AWG size from alternator to B2B Charger

Trying to determine the wire gauge from the van alternator to the Renogy 20A B2B Charger in the back of the van. It is about a 15' run from the alternator to the back where we plan to install the B2B Charger.

Does the amperage from the alternator impact what size wire to use?

According to Blue Sea Systems wire selection chart, it would be <20' and 20A, so 12AWG. Does that sound right?

travisap said:

Trying to determine the wire gauge from the van alternator to the Renogy 20A B2B Charger in the back of the van. It is about a 15' run from the alternator to the back where we plan to install the B2B Charger.

Does the amperage from the alternator impact what size wire to use?

According to Blue Sea Systems wire selection chart, it would be <20' and 20A, so 12AWG. Does that sound right?

Click to expand...

Not quite.

The Renogy Owner's Manual has specific requirements for installation location and for wire lengths and sizes & fuses. Please review these before proceeding here....

OrioN said:

Not quite.

The Renogy Owner's Manual has specific requirements for installation location and for wire lengths and sizes & fuses. Please review these before proceeding here....

Click to expand...

I did and find it very confusing.




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travisap said:

Trying to determine the wire gauge from the van alternator to the Renogy 20A B2B Charger in the back of the van. It is about a 15' run from the alternator to the back where we plan to install the B2B Charger.

Does the amperage from the alternator impact what size wire to use?

According to Blue Sea Systems wire selection chart, it would be <20' and 20A, so 12AWG. Does that sound right?

Click to expand...

You might consider running a bigger cable so you can upgrade to a higher amperage charger in the future. I used 4 gauge for our 60A Sterling DC-to-DC charger and ran it directly from the alternator. Our batteries are under the vehicle just ahead of the rear wheels.

All the best,
Hein
DIYvan
541 490 5098

So from the chart pointed at by john61ct, 6 sq mm is AWG 10.
You're going 15 feet (5 meters)

Not visible from your photo snippet, is whether the Renogy chart is "one way" or "two way"
If it's "one way", then you'd need to double your length to 30 feet (10 meters)

10 AWG is about 0.00102 ohms per foot, so 30 feet of it would put 0.031 ohms into the loop (at 77F).
20 amps * 0.031ohms = 0.62 volts lost in the loop... that's over 5% of 12 volts.

Going up/down three steps in AWG numbers doubles the area and halves the resistance... so AWG 7 (or two parallel runs of 10) would only lose 2.5% of 12 volts.

If you're planning to use the Sprinter's metal as your "negative path", it's hard to give that a "wire size".

As Hein suggests "thicker is better" (and good future planning)

--dick

If you value your time, I would suggest running the biggest wire that fits into the Renogy's input connector or screw or whatever they have. Wire is not that expensive, and it takes as much time to run an AWG4 as an AWG8. It takes twice as much time to run a second wire after you found out the first was not big enough. And, it won't work any worse with the bigger wire. I don't know about Renogy, but Sterling says to NOT use the chassis, run a second wire. I found the chassis to have about 1/2 the resistance of AWG4, but it is not assured to stay that way with corrosion over time, so I ran a ground wire too. All of this rewiring I did was in response to troubleshooting suggestions from Sterling and took me a lot of time. Just trying to save you some.

Regards,

Mark

And note that BSS requires doubling distance to get round trip.

And need to actually measure the wiring path, not use a crows flies guesstimate. A hose works well to take bend radii into account.

Their Circuit Wizard app exposes more variable than the static charts.

That cable size chart is crazy. Their cable voltage drop, based on max length is 1.1V. I presume that’s for only one of the 2 cables. That means if the alt is outputting 14.0V, the B2B sees 11.8V! That’ a real bad design criteria.

The object is to boost alt voltage, not reduce it. The B2B has to work that much harder to get voltage up again.

My motto for battery charging is: go big or go home.

I use 1% voltage drop criteria. 1% of 14V = 0.14V

cable resistance per foot = 0.14V / (20A * 30ft) = 0.23 mohm

4 awg has 0.25 mohm/ft resistance. That would be my choice. 4 awg, 30 ft.

