Anyone else finding cheap MPPT controllers lying about their charging voltages?

I picked up a pair of Renogy Rover 40A units last year to run alongside my existing Victron setup in the workshop, mostly as a backup and to charge a second bank of 200Ah AGMs. Been reasonably happy with them on the whole, but I've started noticing the absorption voltage is consistently reading about 0.4–0.6V lower than what I've set in the controller menu. Set to 14.7V, measuring 14.1–14.2V at the battery terminals with a calibrated Fluke 117. The Renogy's own display is also showing 14.1V, so it's not a wiring drop issue — the controller itself seems to be outputting less than it thinks it is.

I've checked with a couple of mates who have similar budget MPPT units — one with an EPever Tracer 4210AN and another with a generic unit off Amazon — and they're seeing the same sort of drift. Makes me wonder whether this is just a known issue with the cheaper end of the market, or whether these things drift over time and need recalibrating somehow. Anyone know if the Renogy Rover has a voltage offset setting buried in the menus I might have missed? The manual is about as useful as a chocolate teapot.

Would be good to hear if others have found a reliable fix, or whether the honest answer is that you just can't trust the cheaper controllers to hold their set points accurately and should be budgeting for a Victron from the off.

@RetiredElectrician10 this resonates deeply with me.

I ran a cheap MPPT alongside my Victron SmartSolar for about six months in the garden office build — convinced myself I was being clever by "spreading the load." What I actually did was confuse the two controllers constantly arguing over absorption voltage.

The cheap unit kept claiming 14.4V but my Victron's battery monitor told a different story — real voltage at the bank was consistently 0.3–0.4V lower than advertised. Over winter that matters enormously for AGMs.

Eventually I just pulled it and accepted the Victron doing everything. The Bluetooth monitoring alone made the decision obvious — you can actually see what's happening rather than trusting a blinking LED.

Worth running your multimeter directly at the battery terminals during absorption to see what you're genuinely getting versus what the Renogy display claims.

Interesting thread this. I had similar bother with a pair of no-name 30A units I picked up off eBay — advertised absorption at 14.7V but my multimeter was consistently reading 14.4V at the battery terminals. Spent ages thinking I had a dodgy cable before I twigged the controller itself was the culprit.

Worth checking whether the discrepancy is at the controller's output terminals or specifically at the battery — voltage drop across the wiring can account for some of it, especially on longer runs. But if you're seeing it right at the battery terminals with decent cable sizing, then yeah, the firmware on a lot of these budget units just isn't calibrated properly from the factory.

@RetiredElectrician10 have you tried adjusting the charge voltage offset in the settings menu? Some of the Renogy units do allow a manual tweak to compensate.

Really useful thread this. Converting a Transit at the moment and I've been agonising over whether to go all-Victron from the start or mix in something cheaper to keep costs down.

@Spud99 the eBay units are what I'm most wary of — the specs on those listings are basically fiction half the time.

What I'm not clear on though: even with a branded cheaper controller like the Renogy Rovers @RetiredElectrician10 is using, is the voltage drift consistent enough that you could just manually offset the settings to compensate? Or does it wander unpredictably depending on temperature, load, etc.?

Asking because I'm trying to decide whether a Fogstar lithium bank deserves anything less than a Victron SmartSolar watching over it. My gut says no, but the price difference is significant on a tight van build budget.

Really relevant thread for me — I've got a small emergency backup setup and also run 12V on a boat, so voltage accuracy matters quite a bit in both contexts.

Quick question for the group: when you lot discovered the discrepancy, were you measuring at the battery terminals directly or at the controller output? I ask because I'm wondering how much of the error might be cable voltage drop rather than the controller genuinely lying about its setpoints.

Also @RetiredElectrician10 — with your Renogy units running alongside Victron, are you seeing the AGMs getting overcharged because the two controllers are disagreeing on absorb voltage? That's my concern if I ever run mixed brands. Presumably the higher-voltage controller "wins" and the other just sits in float while the batteries cop the excess?

Got this exact issue on the narrowboat. Running a cheap MPPT alongside my Victron SmartSolar and the voltage readings were consistently 0.3–0.4V apart under load. Ended up just ditching the budget unit — not worth the stress when you're trying to maintain a decent battery bank.

@LDVNomad honestly for a van build just go full Victron from the start. The app alone is worth it, and you won't be second-guessing your readings at 11pm in a layby somewhere.

For the garden office I've got a Fogstar 100Ah lithium now with a Victron controller and it's night and day compared to the AGM + cheap MPPT combo I had before. Some kit just isn't worth the saving.

@RetiredElectrician10 great thread. I had something very similar with a pair of no-name 30A units I bought off Amazon — claimed 14.4V absorption but my Victron BMV was consistently reading 13.8-14.0V at the battery terminals even accounting for cable losses. Turned out the controllers were measuring voltage at their output terminals rather than the battery itself, so any resistance in the wiring meant the batteries were genuinely undercharging.

Worth checking whether your Renogys support remote voltage sensing — some of the newer ones do via the app. If not, shortening and upsizing your cable runs between controller and battery made a noticeable difference for me.

@EdCampbell84 narrowboat wiring runs tend to be long, which compounds the problem significantly. Even decent cable over a few metres will drop enough to cause chronic undercharging over time.

@RetiredElectrician10 worth grabbing a decent calibrated multimeter and measuring directly at the battery terminals during bulk/absorption transitions — not at the controller's display. I did this with a budget EPever unit I was trialling alongside my Victron kit and found it was reading 0.4V high at the battery, which on AGMs is genuinely damaging over time. The Renogy Rovers are mid-tier but their voltage sensing is only as good as where you've connected the sense wires. Remote battery voltage sensing (if supported) usually sorts a lot of the discrepancy.