Charging juice to the trailer...
- Thread starter cigarluvr
- Start date
dwh
Tail-End Charlie
Right amount?
The alternator is controlled by the voltage regulator. The voltage regulator will keep the "12v bus" (alternator + battery + all loads) at somewhere between 13.5v -14.5v (thereabouts).
So to charge the trailer battery, you need to tie it into the 12v bus while the engine is running so it gets brought up to the same voltage. Once it's up to that voltage, then it'll need to be kept there for some hours to be completely topped off.
So the question is - How to tie the trailer battery to the 12v bus?
The answer is - It depends.
If your truck has a factory "7-pin" connector, then there is probably a hot wire already there to do it.
The problem is, you don't want the batteries tied when the engine is off so you don't run the engine battery down as you run the trailer battery down.
So you'll either need to rig the hot of the 7-pin to be switched off when the ignition is off, or remember to always unplug the trailer when the truck is parked.
If you don't have a decent sized hot wire to the trailer already, then you'll have to rig one.
It's the same as rigging an aux battery under the hood, except that you have the run the wire all the way to the back.
The main issue is "voltage drop". If the wire to feed power to the trailer battery is too skinny, then the voltage way at the back will be lower than the "12v bus" voltage under the hood - and the trailer battery won't get fully charged.
Voltage drop will also happen any time you use a "diode type" isolator (the ones with the big heat sink that you can find at any auto parts store). For this reason, diode type isolators are inferior to "solenoid type" isolators (a.k.a. "split charge relay").
Here's an example of one sort of solenoid type, with a basic wiring schematic at the bottom of the page:
http://www.powerstream.com/battery-isolator.htm
The one in the above link is just a dumb solenoid with a little "black box" brain added to it so that it won't tie in the aux battery until the engine battery voltage rises up some (quick recharge after engine start). You don't really need that. My truck just has a dumb solenoid wired to tie the batteries whenever the ignition is turned on. Works fine. You don't want to use a Ford starter relay for this - you need something rated for "continuous duty":
http://www.amazon.com/Continuous-Duty-Solenoid-80AMP-12V/dp/B0050I94XG/ref=pd_sim_sbs_auto_1
So you can use a dumb solenoid to feed power to a sufficiently fat wire to the trailer battery and you're good to go. Don't forget to use a fuse to protect any wire that carries "+" (positive).
The alternator is controlled by the voltage regulator. The voltage regulator will keep the "12v bus" (alternator + battery + all loads) at somewhere between 13.5v -14.5v (thereabouts).
So to charge the trailer battery, you need to tie it into the 12v bus while the engine is running so it gets brought up to the same voltage. Once it's up to that voltage, then it'll need to be kept there for some hours to be completely topped off.
So the question is - How to tie the trailer battery to the 12v bus?
The answer is - It depends.
If your truck has a factory "7-pin" connector, then there is probably a hot wire already there to do it.
The problem is, you don't want the batteries tied when the engine is off so you don't run the engine battery down as you run the trailer battery down.
So you'll either need to rig the hot of the 7-pin to be switched off when the ignition is off, or remember to always unplug the trailer when the truck is parked.
If you don't have a decent sized hot wire to the trailer already, then you'll have to rig one.
It's the same as rigging an aux battery under the hood, except that you have the run the wire all the way to the back.
The main issue is "voltage drop". If the wire to feed power to the trailer battery is too skinny, then the voltage way at the back will be lower than the "12v bus" voltage under the hood - and the trailer battery won't get fully charged.
Voltage drop will also happen any time you use a "diode type" isolator (the ones with the big heat sink that you can find at any auto parts store). For this reason, diode type isolators are inferior to "solenoid type" isolators (a.k.a. "split charge relay").
Here's an example of one sort of solenoid type, with a basic wiring schematic at the bottom of the page:
http://www.powerstream.com/battery-isolator.htm
The one in the above link is just a dumb solenoid with a little "black box" brain added to it so that it won't tie in the aux battery until the engine battery voltage rises up some (quick recharge after engine start). You don't really need that. My truck just has a dumb solenoid wired to tie the batteries whenever the ignition is turned on. Works fine. You don't want to use a Ford starter relay for this - you need something rated for "continuous duty":
http://www.amazon.com/Continuous-Duty-Solenoid-80AMP-12V/dp/B0050I94XG/ref=pd_sim_sbs_auto_1
So you can use a dumb solenoid to feed power to a sufficiently fat wire to the trailer battery and you're good to go. Don't forget to use a fuse to protect any wire that carries "+" (positive).
