Robbing Peter to Pay the whole family . . .

[Capt Jon]

I have a charging problem, and I think I now have a way to solve it by shifting around some components, but I would appreciate some feedback from the experts here.

I daily-drive, and play, in a 1990 FJ62. The factory Toyota alternator puts out very low voltages once its warmed up. Often tripping the disconnect on my Blue Sea ACR at stoplights. I supplement with solar and 120v charging to keep the house battery (FLA Group 31 mounted under the hood) charged. But it's an uphill battle.

I recently acquired a new Turtleback Adventure Trailer. I ordered it with an AGM Group 31 and a Redarc 1225 DC-DC charger, but I doubt I will never get 25 amps through the 7-pin plug on the cruiser.

I have very little running on the trailer electrical. The only reason I ordered it with this setup is that I may eventually run a propex heater and/or a second fridge from it. The cruiser carries the fridge now, and I don't anticipate changing that. So I now have a trailer with very little electrical load, but a much better battery and charging system than the one in the truck that actually sees heavier use.

I know that a DC-DC charger should be placed as close to the battery as possible to avoid voltage drop, but that this can be mostly mitigated by larger cable.

My question is this: Can I move the redarc from the trailer to the Land Cruiser in place of the blue sea ACR (thus bumping the charge the house battery in the Cruiser is receiving from 12.8-13.3v to about 14.4), and run a heavy gauge cable from the house battery to the trailer via an Anderson Connector at the bumper (so the trailer battery will see slightly less than the 14.4 the house battery in the Land Cruiser is seeing).

My thought is that this would drastically improve the charging of the house battery in the truck, and only marginally (assuming heavy enough cable) reduce the charge going to the trailer. In fact, the trailer will likely get more through this setup than it would via the 7-pin trailer plug. Though in truth, I will probably run the heavier wire and Anderson connectors whether I move the Redarc or not.

I do pull the trailer with multiple vehicles, so I will need a switch to divert the charge going to the battery from the 7-pin plug or the Anderson connector. It seems to me that allowing them to be connected at the same time would essentially be connecting the house, trailer, and starting batteries together via a small wire and the Redarc. That might release the magic smoke from inside the Redarc, or just set the truck on fire when the insulation melts off the wire on the 7-pin.

Other than the shame of cannibalizing the beautiful work the guys at Turtleback did, what are the drawbacks?

 

[Chris Boyd]

A couple of things :

-Fix the root of the problem and find a better alternator. You really shouldn’t have that kind of voltage drop on the alternator. You might also check the voltage regulator.

-Get a second redarc for the vehicle leave the one in the trailer. Especially if you plan on charging from solar on the trailer directly. Drop in voltage from engine bay to the trailer is really probably too long to be really effective, especially with limited gauge wire in between. Especially since it sounds like you have a second battery on the rig.

-Running 25A through the seven pin connector on the trailer is not advised. Charging the battery from the vehicle should be done on a much thicker gauge wire then likely is in the trailer harness and run dedicated from the redarc you would put on the vehicle. At a minimum, install a dedicated breaker ahead of the trailer connector for the whatever gauge wire charge in the trailer pigtail. If you look at the set ups in Australia they use an Anderson connector on a second harness dedicated just to charging. I’ve had similar issues with charging my trailer, and I am going that route. (And I have a solid alternator). A 25A resettable breaker ahead of a larger gauge dedicated harness is the best route I’ve found to get a lot of amperage back to the trailer.

So you’re on the right track here... But I would run the main lead to the trailer harness from the main battery and not the backside of a redarc in the engine bay to charge your house battery. Otherwise you’re robbing from the house battery charge, and probably confuse the redarc that would be installed in the vehicle.

-Consider reducing loads on the alternator in the vehicle itself. Things like changing lighting to LED, etc.

 

[Chris Boyd]

Oh, and I don’t see an issue running power split to both the 7pin and the Anderson on the vehicle side, just fuse each link on the vehicle with a breaker of appropriate value to the gauge of wire for each harness.

If you run the Anderson harness and don’t want power to flow that way, just remove the charge pin wire from the trailer side 7pin to the battery to make sure no power goes over it on the turtle back, but does for other trailers.

 

[Capt Jon]

Thanks Chris.

I agree that a second redarc would be the best solution. Time and finances will make that happen one day, but my current quandary is how to best deploy the resources I have.

The alternator is functioning as designed. No problems with the alternator or voltage regulator. That era Landcruiser simply doesn't run at very high voltages. I can get a little more out of it by turning on the headlights. My 2015 70-series was the same. I could drive it to work and back for two weeks and never top up the house battery. I hear the Taco guys fight it as well. I'm thrilled to hear that you aren't having the same issue in your Hundy's. I'm trying to talk my wife into one.

The highest voltage I have ever seen on the FJ62 is 13.9, and that's only when its cold. Running down the highway it's 13.1-13.3. There are fixes using something called a shotkey diode. This evidently tricks the alternator into raising the voltage a hair. The Aussies have some plug and play solutions, but won't respond to inquiries about a 1990 FJ62. Here it involves a little surgery.

I have 2/0 cable run to the back of the truck now. My chart says 6 gauge would keep it under 3% (i.e. 14.4 X 0.97 = 13.96v). My big cable should do better than that. I don't think the voltage drop in 20+/1 feet of 2/0 cable is going to be significant enough to warrant a second $400 Redarc.

Any run from the main battery/alternator will suffer from the same drop, but will start at 13.3 or so. i.e the Trailer (sans Redarc) would only get about 12.8. That's not even a trickle charge.

