Charging battery in camper from car auxiliary battery
August 21, 2022 3:30 PM Subscribe
We want to charge the 12v battery in our camper trailer from the car while driving. The car (2018 Nissan Navara - I think it's called the Frontier in some countries) has a 'smart alternator' and already has a 98 Ah AGM auxiliary battery fitted in the rear of the car that charges from the main battery via a DC-DC charger.
We have a solar panel to charge the camper battery while set up. We also want to be able to charge the battery while driving, as we have a couple of trips planned where we will be driving for long periods during the day, so have limited access to power from the sun. It's not practical to fit a solar panel to the camper that would charge while driving. We mostly camp in areas where electricity is not available, so need to be self-sufficient for up to two weeks. Definitely not interested in using a generator.
The camper battery is used for lights, charging devices etc but the main draw is a fridge installed in the camper(this model, without the internal battery). The current battery in the camper is dead and will be replaced by a 150 Ah or higher either AGM or Lithium battery, depending on price and suitable size availability.
My current plan is to install a second DC-DC charger in the car (in the car rather than the camper because of the need for an ignition trigger wire to the charger because of the 'smart' alternator) and draw from the auxiliary battery to charge the camper battery. Effectively, this would mean the main car battery charges the auxiliary battery, which then charges the camper battery. I'd prefer to avoid having to run another set of heavy cables all the way to the front of the car to charge both from the main battery, but will do this if I need to. The electrical connection from car to camper will be via an Anderson plug connection carrying the charge current to the camper battery in addition to the normal trailer wiring connector (7 pin flat). I'm aware of the need for suitably heavy cables etc.
A possible alternative would be to simply connect the auxilliary battery to the camper battery, effectively making them one large battery from the view of the current DC-DC charger, but I think this would require the auxiliary and camper battery to be the same type and capacity to avoid one of them never charging fully.
My main concern/question is whether there is a risk of multiple DC-DC chargers interfering with each other in some way I don't understand. Alternatively, is there an easier way to do this that I haven't thought of?
We have a solar panel to charge the camper battery while set up. We also want to be able to charge the battery while driving, as we have a couple of trips planned where we will be driving for long periods during the day, so have limited access to power from the sun. It's not practical to fit a solar panel to the camper that would charge while driving. We mostly camp in areas where electricity is not available, so need to be self-sufficient for up to two weeks. Definitely not interested in using a generator.
The camper battery is used for lights, charging devices etc but the main draw is a fridge installed in the camper(this model, without the internal battery). The current battery in the camper is dead and will be replaced by a 150 Ah or higher either AGM or Lithium battery, depending on price and suitable size availability.
My current plan is to install a second DC-DC charger in the car (in the car rather than the camper because of the need for an ignition trigger wire to the charger because of the 'smart' alternator) and draw from the auxiliary battery to charge the camper battery. Effectively, this would mean the main car battery charges the auxiliary battery, which then charges the camper battery. I'd prefer to avoid having to run another set of heavy cables all the way to the front of the car to charge both from the main battery, but will do this if I need to. The electrical connection from car to camper will be via an Anderson plug connection carrying the charge current to the camper battery in addition to the normal trailer wiring connector (7 pin flat). I'm aware of the need for suitably heavy cables etc.
A possible alternative would be to simply connect the auxilliary battery to the camper battery, effectively making them one large battery from the view of the current DC-DC charger, but I think this would require the auxiliary and camper battery to be the same type and capacity to avoid one of them never charging fully.
My main concern/question is whether there is a risk of multiple DC-DC chargers interfering with each other in some way I don't understand. Alternatively, is there an easier way to do this that I haven't thought of?
Response by poster: I see there are details I missed including :-(
1. The DC-DC charger already in the car has 25A capacity and the car alternator has, I believe, 120A capacity.
2. The DC-DC charger only operates when the car engine is running, using a trigger from the ignition circuit and doesn't have the ability to draw from the auxiliary battery to the main battery. So there's no way for the auxiliary battery to flatten the main battery.
posted by dg at 4:44 PM on August 21, 2022
1. The DC-DC charger already in the car has 25A capacity and the car alternator has, I believe, 120A capacity.
2. The DC-DC charger only operates when the car engine is running, using a trigger from the ignition circuit and doesn't have the ability to draw from the auxiliary battery to the main battery. So there's no way for the auxiliary battery to flatten the main battery.
posted by dg at 4:44 PM on August 21, 2022
Reason to use a DC-DC chargers:
1. reduces the maximum amp draw during charging. This matters if you have a Lithium battery on a thick wire directly to your alternator - such situations are rumored to be hard on the alternator (though many folks dispute this).