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Awesome, thats what I was thinking. I wanted to try and use the existing 12v hot wire in the trailer harness. So on my rig (Jeep ZJ), if I need to move 12v from the front to the back...
20' @ 90amps (for now)
Looks like anything bigger than 6awg would give me less than a 1 volt drop. Can I accept more? Say use 8 and take the 1.15v drop or even 10awg and take the 1.8v drop? How much is too much drop over this distance?
20' @ 90amps (for now)
Looks like anything bigger than 6awg would give me less than a 1 volt drop. Can I accept more? Say use 8 and take the 1.15v drop or even 10awg and take the 1.8v drop? How much is too much drop over this distance?
dwh
Tail-End Charlie
Voltage drop through wire is a variable which goes up and down based on load. The more amps that are flowing, the more the voltage drop over distance.
So if you used #10 and had (using your numbers above) a 1.8v drop at 90a flow rate - then as the battery fills up and the flow drops to say 5a the voltage drop would be a whole lot less. Eventually, as the flow dropped down to 1a or 2a, the voltage drop would be almost nothing.
Theoretically, even if you used a #14 lamp cord, the remote battery would *eventually* reach the voltage of the 12v bus under the hood.
Voltage drop for DC battery charging is only critical at full load. This is different than using a diode-type alternator which introduces a permanent .5v drop to begin with, and then you've got the wire's voltage drop on top of that - no matter what, the remote battery will always be .5v low...
So the question is: How likely is it that you will actually see 90a flowing to the remote battery?
Answer: Not likely.
The remote battery + the wire that ties it into the 12v bus is a circuit. The resistance of that circuit vs. the voltage of the 12v bus will determine the rate at which amps flow through that circuit.
Even if you had a million amp alternator and million gauge wire - you might never see more than 10a flowing to the remote battery if that's all the battery resistance will allow with a 12v bus (supply) voltage of 13.5v-14.5v.
The battery resistance varies as well. The lower the battery's state of charge, the higher the resistance will be. Also, the higher (closer to full) the battery's state of charge, the higher the resistance will be. Around 40%-50% state of charge will usually have the least resistance.
So you can size the wire based on the max amps that are *expected* to flow. If you had a bank of Odysseys, and could expect 90a to actually flow though the circuit, then go for 90a wire.
For a single flooded or AGM battery and a standard alternator/voltage regulator setup...I'd be comfortable with #10 over a 20' run.
Why? Because a voltage regulated alternator is a bloody slow way to charge a battery in the first place. It'll take so long to fully top off an aux battery that it won't matter if the voltage drops some during the main part of the process. Once the battery hits 80% state of charge, it's going to take hours to get it full anyway, and there won't be a whole lot of amps flowing while that happens; and very few amps flowing once the battery reaches 90%.
EDIT: Thus, as the battery reaches 90% state of charge, and there are few amps flowing, the voltage drop will be negligible.
So if you used #10 and had (using your numbers above) a 1.8v drop at 90a flow rate - then as the battery fills up and the flow drops to say 5a the voltage drop would be a whole lot less. Eventually, as the flow dropped down to 1a or 2a, the voltage drop would be almost nothing.
Theoretically, even if you used a #14 lamp cord, the remote battery would *eventually* reach the voltage of the 12v bus under the hood.
Voltage drop for DC battery charging is only critical at full load. This is different than using a diode-type alternator which introduces a permanent .5v drop to begin with, and then you've got the wire's voltage drop on top of that - no matter what, the remote battery will always be .5v low...
So the question is: How likely is it that you will actually see 90a flowing to the remote battery?
Answer: Not likely.
The remote battery + the wire that ties it into the 12v bus is a circuit. The resistance of that circuit vs. the voltage of the 12v bus will determine the rate at which amps flow through that circuit.
Even if you had a million amp alternator and million gauge wire - you might never see more than 10a flowing to the remote battery if that's all the battery resistance will allow with a 12v bus (supply) voltage of 13.5v-14.5v.
The battery resistance varies as well. The lower the battery's state of charge, the higher the resistance will be. Also, the higher (closer to full) the battery's state of charge, the higher the resistance will be. Around 40%-50% state of charge will usually have the least resistance.
So you can size the wire based on the max amps that are *expected* to flow. If you had a bank of Odysseys, and could expect 90a to actually flow though the circuit, then go for 90a wire.
For a single flooded or AGM battery and a standard alternator/voltage regulator setup...I'd be comfortable with #10 over a 20' run.