The Redarc should not be confused by the second battery. It can only see the combined or "bus" voltage of the two house batteries. It can't tell there are two. It just knows it is connected to a buss with voltage "X." In bulk mode it will run up to whatever the maxV is or 25Amps whichever comes first. In absorb mode, it will try to maintain a constant 14.4 or so to the bus.

My concern is backfeeding that 14.X up the 7-pin to the alternator and voltage regulator. I don't know if the alternator will charge at all if it sees 14.x. If it stopped charging, then the 14.x coming out of the Redarc would go away, and the alternator would start charging again, and the Redarc would produce 14.x, and the alternator would see 14.x and stop charging, . . . endless loop ending in smoke.

Cutting the power lead on the trailer side of the 7-pin doesn't work for me. I won't have the Anderson set-up on all of the vehicles that will tow this trailer. The Fords and Jeeps will push plenty of voltage down that wire to charge the trailer battery. Not at 25 Amps, but enough to charge. I think one of those big red Blue Sea battery switches will work. Or I could just put a breaker on each leg, but that seems excessive.

Thanks for the comments. It makes me think hard, but that's probably good for me.

 

[dreadlocks]

I only see like 5A of power out the 7 pin trailer plug..

Sounds like the solution for now is to move the DC charger up into the engine compartment, use it for charging vehicle house battery.. and set trailer up for its own charging and dont rely on the vehicle for much.. a few amps should keep it fully charged on a large journey even if its running a fridge and stuff.. then if you can keep it charged when not on the road your winning.

My problem with using the alternator is I dont drive enough to recharge, without a DC charger it was taking >8h of driving to get it back full.. with a DC charger I might of got that down to 5-6h, but when I'm only going a few hundred miles before setting up camp again its still a loosing battle like your seeing now.. ended up idling the engine around camp alot and I didnt like that, so now I idle a genset thats cheaper and more efficient than my engine.

 

[dwh]

My question is this: Can I move the redarc from the trailer to the Land Cruiser in place of the blue sea ACR (thus bumping the charge the house battery in the Cruiser is receiving from 12.8-13.3v to about 14.4), and run a heavy gauge cable from the house battery to the trailer via an Anderson Connector at the bumper

Yes.

(so the trailer battery will see slightly less than the 14.4 the house battery in the Land Cruiser is seeing).

That's unlikely, and wouldn't matter anyway. For charging lead-acid, voltage drop is largely a red herring.

Voltage drop is a variable, and a function of load. So the "calculated" voltage drop when 25a is flowing is more than when 10a is flowing. Also the entire charging circuit will be at battery voltage (it's a loop/circuit/circle; voltage does not drop from one end to the other - a circle has no ends; the voltage drops around the entire loop), so the voltage drop at 14v/25a is different than at 14.8v/10a.

A lead-acid battery's resistance rises as it approaches full, so the amp flow tapers off as resistance rises, and the voltage drop becomes less and less until ultimately it becomes so small as to be irrelevant and the voltage of the battery reaches unity with the charge source because the voltage drop has gone away.

Until the battery reaches full charge, the resistance of the charge loop (wire resistance + battery resistance), usually interpreted as a drop in voltage*, actually results instead in a drop in amperage flowing.


* Which is incorrect because the battery is actually regulating the charge loop voltage until the voltage rises high enough for the voltage regulator of the charge source (Redarc) to take over regulating (limiting) the voltage.

 

[Capt Jon]

Thanks dwh.

Given your point that the voltage is the same for the entire loop, what would happen if the 7-pin and the Anderson were both connected to the trailer battery at the same time? (academic question only, I don't intent to wire it this way, but it wouldn't be the first time something didn't go as intended.)

It seems the voltage regulator in the battery would be trying to reduce (i.e. choke?) the voltage down to 13.3 while the voltage regulator in the Redarc was trying to raise it to 14.4 (using current from the alternator). It seems like the alternator's voltage regulator would win and just reduce everything to the voltage its running at now. Then the magic smoke would come out of the Redarc?

 

[dwh]

Nah.

The VR on the engine doesn't reduce or choke. Since the alternator doesn't have permanent magnets, in order to create a magnetic field between the rotor and the stator, you have to feed some power to the rotor using what is called a field coil. Alternators with external regulators have an extra wire besides +/-, called the exciter wire. The voltage regulator controls the power to that wire, using the field coil like a clutch. When the voltage of the bus gets to where it needs to be, the VR starts "slipping the clutch" to reduce the strength of the the field and keeping the voltage from going any higher.


So feeding both the alt output and the Redarc output will combine a bit of extra amperage, basically adding the 7-pin power to the Anderson power.

But only as long as the battery voltage is below the VR setting (13.3v or whatever). Once the battery voltage rises above 13.3v, the 13.3v of the alternator output won't be able to overcome the resistance of the 13.4v (or more) of the battery/Redarc and no more power will flow that direction through the 7-pin.

At that point, the higher voltage POTENTIAL of the Redarc will be feeding power to the whole bus, including backfeeding the engine bay/chassis stuff through the 7-pin...thereby creating a loop where the Redarc is feeding its output back into its input. But the resistance of the 7-pin, being higher than the Anderson, will cause the Redarc to drag down the voltage on the input side, the VR will see a voltage below 13.3v and engage the clutch on the alternator to supply power.


So, basically you are bleeding off SOME of the Redarc's output to feed back in to the input, which will reduce the amperage available to charge the battery by some (good luck with the math) amount - slowing down the charge rate to the battery.


I doubt it would let the magic smoke out of anything, but I might worry about the wire to the 7-pin getting a bit hot.