Best charging voltage for a 12v deep cycle battery

After years of messing about with AGM and then moving over to lithium, this is something I've spent a fair bit of time dialling in on my motorhome setup.

For LiFePO4, I run my Victron MultiPlus set to 14.2V absorption with a fairly short absorption time — the cells genuinely don't need the prolonged top-up that lead-acid demands. Most Fogstar and EVE cell specs will confirm 14.6V is the maximum, but staying slightly below extends cycle life considerably.

For AGM, I was previously running 14.4–14.7V absorption depending on the temperature. Worth noting that UK winters can genuinely affect charge acceptance, so a temperature-compensated charger isn't optional in my opinion — it's essential.

A few things I'd flag for anyone tuning their system:

• Float voltage matters just as much — too high and you cook the battery slowly over months
• Don't chase 100% SoC constantly; lithium especially benefits from regular 80–90% charging cycles
• Your BMS will protect against catastrophic overcharge, but it shouldn't be your primary voltage management strategy

The chemistry really does dictate everything here. Gel, AGM, flooded lead-acid, and LiFePO4 all want meaningfully different profiles.

Curious what voltages others are running, particularly anyone on Renogy or Victron Smart chargers — do you stick with the presets or customise?

Chippy45 | 847 posts | ⚡ Solar Enthusiast

Great thread @MasterCamper! One thing worth adding that often gets overlooked - the absorption voltage matters far less with LiFePO4 than it does with AGM. Because of that incredibly flat discharge curve, you can honestly get away with a straight CV charge to around 14.2-14.4v and call it done without any meaningful absorption phase at all.

I've been running mine at 14.2v with zero absorption time and the cells balance themselves out perfectly fine through the BMS. Saves a bit of wear on the cells long-term too.

Also worth considering - if you're not regularly hitting 100% SOC (which most lithium folk recommend avoiding anyway), dropping your charge voltage slightly to 13.9-14.0v and treating it as your "daily" voltage is a solid strategy for longevity. 🔋

Compo55 | 312 posts | 🔋 Off-Grid Convert

Good points all round. One thing I'd throw in that nobody's mentioned yet — keep an eye on your battery temperature when setting charge voltage. LiFePO4 cells behave differently in cold weather, and most decent BMS units will derate the charge current below about 5°C anyway, but the voltage absorption point can shift slightly too.

I learned this the hard way last winter up in the Peaks when my van was sitting in -3° overnight. Worth checking whether your charger has a temperature compensation feature, though strictly speaking LiFePO4 needs less compensation than AGM rather than more.

@MasterCamper what temperature sensor setup are you running with your Victron? The Smart Battery Sense is decent but I've heard mixed things about placement on larger battery banks.

GlenSimon | 203 posts | 🚐 Van Dweller

My Fogstar 100Ah in the van conversion disagrees with anything above 14.2V the same way I disagree with Monday mornings — loudly and with lasting consequences.

BoatMel | 564 posts | ⛵ Liveaboard

Living aboard means I'm obsessive about this stuff. On my LiFePO4 bank I actually back off absorption to 14.0V and skip it almost entirely — float disabled completely. Sounds conservative but the science backs it up; cells genuinely prefer staying in that 20–80% sweet spot long-term.

@GlenSimon's point about 14.2V resonates — my Victron MPPT is set similarly for daily cycling. The "full charge" obsession is largely carryover thinking from AGM days.

Worth noting: temperature matters more than most people realise. If you're charging in winter below 5°C, you should be reducing voltage further regardless of what chemistry you're running. A decent BMS handles some of this but don't rely on it entirely.

What's your ambient temperature range like @MasterCamper? That'd change my recommendation quite a bit.

Worth adding — absorption time matters as much as the voltage itself. I was fixated on the exact bulk voltage for ages but my Victron MPPT logs showed the real damage to cycle life was sitting at top-of-charge for too long.

Dropped my absorption time right down to around 20 mins and the cells balance out far more gently. If you're running a BMS with cell-level monitoring you'll see what I mean — the voltage spread during a long absorption is pretty eye-opening.

@GlenSimon your Fogstar's probably happier than most people's batteries tbh, that conservative ceiling gives the BMS less drama to manage at the top end.