Why? Because a voltage regulated alternator is a bloody slow way to charge a battery in the first place. It'll take so long to fully top off an aux battery that it won't matter if the voltage drops some during the main part of the process. Once the battery hits 80% state of charge, it's going to take hours to get it full anyway, and there won't be a whole lot of amps flowing while that happens; and very few amps flowing once the battery reaches 90%.
EDIT: Thus, as the battery reaches 90% state of charge, and there are few amps flowing, the voltage drop will be negligible.
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/Hijack, slight
Very interesting, although it runs counter to things I've read about using welding cable, jumper cables, or other larger diameter cable to make it work. My aux battery install has been on hold while I gathered funds for the suprising expense of 1/0 cable. If I can use something like #10, then I can be in business that much sooner.
I have a continous duty solenoid in hand (thanks again to one of your earlier recommendations, although I chose one with a higher amp rating). For the two smaller terminals: does it matter which is ground and which is ignition hot, and is there a recommendation on the required wire size for those smaller terminals?
Thanks again for your insight and advice.
/end hijack
Very interesting, although it runs counter to things I've read about using welding cable, jumper cables, or other larger diameter cable to make it work. My aux battery install has been on hold while I gathered funds for the suprising expense of 1/0 cable. If I can use something like #10, then I can be in business that much sooner.
I have a continous duty solenoid in hand (thanks again to one of your earlier recommendations, although I chose one with a higher amp rating). For the two smaller terminals: does it matter which is ground and which is ignition hot, and is there a recommendation on the required wire size for those smaller terminals?
Thanks again for your insight and advice.
/end hijack
Martyn
Supporting Sponsor, Overland Certified OC0018
The standard is for a 10 gauge wire back from the vehicle to the trailer. The wiring on the trailer is also standard at 10 gauge.
It's typical to run the wire of the primary battery with a fuse or breaker close to the battery all the way back to the 7 pin and from there into the trailer. It provides a slow recharge to the battery in the trailer, which can be just fine depending on usage and consumption.
One of the biggest issues you face is partial recharge of the trailer battery from the vehicle followed by depletion due to usage. It's cyclical, you leave the trailer when not in use and the battery discharges - you hook the trailer up to the vehicle, drive somewhere to go camping, and partially charge the battery - you use a fridge lights etc. when camping and discharge the battery - you hook the trailer up to the vehicle and drive home partially recharging the battery. The end result is sulphur build up on the lead plates and an early death of the battery.
Solutions:
Add a battery charger so that the battery can be fully charged between trips. Stops the sulphur build up.
For faster re charge from the vehicle run 4 gauge wire back from the main battery to the rear of the vehicle. Fit an Anderson connector between the vehicle and trailer, and wire to the trailer battery. Use a resettable breaker in the line close to the main battery.
It's typical to run the wire of the primary battery with a fuse or breaker close to the battery all the way back to the 7 pin and from there into the trailer. It provides a slow recharge to the battery in the trailer, which can be just fine depending on usage and consumption.
One of the biggest issues you face is partial recharge of the trailer battery from the vehicle followed by depletion due to usage. It's cyclical, you leave the trailer when not in use and the battery discharges - you hook the trailer up to the vehicle, drive somewhere to go camping, and partially charge the battery - you use a fridge lights etc. when camping and discharge the battery - you hook the trailer up to the vehicle and drive home partially recharging the battery. The end result is sulphur build up on the lead plates and an early death of the battery.
Solutions:
Add a battery charger so that the battery can be fully charged between trips. Stops the sulphur build up.
For faster re charge from the vehicle run 4 gauge wire back from the main battery to the rear of the vehicle. Fit an Anderson connector between the vehicle and trailer, and wire to the trailer battery. Use a resettable breaker in the line close to the main battery.
dwh
Tail-End Charlie
#10 may not be enough. It depends on the resistance of the circuit (battery and wire).
SmartGauge's "no surge" page explains it very well.http://www.smartgauge.co.uk/nosurge2.html
Now, admittedly, he's talking about the momentary surge when energizing a solenoid and tying two batteries (or banks). BUT, it still applies. The worst case, according to his math is:
"The worst scenario actually occurs with a fully charged engine start battery and the auxiliary battery bank at around 40% charge state. The reason is that, although the terminal voltage falls as the battery approaches totally flat, the internal resistance rises. At 40% state of charge the internal resistance is considerably lower. The result in the above example would be that the initial current surge would be around 40 amps, and even that would only last a minute or so (thus discharging the engine battery by 40A * 1/60th hour = 0.7 amp hours) at the most until the surface charge on the auxiliary bank raised it's [sic] terminal voltage to that of the engine start battery. Once this had happened, the current would be negligible." [emphasis added]
Okay, so that's what happens during the initial surge when the batteries are tied. It's highly unlikely that the alternator is ever going to push amperage through a circuit with a greater shove than a fully charged cranking battery rated at 600 CCA.
