Over charging

Got me thinking about a situation I ran into last spring when I was experimenting with different charging sources on my AGM bank.

Had a DC-DC charger — a Victron Orion in my case — that was pushing voltages I wasn't entirely comfortable with on a sealed AGM. The charger was technically within spec, but it was hitting the upper absorption threshold faster than expected and the battery wasn't responding the way I'd anticipated.

The thing with AGMs is they're far less forgiving than lithium when it comes to overvoltage. Go even slightly above the manufacturer's ceiling consistently and you're gassing the electrolyte away quietly, with no obvious warning signs until capacity starts mysteriously dropping a few months down the line.

What I've found with my setup:

• Always cross-reference the charger's absorption voltage against the battery manufacturer's spec sheet, not just the charger's default profile
• DC-DC chargers often have preset profiles labelled "AGM" that aren't universal — different AGMs have different upper limits
• Some chargers skip bulk if the battery is already at a reasonably high resting voltage, which can look like misbehaviour but is actually normal

The skipping bulk phase is something I've seen confuse a lot of people. It's not necessarily a fault — it just means the battery was already partially charged when the session started.

Has anyone else had their DC-DC charger behave oddly with AGMs specifically? Curious whether this is more of a calibration issue or whether certain battery brands just don't play nicely with certain charger profiles. My Fogstar lithium cells have always been much more predictable in comparison.

@Jock the Orion's absorption voltage for AGM is non-negotiable — stray above 14.7V for any meaningful duration and you're cooking electrolyte, full stop.

Key things worth checking:

• Temperature compensation — AGMs want roughly -4mV/°C/cell, so a warm battery bank needs a lower setpoint, not the same one
• Bulk→Absorption transition — if your current sensor or battery monitor is miscalibrated, the Victron never "thinks" it's full and just hammers away
• Source voltage — a healthy alternator can spike to 14.8V+ momentarily; the Orion should smooth that, but verify your firmware is current

Personally I've seen more AGM casualties from slightly-too-high float voltage sustained for weeks than from brief absorption spikes — the slow cook is the silent killer.

@Jock had similar grief with my AGM bank before I switched to lithium. The problem isn't just the peak voltage — it's duration. Even 14.4V held too long will gas an AGM and cook the water out of it.

Worth checking whether your Orion is actually in pass-through mode rather than actively regulating. Mine was doing something weird where the alternator voltage was basically bypassing the charge profile entirely on a poor connection.

Also — what's your temperature compensation set to? AGMs want roughly -24mV/°C drop from 25°C. Static caravan in a cold spring? You could be pushing higher than you think because the comp isn't dialled in.

@OldSailor is right about the absorption ceiling. Been there, binned two batteries because of it.

@Jock interesting one — what absorption duration were you running? On my boat I found the Orion's default absorption time was way too aggressive for a partially discharged AGM bank. It'd hit target voltage quickly but then keep pumping current in far longer than needed.

Worth checking whether your Orion is in fixed absorption time mode versus adaptive. The adaptive setting backs off once current tapers, which is much kinder on AGMs.

Also — what's your battery temperature situation? Batteries sitting in a warm engine bay versus a cool locker can shift the safe ceiling noticeably. Did you have a temp sensor fitted?

@Jock the piece nobody mentions with the Orion and AGM is temperature compensation. If your battery bank is sitting in a cold locker — and in a UK spring that's entirely plausible — the Orion's fixed absorption voltage looks fine on paper but your actual battery is asking for more voltage than ambient suggests. Then on a warm afternoon the same setting becomes genuinely aggressive.

Ran into exactly this on my old motorhome before I ditched AGM entirely. The Orion Smart at least has Bluetooth logging, so I pulled the charge curves afterwards and could see the absorption phase dragging on because the battery never quite hit the acceptance current threshold.

Worth checking whether you've got the Smart variant — the dumb Orion gives you zero visibility into what's actually happening, which is half the problem right there.

@Jock classic case of the charger being too keen. Like hiring a personal trainer who doesn't know when to stop — your batteries just wanted a gentle jog and instead got a full bootcamp session.

One thing nobody's touched on yet: are you running multiple charging sources simultaneously? I had a solar MPPT and an Orion both

@BoxerProject that analogy is actually spot on 😄

Worth adding — are you monitoring voltage at the battery terminals rather than at the charger output? On my boat I had a decent chunk of voltage drop in the cabling which meant the Orion thought it was hitting absorption threshold when the batteries hadn't actually got there yet. Went the other way to your issue but same root cause — the charger's flying blind.

Solved it by running a proper sense wire direct to the battery. Night and day difference. Victron's remote sense feature is there for a reason and I'd ignored it for months like an idiot.

Cheap multimeter permanently clipped to the terminals told me everything I needed to know before I bothered with anything fancy.