Insufficient Charging Wire Size

TNOutback

I know this topic has been widely discussed on this forum, but please humor a guy trying to do some electrical engineering math. I have a 2020 Tab 400 with the 6V batteries in the extreme back of the trailer and the power distribution center (with charger) underneath the front right bench seat.  Conservatively, there is 25 feet of 8 AWG wire for the charging circuit between the distribution center and the terminal block close to the batteries.  Multiple occasions have shown it to be impossible for me to bulk charge the batteries with a generator while boondocking.

Because of the wire size used and the distance to the batteries, I would never get anywhere close to the 55 amp capability of the charger to the batteries, and the charger cannot properly sense the voltage of the batteries, so the charger never enters Bulk mode charging.  Yes, I have seen other posts about people relocating the charger next to the batteries, but gosh darn it, we should not have to do that, and it comes with some trade offs I would prefer to not have to deal with.

In order to see what wire gauge would be needed for the batteries to get the maximum out of the charger, I did some math to calculate the CM (circular mils) of the 12V wire needed to deliver 12V 55 amps approximately 50 feet (distance of the + and - wires) assuming an allowed 3% voltage drop per code:

CM = (12.9 x 55A x 50’) / (3% x 12V) = 98,542 CM

Then cross-referencing the CM with AWG chart, it says I would need 1/0 AWG wire for that amperes/distance/V drop !  If one looks at the chart for the 8 AWG wire actually used in the camper, the max amps allowed for that length of circuit would be 10 amps.

Any EEs see anything wrong here?

Dutch061

I believe that your calculations are probably more than accurate enough. The 8 gauge wire and 40 amp breaker that NuCamp uses is more than adequate for the load that would ever be present for everything in the camper. Even with everything turned on, fans, lights, refrigerator (12 volt compressor type) you will never come close to half of that. In fact it will be around 15 amps if I were to guess because I haven't turned everything on to see what it would actually draw. 

When it comes to the charging time issue (because in reality the WFCO charges the batteries, it just takes extended time to do so), there are several factors at work.

I am attaching a copy of the post that @rh5555 made several months ago regarding the issues and how to overcome this. Roger done all of the hard work, I done the simple task of buying a Progressive Dynamics converter from Amazon along with the removal of the OEM WFCO. 

If you spend all of your time camping with electricity available to you, this upgrade (or fix) will mean nothing to you. For those of us who like to be "remote", it makes all the difference to be able to charge batteries with a generator in 3 or 4 hours VS. 24 to 48 hours. 

My estimate is that you will spend around $275.00 to resolve this and would include the following items (or similar):

PD9260CV Converter with remote pendant - Amazon
1 x 4 gauge black battery cable with a 3/8" lug - Auto Parts store or Amazon 
1 x 4 gauge red battery cable with a 3/8" lug - Auto Parts store or Amazon 
1 x 120 volt dual RV outlet (they clamp on the wire, I got mine at camping world because I was there)

Installation is simple, there are 5 steps. 

Replace the single outlet on the curb side under the bed with the dual
Mount the new converter to the floor with 3 screws
Cut the red and black cables to the proper length to reach the battery junction block
Modify the battery junction block cover to make room
Remove the factory converter form the power center, there are 2 screw and 5 wires to disconnect

Start to finish it will take you about an hour and will resolve your "charge time" issues 100% regardless of the battery chemistry you are using.

The only reason I suggest the PD9260CV is that in the future if you wish to upgrade to lithium battery technology (once again, only needed for your remote campers) you will not have to change the converter again. You merely force the Boost Mode with the pendant to charge your lithium batteries. 


Brad

rfuss928

Most lead acid and AGM batteries are limited to a maximum charge rate ~20% of their capacity.  55 amps would be the max rate for ~275 amp-hour battery bank.  IIRC, the 2 - 6v battery set up in a 400 is about half that.
If your system requires/accepts very high currents, many high rate charging systems have a remote voltage sense feature independent of the current carrying cables allowing monitoring and control based on the voltage at the battery (or load) terminals.  This feature allows the control of voltage at the battery independent of any losses in the power cabling.

Dutch061

@rfuss928, It's great that you bring this up; thank you! You are most correct if there were a secondary circuit for "voltage sensing" to adjust charging rate independent of the charging circuit that it would eliminate this issue. I am not sure it exists in the RV industry or at least in the product range that is in my budget. 

