second 12 volt battery - 2020 edition

Been there since Gen 1. I think there might be warmer climes that don't get them. Gen 1 (and maybe Gen 2) split it into two electric elements, one of which was always there and the other just in cool markets. In Gen 3 there's just one element (capable of three levels of output, you can play in Techstream).

 

Right, it's better if they publish something like this, as Engel does:



One could play with the 61kWh/annum and make it look like 0.54 Ah/h at 12.8 volts, but I question it; too low. We can roughly compare the CFX3 35 and Engel figures above for around 90℉ ambient and 39℉ inside, but to get that much lower number for the CFX 28, I would have to guess they picked less challenging temperatures.

 

It's hard to be sure what's driving you in that direction. Around a kilowatt is as much as you're going to get. You can hang excess capacity out there, but when it's "drawn upon", it will not be forthcoming. You won't get a fiery camping incident (assuming you're competent about wiring and fusing), you will just see the DC/DC converter output at first droop some (Bob Wilson posted results some years ago) or, if aggravated, code and turn off.

If you put in an "1100 watt" rated unit because you found happened to find a better deal on one than a "1000 watt" one, well sure, that's understandable, as would be a "1500 watt" one if that's how it happened. But I'd advise if you're thinking to actually draw those levels of power in practice, maybe wait around and see what products Randy Bryan is going to offer through his recently-reconstituted company, for connection to the traction battery.

As I understand it, it's usually the case that an inverter won't see its best efficiency at low fractional loads ... so if you're choosing between a 1000 watt and a 1500 watt inverter and it's mostly going to run your laptop, neither one is going to hurt anything, but the bigger one will be wasting more juice.

 

"Cool. Thanks. Exactly what I was wondering."

Yeah, he's kinda good that way...

 

Mine just sits on the spare tire, underneath the storage box, so it's completely out of the way. The spare tire angles forward a bit, so there's some clear height at the forward part. That space under there is better ventilated than the box itself; the cooling air for the traction battery passes under the box on its way to the exit vents hidden by the bumper cover. I didn't add any extra fans; there is one in the inverter itself that will start if it wants to. I have never had it complain about overheating. If it does, I can think about further measures then.

For accessibility, I just used the end of a three-outlet extension cord, and ran the remote circuit up to the dash.

 

True, I didn't buy a 1200W for an application where I knew 1000 would be pushing it.

You can ground to whatever good ground point is near your location. The wiring diagram shows where all the designed ground points are. Those use specially formed screws to function best as electrical grounds. If there isn't one nearby, an ordinary bolt that mechanically mounts something can be ok, depending on the paint situation. You might check that the conductivity is good.

Not that I would recommend. If you examine the positive battery clamp carefully, you'll see there is a fusible link in it, and the end of that link where you want to attach is the end down over the side of the battery where the cable attaches to it. That puts you one fuse drop less far from the DC/DC converter at the front of the car, which is where your power is coming from (you are not planning to run a 1200W inverter without your car in READY, I trust), and better protects from the battery's fault current capacity in case of damage to your wiring.

Speaking of damage to your wiring, be sure to protect it from pinching or abrasion anywhere it goes under the storage box edges, over the edge of the spare tire rim, etc.

If you look in this image, the fat wire coming up from behind (encased in black loom), joining to the right (common) side of the MAXI fuse block, is what's coming from the main cable side of the battery clamp. The takeoff from the 80A fuse is going to the inverter, the other 20A one is feeding the other stuff. (Photo was taken before adding the air compressor, fed by another 30A fuse in the MAXI block.)

 

I grounded the inverter to a tie-down bolt. But I hooked the negative terminal of the inverter to the negative post of the 12V battery. I want the most unrestricted power to be able to flow as possible, without wondering whether the bolt on the body is enough.

Yes, you can (and in my mind) should connect the positive terminal of the inverter to the positive terminal of the battery, for the same reason as above. BUT, add a 100A fuse to protect the battery. If you connect to the bolt of the grey plastic you are skipping the 140A fuse that's built into the positive battery connection, so you're hooked directly to the DC/DC inverter, which is not a good idea, unless you have a fuse between your 1200W inverter and the DC/DC inverter. I like to think of the 12V battery as a "sump" which can keep current happy.

In the end, the power is coming from the DC/DC inverter; you should be fine, until that one time you push things a little too far...

 

Because I push my inverter pretty much to the max (20 min for the rice cooker, 5 min for the microwave, and when connected to things in our house, hours), I bring the inverter out from the storage box, to give it all the air circulation it can get. The fans in the inverter usually ramp up when I'm using it.

Very neat remote switch!

 

Very neat looking holes! How do you cut them?

 

When you do that, you are adding the battery negative post and terminal, its cable, and the bolt on the body where that cable attaches, into the length of the path between you and the DC/DC converter up front, which is where your power is coming from.

Again, doing so puts your connection farther from the power.

You may not have spotted that there already is a high-current fuse link both at the battery and at the converter end.




Right in between those two fuse links is the happiest place to add your tap: (a) you are protected against excessive fault currents from both ends; (b) you are not adding the fuse drop of the 140 A fuse into the power path from the converter to your inverter input.