Wondering what limits the draw on a circuit used as charge for a toad ? Thinking about wiring into the taillight circuit but don’t know what the draw will be on the circuit.

  On 11/10/2019 at 10:48 PM, dewilso said:

 Thinking about wiring into the taillight circuit

Don't. You'll end up with taillights that are on all the time, and run the risk of a dead battery if you forget to unplug overnight. 

ETA: The charge current is limited by the demand from the battery. A low battery will pull a higher current than a nearly full one.

Edited November 10 by Darryl&Rita

Wire size and length equels line loss, those are your limits. For long runs you need larger, smaller #, wire. I have never tryed to charge a toad, so I dont know how much the brake unit  draws,  but have keept many house batterys in horse trailers charged.  

With taillight wire or even a dedicated hot, that may come with your vehicle, you will get some charge for your toad but it will be limited. It may be all you need, that will probably depend on how often you run the toad. You can run a dedicated wire that is the corect gage, there are charts on line, and it will deffinatly keep the battery full even if you dont run the toad for weeks. I asume that a day or 2 of use will leave you ok with just the tail light wire charging the battery. By the way I would try for the dedicated charge wire to protect the tail light wires on your horse most rvs and trucks come with one. This would be different than a large wire you install your self. Either way you need to protect the toad from over charging,  a very young me found that out 30 years ago. Good luck I am sure someone with spacific experiance will chime in to help you.

  On 11/10/2019 at 11:55 PM, Lance A Lott said:

Wire size and length equels line loss, those are your limits.

?

  On 11/10/2019 at 10:48 PM, dewilso said:

Wondering what limits the draw on a circuit used as charge for a toad ? Thinking about wiring into the taillight circuit but don’t know what the draw will be on the circuit.

Good questions, charging a toad via the tow vehicles alternator is a good scheme,  I've done it successfully for years.

1) The current draw would be (even if nothing else)  LIMITED by the fuse or breaker feeding the toad charging circuit

2) The current draw is further LIMITED by whatever capacity the circuit/source providing the charging is capable of providing.

3) The current draw can be limited due to the current rating and resistance (line voltage drop) of the Tow vehicle to Toad plug/connector PLUS is subject to the Voltage Regulation scheme on the tow vehicles charging system.

4) The amount of charging current the toads battery might attempt to draw is a function of: The toad battery voltage and state of charge,,,,,,,,,,,The resistance and resultant line voltage drop in the circuit FROM tow vehicles charging circuit TO the toads battery.

5) To provide charging current FROM tow vehicles charging system TO the toad battery one should reduce line voltage drop (bigger wire and/or short distance as possible) and tap off a circuit or location that has the capacity to deliver as much charging current as possible yet NOT overtax source capacity .

THAT BEING SAID

A "tail light circuit" would be a poor choice due to its limited amount of current capacity (wire size and fuse/breaker) it lacks capacity to  supply much current to the toad in the first place and if were exceeded the fuse would blow.

A "better" method would be connecting to the tow vehicles battery with a fuse and run a circuit and connectors with sufficient ampacity back to the toad  battery via another fuse located there.

Other methods might involve the use of sold state or mechanical relay battery isolators for connection or disconnection of the tow vehicle and the toad batteries  

To prevent backfeed from toad to tow vehicle some install an in line Diode which works but does have a 0.6 voltage drop across it

Others install a relay in the charge circuit such that the remote charging circuit is closed when the tow vehicle is running but opens otherwise.  

One fairly common even if NOT ideal or best or perfect is to attach say a 10 or 8 or even larger wire and fuse at the tow vehicle battery, run it back to an auxiliary terminal on an RV 7 pole round connector, and run a wire from the toad battery and a fuse to the cord and plug. Such could contain a diode as discussed above or a relay to prevent the toad battery backfeeding the tow vehicle. I have seen  others use heavy wire from tow vehicles battery, a high current rated toad to two vehicle connector,  and perhaps a relay or other isolation or back feed prevention device.  

 Some don't use any diode or relay or isolation scheme but instead rely on simply disconnecting the toad when not in tow. 

BOTTOM LINE to maximize the charging current to a toad you need a high current capacity source, adequate overcurrent protection  at BOTH toad and tow vehicle, high ampacity wiring to limit voltage drop in the charge circuit, back feed prevention and isolation methods so toad can receive charging current yet not reverse discharge tow vehicle, sufficient ampacity connection methods from toad to tow vehicle.

I would consider at the very minimum 8 or 10 gauge fuse protected wire from tow vehicle battery back to an aux terminal on a 7 pole round connector with a wire up to toads battery using  a diode or relay isolation method to prevent backfeed or else just unplug when not under tow. Better if bigger wire and adequate connection methods with idiot proof isolation and back feed protection.

NOTE The source voltage and regulation scheme, the Voltage and SOC of the toad battery along with the resistance and voltage drop of any wiring plays a HUGE role in just how many actual charging amps may or may not get delivered to the toad

NOTE Despite big enough wire and sufficient rated plugs and cords and connectors and a good source these methods aren't perfect but as long as they keep the toad battery sufficient charged GO FOR IT. There are a ton of 12 or 10 gauge charge wires using the RV 7 pole round RV connector ran from tow vehicle to toads battery  with or without diodes or relays out there that get a person by.

NOTE one might also purposely limit the max charging current the toad battery can receive and/or provide max current protection so as to NOT over tax the tow vehicles alternator !!!!!!!!!!!!!

NOTE The above methods, especially if smaller wire and using the RV 7 pole round connector, I consider a maintenance to a trickle charge NOT any high current heavy duty full battery charge.

John T        Coming to you live in the RV currently dry camped near Holland Indiana cold brrrrrrrrrrrrrrrrrr furnace and LP supply is getting a workout

Edited November 11 by oldjohnt

Grest explanation John!

