Wiring question

[AJMike]

A simple wiring question. I think I know the answer but the more I think about it the less sure I am that what I believe is correct.

The wiring table for the DC-DC charger in my RV says that I need 4 AWG wire for the 11 foot distance from my starter battery to the charger for the Lithium batteries. The guy who did the installation used 4 AWG for about the 7 feet from the starter battery to the junction post under the coach step, then added an inline MAXI fuse holder with 8 AWG wire and then ran 3 feet of 6 AWG wire to the charger. So here is my question.

I assumed that if the table said I needed 4 AWG for 11 feet to hold the needed current it meant that I needed 4 AWG the entire distance. My understanding is that the larger wire is needed to reduce the resistance and keep the wiring from getting too hot and affecting the impedance, and I just assumed the entire run had to be 4 AWG. But if the issue is to keep the wiring from getting too hot perhaps it is OK to add short pieces of 6 AWG or 8 AWG since they would not add too much resistance to the total length.

The more I think about it the less sure I am that what I believed was right. I was going to have the wiring re-run using 4 AWG everywhere, but do I need to do that?

And a second question. The wiring from the starter battery to the charger, and from the charger to the batteries, both have the hot and ground. That is, there are 2 wires to the charger and 2 more from the charger to the house batteries. The guy who gave me the estimate for replacing all of the wiring with 4 AWG told me I only needed to do the hot wire, not the ground. That is, I could leave the current mix of 4 AWG, 6 AWG and 8 AWG in the ground wire and only needed to upgrade the hot wire. Is that right? I would have thought that both needed to be replaced, but since he is saving me money I know he is not trying to over-sell me. But is he under-selling me? Do I need to upgrade both the hot and ground wire?

Thanks.

[Goodspike]

I don't know the answer to all your questions--or any of them really. But I do have some concerns and suggestions.

The size of wire is not just about current, it's also about voltage drop. So for example, some early USB cables are too small of gauge to allow a phone to charge after a 3' run! And voltage drop may be the reason for the size requirement more than current.

You can calculate the voltage drop at this site, but it will be a bit tough because you'll have to start with the length of the first piece using 12 volts as the start, the second piece will start with whatever the first piece ended up with, and the third whatever the second ended up with. You could compare that number to whatever the voltage drop would be if it were a length of the proper size.

https://www.calculator.net/voltage-drop-calculator.html

Oh, and I'm not 100% certain all that is accurate. Actually measuring it over the distance with a multimeter and then comparing it to the calculated value on the website with all proper size would be better. For one thing, the connections might add to the voltage drop.

But really I have to add, this job sounds terribly amateurish. They didn't have the right materials so they just used what they had. And who runs a 7' run and then adds a fuse at that point? I wouldn't be happy.

As to the return trip, I'm not sure why you couldn't just ground to the frame nearby rather than running wire the entire length. I'd be interested in what others have to say about that.

And finally, some vehicles use a shunt to determine battery usage and charging from the alternator, so connecting directly to the positive or negative terminal might throw those systems off, leading to a dead starting battery eventually. I'm not sure where you would find that sort of information for your vehicle, but being a 2018 that is very possible. My suggestion of grounding to the frame would make this less likely if the shunt is on the ground side of the battery.

[Goodspike]

Originally Posted by Goodspike

Oh, and I'm not 100% certain all that is accurate. Actually measuring it over the distance with a multimeter and then comparing it to the calculated value on the website with all proper size would be better. For one thing, the connections might add to the voltage drop.

I should add you'd need to take this measurement when you knew the device was at full draw, because the amount of voltage drop is related to current.

[AJMike]

Originally Posted by Goodspike

But really I have to add, this job sounds terribly amateurish. They didn't have the right materials so they just used what they had. And who runs a 7' run and then adds a fuse at that point? I wouldn't be happy.

Exactly.

When I upgraded the batteries in the RV I decided to go with 2 100 AH Battle Born batteries and I contacted them to ask if they could recommend any local installers where I live. They sent me 2 names. One of those has a shop front but when I talked with them they wanted to use a BIM to control the current from the alternator and that is something that BB specifically recommended against, so I chose the other guy.

He operates like the old "shade tree mechanics" used to - no store front, just his own place - and he did the work. I had no idea about this miso-mash of wiring until I started having issues with the charger overheating because of its location and lack of ventilation. That made me move the unit and exceed what I assumed to be the maximum length for the wire gauge I could see, and that caused me to contact someone about upgrading the wiring. He examined the setup and let me know what he found, and that is what I posted.