The OEM installed WFCO is rated at 55 Amps even though in the NuCamp build along with the operating characteristics of the the WFCO it will never enter the Bulk Charging Rate. The Bulk Charging Rate is 24.55% of the 224 Amp AGM battery capacity.

The PD9260CV is rated at 60 Amps, so the Bulk Charging Rate is 26.78% of the 224 Amp AGM battery capacity. With locating the converter closer to the batteries and lower resistance in the charging circuit the converter can use the Bulk Charging Rate. The second thing is with the pendant and Boost Mode, you can force the Bulk Charging Rate anytime for up to 4 hours. It is this feature that will allow this converter to be used with lithium batteries if an upgrade is made. 

I have looked at several studies that compare using 20% to 40% of Amp Hour charging rate of the batteries being charged. I think this is more of an issue with lead acid batteries due to the potential of gassing than with AGM. And of course, as the lead acid batteries degrade with age and use this becomes a much greater issue to contend with.

I have not reached out to Harris regarding this upgrade or their allowable Charging Rate but I suspect that when NuCamp spec'd the components for their build that their engineering team did. With this being said, I wouldn't lose any sleep over adding an additional 5 amps of Charging Rate.

In closing, I would like to stress again it is my opinion that this upgrade, modification, or fix only benefits those who camp without utilities IE boondock. If you are primarily stay at resorts, parks, or campgrounds with electricity there is minimal benefit in making this change or upgrading the batteries to Lithium. 

Brad

TNOutback

Thanks for your input.  I don’t know where the power center is located in other 400 models, but in my 2020, with the way it is wired, it’s not a reasonable boondocking solution as others have pointed out.  For probably the majority of the owners who only camp where there are hookups, then it really is a non-issue.  The particular WFCO charger used is not particularly well- suited for AGM batteries either, so a new/improved solution is needed for boondocking.

I was considering if it made any sense to leave an upgraded charger in the distribution center and upgrade the wiring back to the batteries.  Apparently the wire size required for that distance/amperage would be too large for the lugs on the charger.

rfuss928

TNOutback
Keep in mind that the higher the rate of charging the less efficient the uptake of stored energy.  One reference I found stated 5 hours at 20% charge rate only yielded 60% SOC in AGM batteries.  Once the batteries reach "absorption  voltage" charge rate becomes voltage limited rather than current limited. 
I gather when you refer to boondocking requirements, you are referring to generator charging through the onboard converter and are trying to limit run time.  Possibly a sperate input port for the 12v output from the generator would be the most effective solution.

This is an interesting,  well documented series of experiments a sailor did that provides some practical  
https://pbase.com/mainecruising/effect_of_charge_rate_on_agm_battery&page=2

TNOutback

@Denny16 the charging circuit in my 400 runs from the distribution center under the front door side bench across the front of the camper, then down the street side of the camper, then all the way back across the camper to the battery cutoff switch inside the outside storage compartment, then doubling back across the camper to the distribution block ahead of the batteries in the back.  Conservatively, that’s about 33 feet of positive wire, and you size wire based on both the positive and negative runs, so with both + and - runs, it is at least 55 feet of wire in the charging circuit; maybe a bit more. Also the rated converter output in the WFCO 8955 is 55 amps at 13.6VDC.  The design of the system is not capable of anywhere close to that, and it’s been documented on this forum the charger can never enter bulk mode. 

I do have a portable charger that I intend to wire directly to the batts for the next boondocking trip, and I will likely bite the bullet later this year for a remote converter + BB lithium solution as it is not feasible to upsize the wire enough to overcome the resistance/voltage drop from the distribution center.

Dutch061

@Denny16, not trying to start an argument but I am curious where you get your data on "The TaB WACO is only putting out a small 15 amp or less charge current". Please provide sources if you have them. 

Here is a link to their website for WFCO (sorry not WACO that is a town in Texas) Model WF-8955 that was installed in our 2020 400 BDL from NuCamp.

https://wfcoelectronics.com/product/wf-8955-55-amp/

It is the resistance as per the my earlier post that is the root cause coupled with the programming of operation of the WFCO that is the limiting factor on amperage and NOT the converter.

