Anyone else struggling to balance a 200Ah lithium bank with two different solar controllers?

I've got a bit of a head-scratcher going on with my setup and wondered if anyone's been in a similar boat. I'm running a 200Ah LiFePO4 battery (Fogstar Drift) with two separate MPPT controllers — a Victron SmartSolar 100/20 on a 200W panel on the roof, and a cheaper Renogy 40A unit connected to a 400W ground array I put together for when I'm parked up for longer stints. Both controllers are set to the same charge profile (14.2V absorption, 13.6V float), but the battery never seems to settle properly.

The problem is the Victron finishes absorption and drops to float, but the Renogy seems to lag behind and kicks back into absorption again, which then confuses the Victron into following suit. The result is the battery's getting cycled through absorption repeatedly on sunny days and I'm worried about long-term cell stress. I've checked the wiring and both controllers are connected directly to the battery terminals with decent cable runs.

Has anyone managed to get two controllers playing nicely together on a lithium bank without going full Victron ecosystem? I can't justify the cost of replacing the Renogy right now. Is the only real fix to wire them in a way that one is essentially "master," or is there some setting tweak I'm missing?

@Mike1997 Classic issue with dual MPPTs on a single bank — if they're not coordinating charge states, you'll get one controller constantly "winning" and the other either backing off too early or fighting it.

Key question: are both controllers talking to the same BMS data, or are they operating blind? If you're using Victron kit, linking them via a shared VE.Network or running both through a Cerbo GX largely solves this — they'll negotiate absorption/float together rather than independently.

If they're different brands, you'll need to manually match the charge profiles precisely and accept some inefficiency. Even small voltage calibration differences between controllers cause one to perpetually lead.

Also worth checking: are both MPPT cable runs similar length/gauge? Voltage drop discrepancies will compound the problem significantly.

What controllers are you actually running? That'll determine whether a software fix is even possible.

@Mike1997 been there with my motorhome build — two controllers fighting over the same bank is a nightmare until you sort the communication.

The root issue is usually that each MPPT thinks it's the primary charger and they end up disagreeing on absorption voltage, one backing off while the other pushes. Classic if they're different brands.

If both are Victron, get them talking via VE.Smart Networking — game changer, they'll actually coordinate charge phases properly. Mixed brands though, you're stuck doing it manually: set both to identical voltage profiles and slightly stagger their absorption timers so they're not competing at the same moment.

The Fogstar Drift is pretty tolerant but it deserves better than two controllers arguing over it constantly. What controllers are you actually running?

Turbo43 | 47 posts

@Mike1997 you've cut off mid-sentence there mate, but I think I can guess where you're going! Classic issue with dual MPPTs feeding the same bank — if they're not communicating with each other, they'll often have slightly different charge voltage calibrations, which can cause one to think charging is complete whilst the other keeps pushing.

What I'd suggest first is measuring the actual output voltage of each controller independently and making sure they're set identically — even a 0.1V discrepancy can cause odd behaviour with LiFePO4 given how flat the voltage curve is.

Also worth checking whether your Fogstar has a built-in BMS that's doing any balancing itself — most Drift units do, so it might be quietly sorting things out anyway.

Finish your post and we can dig into the specifics! 👍

@Mike1997 two MPPTs bickering over one Fogstar is basically a digital custody battle — stick them both on the same Victron VE.Smart network and let them share charge state via Bluetooth like civilised adults 📡

AshHermit | 203 posts

@Mike1997 Worth checking whether either of your MPPTs supports a shared battery voltage sense wire — if both controllers are reading from the same point on the bank rather than their own terminals, they'll at least be working from identical data. Also, have you looked at setting one controller as the "master" with a slightly lower absorption voltage? Even a 0.1–0.2V difference can effectively let one lead and the other follow, reducing the fighting. What brands are the two controllers? Some have network/comms options (Victron's VE.Net for instance) that let them coordinate properly. That's the cleanest solution if the hardware supports it.