Originally Posted by Goodspike

As to the return trip, I'm not sure why you couldn't just ground to the frame nearby rather than running wire the entire length. I'd be interested in what others have to say about that.

The wiring diagram for the charger specifies that the negative from the starter battery should be wired directly to the negative on the charger input and the negative from the house battery should be wired directly to the negative on the charger output. The same for the positives, so that is how I had it wired.

This charger is the Renogy charger. Victron makes 2 30 amp chargers, one using the ground connection and one using a direct negative wiring like the Renogy.

Originally Posted by Goodspike

But really I have to add, this job sounds terribly amateurish. They didn't have the right materials so they just used what they had. And who runs a 7' run and then adds a fuse at that point? I wouldn't be happy.

Exactly.

When I upgraded the batteries in the RV I decided to go with 2 100 AH Battle Born batteries and I contacted them to ask if they could recommend any local installers where I live. They sent me 2 names. One of those has a shop front but when I talked with them they wanted to use a BIM to control the current from the alternator and that is something that BB specifically recommended against, so I chose the other guy.

He operates like the old "shade tree mechanics" used to - no store front, just his own place - and he did the work. I had no idea about this miso-mash of wiring until I started having issues with the charger overheating because of its location and lack of ventilation. That made me move the unit and exceed what I assumed to be the maximum length for the wire gauge I could see, and that caused me to contact someone about upgrading the wiring. He examined the setup and let me know what he found, and that is what I posted.

Originally Posted by Goodspike

And finally, some vehicles use a shunt to determine battery usage and charging from the alternator, so connecting directly to the positive or negative terminal might throw those systems off, leading to a dead starting battery eventually. I'm not sure where you would find that sort of information for your vehicle, but being a 2018 that is very possible. My suggestion of grounding to the frame would make this less likely if the shunt is on the ground side of the battery.

I have the Victron 712 BM installed and use it to determine what the batteries are doing - charging or discharging.

[Goodspike]

Originally Posted by AJMike

I have the Victron 712 BM installed and use it to determine what the batteries are doing - charging or discharging.

I was referring to your starter battery possibly discharging.

Again I don't know if this direct connection would be an issue with your vehicle, but it would be with my Chevy truck, and for my vehicle it's the negative at issue. So grounding to the frame would be okay, but not the battery terminal.

You could just monitor your starter battery voltage to see if a problem develops. I'm not even sure how you'd research what your vehicle does. If there is a problem hopefully it's on the ground side. If it is you'd need to contact the manufacturer of that device and find out how important that direct connection is. I could see it might be on the positive side, but I don't see it at all on the ground side.

[AJMike]

Originally Posted by Goodspike

I was referring to your starter battery possibly discharging.

Again I don't know if this direct connection would be an issue with your vehicle, but it would be with my Chevy truck, and for my vehicle it's the negative at issue. So grounding to the frame would be okay, but not the battery terminal.

You could just monitor your starter battery voltage to see if a problem develops. I'm not even sure how you'd research what your vehicle does. If there is a problem hopefully it's on the ground side. If it is you'd need to contact the manufacturer of that device and find out how important that direct connection is. I could see it might be on the positive side, but I don't see it at all on the ground side.

I don't pretend to know, but once I got the charger moved to where it had sufficient ventilation and it was not overheating it seemed to work properly and I had no issues with the starter battery.

Here is the wiring diagram from the manual so I assume this is how it is supposed to be installed.

[Goodspike]

I think the reason my truck is the way it is related to fuel mileage and GM trying to squeeze out better MPG ratings. That same concern probably doesn't apply to motorhomes, so yours may be a more old-style system.

[BobC]

Your diagram appears to be for a Renogy DC to DC charger. I have the 20A version. From your comments about AWG, I'm guessing your's is possibly a 40A charger.

First, your instincts about AWG are correct. If 4 AWG is spec'd, it should be 4 AWG for the entire run. However, if your charger is a 40A model, the specs show either 6 AWG or 4 AWG for an 8' to 16' run. Your 7' of 4 AWG and 3' of 6 AWG definitely wouldn't be an issue and the 1' of 8 AWG might not be an issue since it's so short. This is academic since the fuse should be close to the chassis battery, in which case the cable between it and the battery should be either 4 or 6 AWG.