Well the whole reason for getting B2B chargers is often to allow the use of thinner wires.

Sometimes a $600 charger setup can prevent having to spend $1500 on heavy copper.

Just the added weight can be a critical issue.

Obviously away from this context of a little van. . .

john61ct said:

Well the whole reason for getting B2B chargers is often to allow the use of thinner wires.

Sometimes a $600 charger setup can prevent having to spend $1500 on heavy copper.

Just the added weight can be a critical issue.

Obviously away from this context of a little van. . .

Click to expand...

4/0 vs. 4 AWG @20' = 9.8 lbs & $67 more.

Are you nuts? $1500 on “heavy” wire???

4 awg wire costs $1 per foot. We’re talking $30 for a solid design.

And NO, no one is spending big bucks on a B2B just to skimp on wire. The alt doesn’t do an adequate job charging the house battery. It never gets the battery to 100% soc, and in addition, alt charging quickly tapers, making for a slow charge.

john61ct said:

Well the whole reason for getting B2B chargers is often to allow the use of thinner wires.

Sometimes a $600 charger setup can prevent having to spend $1500 on heavy copper.

Just the added weight can be a critical issue.

Obviously away from this context of a little van. . .

Click to expand...

OrioN said:

4/0 vs. 4 AWG @20' = 9.8 lbs & $67 more.

Click to expand...

Yes, as I said, this context makes the issue moot.

Just explaining the issues in a more general sense for those not just working on vans.

Where plenty of alternator setups charge any chemistry just fine using specialist external VRs

where B2B chargers are fed from solar, wind, hydro or DC genset inputs, including several rigs I've worked on putting out over 12kW

and indeed many B2B are sold precisely to overcome voltage drop and save on wiring costs.

A few hundred extra is not "expensive" in that context.

As described in my Sterling thread:

https://sprinter-source.com/forum/showthread.php?t=75471

That one certainly is running hot and dissipating a lot.

These are switching power supplies. The voltage difference between input and output is not directly responsible for the losses. Theoretically, it should be able to be efficient no matter what the voltages are. They make switching supplies that transform much different volatages than this and they can be 90% efficient or more. I am not an expert on these supplies, however. I am sure the efficiency calculations are complicated and depend on a lot of factors.

I measured the most recent Sterling I got and it actually was more efficient with lower input voltage and higher output voltage.

In V____In I (A)___Out V___Out I (A)_____Efficiency %_____Heat Loss (W)
14.0_____60.1_____13.5_____46.1__________74_______________221
12.9_____63.2_____13.6_____46.7__________78_______________180
12.5_____62.7_____13.62____46.9__________82_______________145

From what I can tell, it works better when it is hot. When doing my temperature / cooling tests, the output current slowly increased up to 49 A before it overheated and cut back to 1/2 power.

Regards,

Mark

bla, bla, bla,

Just be done with it and do it at a value... spend the money for 6g wire. That's exactly what I did and it is working beautifully. My run is a little closer to 20 ft though. Same B2B your using.

Dale

Using 4 AWG for that alternator to B2B charger and some other items requiring a larger gauge. Trying to size wires appropriately but also trying to keep wiring system simple. Using 12/14 gauge for 12v loads.

Thanks everyone!


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Let us know how it works out. Can you post measurements (like Marks) showing in and out V & I?

Here is a link to the wiring diagram on Google Draw:

https://docs.google.com/drawings/d/1NGxXI0dFyKun5yCQs2eWEL4kIryEz_V0Y3I9HR9h0-4/edit?usp=drivesdk


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Here is Travisap's drawing as an attachment:



--dick

Does the B2B actually operate down to 8V? Spec: Input voltage range = 8 - 16V. Chassis battery must be disconnected long before it gets dragged down to 8V. Perhaps a battery isolator relay is needed?

OP should add a solar panel disconnect switch. Solar should be disconnected before house battery is disconnected.