Efficiently charging 36V E-bikes from a 12V DIY House Battery (No Inverter needed)

Been doing exactly this for the past two summers on the moors and it's a proper rabbit hole worth diving into.

The key bit most people miss is that a 36V e-bike pack is typically charged to around 42V, so you're not just doing a simple 12V-to-36V step-up — you need a decent DC-DC boost converter that can handle the full charge profile properly. Cheap eBay units will get you somewhere near there but won't do a proper CC/CV curve, which is quietly murdering your cells over time.

What's worked well for me is pairing a programmable boost converter (I used a DPS5020 for a while) with a decent BMS on the bike side that handles the termination. Not elegant, but it does the job without the massive efficiency losses you'd bleed through an inverter + AC charger route.

Rough numbers from my setup:

• ~12.5V input from a 200Ah LiFePO4 (Fogstar Drift cells)
• Boost to 42V output
• Typically pulling 8-10A on the 12V side
• Charging a 10Ah 36V pack in around 90 minutes

Efficiency sits somewhere in the 88-92% range depending on load, which beats an inverter chain by a fair margin.

Curious whether anyone's tried the Victron Orion-Tr in this application — technically it's designed for 12V-to-24V vehicle stuff but I've wondered if the Smart variants could be configured creatively here.

Also, does anyone have experience with purpose-built e-bike DC chargers that accept a low-voltage DC input? Feels like a gap in the market nobody's properly filled yet.

@MoorHamish — you've cut off mid-sentence there mate, but I think I know where you're going with this.

The 36V pack charges to 42V (10S lithium chemistry), which means you need a proper boost converter capable of stepping 12V up past that threshold with enough current to actually be useful.

Been using a Victron Orion-Tr smart DC-DC for similar applications on my van build — not cheap but the charge profiles are actually configurable which matters if your e-bike BMS is fussy.

Worth noting:

• Efficiency on boost converters drops off noticeably at higher step-up ratios
• Your 12V bank needs to handle the input current spike — mine's a 200Ah Fogstar Drift LiFePO4 and it barely notices, but lead-acid would struggle

What's your source voltage actually sitting at under load?

Both of you cut off mid-sentence and somehow I still learned more than from three hours of YouTube.

@MoorHamish @LindaJones69 — right, I'll attempt to rescue this thread from its mysterious case of Simultaneous Truncation Syndrome.

A 36V nominal pack charges to 42V fully topped off (10S lithium chemistry, 4.2V per cell). So what you actually need from your 12V system is a DC-DC boost converter — something like a Victron Orion or a decent programmable unit — set to output a steady 42V at whatever current your charger port will accept.

The pedantic bit everyone skips: verify your e-bike's charge port accepts direct DC input before you fry the BMS. Some cheaper bikes expect a specific CC/CV profile from a dedicated charger, not raw voltage from a converter.

Running this exact setup off a Fogstar 100Ah in my shepherd's hut for emergency bike top-ups. Works a treat, no inverter humming away wasting 20W just to exist.

@CopperWelder genuinely laughed out loud at "Simultaneous Truncation" — that wants to be a band name.

Right, I'll fill in the blanks from actual experience. Running a Victron Orion-Tr Smart 12/36-15A DC-DC boost converter off my 280Ah Fogstar Drift LiFePO4 bank, I charge both my wife's and my 36V Pendleton packs directly — no inverter, no faff.

The bit @MoorHamish was presumably getting to: a "36V" pack charges to 42V, and the Orion handles that natively once you set the absorption voltage correctly in VictronConnect.

Efficiency-wise you're looking at roughly 93–95% conversion loss versus the 85–88% you'd bleed through an inverter-plus-charger chain. Over a Scottish summer that adds up to a meaningful difference in solar budget.

Only gotcha — check your e-bike's charge port polarity before connecting anything. Learned that the exciting way.

@SmartSolarGuy get a room, both of you — right, the actual useful bit: a 36V pack charges to 42V fully, so what you actually need is a proper DC-DC boost converter rated above that, a decent Victron Orion won't do this particular job (it's a buck converter stepping down), so you're looking at a dedicated boost unit — search "10A 12V to 48V boost converter" on Amazon, dial it back to 42V output with a multimeter before you connect anything expensive, set current limit conservatively around 5-8A, and jobs a good'un. Running this off my static van's 12V Fogstar lithium bank works a treat as emergency backup when the bikes come home half-dead. Don't skip the multimeter step or you'll be buying a new BMS.

@ForestDaz right so 42V target, got it — the bit I always trip over is the actual converter choice. Been running a Victron 12V system in my shepherd's hut and I keep eyeing up those DC-DC boost converters on Amazon but the quality is all over the shop.

Anyone actually using a proper branded unit for this rather than some random 15 quid module? Worried about efficiency losses and also just... fire. Mostly fire.

My solar input varies quite a bit on overcast days so wondering if the converter needs a stable 12V input or whether it handles a bit of fluctuation from the battery without drama. Running 100Ah Fogstar cells so it's not like I'm scraping the bottom constantly but still.