In any case the wiring diagram (both your's and the one in the manual) doesn't seem to match the specs in the manual. The wiring diagram shows a fuse between the chassis battery and the charger but not between the charger and the house battery. It does show a fuse between the alternator and the chassis battery but that should already be there as part of your vehicle's charging system.

However, the specs (page 14 of the installation manual) gives ratings for two DC-to-DC charger related fuses, one between the chassis battery and charger and a second one between the charger and the house battery.

https://www.renogy.com/content/RNG-D...060-Manual.pdf

The one on the input side should be as close to the chassis battery as possible. The one on the output side should be as close to the charger as possible.

I hope this helps.

[AJMike]

Originally Posted by BobC

Your diagram appears to be for a Renogy DC to DC charger. I have the 20A version. From your comments about AWG, I'm guessing your's is possibly a 40A charger.

Yes. That is correct, although my 40 amp charger has stopped working and I have temporarily replaced it with the 20 amp version until I can get it fixed or replaced. It should still be under warranty.

Originally Posted by BobC

First, your instincts about AWG are correct. If 4 AWG is spec'd, it should be 4 AWG for the entire run. However, if your charger is a 40A model, the specs show either 6 AWG or 4 AWG for an 8' to 16' run. Your 7' of 4 AWG and 3' of 6 AWG definitely wouldn't be an issue and the 1' of 8 AWG might not be an issue since it's so short. This is academic since the fuse should be close to the chassis battery, in which case the cable between it and the battery should be either 4 or 6 AWG.

I am talking about the input to the charger so 14 feet should be 4 AWG as far as I can tell from the table.

What I don't see is why the 7' of 4 AWG and 6' of 6 AWG (I had to add another 3 feet to move the charger to a location that was properly ventilated. The installer left it in a non-ventilated space and it kept overheating) would not be an issue. I believe what you wrote, but I don't understand why since the table calls for 4 AWG. Or were you referring to the output side of the charger which has a smaller wire specification?

Originally Posted by BobC

In any case the wiring diagram (both your's and the one in the manual) doesn't seem to match the specs in the manual. The wiring diagram shows a fuse between the chassis battery and the charger but not between the charger and the house battery. It does show a fuse between the alternator and the chassis battery but that should already be there as part of your vehicle's charging system.

However, the specs (page 14 of the installation manual) gives ratings for two DC-to-DC charger related fuses, one between the chassis battery and charger and a second one between the charger and the house battery.

The one on the input side should be as close to the chassis battery as possible. The one on the output side should be as close to the charger as possible.

The installer added 2 fuses, one between the house batteries and the wire that goes to the output of the charger and one between the 7' run of 4 AWG wire and the remaining 6' of 6 AWG wire. From what you are saying they are in the wrong place.

Originally Posted by BobC

I hope this helps.

It does, but I do have one more question.

Before the 40 amp charger stopped working I noticed that it would start charging at 40 amps but then quickly start dropping down to about 35 amps, then close to 30 amps, then maybe 25 amps as it charged the batteries. I am not referring to a drop down when the batteries were close to being charged, but early in the charging process, and the values that I gave of 35, 30 and 25 are not exact, and the current would jump around as well, usually down, but sometimes up as well.

So, for example, the Lithium batteries might be down to 50%. The charger would output 40 amps for perhaps 20 minutes (and the batteries are now up to perhaps 65%) and then drop down to 35 amps. Another 15 minutes (now at perhaps 70 or 75%) it would drop down to 30 amps. I thought this might be due to heat buildup due to the wires not being sized correctly, but perhaps this is the way the charger works. I just don't know.

Have you seen this with your 20 amp charger?

[AJMike]

Originally Posted by BobC

I hope this helps.

One more question.

The guy who looked at the wiring and told me about the mixed wire usage said that if I wanted it rewired all 4 AWG I only needed to do the positive side, not the ground. I don't understand why that would be since I assume the current needs to make a complete circuit, hot and ground.

He would be saving me money so I know he is not trying to over-sell me, but I don't understand why I would only need to have the positive side rewired. Is that correct?

[BobC]

First, according to the table, for a 16' run the requirement is "16FT/6 - 4 AWG". Therefore your combination of 7' 4 AWG and 6' of 6 AWG should be more than adequate.

There are calculators and tables online that will tell you what AWG you need in terms of length and max amps. Here are a couple of links:

https://www.wirebarn.com/Wire-Calculator-_ep_41.html

https://www.bluesea.com/resources/1437

Note: Your situation would be what Blue Sea classifies as critical at 3% (the Wirebarn is based on 2% which is more conservative).

