Wire size matters

JPS350

Senior Member
Joined
Jan 22, 2023
Messages
142
And apparently Grand Design never got the memo.

This is related to a couple of existing posts, but thought I'd add my experience with our 2021 imagine XLS 22mle. We've had it a couple of years and I noticed that the stock charging inverter does a really slow job of charging of the batteries. I have 2x 6v golf cart batts in series . It's not a huge problem when plugged in, but is a pain when off grid and relying on generator for charging. Looking through some of the historical posts here I've noticed that many people with battery issues are recommended to just go to LiFePo4 batteries for faster charging among other benefits. I'm not against the advice, but if you think you're going to fix a stock setup with a simple battery swap, think again.

I can't speak for any other models or brands, but my trailer is set up with 8 awg wiring for the battery main wiring. I was amazed when I saw this (for lack of a better word that doesn't involve a 4 letter word). I estimated that the main power cable has about a 35' run when it heads back into no man's land to the power switch, and arrives at the charger inverter toward the back of the trailer. So out and back, that's approximately a 70' run.

The company installs a 55A charger and expects it to charge the battery. Using a wire size calculator (I like the Blue Sea circuit wizard online, as I'm a boat guy and have done a bit of marine electrical work), if you plug in the numbers, the recommended wire size for that distance and 55 amps would be around 2/0 wiring. The voltage drop using 8 awg will be close to 25%!

So I did a little test. I have a battery monitor shunt installed at my batteries and ran them down below 75% state of charge so I could see just what the charger sent to the batteries. At the start of charging it only put out 16 A. Within several minutes the charge dropped to around 8 A and stayed there for the duration.

So what happened? My theory. First, there's no way it could ever send 55A to the batteries with that small gauge wire. Second, due to the resistance, when charging, the charger is seeing a much higher voltage than the battery and it goes into a variable current mode very early. In essence, it behaves no better than a 10A trickle charger.

The fix? Either replace all the wiring (huge job), move the charger inverter to a location near the battery (another big job) or just install a second good quality smart charger right near the battery (my solution). I plan on putting a Victron 30 A charger in the pass through and plugging it into the existing 120 v outlet there. Currently waiting to see if there's any kind of black Fri. deal coming up, but not holding my breath.

Most owners just plug their trailers in all the time at camgrounds, so they'll likely never notice this charging problem. Lead acid batteries should be charged up to an absorption voltage and held in that phase for a while when charging. It affects longevity of the battery when they aren't. This will never happen with the stock setup, at least on my trailer, as I've never seen battery voltage over about 13.8 v. Hope this bit of info may be helpful to someone else.
 
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1st off, I hope your 2, 6 volt batteries are hooked up in a series, not parallel.

I understand your concerns.

I wonder if the drop in charge amperage is because of the converter switching between modes (boost to normal) as the batteries charge some ??

Also, since these trailers are only 26’ long total, I would think that the 35’ runs you mentioned might be a bit longer than actual.

So far, it seems like our 22 MLE the batteries are charged great, although we might have a thicker wire gauge since we have a 2024.
 
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I don't suppose you could move the charger to the front near the battery and just jumper the rear 8 gauge wires to each other. That's probably over simplified, I expect there is something in the rear that needs power. But maybe run 12v to the back with the converted moved to the front and run a charge wire to the battery separately. I'm trying to figure out a way to avoid having to buy another charger.

At the very least, maybe put a stand alone battery charge (high amp one) on the partially discharged battery and see how many amps that battery arrangement pulls before making a bunch of changes that won't be cost effective.
 
Yes, typo, fixed, thx. Concerning length. An estimate using a tape measure, but battery cables go into the floor on right side of trailer, at some point, over to the battery switch on the other side back to the charger inverter on the other side of the trailer So I doubt I'm off by much.

The drop in current took minutes. Any other smart charger would have given it full current at that state of charge until it hit absorption voltage then dropped the current.

