DC to DC two way charging

Been down this rabbit hole myself recently, so thought I'd share what I've learned.

I've got a mixed bank situation on my boat — a pair of older AGMs that I wasn't ready to ditch, but I wanted to start migrating toward LiFePO4 without a full rip-and-replace job. The obvious answer seemed to be a bidirectional DC-DC charger sitting between the two chemistries.

The problem is that "two-way" DC-DC units aren't as common as you'd think, and the ones that exist vary wildly in how clever they actually are. Some are little more than glorified voltage followers — not what you want when your AGM and lithium have completely different charge profiles.

What I've been looking at:

• The Victron Orion-Tr Smart — not natively bidirectional, but some folks are running two back-to-back, which feels like a bodge
• Sterling Power make some interesting bits for exactly this kind of hybrid setup — very boat/vehicle focused, worth a look
• Renogy have a DC-DC unit but I'm not convinced it handles the edge cases well

The real question I keep coming back to is: does the BMS on your lithium side need to see both chargers independently, or can the DC-DC unit act as a buffer? That changes everything about how you wire it.

Anyone here actually running a working hybrid AGM/LiFePO4 setup long-term? Particularly curious if you're managing this on a narrowboat or motorhome where alternator charging complicates things further.

Would love to hear what's actually working in the real world rather than just what looks good on a spec sheet.

@OffGridTerry interesting setup — bidirectional DC-DC is something I've been looking at closely for EV-to-system integration actually.

Worth flagging that most Victron Orion-Tr Smart units are unidirectional, which catches people out. For genuine two-way flow between chemistries you're typically looking at the Orion XS (newer, bidirectional-capable) or third-party units like the Renogy DC-DC 40A.

Key consideration with mixed banks: the DC-DC isolates the charge profiles completely, so your AGMs aren't seeing lithium bulk voltages — that's the point, not a limitation.

One gotcha I hit: make sure your BMS can communicate a load-disconnect signal to pause the DC-DC before it trips on undervoltage. Otherwise you get a nasty cascade. Victron's VE.Direct ecosystem handles this cleanly if everything's playing together, but mixing brands needs careful relay logic.

What BMS are you running on the lithium side?

@OffGridTerry done similar in the motorhome — separate AGM starter bank and a Fogstar lithium leisure bank. Victron Orion-Tr Smart handles it well one-way but the bidirectional stuff gets complicated fast.

Main thing I'd flag: your BMS comms matter a lot here. If the lithium side cuts out mid-charge cycle the AGMs cop a voltage spike. Learned that the hard way.

@ThistleTel EV integration is a whole other headache — the CAN bus stuff on most EVs doesn't play nice without a proper gateway. Not cheap either.

Honestly for most setups I'd question whether true bidirectional is worth the hassle vs just two separate DC-DC units running opposite directions with proper isolation. Less elegant but far easier to fault-find at 11pm in a layby.

@DownsCruiser the Orion-Tr Smart is doing serious legwork in these setups — but worth flagging for anyone reading: the bidirectional variant (Orion XS) is a different beast entirely and actually allows proper two-way flow rather than just intelligent one-way charging.

I've got an XS bridging my tiny house's Fogstar lithium bank to a small AGM buffer for legacy 12V loads, and the configurability via VictronConnect is genuinely impressive — you can define which direction takes priority under what voltage thresholds.

Key gotcha nobody mentions: your BMS communication matters enormously here. If your lithium BMS pulls the plug mid-transfer without telling the DC-DC converter gracefully, you can get some unpleasant transient spikes. Make sure your BMS has a proper charge-disconnect signal the Victron can actually listen to, not just hard-cutting the circuit.

@Titch ran the same Orion-Tr Smart between my shepherd's hut cabin battery shed and a spare AGM I refused to part with — basically a two-way therapy session for incompatible chemistries, and honestly it's been rock solid for two years without me having to think about it once.

@BatteryPaula a spare AGM you refused to part with is just a starter battery with commitment issues