2. provides the correct DC charging voltage. This matters if the wires are thin and long, so you have a lot of voltage sag. If so, you would want the DC-DC charger mounted in the travel trailer as close to the "leisure"* battery as possible - that way the voltage sag is beore the DC-DC charger, which can compensate, so your "leisure" battery gets the proper voltage.
* Since you have 3 batteries, it helps to name them :-) and I've learned that in USA, a RV trailer battery is called a "house" battery, but in UK it is a "leisure" battery. The latter makes me smile.
posted by soylent00FF00 at 4:58 PM on August 21, 2022 [1 favorite]
1. reduces the maximum amp draw during charging. This matters if you have a Lithium battery on a thick wire directly to your alternator - such situations are rumored to be hard on the alternator (though many folks dispute this).
2. provides the correct DC charging voltage. This matters if the wires are thin and long, so you have a lot of voltage sag. If so, you would want the DC-DC charger mounted in the travel trailer as close to the "leisure"* battery as possible - that way the voltage sag is beore the DC-DC charger, which can compensate, so your "leisure" battery gets the proper voltage.
* Since you have 3 batteries, it helps to name them :-) and I've learned that in USA, a RV trailer battery is called a "house" battery, but in UK it is a "leisure" battery. The latter makes me smile.
posted by soylent00FF00 at 4:58 PM on August 21, 2022 [1 favorite]
Can the solar panel be on the roof, charging while you drive?
posted by theora55 at 5:15 PM on August 21, 2022 [1 favorite]
posted by theora55 at 5:15 PM on August 21, 2022 [1 favorite]
We have one of these in our sprinter, charging 224 aH of AGM battery while we drive. Works great, easy to install. High output alternator. Our house battery is also charged by solar on the roof, but this does a great job bulk charging while we drive.
https://www.bluesea.com/products/7610/SI-ACR_Automatic_Charging_Relay_-_12_24V_DC_120A
posted by Pantengliopoli at 9:03 PM on August 21, 2022 [2 favorites]
https://www.bluesea.com/products/7610/SI-ACR_Automatic_Charging_Relay_-_12_24V_DC_120A
posted by Pantengliopoli at 9:03 PM on August 21, 2022 [2 favorites]
Response by poster: Thanks, but a relay system won't work with the 'smart' alternator. These monitor the engine battery and turn the alternator if the battery voltage reaches a certain level so, as soon as the engine battery is charged, nothing goes to any other battery.
posted by dg at 12:15 AM on August 22, 2022
posted by dg at 12:15 AM on August 22, 2022
Best answer: I think you're right to worry about the two DC-DC chargers interfering with each other. The scenario I see being problematic is when both the auxiliary and trailer batteries are low. The existing 25A DC-DC unit will be trying to charge the auxiliary battery, but the DC-DC for the trailer might keep kicking in, dragging down the auxiliary battery voltage and taking up the available current from the first charger. You might get a situation where the trailer charger keeps shutting down as the auxiliary battery voltage drops. Or where the auxiliary charger times out and gives up because it's not succeeding at raising the voltage of the auxiliary battery.
It should work OK if the second DC-DC has a lower charge current than the first one, because that means that even if the trailer charger is pulling say 15A, you still have 10A from the first charger going into the auxiliary battery. But you may still get some weird interactions.
And you run into the next problem - 25A total just isn't that much charge current for what's going to be a total of at least 250Ah of batteries. Even assuming you could charge at 25A continuously, that'd mean a 10h drive to fully recharge both, and in practice the charge current has to be reduced as the batteries top up, so you're looking at a lot longer than that.
So my suggestion is that you install the new DC-DC charger with its input supply in parallel with the first one, so that they can each charge one battery independently, and so you can take advantage of more of your 120A alternator capacity. That doesn't necessarily mean running more heavy cables between the front and back of the car - you could mount both DC-DCs in the rear of the car and use your existing cables to supply them both, if big enough.
You also need to give some serious thought to safety and protection of the charge cables between the car and the trailer. They're going to be very vulnerable to wear and damage. A short between them, or to the car or the hitch, could dump thousands of amps out of the trailer battery. You'll want a protective sleeve around the cables, and a fuse at the trailer end, as close as possible to the battery, rated for a bit above the charge current you plan to use. Also, the common Anderson connectors aren't waterproof. Use some IP65+ rated ones, or the contacts will get wet and corrode.
posted by automatronic at 2:19 AM on August 22, 2022 [2 favorites]
It should work OK if the second DC-DC has a lower charge current than the first one, because that means that even if the trailer charger is pulling say 15A, you still have 10A from the first charger going into the auxiliary battery. But you may still get some weird interactions.