What does charged great mean? If I were always plugged in I might think the same as the battery is always fully charged per a voltage vs state of charge chart. Charged correctly is another matter and speed of recharge too. Neither can happen with my setup as is.

I do hope they made some changes by 2024. The smartest change would have been to locate the charger inverter unit close to the battery. Mine is back near the bathroom.
 
You aren’t going to get those batteries to take 55A regardless of wire size unless you raise voltage well beyond what the charger can produce or the battery can tolerate. You’ll also likely find a 30A breaker in that chain somewhere and you’ll never see charging current approach that. But you may see higher currents if you are both charging a deeply discharged battery and using a significant amount of 12V power in the trailer.
 
I don't suppose you could move the charger to the front near the battery and just jumper the rear 8 gauge wires to each other. That's probably over simplified, I expect there is something in the rear that needs power. But maybe run 12v to the back with the converted moved to the front and run a charge wire to the battery separately. I'm trying to figure out a way to avoid having to buy another charger.

At the very least, maybe put a stand alone battery charge (high amp one) on the partially discharged battery and see how many amps that battery arrangement pulls before making a bunch of changes that won't be cost effective.
It really wouldn't be practical to try and move it. All the 12v fuses and 12v breakers are on it.

Yes, putting another charger on it would verify that. No real need in my case as it can't possibly work correctly as designed and I have a pretty good feeling for how lead acid batteries should behave. I have a decently large battery bank and solar system on my sailboat (and previous boats) and an off grid solar system on my cabin with a 24v lead acid battery bank. Some argue I'm crazy for sticking with flooded lead acid, but treated well, they've served me fine for many years.
 
You aren’t going to get those batteries to take 55A regardless of wire size unless you raise voltage well beyond what the charger can produce or the battery can tolerate. You’ll also likely find a 30A breaker in that chain somewhere and you’ll never see charging current approach that. But you may see higher currents if you are both charging a deeply discharged battery and using a significant amount of 12V power in the trailer.
I agree, they probably wouldn't take 55 amps. They should be able to take 15% of their rated capacity (in my case 215 amp hour capacity), so they should be able to take every bit of what a Vicron 30 a charger can put out during bulk mode, provided it's wired right. This is a far cry from the 8 amps the stock charger will supply due to the poor wiring setup.
 
I discovered the same with mine. I’m moving the converter to the front bay of my 5vr. I think I’m getting > 0.6V drop so even with dead batteries, the converter never goes into bulk mode and in absorption mode I’m only getting about 13V at the batteries when discharged
 
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You sound like you got a grasp of the situation. As you figured out, there is no easy answer. Maybe a DC/DC charger next to the battery would help. I believe that is what they are designed to do.

Do you have a charger/inverter or just a charger [converter]? I cannot imagine an inverter running with 8ga wires. At least an inverter that would run more than a laptop.

Since most campers never use the battery like some of us, Grand Design put the right size wire for the average consumer and what the average battery is installed at the dealership. There is no reason to upsize the wiring for the few 10% that would use it.

If you are the type that likes boon docking, you are responsible for looking at that kind of stuff before you put your hard earned money down for the unit. I know, my main priority was accommodations. I would handle the upgrades myself. I prefer it that way since nobody does it better than me [so I think].

I've mentioned before, if RV manufacturers built them like some of us think, the cost would be so high, none of us would be able to buy them. The additional weight would also be so much, we could not tow them either. The way I see it, as long as the basics are built to last a few years, it's my job to build the rest for my needs.
 
Inverter (1000W) and converter are separate units in mine. The inverter seems to only power the residential fridge and a couple of outlets nearby. The inverter is getting power from the batteries through the 30’ of #6. My rewire will make it less than a foot.
 
I agree, they probably wouldn't take 55 amps. They should be able to take 15% of their rated capacity (in my case 215 amp hour capacity), so they should be able to take every bit of what a Vicron 30 a charger can put out during bulk mode, provided it's wired right. This is a far cry from the 8 amps the stock charger will supply due to the poor wiring setup.
With factory wiring, I can supply 36A to my LiFePO battery. It’s not the wiring. It’s the battery that limits charging current.
 