Here I quote from Rogers extensive testing (@rh5555) the first few sentences of the test results. 

WF-8955 Operation in the 2020 TAB400

"There are 3 modes in which the WFCO WF-8955 converter can function, “bulk”, “absorption” and “float”. In any of these modes, the converter will provide up to 55 Amps of charging current, but the 3 modes are differentiated by how high the drive voltage of the converter can get:

Mode	Maximum Voltage
Bulk	14.4V
Absorption	13.6V
Float	13.2V

 

The charging current is split between any 12V peripherals that are currently being powered and charging the battery. 

When 120V is applied, the WF-8955 converter senses the battery voltage before applying any charging current and makes a decision based on this voltage: If it is below 13.2V (which will almost always happen), then the converter attempts to initiate "Bulk" charging. In this mode the converter will push up to 55 Amps of current into the battery (while not exceeding 14.4V drive) and measures the battery voltage at the converter. If the charge current has not reached the converters limit (~55 Amps) or the voltage at the converter is above 12.8V the converter immediately moves to “Absorption” charging." 

At the lower drive voltage, the amperage drops off like a rock due to the fact that the "sensed voltage" is close to the drive target which is interpreted as being nearly fully charged. 

As far as using a 12 volt output from a generator both generators I have owned were rated at 8 Amps maximum output and NOT regulated, which means you would need to use a charge controller if you were to attempt this type of connection. Although I have owned Predator only, the Honda generators are set up the same and I would suspect that the others (champion, etc.) are similar also. So, you can use 8 amps and run forever to try and charge batteries or plug it in to a system that has the capability to charge at a higher rate. This is also where you are mistaken, I can easily get 58 Amps from my converter going through the shunt for the BMV-712, which completely busts your comment "Only the big industrial type quick chargers put out 50 or 60 amp charging amps."

For our use, once again we rarely have access to shore power making this simple and relatively inexpensive change to the converter was a game changer. We regularly end up with lots of tree cover (on purpose) and don't get enough sun on the solar panels to make it up daily.

At the end of the day, everyone needs to make a decision based on their use case scenario and more importantly correct and factual information. I would encourage anyone contemplating this to read the attachment in the earlier post that I made, again kudos to @rh5555 for doing the hard work of the testing and documentation. I didn't do anything but read his results and make a decision based on the "correct factual information" provided.


Brad

rh5555

A little more background:  My 2020 TAB400 also has the converter by the door (under the seat).  The wiring that NuCamp installed between the converter and the battery (including switches and fuses) had a resistance of 0.0375Ω.  This may not sound like very much, but when you try to push 55 Amps through these wires, you get a voltage drop of over 2 Volts (good old Ohm's law).  This is why the standard converter (which measures the battery voltage at its output) can never enter bulk charge mode, and why in Absorption mode it can only deliver about 10 Amps of charging current.

If you need to be able to recharge your AGM batteries fast (using a generator, for instance) the best, neatest and most economical way to do this is to install a stand-alone converter as close to the batteries as possible, and wire it up with the heaviest wire you can handle.  I did this with a stand-alone WFCO unit (which NuCamp got me for free), but I wouldn't recommend it:  It is still a challenge to get the converter to enter Bulk charge mode.  The Progressive Dynamics unit that @Dutch061 installed is significantly better because you can force it into Bulk charge mode at the press of a button.

As far as charging rate is concerned, I believe Rolls battery when they say for AGM batteries: 10% of C20 (minimum), 20% (recommended), 30% (maximum).  For our AGM batteries, this converts to 22 Amps (minimum), 44 Amps (recommended) and 66 Amps (maxmum).  The standard converters in our TABs are charging at rates lower than the recommended minimum.  I have no idea if this is a bad thing...

Denny16

Brad, my poor estimate was based on what I was seeing on the battery monitor on my 2018 TaB400, which has a much shorter run from the converter to the battery.  I edited my original post to remove any confusion on this.  Yes smaller generators are not going to run a large battery charger or provide a high amp 12VDC battery charge circuit.
  

Regarding solar charging, my comments are based on my actual experience, parking the TaB under a tree for mid day shade and partial afternoon shade.  I was initially concerned with how well the solar would work BoonDock camping, and was surprised at how well it actually works out.