And you run into the next problem - 25A total just isn't that much charge current for what's going to be a total of at least 250Ah of batteries. Even assuming you could charge at 25A continuously, that'd mean a 10h drive to fully recharge both, and in practice the charge current has to be reduced as the batteries top up, so you're looking at a lot longer than that.
So my suggestion is that you install the new DC-DC charger with its input supply in parallel with the first one, so that they can each charge one battery independently, and so you can take advantage of more of your 120A alternator capacity. That doesn't necessarily mean running more heavy cables between the front and back of the car - you could mount both DC-DCs in the rear of the car and use your existing cables to supply them both, if big enough.
You also need to give some serious thought to safety and protection of the charge cables between the car and the trailer. They're going to be very vulnerable to wear and damage. A short between them, or to the car or the hitch, could dump thousands of amps out of the trailer battery. You'll want a protective sleeve around the cables, and a fuse at the trailer end, as close as possible to the battery, rated for a bit above the charge current you plan to use. Also, the common Anderson connectors aren't waterproof. Use some IP65+ rated ones, or the contacts will get wet and corrode.
posted by automatronic at 2:19 AM on August 22, 2022 [2 favorites]
Response by poster: Thanks automatronic - you've triggered the exact thing I couldn't think of that was worrying me. A DC-DC charger will turn off when the battery it's charging from drops below a certain voltage (11.9v for the one I already have) So, if the auxiliary battery is below that, the camper battery won't charge until the auxiliary battery reaches 12.2v. Combined with the charger for the auxiliary battery turning off until the main battery reaches 12.2v, there are a lot of dependencies in play in that setup.
posted by dg at 3:37 PM on August 22, 2022
posted by dg at 3:37 PM on August 22, 2022
The forums at CheapRvLiving.com may be really helpful; vandwellers can be quite resourceful.
posted by theora55 at 10:30 AM on August 23, 2022
posted by theora55 at 10:30 AM on August 23, 2022
Response by poster: Thanks for your input everyone.
I went for this DC-DC charger mounted in the back of the car adjacent to the existing (identical) one that charges the car auxiliary battery. I mostly used this one because it's identical to the existing one and that made it easier to install in the built-in cabinetry in the back of the car.
I spliced the inputs to the cables that come from the main car battery and connected the alternator output (and ground) to an Anderson plug, replacing the existing trailer plug with this one to keep it low profile and avoid anything hanging down below the tow bar where it will get hung up when off-roading.
I decided on a 145Ah AGM deep cycle battery in the camper. I would have preferred a Lithium battery, but the cost was so much higher that I considered the benefits didn't justify the extra cost. If I had been able to find one that fitted in my preferred location, I probably would have gone Lithium, but the slimline batteries (the only ones that would fit in terms of width) are all too tall to go in that compartment. Also, my wife was a bit concerned about the risk of Lithium batteries catching fire.
It's all installed and works as expected. There was a nervous wait when I plugged the camper in and started the car up, because it takes a little while for the DC-DC charger to kick in while it waits for the car main battery to charge after starting. Once it kicked in, it happily charged at 21 Amps, then reduced to 7 Amps after about 10 minutes and the other DC-DC charger happily did its usual thing alongside it. After another 10 minutes or so, it switched to 'float charge' mode.
Only one tiny surprise in the whole thing - when I removed the existing (7 pin flat) trailer wire plug, I discovered it had 9 wires in it. It turns out the other two are for a switch that disconnects the reverse parking sensors when a trailer is plugged in. Our car doesn't have reverse sensors but, if it did, I wouldn't have been able to use the combined trailer plug.
There's also one thing that isn't yet tested - there is a 50 Amp fuse close to each of the three batteries and, if both DC-DC chargers were to be charging at their maximum 25 Amp capacity simultaneously, this could blow the fuse at the main car battery, but I'll see how that goes. I don't really want to increase the size of that fuse unless I have to.
posted by dg at 7:26 PM on September 5, 2022
I went for this DC-DC charger mounted in the back of the car adjacent to the existing (identical) one that charges the car auxiliary battery. I mostly used this one because it's identical to the existing one and that made it easier to install in the built-in cabinetry in the back of the car.
I spliced the inputs to the cables that come from the main car battery and connected the alternator output (and ground) to an Anderson plug, replacing the existing trailer plug with this one to keep it low profile and avoid anything hanging down below the tow bar where it will get hung up when off-roading.
