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

Same issue here with my motorhome setup. Running a 100Ah Fogstar Drift and through November/December I was finding it barely getting past 80% after a 45-minute run. Alternator charging just isn't cutting it in winter — shorter daylight, higher loads from heating, and the alternator itself isn't always putting out full current until it's warmed up.

A few things that helped me:

• DC-DC charger (B2B) — fitted a Victron Orion 30A and it made a noticeable difference. Charges properly rather than the alternator "seeing" a full battery and tapering off early
• Mains top-up when possible — even a couple of hours on hook-up sorts it
• Being more disciplined about loads — I stopped running the diesel heater flat out all night

The B2B was honestly the biggest gain. Before that I had a direct split-charge relay and it was pretty ineffective with lithium because of how the voltage profile works.

Worth checking what charging setup you've actually got, because a lot of motorhomes come with relay-based systems that were designed for lead-acid and don't really suit lithium properly.

Anyone else tried the higher-amp B2B units — the 30A versus 60A — is the upgrade worth it for a single 100Ah battery, or is 30A the sweet spot?

@Louise1984 Classic winter alternator woes. A few things worth checking:

1. What's your B2B charger rated at? A 30A unit will only push ~360Wh into a 45-min drive — not much headroom once you factor in van loads running simultaneously.

2. Alternator temperature matters — cold engines take longer to reach full output, and modern smart alternators throttle back aggressively in the first 20 minutes.

On my motorhome I swapped to a Victron Orion-Tr Smart 30A and saw a noticeable improvement, partly just from the proper absorption profile it runs compared to my old dumb isolator relay.

Also worth checking your cable sizing between the batteries — undersized cabling will cost you more than people realise, especially on longer runs from cab to habitation battery.

What B2B (or isolator relay) are you currently running?

@Louise1984 Worth adding to what @MrBodge65 is saying — ambient temperature plays a bigger role than most people realise with lithium. The Fogstar Drift has a low-temp charging cutoff, so if your hab area is cold when you set off, the BMS may actually be throttling or pausing charging entirely for the first stretch of your journey until the cells warm up a bit. That alone could easily account for the disappointing SOC after 45 minutes. Do you notice any difference if you pre-warm the van before driving? Even running a small heater for 20 minutes beforehand can make a noticeable difference to how quickly charging kicks in properly during winter runs.

Good points from @MrBodge65 and @LesCrane90 already, but one thing neither has mentioned: your alternator's voltage regulation in winter.

Many modern vehicles run a "smart" alternator that deliberately drops charge voltage when the ECU decides the battery is "full" — it's reading the starter battery, not your leisure setup. A B2B charger (Victron Orion-Tr Smart is my go-to recommendation) partially solves this, but if the alternator is only outputting 13.2V rather than 14.4V+ because of ECU intervention, even a good B2B struggles to source enough current to work efficiently.

Worth logging your actual alternator output voltage whilst driving using a Victron SmartShunt or similar. On my narrowboat I was shocked to find mine sitting at 13.6V for most of a run — looked fine on a basic meter but was starving the B2B.

Something nobody's flagged yet — how old is your vehicle's alternator? On older motors they're often already running below rated output before temperature derating even kicks in. Had this exact problem with my setup last winter; turned out my alternator was only pushing about 65% of its stated capacity.

Worth grabbing a clamp meter and actually measuring what's going into your B2B input rather than guessing. I was shocked at the difference between what I assumed I was getting versus reality.

Also on the Fogstar Drift specifically — check your BMS charge current limits are set correctly. Mine defaulted conservatively out of the box and I was leaving free capacity on the table for months before I noticed.