What gauge wire?

assuming the frame is a decent ground, equal to #0wire or so, which with a thick welded frame is highly likely, then with #8 wire at 40A you would see about a 0.45V drop in the wire. Going to #4 would drop that to 0.23V.

for comparison I have a 45A PD converter in the WFCO box with 30ft of #4 cable and chassis ground. voltage drop at 45A is about 0.3-0.4v about what I calculated. The PD has no problem going into bulk mode charging on startup with a pair of GC batteries even at 80%SOC. It will start and hold 45A quite a while.
your shorter run with #8 should yield similar results. going to #4 will hold 45A longer, but I think you'll find the difference in charge time from say 50% to 90% won't change by a huge amount.

Now if your frame is thin beams rather than std I beams and bolted rather than welded together, ground return resistance may be high enough to alter your results.

My frame is much better than #4/0 wire based on some measurements I did, roughly 1/3 the resistance of #4/0 wire.

smkettner wrote:

pasusan wrote:
Using the wizard mode button I put it in boost mode which is supposed to be 14.4V. My remote display read 14.49 and my multimeter read 14.43 on the battery posts.

Repeat this test with battery starting below 12.2 volts.

using two different meters of unknown accuracy to try to measure voltage difference in the 10's of milivolts, is questionable. Try to use the meter at the battery posts AND converter output. you can then rely on getting a pretty accurate voltage drop measurement.

Further to what SMK said, the thing that causes the voltage drop in the wire is high current. Just because you have the voltage at 14.4v doesn't mean the current will be high. You need a discharged battery so the battery will accept more current.

Ohm's law: voltage = current x resistance ( E=IR ).

X2 on testing with low battery SOC.

pasusan wrote:
Using the wizard mode button I put it in boost mode which is supposed to be 14.4V. My remote display read 14.49 and my multimeter read 14.43 on the battery posts.

Repeat this test with battery starting below 12.2 volts.

That test shows the converter does do its stages, but does not show what happens when you try to recharge low batteries using the 14.4v stage.

One test would be to start with low batteries (12.2v say) and not use the Wizard, but just see if the converter will go into the 14.4v stage by itself. If it won't, and you need to use the Wizard to make it do 14.4v, then you know you need fatter wire---if you want more amps at 14.4

So the next thing is to measure how many amps you are getting on low batteries with the existing wire after you force it into 14.4, and then decide if that is enough amps.

The big selling point of the PD IMO is that, unlike other some other converters, it does have the CW so you can force it to do 14.4 when it (and the some others) would not do so on its own. This can make up for having thin wires to some extent. Is that extent enough in your case for your needs?

If you don't have access to an ammeter that will measure 45 DC amps or so, then you would have to do some timing tests to replicate camping recharging but this can be a pain.

Another problem with backyard testing of converters is the 120v input you use. You can have lower voltage on an extension cord from the house than the higher voltage you get from your generator. Low 120v input voltage hurts the converter's DC output. You might need to run the gen for hours in the backyard to get the true picture.

OP here.... I checked with my multimeter...

In Storage mode which is supposed to be 13.2V, the remote display read 13.35, and my multimeter read 13.28 on the battery posts.

Using the wizard mode button I put it in boost mode which is supposed to be 14.4V. My remote display read 14.49 and my multimeter read 14.43 on the battery posts.

At this point I'm not worried and maybe I won't need to change my wiring - but I will definitely be calling Randy at Best Converters next week and check with him.

As I see it, #8 is safe. If you are happy with the charging time, leave the #8 wire as is. If you want faster charging, replace the #8 with a larger wire. If you are replacing the wire you might as well use #4. It doesn't cost much more than #6 and will reduce the charging time a little more. Basically what SMK said earlier. JMHO

If your 45amper acts like a 35 amper on a 220AH bank if will take approx. 30 min longer to do a 50-90. (3 hrs vs 2.5 hrs)

If you upgrade the wire and get it to act like a 40amper (best you can hope for with w 45a PD--which are known not to do full amps at 14.4v anyway) you might save 15 min of that.

So it is all about that 15 minutes and how important that is for deciding what is "worth it."

Besides the "PD issue", to get full amps from any converter you have to mount them quite close to the battery with fat wire. Even then it is very sensitive what you get. On a 60 amper I got 59a with it close but when even closer (under two feet of #4) I got 63a.

If you mount the converter in with the power centre somewhere back in the rig, you are not going to get full amps even if you use a fat wire (where fat is as fat as is reasonable to work with.) You really need to do a deck mount within say two feet of the battery bank.

I can only tell you that the PD 45 amp on 2 GP 27s @ 50% soc, is going to spent most of its time at 30-35 amps and less. The short time it will spend at 40-45 amps, is not worth haggling over when choosing wire size. Personally I think 4 ga is way overkill.
But then, I've read of 4 ga coming down off the roof for a few hi voltage solar panels, so there must be more to it than what I know.