I decided on a 145Ah AGM deep cycle battery in the camper. I would have preferred a Lithium battery, but the cost was so much higher that I considered the benefits didn't justify the extra cost. If I had been able to find one that fitted in my preferred location, I probably would have gone Lithium, but the slimline batteries (the only ones that would fit in terms of width) are all too tall to go in that compartment. Also, my wife was a bit concerned about the risk of Lithium batteries catching fire.
It's all installed and works as expected. There was a nervous wait when I plugged the camper in and started the car up, because it takes a little while for the DC-DC charger to kick in while it waits for the car main battery to charge after starting. Once it kicked in, it happily charged at 21 Amps, then reduced to 7 Amps after about 10 minutes and the other DC-DC charger happily did its usual thing alongside it. After another 10 minutes or so, it switched to 'float charge' mode.
Only one tiny surprise in the whole thing - when I removed the existing (7 pin flat) trailer wire plug, I discovered it had 9 wires in it. It turns out the other two are for a switch that disconnects the reverse parking sensors when a trailer is plugged in. Our car doesn't have reverse sensors but, if it did, I wouldn't have been able to use the combined trailer plug.
There's also one thing that isn't yet tested - there is a 50 Amp fuse close to each of the three batteries and, if both DC-DC chargers were to be charging at their maximum 25 Amp capacity simultaneously, this could blow the fuse at the main car battery, but I'll see how that goes. I don't really want to increase the size of that fuse unless I have to.
posted by dg at 7:26 PM on September 5, 2022
Best answer: Don't worry about the 50A fuse. Fuses aren't precise devices. If you look up the datasheet for a fuse you'll see a time-current curve.
A 50A fuse is required to handle 50A continuously without failing. If you put 60A through one it would get hot, but only very slowly. It might blow after a few hours, or maybe not at all in cold conditions. Take it to 80A or so and it will blow in a few minutes. At 100A it should blow within a few seconds. It takes hundreds or thousands of amps to make one blow suddenly. You're not going to blow a 50A fuse with two 25A chargers. It's there to save you from a short circuit with a huge fault current, and that's pretty much all.
posted by automatronic at 6:39 PM on September 6, 2022 [1 favorite]
A 50A fuse is required to handle 50A continuously without failing. If you put 60A through one it would get hot, but only very slowly. It might blow after a few hours, or maybe not at all in cold conditions. Take it to 80A or so and it will blow in a few minutes. At 100A it should blow within a few seconds. It takes hundreds or thousands of amps to make one blow suddenly. You're not going to blow a 50A fuse with two 25A chargers. It's there to save you from a short circuit with a huge fault current, and that's pretty much all.
posted by automatronic at 6:39 PM on September 6, 2022 [1 favorite]
Response by poster: Thanks automatronic - that's really the only reason the fuses are there - to protect against a short circuit. I wasn't really expecting it to be a problem, but I appreciate the reassurance.
posted by dg at 8:21 PM on September 6, 2022
posted by dg at 8:21 PM on September 6, 2022
This thread is closed to new comments.
The auxiliary battery almost certainly has automatic disconnects to prevent it from drawing down the engine battery, as well as a manual connect so you can use the auxiliary battery to boost starting power of your engine battery. This is added complexity for your planning that you should bear in mind.
You certainly don't want to be drawing down batteries in the vehicle (neither the auxiliary or the primary) from the coach, ever, when the engine isn't running. I suggest an RV battery isolator connected between the positive pole on the engine battery, continuing on to the positive pole of the coach, with ground return through the normal ground circuit of your vehicle. As you said, you have to use heavy cables for the power and ground connections to and from the trailer.
To ensure that this configuration works, your alternator has to have enough capacity to charge your very discharged coach battery and a very discharged alternate battery, as well as all the other loads in the car. What is the maximum current deliverable by your "smart alternator? (they're all pretty smart, varying the voltage as needed. A forum said that this smart alternator relies on a current shunt in the ground circuit, and that connecting to the wrong side of the shunt is problematic, hence the use of the chassis ground circuit for your power connections).
One concern about the shunt. Old-style shunts have a maximum current that they are rated for, which if exceeded causes heating, and perhaps permanent alteration of the accuracy of the shunt. Make sure that the shunt is up to the job with the maximum added load of a very discharged coach battery.
You'll want to install a resettable overcurrent protection device in your coach battery charge circuit (and some current protection in your solar panel, as well, if you don't have any already).
Salt this liberally, be cautious, and have fun.
posted by the Real Dan at 4:30 PM on August 21, 2022 [2 favorites]