Parasitic drain on Main AGM Battery after Mechanic installed a High Output Alternator & 3-wire kit

[George Perry - MB]

Parasitic drain after mechanic installed a high output alternator.
I'm no mechanic, yet a persistent problem solver, but I lack the tools and space to freely work this problem.
Its been a journey.
I have a 2012 LR4, 5L NA.
I completed a dual battery setup: AGM main X2 PWR (80 AH) + DL+ 135 AH LPF 2nd battery. I connected them with a Victron DC-DC charger 50A.
All was well, but because of the accessories it made sense to upgrade to a bolt compatible high output Alternator (JS Alternators - 170 A --> 340 A).

Now the issue.
Thursday the area Land Rover specialist (non dealer) replaced the OEM 170A with the after-market ** alternator and a 3-wire kit.
Aside from a few faults, which I cleared with my GAP IIX G3 BT all seemed well.
I returned to my base camp for the 4th July and started the Rover back up lunch time on Sunday - it was DEAD!
I had to jump start it. GAP tool showed 7% SOC for the main battery. I also have a Victron smart shunt for the LPF, it showed 100% SOC.
I had to go to a volunteer shift, so I left it idling and checked on it through out for a couple hours. Eventually it returned to 100% SOC for the main AGM.
This was last night JUL-6th (Sun), 2025.
Now this morning at 7AM my main AGM battery per GAP IIX tool showed 60% SOC. I'm now charging it back up with a 3A AGM / LPF charger.
Notes:
1. The mechanic connected one of the 3-wire directly to the LPF 2nd battery from the alternator.
2. My first post months ago was wrestling with a LPF main (single battery) which created all sorts of havoc. I abandoned the idea, went with the dual battery thanks to some of you. But now I'm stuck

The area mechanics aren't used to dual battery set-ups and aren't concerned with the different battery chemistries. This worries my because online and my own experience shows it does matter.
My GAP tool showed stable voltage and current prior to the upgrade.
Now the same live values show some fluctuations - is this a sign or to be expected?
I have a scheduled return visit to the area mechanic this Thursday. They couldn't figure out my earlier charge fault problem, but ultimately I caused it when I put the TSB brown-blue BMS wire on the negative pole of the main battery, essentially not powering up the Rover's BMS. Once that was correctly wired, the original alternator went from stable 14.0V to stable 14.8V. This cleared the fault and improved overall charging performance allowing the dual battery set-up to equalize and top of the charging.

The upgraded alternator has a smaller pulley and a 1/2 inch shorter belt was needed, otherwise it was bolt compatible and connections equivalent (Likely took stock case, upgraded windings and internals to improve output performance).

I think the trouble is the direct connection from the alternator to the 2nd (LFP) battery which effectively bypasses the victron dc-dc charger that mitigates the profile differences between LA / AGM & LFP battery chemistries.
But I don't understand the theory of how a powered down car can lose that much energy in such a short time by having that 2nd positive 1-guage wire. Chat GPT or other online answers seem to say this is the likely cause.
Any thoughts on how to proceed?

 

[greiswig]

It's hard to picture what you're describing without a diagram or something, and I don't know what an LPF battery is. But if they hooked it up so that the positives of both batteries are connected to one another, then one is likely discharging through the other. That happens even with "identical" batteries, because they aren't identical.

 

[George Perry - MB]

It's hard to picture what you're describing without a diagram or something, and I don't know what an LPF battery is. But if they hooked it up so that the positives of both batteries are connected to one another, then one is likely discharging through the other. That happens even with "identical" batteries, because they aren't identical.

Lithium Iron Phosphate (LiFePO4) aka "LFP" its a Dakota Lithium (MFg# 12V135BV2A) brand.
Imagine an alternator with (2) positive leads connected, one to each positive terminal of the two batteries. AGM (Main) & LFP (2nd).
I'm running an hourly list of voltage measurements and SOC while running and off over the next two days.
If correct, the GAP IIX ODB tool shows a -2.0A draw while everything is shutdown. This makes sense why my all afternoon charging with a 3A battery tender is going backwards, because I think the draw is greater than the low grade battery charger is providing.
Yesterday afternoon, I ran the vehicle for probably 2.5 - 3 hours to go from 7% to 95% SOC with a battery current registering 53A from the GAP Tool.
Lastly, I was hearing a 'clock clicking' and saw my two instrument gauges pulsing, which supports the idea of residual, unwanted current flowing through the system. And I have to wait until Thursday like this - messed up.

 

[Rover Range]

Like greiswig said, some sort of wiring diagram is need to see where the issue is.
The OEM alternator is controlled by the powertrain control module. Circumventing that circuit will certainly cause issues.

 

[Land Rover Joe]

All was well, but because of the accessories it made sense to upgrade to a bolt compatible high output Alternator (JS Alternators - 170 A --> 340 A).

Does anyone have experience with JS alternators?

I ask because I had a bad sales experience with them and despite wanting an "upgrade"........went with a new OEM (sourced from Atlantic British).

Could this be an issue (either the alternator or the wiring-up)?

 

[wills1969]

This sounds like EXACTLY to a T, what I experienced recently. I’m going to give the very short version, not the one that describes countless hours trying to hunt down the problem. Turned out the incorrect alternator (non-Denso), wreaked havoc in my system. It ended up frying my mega fuse though I didn’t know it until a replacement (proper OE Denso), was installed. The parasitic drain went away BUT I was getting a 14.4 at the alternator and 12.2 to 12.5 at the batteries, and it required a jump EVERY SINGLE TIME to start it after 1 proper start.

Someone suggested the mega fuse. I questioned if the vehicle would even start with a blown mega fuse. Turns out…it can. Once. Turned out to be EXACTLY THAT… a 400 amp mega fuse, from Littlefuse (had no idea it even had one of these). Part number LR080537. LR original-$100(Littlefuse). $5 from Amazon also Littlefuse. Sincei had already spent THOUSANDS trying everything else, figured an $5 fuse wouldn’t hurt. Instantly solved. I mean…INSTANTLY.

I’m not sure this is your issue but it sure sounds close enough to take a look. Rather than take every electric component apart, clean every cable, earth point, plug…. Yeah. Should have started with the mega fuse, but didn’t even know it was a “thing”.

Good luck.

 

[powershift]

I've also read about aftermarket alternator problems. But aside from that, if I left my rig from Thursday to Sunday while camping I'd expect the battery to be too low (11.x V) to start the engine. The systems use juice. Parking it and leaving it alone from Thursday to Sunday would be different, but opening a door creates more drain until it sleeps again then it is still draining, but less fast.

 

[CRYA]

I'm no expert, but I'd question connecting the house battery to both the alternator AND the Victron Dc/DC. Doesn't seem like it's supposed to work that way, especially given different chems.