If you're ground runs to your frame and not back to your battery, the AWG can be smaller (higher number) depending on its length. If it runs all the way back to your starting battery, it needs to be the same AWG as the positive run. The Wire Barn calculator will tell you what's required based on the length of the run to your frame.

Secondly, I only installed a battery monitor yesterday and my house battery (AGM, not LiFePO4) was almost full so, after starting the engine, it showed something in excess of 10 amps that dropped to around 9 amps in a few seconds. My Renogy experience is limited, however, my experience with solar charge controllers has taught me that the solar controller is only going to provide what the batteries need in terms of amps and the DC to DC charger is going to operate the same. My impression is it's reacting quickly based on the feedback it's getting from the battery. I think it's unlikely that temperature is a factor unless it's unusually warm or cold.

[AJMike]

Originally Posted by BobC

First, according to the table, for a 16' run the requirement is "16FT/6 - 4 AWG". Therefore your combination of 7' 4 AWG and 6' of 6 AWG should be more than adequate.

That is really odd.

I have looked at that table at least a dozen times in the last couple of days and I never noticed that it said 6-4 AWG. I only saw the second line that contained the 4 AWG information. I am just sitting here shaking my head wondering how that happened.

Originally Posted by BobC

If you're ground runs to your frame and not back to your battery, the AWG can be smaller (higher number) depending on its length. If it runs all the way back to your starting battery, it needs to be the same AWG as the positive run. The Wire Barn calculator will tell you what's required based on the length of the run to your frame.

That makes perfect sense to me. In this case the ground wire runs from the starter battery to the input to the charger, as it does in your version as well, so I assume that means that if the hot wire is upgraded they both should be.

So I assume that probably means that the wiring is OK and does not need to be upgraded but, if I do upgrade it, I should upgrade both the hot and ground wires.

Originally Posted by BobC

Secondly, I only installed a battery monitor yesterday and my house battery (AGM, not LiFePO4) was almost full so, after starting the engine, it showed something in excess of 10 amps that dropped to around 9 amps in a few seconds. My Renogy experience is limited, however, my experience with solar charge controllers has taught me that the solar controller is only going to provide what the batteries need in terms of amps and the DC to DC charger is going to operate the same. My impression is it's reacting quickly based on the feedback it's getting from the battery. I think it's unlikely that temperature is a factor unless it's unusually warm or cold.

I had made the initial assumption that the wiring was the problem and that I was seeing the current jump all over the place due to too much resistance in the wire and I further assumed that I needed to upgrade the wiring as a safety measure. I guess all of that was wrong.

Thanks for the posts. You have been a big help.

[BobC]

The one thing you still need to address is the location and wire gauges of the fuses. The fuse between the charger and starter battery should be close to the starter battery. Since you already have 4 AWG wire to the battery, I'd recommend a short piece of 4 AWG between the fuse and the battery. Personally I'd prefer a re-settable 60A breaker but it's not necessary.

The same goes for the output fuse, it should be close to the charger (ie. source) and should be wired with 8 or 6 AWG cable, depending on the length of the run. Again a re-settable breaker (in this case 50A) would be preferable but not necessary.

[Goodspike]

Originally Posted by BobC

The one thing you still need to address is the location of the fuse between the charger and starter battery. It should be close to the battery and should be connected via 4 or 6 AWG cable. Personally I'd prefer a re-settable 60A breaker.

That was the part I highlighted in my first post. I cannot imagine what they were thinking with the fuse in that location. I wonder what size that fuse is they put in?

I do find those wire gauge specs odd. Who specifies a minimum as this or that? "You should drink a minimum of 128 or 96 ounces of water a day." Usually a minimum is one number.

[AJMike]

Originally Posted by BobC

The one thing you still need to address is the location and wire gauges of the fuses. The fuse between the charger and starter battery should be close to the starter battery. Since you already have 4 AWG wire to the battery, I'd recommend a short piece of 4 AWG between the fuse and the battery. Personally I'd prefer a re-settable 60A breaker but it's not necessary.

The same goes for the output fuse, it should be close to the charger (ie. source) and should be wired with 8 or 6 AWG cable, depending on the length of the run. Again a re-settable breaker (in this case 50A) would be preferable but not necessary.

I assume both fuse holders are in the wrong place.

The starter battery in the Winnebago Fuse is under the driver's seat so I guess that is where the fuse ought to go. The charger is located in the dinette table seat and it should be easy to locate a fuse there, and a circuit breaker could be mounted on the back of the seat in the house entrance area.