FWIW, it will be 49/16 here tomorrow, snow,rain, ice mix. Tue 26/17, and Wed 35/30.Hope you are headed South; wish we were___.

Edited November 11 by Ray,IN

Thanks Ray, yep we got outa Dodge heading South lol

John T

  On 11/11/2019 at 12:18 AM, oldjohnt said:

Good questions, charging a toad via the tow vehicles alternator is a good scheme,  I've done it successfully for years.

1) The current draw would be (even if nothing else)  LIMITED by the fuse or breaker feeding the toad charging circuit

2) The current draw is further LIMITED by whatever capacity the circuit/source providing the charging is capable of providing.

3) The current draw can be limited due to the current rating and resistance (line voltage drop) of the Tow vehicle to Toad plug/connector PLUS is subject to the Voltage Regulation scheme on the tow vehicles charging system.

4) The amount of charging current the toads battery might attempt to draw is a function of: The toad battery voltage and state of charge,,,,,,,,,,,The resistance and resultant line voltage drop in the circuit FROM tow vehicles charging circuit TO the toads battery.

5) To provide charging current FROM tow vehicles charging system TO the toad battery one should reduce line voltage drop (bigger wire and/or short distance as possible) and tap off a circuit or location that has the capacity to deliver as much charging current as possible yet NOT overtax source capacity .

 

THAT BEING SAID

A "tail light circuit" would be a poor choice due to its limited amount of current capacity (wire size and fuse/breaker) it lacks capacity to  supply much current to the toad in the first place and if were exceeded the fuse would blow.

A "better" method would be connecting to the tow vehicles battery with a fuse and run a circuit and connectors with sufficient ampacity back to the toad  battery via another fuse located there.

Other methods might involve the use of sold state or mechanical relay battery isolators for connection or disconnection of the tow vehicle and the toad batteries  

To prevent backfeed from toad to tow vehicle some install an in line Diode which works but does have a 0.6 voltage drop across it

Others install a relay in the charge circuit such that the remote charging circuit is closed when the tow vehicle is running but opens otherwise.  

One fairly common even if NOT ideal or best or perfect is to attach say a 10 or 8 or even larger wire and fuse at the tow vehicle battery, run it back to an auxiliary terminal on an RV 7 pole round connector, and run a wire from the toad battery and a fuse to the cord and plug. Such could contain a diode as discussed above or a relay to prevent the toad battery backfeeding the tow vehicle. I have seen  others use heavy wire from tow vehicles battery, a high current rated toad to two vehicle connector,  and perhaps a relay or other isolation or back feed prevention device.  

 Some don't use any diode or relay or isolation scheme but instead rely on simply disconnecting the toad when not in tow. 

BOTTOM LINE to maximize the charging current to a toad you need a high current capacity source, adequate overcurrent protection  at BOTH toad and tow vehicle, high ampacity wiring to limit voltage drop in the charge circuit, back feed prevention and isolation methods so toad can receive charging current yet not reverse discharge tow vehicle, sufficient ampacity connection methods from toad to tow vehicle.

I would consider at the very minimum 8 or 10 gauge fuse protected wire from tow vehicle battery back to an aux terminal on a 7 pole round connector with a wire up to toads battery using  a diode or relay isolation method to prevent backfeed or else just unplug when not under tow. Better if bigger wire and adequate connection methods with idiot proof isolation and back feed protection.

NOTE The source voltage and regulation scheme, the Voltage and SOC of the toad battery along with the resistance and voltage drop of any wiring plays a HUGE role in just how many actual charging amps may or may not get delivered to the toad

NOTE Despite big enough wire and sufficient rated plugs and cords and connectors and a good source these methods aren't perfect but as long as they keep the toad battery sufficient charged GO FOR IT. There are a ton of 12 or 10 gauge charge wires using the RV 7 pole round RV connector ran from tow vehicle to toads battery  with or without diodes or relays out there that get a person by.

NOTE one might also purposely limit the max charging current the toad battery can receive and/or provide max current protection so as to NOT over tax the tow vehicles alternator !!!!!!!!!!!!!

NOTE The above methods, especially if smaller wire and using the RV 7 pole round connector, I consider a maintenance to a trickle charge NOT any high current heavy duty full battery charge.

John T        Coming to you live in the RV currently dry camped near Holland Indiana cold brrrrrrrrrrrrrrrrrr furnace and LP supply is getting a workout

 

I wish I could get my thoughts on paper like this!!?

  On 11/11/2019 at 11:15 PM, Lance A Lott said:

I wish I could get my thoughts on paper like this!!?

Yea, I know what you mean..........? Just not articulate with a keyboard

Edited November 12 by Mntom

  On 11/11/2019 at 11:15 PM, Lance A Lott said:

I wish I could get my thoughts on paper like this!!?

Lance, thanks it comes from yearssssssss of technical and spec writing. At our huge Naval Industrial Base we did our own electrical engineering for years and later on had to write even more when we advertised a job for A & E firms to submit proposals, it was  more fun when we did our own work in house …………...Farm it out to private industry became popular before I retired. Oh well such is "progress" ya reckon  ??

John T 

Edited November 12 by oldjohnt

Your best solution is a DC to DC charger. These used to be huge $$ but Renogy recently introduced one that would be perfect for charging a toad at $129. They make two units currently. A 20AMP and a 40AMP. The 20AMP would be all you need. Do a YouTube search on the Renogy DC to DC Charger. Great product at an affordable price and no mickey mouse relays or screwing up your main engine battery.

And....Many modern vehicles have the alternator controlled by the computer. They can be damaged with a simple relay solution and you certainly don't want to just parallel the toads battery.

Edited November 12 by rbertalotto