Now, to increase the amp flow, you can either increase the supply voltage - which a normal voltage regulated alternator setup is NOT going to do - or you can decrease the resistance of the circuit. Increasing wire size, as Martyn pointed out, is one way to decrease the resistance.
However, increasing wire size quickly reaches a point of diminishing returns, because as soon as the secondary battery voltage reaches parity with the 12v bus - then the resistance of the battery controls the flow, and with a constant voltage supply (even the slight 13.5v-14.5v variation of a voltage regulated alternator is still considered "constant voltage") there won't be a lot of current flowing.
- So, you turn on the ignition and energize the solenoid.
- There is a slight surge toward the aux battery, but with a single aux battery it will almost certainly be less than 30a.
- Then the engine starts, and the voltage regulator will switch the alternator on and off to keep the bus voltage between 13.5v-14.5v.
- Current will flow from the 12v bus to the aux battery, and the aux battery will draw the 12v bus down below 13.5v - and KEEP drawing it down.
- This will tend to keep the alternator switched on, BUT it will only supply however many amps are being drawn out of the bus.
- At the voltage differential between the 13.5v bus, and the...say 11.5v...battery, the amp flow will almost certainly be less than 30a.
- Then, once the aux battery reaches 13.5v the voltage regulator will continue to keep the alternator turned on, until the whole shooting match reaches 14.5v.
- At that point, the voltage regulator is holding the bus between 13.5v-14.5v and the aux battery will slowly absorb power until its chemistry is completely activated (saturated).
Now, with a really good low resistance battery such as an Odyssey or Optima, more amps can flow - but not for long; the battery will reach parity voltage with the 12v bus even faster than a battery with higher resistance - and at that point, it's still just a constant voltage trickle charge situation...which is really all a voltage regulated alternator is anyway.
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Martyn
Supporting Sponsor, Overland Certified OC0018
Good points made so far. My real world experience is that the issues only start if the auxiliary battery is drawn down below 11.75 volts or 40% charge. Recharging the auxiliary battery using the tow vehicle and standard 10 gauge wire and getting it back up to 12.4 volts or an 80% charge is feasible after a nice long drive. It's when the voltage drops to 11.30 volts or lower that there can be issues.
Using 4 gauge wire between the tow vehicle and the trailer usually allows you to get the auxiliary battery charge back up to 12.7 volts, 100% charge much quicker. You'll even read 13.1 to 14.0 volts on the battery right after the vehicle has been turned off and before it goes to it's rested state.
The best thing you can do is set up a volt meter on the auxillary battery and monitor it. As soon as you hit 11.75 turn the vehicle on or plug in the solar panel to recharge it (if you have a solar panel leave it connected all the time and allow the controller to monitor the current flow). It's the constant depletion of the battery to 11.3 volts or less followed by and incomplete charge that's the normal issue.
Using 4 gauge wire between the tow vehicle and the trailer usually allows you to get the auxiliary battery charge back up to 12.7 volts, 100% charge much quicker. You'll even read 13.1 to 14.0 volts on the battery right after the vehicle has been turned off and before it goes to it's rested state.
The best thing you can do is set up a volt meter on the auxillary battery and monitor it. As soon as you hit 11.75 turn the vehicle on or plug in the solar panel to recharge it (if you have a solar panel leave it connected all the time and allow the controller to monitor the current flow). It's the constant depletion of the battery to 11.3 volts or less followed by and incomplete charge that's the normal issue.
This. The side benefit is that you can also wire other high-current devices (air compressor, water pump, margarita blender) with an anderson connector and be able to plug it in and use it off your truck (while running, so you don't wear down your starting battery). The advantage of using a standardized Anderson connector is that if you wire all your other devices with an Anderson plug, you can run it off your truck or your trailer battery without having to change the connector. You can get cheapo 4 gauge jumper cables from Wal-mart for $20-30, cut the ends off and you have your power wire from the engine bay to the rear of your rig. Then order the Anderson connectors cheap from All Battery Sales and Service and for <$50 you have a full high-current connection system for your trailer and all your accessories