Everyone needs to make a choice based on how the use their camper.
cheers

Dutch061

Denny16, I agree 100%. Each person needs to make decisions based on how they use their RV. As mentioned, no offense intended, just curious as to your source of information. It appears that at least in the 2018 model year that the power center may have been in a different location to the battery, I say battery because it is my understanding from other posts that there was only 1 in 2018 and possibly 2019. So I understand the source you were using and that is important. 

My goal is to always try and provide as accurate of information as possible when posting on electrical-mechanical so that other forum members can use that to aid in making decisions or repairs that are appropriate for their needs. I am far from being always correct both in content and political correctness and I have been known through my working career (recently retired and I highly recommend retirement) for being honest, open and unfortunately blunt sometimes.

Brad

Denny16

No worries Brad, I share the same viewpoint and you do.  My DC electrical experience is mostly from Marine Industry, rewiring small yachts, and upgrading electrical systems.

The 2018 TaB400 came originally with one 200/224 Amp battery, located in the front of thr pax side rear storage locker (under the bunk next to the galley).  The WACO is under the wardrobe closet.  Sometime after Jan 2018, nüCamp switched to two 6VDC batteries in series to get 12VDC, so 2019 models got a sealed two battery locker in the same place as the single battery in 2018.  When nüCamp switched to the Air8 in 2020 model, the batteries were relocated to the center rear under the bunk, behind the Air8 unit which was under the center part of the bunk (previously a storage area).  

The power distribution WACO unit was moved to the front of the trailer under the pax side dinette seat.  I keep forgetting about this location for the power unit, which was moved back to the galley area in 2021.  So the 2020 TaB400 had the longest battery to charger/converter run, and nüCamp should have used a larger size wire for the bus run between the two.  The 2021 TaB400 has a shorter run, but is still longer than the 2018/2019 battery cable run.

In a boat, we would use AWG No 4 or 2 wire for battery to power buss panel runs, and for larger system even 2/O cables.  Given the smaller power loads in the TaB, this would be larger than necessary.  

But the long distance run and nüCamp’s use of AWG 8 wire for the battery to converter/charger connection has been an issue for some 2020 owners since late 2019, and the subject of numerous posts here.  While the WACO can do a larger Bulk charge, the way nüCamp has wired the system, and the bulk charge cut off voltages keeps the unit from going to a bulk charge, from what other TaB400 users have reported here.
I agree we should always try and provide as accurate of information as possible when posting info here, and when I get a bit wrong, I appreciate being corrected.
Electrical systems are not something you want to get wrong, or major damage could result.  

cheers

TNOutback

Denny16 said:

My original post was just pointing out the OP was not calculating the cable length correctly, thinking the load had to make a round trip, so he X2 the 30-foot run thinking he needed a cable spec’d for 60 feet.  The actual load cable length is determined by only using the actual distance one way for the run, to determine the correct AWG wire size.  There are numerous charts showing this info available on line.
cheers

@Denny16 I am the OP and I would like to see your reference for that, because every resource I found says you must include the length of the entire circuit - the round trip distance - to the power source and back when making wire size determinations. that means the length of the positive and negative runs from the charger/converter to the battery and back.  In a 2020, that would be somewhere around 50-60 feet of wire.

This from Bluesea, a marine electrical supplier:

B  Find circuit LENGTH IN FEET along the left side of the chart. Note that the total length of the circuit is the roundtrip distance from power source (usually the battery) to the product and back.

This from Enginering Toolbox:

Wire Gauge Design Procedure

1. calculate total length of wire from source to device and back again
2. determine amount of current in wire
3. determine correct wire gauge in the intersection of amps and feet

TNOutback

Here is an approximation run and location of the charging circuit in the 2020 (positive red, negative black)

Denny16

OK, I see where you are getting your info.  The charts I used already factored in the return negative circuit run.  The actual load is the distance from the source to the load and back.  Sorry for the confusion.
cheers

billmaghan

Charging voltage is seldom greater than two volts.  Sure, it might be 14 volts relative to ground, but relative to the battery being charged it's two volts at best.  Even with a nice thick wire two volts won't move much current through ten feet of wire.  The charger has to be near the battery to realize a strong bulk charge.