I assume one good thing about the fuse holders is that since they are not circuit breakers it was easy to replace the 60 and 50 amp fuses with 30 and 25 amp fuses when I installed the 20 amp charger. If they had been circuit breakers it would have been an issue since the 20 amp charger requires half the protection values of the 40 amp charger.

If I can get the 40 amp charger repaired, either under warranty or outside of it, I expect to replace the 20 amp charger with the 40 amp version since it would make charging the 200AH of battery easier.

[Goodspike]

Originally Posted by AJMike

I assume one good thing about the fuse holders is that since they are not circuit breakers it was easy to replace the 60 and 50 amp fuses with 30 and 25 amp fuses when I installed the 20 amp charger. If they had been circuit breakers it would have been an issue since the 20 amp charger requires half the protection values of the 40 amp charger.

I suppose, but fuses are generally based on the size of the wire, not the actual demand typically placed on the wire. So for example, in today's homes a typical 120 volt circuit might have 14 watts of LED lighting and a couple of wall wart phone chargers, but it still has a 15 amp breaker.

But yeah, if you thought this device might malfunction and start drawing more power than it should . . .. it certainly wouldn't hurt anything.

[BobC]

Originally Posted by Goodspike

That was the part I highlighted in my first post. I cannot imagine what they were thinking with the fuse in that location. I wonder what size that fuse is they put in?

I do find those wire gauge specs odd. Who specifies a minimum as this or that? "You should drink a minimum of 128 or 96 ounces of water a day." Usually a minimum is one number.

It is a bit strange since any gauge larger than the minimum would be fine. There'd be no downside, other than cost, to put in 2 AWG instead of 4 or 6.

[AJMike]

Originally Posted by BobC

It is a bit strange since any gauge larger than the minimum would be fine. There'd be no downside, other than cost, to put in 2 AWG instead of 4 or 6.

One of the issues I have tried to keep in mind is the question of what charger I should use. I had the Renogy 40 amp charger installed until it failed, and I wanted to size the wire and circuit protection for a new Rengoy 40 amp charger (60/50 amp fuses) but alway thought that if I could not get the 40 amp charger fixed or replaced and if the 20 amp that I have in there now turned out to be too small I might want the Victron 30 amp.

Upgrading the wiring would be fine with a smaller charger but I also would need to be able to downsize the circuit protection and that is easier with a fuse than with a circuit breaker.

[BobC]

Originally Posted by Goodspike

I suppose, but fuses are generally based on the size of the wire, not the actual demand typically placed on the wire. So for example, in today's homes a typical 120 volt circuit might have 14 watts of LED lighting and a couple of wall wart phone chargers, but it still has a 15 amp breaker.

But yeah, if you thought this device might malfunction and start drawing more power than it should . . .. it certainly wouldn't hurt anything.

In my case, the cable from the starting battery to my charger was a pre-existing 2 AWG cable serving the wheel chair lift. The specified input fuse size for my 20A charger is only 30A, which is way less than the 2 AWG cable can handle. Therefore, I connected the 2 AWG cable to an insulated terminal block near the charger and put the fuse between it and the charger, using 10 AWG wire since it's only a 1' run from the terminal block to the charger.

[BobC]

Originally Posted by AJMike

One of the issues I have tried to keep in mind is the question of what charger I should use. I had the Renogy 40 amp charger installed until it failed, and I wanted to size the wire and circuit protection for a new Rengoy 40 amp charger (60/50 amp fuses) but alway thought that if I could not get the 40 amp charger fixed or replaced and if the 20 amp that I have in there now turned out to be too small I might want the Victron 30 amp.

Upgrading the wiring would be fine with a smaller charger but I also would need to be able to downsize the circuit protection and that is easier with a fuse than with a circuit breaker.

I suppose it all depends on the size of your battery bank, how many amp hours you expect to use in a day and how long you'd like to be able to charge it. The output of your alternator would also be a factor in terms of how much it can handle over and above the needs of your vehicle.

If I were you, I'd gather all the info and contact Renogy for their advice. Although there's no downside of having a larger charger than necessary, other than cost, why spend more than necessary?

If you plan on installing solar, Renogy makes a combo DC to DC Charger/MPPT Solar Controller combo unit:

https://www.renogy.com/dcc50s-12v-50...ger-with-mppt/

Also, here's a review:

https://diysolarforum.com/threads/re...r-w-mppt.1406/