You don't have to buy a smart alternator regulator to charge a lithium bank. You can put something in between instead — and end up with more control than the smart regulator would have given you.
Why you can't just wire the alternator to lithium
A standard internally-regulated alternator — a "dumb" one — does one thing: it tries to hold its output at a fixed voltage, somewhere around 14 volts for a 12 volt system. It has no idea what it's charging and no way to be told.
Connect that to lithium and you have two separate problems.
Lead acid protects an alternator without anybody noticing. As the bank fills, its charge acceptance collapses and it stops asking for current, so the alternator gets to cool down.
Lithium never backs off. It will take everything the alternator can make, for as long as the engine runs. A standard alternator at full output continuously will cook its windings and diodes. It was never designed for a load that doesn't taper.
This is the one that kills alternators outright.
A BMS protecting its cells opens the circuit. Instantly, completely, and without telling anything upstream. If the alternator is pushing 100 amps at that moment, the current has nowhere to go, the field collapses into an open circuit and the voltage spike takes out the diodes.
An alternator must always have somewhere to push. Everything below is really about honouring that one rule.
Two valid routes
Replace the alternator's internal regulation with an external one that has a temperature sensor on the alternator case, talks to the BMS, and ramps output up and down intelligently.
Proper, elegant, and the right answer for a lot of boats. It's also expensive, it means opening up the alternator, and when it fails you have one unfamiliar box between your engine and your batteries.
Leave the alternator exactly as it is, charging a lead-acid buffer bank the way it has always charged lead acid. Then take power out of that buffer with DC-DC chargers, which do the lithium-aware part.
The alternator never sees lithium. It never sees a BMS disconnect. It sees a lead bank, which is all it was ever designed to see.
The second one isn't a compromise. It's a different architecture, and it has a real advantage: the DC-DC chargers can be stepped up and down. You get a volume knob on your alternator load, which no internal regulator was ever going to give you.
Alternator → buffer → converters → lithium
A Niehoff N1387, rated 210 amps, on a Ford Lehman 120. Standard internal regulation. Nothing about it has been modified.
Two 12 volt telecom batteries in series, making a 24 volt, 200 amp-hour lead-acid bank. Telecom cells are designed to sit on float for years and take the occasional hard discharge, which is exactly this job.
This bank is the shock absorber. The alternator charges into it and nothing else. If every DC-DC charger downstream switched off at the same instant, the alternator would simply be charging a lead bank — which is fine.
Victron DC-DC units take 24 volts off the buffer and feed the lithium banks. On Exodus there are six of them, in three groups:
| Group | Does what | How it switches |
|---|---|---|
| 24 → 24 V, smaller | Buffer to the 24 V house lithium | Relay — comes on with the engine |
| 24 → 24 V, larger | Buffer to the 24 V house lithium, in bulk | Switched by hand |
| 24 → 12 V | Buffer to the 12 V nav lithium | One on the relay, one by hand |
The relay-driven ones come on automatically with the engine, so basic charging needs no thought. The hand-switched ones are the extra load you add when you want it.
Six converters, switched in groups, means you can add or remove alternator load in steps while the engine is running.
Motoring in cold weather with plenty of airflow? Switch them all in and take everything the alternator will give. Alternator running hot, or the belt starting to complain? Switch a couple out and it immediately backs off. Nearly full and only topping up? Leave the automatic ones on and forget about it.
That is a crude version of what a smart regulator does automatically — but it's crude in a way you can see, understand and fix with a switch.
The Victron DC-DC units let you define your own charge profile — absorption voltage, float voltage, absorption time, current limit — rather than picking from a list of preset battery types. For lithium that matters, because the presets are all written for lead chemistries and none of them are right.
Setting them is done over Bluetooth from the Victron app, and the numbers come from your cell datasheet. Which numbers to use is a per-installation question and is not something to copy off a website — including this one. If you want to talk through how ours are set up, come and ask on Discord.
Four of them, and what each one is
Terminal markings vary by manufacturer. The names below are the common ones, but your alternator may use different letters, and some use the same letter for a different job. Identify every terminal against your own alternator's documentation and a meter before you connect anything. Getting the excite wire wrong can destroy a regulator.
| Wire | Usually marked | What it is |
|---|---|---|
| Charge output | B+ |
The big stud. All the current comes out of here. On this boat it goes to the buffer bank and nowhere else. |
| Ground | B− / case |
The return path. Often through the engine block, but a dedicated cable back to the negative bus is better — a ground through a rusty mount is a charging fault that looks like a dead alternator. |
| Excite / field | D+, IG, L, I |
The small wire that tells the alternator to start working. No excite, no output, no matter how fast the engine turns. Usually fed from the ignition switch, sometimes through the charge warning lamp. |
| Ignition sense | from the key switch | Not an alternator terminal — this is the wire that goes live when you turn the key. It's what we use to tell everything else that the engine is running. |
That ignition wire is the whole trick. It is the one signal on the boat that reliably means the alternator is now making power. Tap it and you can switch anything you like off the engine running.
A small signal switching a big one
A relay is an electrically operated switch. A small current through a coil pulls a contact closed, and that contact can carry far more current than the signal that operated it. That's it. That's the whole device.
The standard automotive relay has five pins, and they are numbered the same on almost every one you'll buy:
| Pin | Connect to |
|---|---|
86 | The ignition wire. Live when the key is on. |
85 | Ground. |
30 | Your power feed, through a fuse. |
87 | The thing you want switched on with the engine. |
87a | Live only when the engine is off. Usually left empty. |
Pins 85 and 86 are the coil and the current through them is tiny. Pins 30 and 87 carry the real load. That separation is the point: your ignition switch, which was never designed to carry charging current, gets to control something that is.
One relay per automatically-switched DC-DC charger, coils fed from the ignition circuit, contacts carrying the enable signal to each charger. Turn the key, the engine starts, the alternator begins charging the buffer, and the relays bring the converters up behind it.
Turn the key off and it all drops out in the same order. No configuration, no software, nothing to go wrong on a wet night.
Both directions
| In favour | Against |
|---|---|
| The alternator is untouched and stays simple | More boxes, more wiring, more to install |
| A BMS disconnect cannot reach the alternator | Conversion losses at every DC-DC stage |
| Load is adjustable while the engine runs | Some of that adjusting is manual — you have to pay attention |
| Buffer bank keeps working if a converter fails | A lead bank to buy, site, and eventually replace |
| Every component is ordinary and replaceable anywhere | Not as elegant as one well-chosen regulator |
If you're fitting out from scratch with a budget, a proper external regulator with alternator temperature sensing is a clean answer and we wouldn't argue with it.
If you already have a perfectly good alternator you'd rather not open up, this gets you there with parts you can buy anywhere and understand completely. That was worth more to us than elegance.
Exodus