Charging from a tow vehicle

[AaronD]

Is that because of the potential current draw?

I'm interested in this too. @mermike - could you elaborate so we all learn a little more? My instinct is to note that Powerpole connectors are a more reliable electrical connection for relatively high-amperage draw. Are there other motivating factors too?

FWIW - I finally did enough digging to find that my Sierra 2500HD's 7-pin connector is fused at 40A (that surprised me - I would have expected less).

 

[mermike]

Is that because of the potential current draw?

Yes, exactly. If you check the wire gauge on the 7-pin connector, you'll likely find 12-14 AWG. That is not heavy enough to support enough current to charge the house bank, which is the most common scenario. The 7-pin connector itself is not likely to be rated for much more than 15A.

 

[mermike]

I'm interested in this too. @mermike - could you elaborate so we all learn a little more? My instinct is to note that Powerpole connectors are a more reliable electrical connection for relatively high-amperage draw. Are there other motivating factors too?

FWIW - I finally did enough digging to find that my Sierra 2500HD's 7-pin connector is fused at 40A (that surprised me - I would have expected less).

40A? Holy cats that seems a bit excessive, I would be interested to know what the wire gauge is.

To elaborate further, most installations that I do use the Orion DC-DC 12/12-30 charger, which draws a little over 30A at 25°C. Given a 60' round trip cable length between the vehicle battery and the charger, you'd want 2 AWG cable. There's no way that the 7-pin socket can be used safely.

 

[AaronD]

40A? Holy cats that seems a bit excessive, I would be interested to know what the wire gauge is.

I think the 12v line is 12 AWG and the ground is 10. I'm not 100% certain (the wire is probably original with the 2001 truck, so no chance of reading the labeling, and without cutting it, no way to be sure just how thick the insulation is - I supposed they might be 10 and 8). But it looks like 12 and 10 to me.

And I guess a 40A fuse is appropriate (per wire ampacity ratings like Max Ampacity Chart - Non-Bundled Wire by Compass Marine How To). I.e., it will keep the wire from burning (as a fuse ought to do). But you'd sure never want to pull that much current through it! Thus my attempt to find a <10A charger.

To elaborate further, most installations that I do use the Orion DC-DC 12/12-30 charger, which draws a little over 30A at 25°C. Given a 60' round trip cable length between the vehicle battery and the charger, you'd want 2 AWG cable. There's no way that the 7-pin socket can be used safely.

2 AWG! Could you elaborate again? For 60' round-trip, 14.4v input, 2 AWG, 30A, I get 1.94% voltage drop. For this application, with a DC-DC charger to bump the voltage back up, can we not afford a larger drop than that? E.g., something like 10% (AWG 8 or so)? I based that on advice like "...items which affect the safe operation of the boat and its passengers...use 3%; all other loads use 10%..." (from Blue Sea, who seems pretty reliable).

I know 10% is wasteful, but we won't be pushing the full amperage for long, and the efficiency will improve as the amperage drops. Or so says my logic, but I'm happy to be corrected by someone with more experience.

I did similar calculations for the 18A charger; 75' total, 14.4v, 10 AWG, 18A: and I get a 9.3% voltage drop. That seemed marginal to me, but possibly doable. But I might be missing something.

PS - And yes, I have some AWG 2 on my little boat (for sustained 40A to an electric outboard with minimal loss), but it seemed overkill in this application.

 

[mermike]

I used the Blue Sea Circuit Wizard.

 

[AaronD]

I used the Blue Sea Circuit Wizard.

That makes sense. They default to the conservative 3% drop, which they recommend for "items which affect the safe operation of the boat and its passengers (running lights, bilge blowers, ...)". I think this case falls under the latter half of that sentence: "...all other loads use 10%...". If you run their calculator at 10% drop, 18A, 60', you get AWG 10. Even a little more than 10% drop might be OK, since we're feeding a DC-DC converter with a minimum input voltage of 8v.

My understanding is that 10% is recommended to 1) Operate reasonably efficiently (not losing 20% of your power to cable losses); and 2) Avoid under-voltage drop-outs in operating equipment (flickering lights, stalling pumps, random computer glitches, etc.). In this case, we'll only draw the max amperage for a few minutes (when we first start up with somewhat depleted batteries), so the inefficiency isn't a huge issue (as the batteries charge, the loss will decline along with the current, per Ohm's law). And we're using a DC-DC converter designed for low(ish) input voltages to address #2.

 

[mermike]

As a practical matter, the installations I do generally have large house banks and a 30A DC-DC charger is relatively undersized. For the very minor additional cost of heavier gauge wire, especially in the context of a large upgrade, I can safely deliver the full rated charger current to the bank.

On a side note, I am pretty sure a Victron DC-DC charger will not operate with an 8 VDC input, so what charger are you using?

 

[AaronD]

As a practical matter, the installations I do generally have large house banks and a 30A DC-DC charger is relatively undersized. For the very minor additional cost of heavier gauge wire, especially in the context of a large upgrade, I can safely deliver the full rated charger current to the bank.

On a side note, I am pretty sure a Victron DC-DC charger will not operate with an 8 VDC input, so what charger are you using?

I'm (naively?) believing Victron's specs. The 18A is spec'd for 8-17v and the 30A for 10-17v (page 12 of
https://www.victronenergy.com/uploa..._Isolated/Orion-Tr_Smart_DC-DC_Charger-en.pdf). My reading of the footnote is that the 18A will put out 12.2v from an 8v input, and a 14.3v absorb at ~9.4v in (and, again, when we approach absorb stage, current has dropped a lot, and so has the voltage loss). But maybe their specs are optimistic? I've heard a lot of recommendations for Victron, but have no personal experience.

Re bank and charger size: my bank isn't all that big, so even 18A is overkill. Sounds like your normal use cases are a bit larger than mine.

 

[mermike]

I don't doubt Victron's specs. I missed that you were using the smaller charger.