With factory wiring, I can supply 36A to my LiFePO battery. It’s not the wiring. It’s the battery that limits charging current.
Not necessarily. Depends on wiring configuration. Factory configurations on mine has 30’ of #6 between the converter and batteries. I only get about 10 amps into 3 batteries due to the voltage drop from the added resistance. Thats only 3.3 amps per battery. Surely they can take more than that
 
With factory wiring, I can supply 36A to my LiFePO battery. It’s not the wiring. It’s the battery that limits charging current.
Your wiring may be able to support that much current. Do the math on my setup as I mentioned it above and you'll see it isn't possible.

On your second comment, that's not quite how it works. For lead acid batteries, all 2 and 3 stage smart battery chargers have an initial bulk mode that is constant current until the absorption voltage is reached. That will be either the rating of the charger, or another value if the charger lets you program it in.

This is why it's important to check the specs on your battery to see what the recommended bulk current is, so not to overheat and boil it. The "rule of thumb" for lead acid is 30% of capacity. A check of the spec sheet on my pair of 6v golf carts shows it is 20% for mine. So for my 215 amp hour capacity, that means the max bulk current for mine is 43a. They can take that and more, if you want to overheat them. Once they reach absorption voltage (14.7v is the spec for mine), current is slowly reduced while the voltage remains at absorption voltage until they are fully recharged, then floated at a lower voltage to remain charged. Bulk mode is responsible for around 90% of the charge, so the battery does not limit the charging current during charging.

That said there are big differences in charging rate between lead acid and lithium. Lithium can take at least 50% of its capacity in charging current and I've seen it mentioned that it can take 100%. It charges at fixed current until around 14.2 -14.4 volts and then it's pretty much done, though there may be a fairly short absorption mode (fixed voltage variable current) on the charger.

This is why, when I see people jumping on the lithium band wagon, they really should look at the whole picture and do the lithium battery bank justice by taking care of the issue of the inadequate wiring size and poor charging performance of the stock converter chargers. If you care about the increased charging speed of lithium, why charge a 200ah bank at 10a (my trailer) or 36a (your setup)?
 
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This is why, when I see people jumping on the lithium band wagon, they really should look at the whole picture
I did and that is why I eventually purchased a 125A converter [1200ah batteries]and swapped out the breaker in the trailer to 20A. Now, I just need to change the breaker/wiring in the shop so it does not blow. Might as well throw away the 15A cord adapter too.

It's a never ending ordeal with LifePo4 batteries. It's just better to stick with LA. :p
 
An update on my setup. I installed my 30a Victron IP22 charger today. Put it in the pass through with 9 1/2' of 6 awg cable run directly to the battery.

With the battery bank at 75% state of charge (same as I used with my initial test), charger went into bulk mode at 30a and stayed there until the 14.7 absorption voltage was reached This took around an hour and a half. At this point it was just shy of 92% state of charge. It's currently less than 2 1/2 hours into the charge. Still in absorption mode and state of charge is 97.4% and around 11 a at 14.7v. I'm seeing a voltage drop across the new cables at around 1.2% so they are sized right.

This is the way it's supposed to work. Running on generator will work out a whole lot better now.
 
I just purchased a 600AH LiFePO4 battery and installed in the front bay (55A converter is in the basement with #6 running to the power distribution panel then to the battery or about 30’ of #6). Plugged the TT into shore power and only getting 10A into the battery with 0.6V drop from converter to battery. Moved the battery into the basement beside the converter with direct connection. Plugged the TT into shore power. Now getting 57A from the converter into the battery. Second benefit is battery is now in the heated basement so won’t see freezing temperatures.
 
One last update. You've gotta love the Victron products. Here's the nitty gritty from the charge cycle from the Victron app. for any geeks out there.

