Anyone else struggling to keep a 100Ah lithium topped up on short winter drives?

My shepherd's hut runs a similar setup to a campervan in terms of charging sources, so I've wrestled with this exact problem from a slightly different angle — but the physics is the same.

Short drives in winter are genuinely brutal for lithium replenishment. The alternator barely gets a chance to breathe before you're parked up again, and with a 100Ah bank you're essentially trying to fill a bucket with a teaspoon during the commute.

A few things that shifted the needle for me:

• B2B charger — non-negotiable if you ask me. Running a Victron Orion-Tr Smart meant the alternator was actually working for the battery rather than just tickling it
• Managed expectations around BMS acceptance rates — lithium won't gulp charge the way AGM will, especially cold. Below about 5°C your cells are likely throttling acceptance anyway
• Pre-conditioning before a drive — if I could top up via shore power or even a small Renogy panel the night before, the alternator had far less to do

The dirty secret nobody mentions is that 100Ah lithium in a UK November is effectively more like 70-75Ah usable once you factor in cold-weather performance and the fact you probably shouldn't be regularly dragging it below 20%.

Has anyone tried running a dedicated DC-DC with a higher output — something in the 40A range rather than the standard 18-20A units? Curious whether the diminishing returns kick in quickly on shorter runs or whether it genuinely shifts the equation.

Would love to know what others are actually seeing on their Victron app data during winter drives — real numbers tell a better story than specs ever do.

@CliveKnight interesting angle coming at it from a shepherd's hut setup — how are you handling the charging side when it's static? I'm in a similar boat with my static caravan and garden office combo, both running Fogstar 100Ah cells, and I'm always curious how people bridge the gap when solar's basically useless and you're relying on alternator or shore power.

With the caravan though I've noticed even a decent MPPT like my Victron 75/15 barely tickles the battery on a grey December day — are you finding the same? What's your primary charge source when parked up for winter? Do you have any kind of B2B charger in the mix or are you purely solar/mains? I keep going back and forth on whether a Renogy DC-DC unit would actually make a meaningful difference for short inputs.

@CliveKnight that's a good point about the physics being identical regardless of the application — I ran into exactly this with my motorhome over the last couple of winters.

The short drive problem is essentially a bulk charging issue. Your alternator barely gets past the absorption threshold before you've parked up again. A DC-DC charger (I use a Victron Orion-Tr Smart 30A) transforms this completely — it actively pulls current rather than relying on voltage differential, so even a 20-minute run actually shifts meaningful charge.

Also worth considering:

• Solar panels even in winter contribute more than people expect on clear frosty days
• State of charge awareness — Fogstar lithiums often arrive partially discharged, so calibrate your shunt properly before drawing conclusions

Curious what your shepherd's hut uses as the primary top-up when there's no vehicle alternator in the picture?

@CliveKnight and @OldSparky — to answer the unfinished question, the real bottleneck on short winter drives isn't just the alternator output, it's the battery's acceptance rate when it's cold. A lithium at 5°C might only accept 20-30A even if your B2B charger is capable of pushing 40A+. I spent two winters fiddling with my shepherd's hut setup before I twigged that warming the battery space slightly before charging made a genuine difference. Even a small ceramic heater running on hook-up overnight transformed acceptance rates the following morning. Worth checking what your BMS is actually throttling back to in cold conditions — most decent ones will log this. What B2B charger are you running, @CliveKnight? Some of the cheaper units don't handle cold battery profiles particularly well in my experience.

@ExFarmer18 nails the bottleneck point — and it's worth spelling out why for anyone new to this: a lithium at, say, 20°C will happily gulp 40–50A from your alternator, but the B2B charger is the gatekeeper. Most 30A units (Victron Orion-Tr Smart being the obvious choice) will deliver maybe 360Wh on a 45-minute run. That's genuinely not bad, but if you've run the battery down overnight on a frost-warning night with a diesel heater cycling constantly, you're playing catch-up against physics.

My shepherd's hut leans heavily on solar as the primary source, with the alternator connection as backup — completely flips the dependency compared to a motorhome. But I did briefly run a 40A Victron Orion, which made a noticeable difference over the 30A version. The £30 price difference is irrelevant over the lifetime of the install, frankly.