Anyone else running a cheap 100Ah LiFePO4 alongside an old AGM to stretch the budget?

So I've been gradually upgrading my van setup without doing a full rip-and-replace, and I'm currently running a 100Ah LiFePO4 (one of the Fogstar Drift cells, picked up for about £89 on offer) alongside my existing 110Ah AGM that came with the van. I know mixing battery chemistries isn't exactly textbook, but I've got them wired in parallel through a battery-to-battery charger feeding the lithium separately from a 40A Renogy unit, so in theory they're not directly fighting each other.

Day-to-day it seems to be working — the LiFePO4 does most of the heavy lifting for my 12V compressor fridge (around 3-4A draw) and a few USB loads overnight, while the AGM sits mostly as backup. I'm pulling about 180W of solar through a cheap 30A PWM controller into the AGM side, which I know is inefficient but the panels were free off Facebook Marketplace so I'm not complaining.

What I'm not sure about is whether my AGM is quietly suffering because of this arrangement, or whether the B2B charger is genuinely keeping things isolated enough that it doesn't matter. The AGM is about 3 years old and still reads 12.6V resting, so it doesn't seem unhappy yet.

Has anyone else bodged together a mixed-chemistry setup like this on a budget rather than going all-in on lithium straight away? Curious whether people have made it work long-term or whether the AGM eventually just gives up the ghost.

Chalky30 | Posts: 847

@JennyParker great shout on the Fogstar Drift, cracking value those things. One thing worth watching with mixed chemistry setups is your charging profile - AGMs typically want around 14.4-14.7V absorption whereas LiFePO4 is happier around 14.2V. Running them in parallel means you're compromising somewhere.

What I did was put a cheap DC-DC charger (Victron Orion-Tr 30A, though the Sterling ones are decent too) between my alternator feed and the LiFePO4, keeping them effectively isolated during charging but still letting the LiFePO4 supplement loads. Costs a bit upfront but saves you cooking either battery.

Also worth checking your AGM's actual capacity before assuming it's contributing much - older AGMs often drop to 60-70% of rated capacity and can drag things down unexpectedly.

What's your primary charging source, solar or alternator?

StormySocket | Posts: 1,203

@JennyParker ran almost this exact setup on my narrowboat for about eight months before I finally swapped the AGM out entirely. The thing nobody mentions is the charging priority battle — your charger will keep topping up the AGM long after the LiFePO4 is sitting happy, because the voltage curves tell it to. Ended up with my Victron SmartShunt showing the lithium barely getting touched during shorter runs.

Worth looking at whether your B2B or alternator charger has a lithium-specific profile you can actually assign to one output. Some of the cheaper units just don't. Renogy's DC-DC chargers let you set this properly — made a real difference once I sorted it.

Mixed chemistry setups can work, but they need a bit of deliberate wiring thought rather than just bolting things in parallel.

Turbo43 | Posts: 412

@JennyParker nice setup! One thing I'd flag that nobody's mentioned yet — watch your charging voltage carefully. Most standard alternators and solar controllers will default to AGM profiles (around 14.4-14.8V), which is fine for the LiFePO4 but could be pushing your AGM a bit hard long-term depending on its age.

Also worth thinking about discharge behaviour — the LiFePO4 will happily dump current right down to its cutoff whilst your AGM voltage sags much earlier, so if they're on the same bus without isolation you might find the AGM doing more work than intended. A simple battery-to-battery charger between them can solve a lot of headaches. Decent B2B units have come down in price massively lately. What's your current charging setup — solar, alternator, or both?

OffGridPete | Posts: 2,156

Running a similar hybrid on the narrowboat — LiFePO4 paired with an older 110Ah AGM. The thing nobody really talks about is the resting voltage mismatch once both banks are partially discharged. Your LiFePO4 will sit at a higher voltage and effectively stop accepting charge while the AGM is still thirsty. Depending on how your isolator or combiner is wired, you can end up with the AGM chronically undercharged over time.

Worth picking up a cheap Victron Battery Protect or even just a voltage-sensing relay to manage which bank gets priority from solar. Keeps the AGM honest.

@StormySocket curious what you ended up doing on the narrowboat — did you go split charging or just run them in parallel and accept the compromise?