Bulk mode took just a little over an hour. At that point, the batteries had charged from around 75% state of charge to about 92%. Yes, it takes quite a while to get that last 8%, but in reality most of that was accomplished within the first couple hours of the absorption charge when current was higher. The portion labeled "rec" is an equalization cycle I set up for this first charge (not a usual part of the charge cycle).

The interesting thing for me is the amount of time the charger spent in absorption mode at 14.7 v. The stock converter chargers in the trailers don't charge that way and likely won't ever get the battery above 13.6-13.8 v especially with the voltage drop from the trailer wiring. The battery never gets fully charged. This matters with lead acid batteries, as the higher voltage charge cycle helps keep them from sulfating and increases battery life. I would guess the average trailer owner only gets a couple of years out of their lead acid battery bank. I just replaced a battery bank on my sailboat that was 8 years old, and that's been my experience with flooded lead acid batteries. Not sure how critical this is with LiFePO4 batteries, but personally I'd want them charged right if I spent the money for one.


Screenshot_20241127-073055.png
 
The interesting thing for me is the amount of time the charger spent in absorption mode at 14.7 v. The stock converter chargers in the trailers don't charge that way and likely won't ever get the battery above 13.6-13.8 v especially with the voltage drop from the trailer wiring.
Not sure I'd like that 14.7v..but I'm still a LiFePO4 newbie. I only regularly charge my lifepo4 batteries to 13.75 most of the time..occasionally 14.4-14.6 to balance.
 
One last update. You've gotta love the Victron products. Here's the nitty gritty from the charge cycle from the Victron app. for any geeks out there.

Bulk mode took just a little over an hour. At that point, the batteries had charged from around 75% state of charge to about 92%. Yes, it takes quite a while to get that last 8%, but in reality most of that was accomplished within the first couple hours of the absorption charge when current was higher. The portion labeled "rec" is an equalization cycle I set up for this first charge (not a usual part of the charge cycle).

The interesting thing for me is the amount of time the charger spent in absorption mode at 14.7 v. The stock converter chargers in the trailers don't charge that way and likely won't ever get the battery above 13.6-13.8 v especially with the voltage drop from the trailer wiring. The battery never gets fully charged. This matters with lead acid batteries, as the higher voltage charge cycle helps keep them from sulfating and increases battery life. I would guess the average trailer owner only gets a couple of years out of their lead acid battery bank. I just replaced a battery bank on my sailboat that was 8 years old, and that's been my experience with flooded lead acid batteries. Not sure how critical this is with LiFePO4 batteries, but personally I'd want them charged right if I spent the money for one.


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Please help me understand. I thought 14.7V was bulk mode and 13.6V was absorption mode?
 
Please help me understand. I thought 14.7V was bulk mode and 13.6V was absorption mode?
13.6 is a converter mode rather than a charging mode.

The idea that there is a difference in charger behavior for bulk and absorption in a so-called smart charger is marketing by charger manufacturers. The charger makes only one change to its output during the charging process, and that is at the end when it sees a low enough current draw that it switches to float voltage.

The idea that the charger is in constant current or constant voltage mode is like saying a vehicle is in constant speed mode when the pedal is on the floor.

The charger is a DC power supply with output voltage in this case set at 14.7V. When it first comes on,it doesn’t have enough current capacity to raise the battery to 14.7V, so it runs at max current while battery voltage increases as it charges. Then, once the battery charge increases enough, the charger is able to hold it at 14.7V with less and less current, so the current falls. The charger never, ever changes its output in any way until it sees that low current draw and switches to float mode. The fact is, it is physically impossible for a charger to change voltage or current independently.

Converters have an intermediate mode where they see usage from the RV and jump up a little above float.

There is no such thing as bulk charge, and there is no such thing as absorption charge except as a description of what’s going on in the battery. At all times while the battery is charging the charger acts as a 14.7V fixed output DC power supply and never changes during the charge.
 

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