for the 20 Amp Blade-Style Fuse, may be more appropriate.
As it would easily handle any initial inrush, without dropping the voltage like a resistor.

Also, a Slow-Blow fuse will allow an initial in-rush, without dropping the voltage like a resistor.
 
Last edited:

crutschow

Joined Mar 14, 2008
23,335
A 1/2 Volt drop may be too much, such that the Trailer Battery never fully charges to 100%.
The battery is only momentarily used to operate the 15A jack motor, so it won't take much current during the travel time to recharge the battery.
I think you are proposing a complex solution to a simple problem.
 
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