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About to install a second 12v battery, what do you guys think of this setup? Must say, I’m still a bit confused on what exactly I need, but from what I gathered this is it?


So, this would work, but not as well as it could.

Ultimately all the power comes from the DCDC converter up front, through the white wire that hits the + terminal of the vehicle battery on the right. (You’ve got it labeled “starter” which isn’t quite accurate; it boots the computers and allows the traction pack contactor to close, but the Prius doesn’t have a conventional starter so there’s never high starting current being drawn from it. But I digress!)

Then you’ve got it going through another set of fuses and wires, and a battery isolator, over to the leisure battery (I love this name!), and another set of fuses and wires to the inverter. That’s a long trip, and while at steady-state everything would eventually reach equilibrium with the same voltage everywhere, in practice that’s rarely the case, and you’ll see voltage drops along those wires according to how much current is flowing. If you’re getting much use out of the system at all, the leisure battery will be chronically undercharged.

Let’s imagine the system in three states:

  • Car ON/READY, inverter running a load and leisure battery trying to charge

  • Car off, inverter running a load

  • Car ON/READY, inverter not running but leisure battery charging

These approximately represent a daily drive cycle.

First: Car on, inverter running a load. There’s current flowing in every wire, and the inverter is at the end of the line. It sees the lowest voltage out of everything, but that’s probably harmless as it’ll turn itself off if it gets too low. More concerning is the leisure battery, which, because of that voltage drop, won’t likely see the proper float/absorption voltage to charge itself fully.

Second: Car off, inverter running a load. The isolator sees the system voltage drop below 13.8-ish and opens up, so there’s no current flowing except from the leisure battery to the inverter. This is normal, and we’re simply pausing to consider this because it means the leisure battery gets significantly discharged by the time you turn the car on again.

Third: Car on, inverter not running a load. The DC-DC converter is pouring current into the vehicle battery, the voltage comes up, and the isolator closes so now the leisure battery is charging too. In the first few hours, the leisure battery will absorb a lot of charge, and as its SoC comes up, the current falls off, meaning the voltage drop across the wiring decreases, and if it remains in this state long enough, the leisure battery will approach a healthy float charge. If not, it’ll be chronically undercharged.

So, how bad is it? Let’s do some math.

I’m going to run the numbers twice, with #6 AWG and #2 AWG wire, which are pretty typical for such installations. I’m going to assume MRBF fuses near each battery, which are compact and incur negligible voltage drop of their own (I’ll model each fuse as an additional foot of wire), and a Victron Cyrix ct-120 battery isolator, which has very low voltage drop as well (one more foot for this). Assume the negative wire from the leisure battery runs back to the same body ground that the vehicle battery connects to. Assume the run is about 5 real feet, plus roughly a foot on each end for service loop and installation flexibility, plus the mentioned device allowances, and we get 10 feet of red, 7 feet of black. (The negative wire is unfused and bypasses the isolator.)

Scenario 1: With roughly a 500-watt load on the inverter and thus roughly a 50-amp draw on the 12-volt side, the apparent voltage drop over 17 feet of #6 AWG is 0.83 volts. That’s enough that, when the DC-DC is keeping the vehicle battery at 14.5v, the leisure battery is only seeing 13.67, well below what’s needed for proper charging. If the battery started with a low SoC, it’s not going to gain much/any during this usage. Doing it again at #2 AWG, the drop is 0.32 volts, and the leisure battery thus sees 14.18, vastly better. It’ll charge while running the load.

Scenario 2: no modeling here

Scenario 3: There’s no continuous load so the only voltage drop comes from current absorbed by the battery as it charges. It’s a bit hard to predict how fast it’ll charge in this case because we don’t know its internal resistance or structure, but 20A is probably a reasonable max assumption, falling off as it charges. With #6 AWG, you’ve got 0.33v drop, so 14.5 drops to 14.17 which is fine and dandy, and with #2 AWG you’ve got 0.13v drop, so 14.5 drops to 14.37, beautiful.

So, as long as the battery gets a few hours of “alone time” to recharge with the car running, after any time it’s discharged, it should be alright. But if those opportunities are scarce, it’ll rarely reach 100% SoC, and die prematurely. (Dozens of cycles rather than hundreds.)

So, what could be improved?

Ideally, we’d power the inverter from the front, right off the terminal on the side of the DC-DC, to completely separate the inverter’s voltage drop from the batteries’ voltage drop. That’s not practical, so it won’t be considered further.

But one very easy thing you could do, is move the inverter to the right side. Get its draw “before” most of the leisure battery’s wire length. This sounds counterintuitive because inverter manuals always implore you to use the shortest and most direct connection possible, but they’re not considering charging!

Drawing: https://i.imgur.com/EoiMxn1.jpeg

Let’s run the model again.

This time in scenario 1, the 50-amp load is only incurring 2 feet of voltage drop (the fuse and the isolator; assume the wire between them is negligibly short), and the battery “sees” the voltage at the left-hand terminal of the isolator. With #6AWG, that’s 0.098 volts of drop, or 14.5 drops to 14.4, and the leisure battery is then a passive observer; the only current in its wiring is charging current, so even if it’s absorbing 20A, the other 15 feet of wire only have 20A across them, and the 0.29v drop means the battery sees 14.11 for bulk charging, tapering up to 14.4 as SoC climbs and charge acceptance falls. (So unlike the old wiring, this means the battery will absorb a reasonable amount of charge even while the inverter runs a load.) With #2AWG, the 2-foot drop is 0.038v so the left terminal of the isolator is at 14.46, with 20A in the other 15 feet the battery sees 14.35 tapering up to 14.46. Beautiful.

Skipping scenario 2 again.

Scenario 3 is identical to the old wiring scheme, because the inverter’s load is not a factor so it doesn’t matter where it’s attached.

I hope this all makes sense! If not, ask lots of questions. 🙂

Okay so, a few things… first, thank you so much for taking the time to reply so elaborately and details. It means more than you know, if there’s anything that confuses the living hell out of me, it’s electrical work. – if I understood correctly.. I suppose I just copied what you wrote down, my set up would look like this? would it be okay if I connected the ground on the other side of the car?

The only issue I see with this, and maybe I’m not understanding completely, but, I’d need to use the leisure battery to power the inverter, otherwise would I really have a use for the second battery? I drew arrows in my drawing showing the current travel direction. Would there ever be an instance where the leisure battery would power the inverter? Sorry for all the questions, I’ve already spent so much on custom made wires, only to not use them and left confused yet again. I want to make sure I get this right this time.

Yeah, fuse on the + line, as close to the starter battery as possible. Fat gauge cables. Starter battery ground should attach to the point on the chassis where the battery is grounded.

What chemistry is your second battery? It used to be assumed to be a sealed lead acid, but lithium iron phosphate has been getting good and being adopted these days.

With an LiFe battery, you would want a specialized charger in place of the isolator.

I recommend that system if you got the dough for it.

I’ll be using two of these batteries – one as the starter, the other as the leisure

Look up on YouTube a channel called PSC camper conversion. He had a video on how he installed a second battery in the back

Found it: https://youtu.be/yvq6ObcdFtY

Look up “automatic charging relay” they’re big in marine use because it will let you drain the battery without draining your starter battery, then it will automatically charge your battery when your car is running. When I did my boat I did it with a switch that had batt1, batt2, off, and All. It was easy, but not as easy as an acr would’ve been.

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