Midge66 | 89 posts

@Mike1997 the cut-off is killing me, but I'll take a punt — are you seeing one controller backing off whilst the other hammers away, leaving the bank unbalanced across a charge cycle? I had something similar with a Renogy and a Victron sharing a 100Ah LiFePO4. The fix for me was setting identical absorption voltages and float voltages on both units to the decimal, because even a 0.1V difference means one controller thinks it's done whilst the other keeps pushing. Also worth staggering the cable runs so both controllers see similar resistance to the battery terminals. @AshHermit's point about shared voltage sense is spot on too if your controllers support it. Finish your original post though — the specifics of which two MPPTs you're using make a big difference to what's actually possible! 🙂

If both MPPTs are fighting over who gets to top up the Fogstar, just wire a Victron VE.Smart network dongle on each one and let them gossip wirelessly until they agree — works a treat on my narrowboat where arguing controllers were the second most stressful relationship I had that winter.

LazyWarden | 47 posts

@Mike1997 Had something similar when I added a second MPPT for my garden office panels — one controller kept pushing the bank into absorption while the other was still in bulk, and they were basically arguing with each other constantly.

What sorted it for me was setting one controller as the "master" with proper absorption/float voltages, and deliberately detuning the second one to float-only. Not elegant, but it stopped the confusion.

@Tommo38 makes a fair point about VE.Smart networking though — is at least one of your MPPTs a Victron SmartSolar? That'd make the whole thing much cleaner.

HeathGazer | 312 posts

Welcome to the forum @Mike1997 — great first post, you've clearly done your homework already.

One thing worth checking that hasn't been mentioned: are both controllers set to identical charge profiles? Even a 0.1V difference in absorption voltage between the two can cause one to constantly "win" and the other to hover in bulk indefinitely, which looks like a balancing issue but is really just a settings mismatch.

On my narrowboat I had two Victron units doing exactly this until I standardised the profiles properly. Once matched, they played nicely together without needing any networking solution.

What are your current charge voltage settings on each controller?

BoatSteve | 184 posts

Running a similar dual-MPPT setup on my shepherd's hut — two Victron SmartSolars into a Fogstar Drift 200Ah. The key thing nobody's mentioned yet is absorption voltage mismatch. If your two controllers have slightly different absorption setpoints (even 0.1V apart), one will always "win" and the other backs off thinking the battery is full. Worth pulling up the Victron Connect app and making sure both are using identical charge profiles. Easiest fix I found was setting the secondary controller's absorption voltage fractionally lower so it deliberately yields — keeps everything predictable rather than the two constantly jostling for control.

GoldenBodger | 1,204 posts

@Mike1997 Worth checking whether your two MPPTs have adjustable charge current limits — if you can dial each one back slightly from its maximum, you'll reduce the chance of them both hammering the battery simultaneously and confusing the BMS. Also, what's your absorption voltage set to on each controller? If they're even slightly mismatched (say one's at 3.45V/cell and the other 3.50V), one will effectively always "win" and the other just idles, which might actually explain any balancing oddities you're seeing. Get those voltages identical first before anything else.

EcoFlow_Queen | 847 posts

@Mike1997 the key thing nobody's mentioned yet — both MPPTs need identical absorption and float voltage settings, not just similar ones. Even a 0.1V discrepancy causes one controller to "win" and the other to back off completely, leaving your bank perpetually unbalanced. In my garden office setup I run two Victron SmartSolars paired via VE.Direct into VictronConnect; the network synchronisation feature handles this automatically through Bluetooth. If your controllers don't support networked charging, set them both to the same fixed voltages manually and verify with a decent multimeter rather than trusting the controller readouts alone.

SarahLewis | 312 posts

@Mike1997 One thing worth adding to what's already been said — if your two controllers aren't the same brand, getting them to hand off from bulk to absorption at exactly the same moment is virtually impossible. What tends to happen is one finishes bulk first and backs off, leaving the other doing all the heavy lifting unevenly. If VE.Smart networking isn't an option across different brands, some people have had decent results slightly lowering the absorption voltage on the more aggressive controller so they step down together more gracefully. Bit of trial and error but it can help enormously.

MarineSam | 2,089 posts

@Mike1997 Coming at this from a marine angle where dual-controller setups are pretty common — one thing I'd add is to check whether either MPPT has a "shared battery sense" option or supports a shared comms bus (Victron's VE.Smart Networking is brilliant for this if both are Victron). Without shared voltage sensing, each controller is essentially guessing at battery state independently, which causes exactly the hunting behaviour you're likely seeing. Also worth temporarily disconnecting one controller entirely just to confirm which one is behaving oddly — helps narrow things down quickly.