Alternate Charging: the DC-DC, the Genny and the Honest Row
You’re driving, the battery’s climbing — and a solar app that only understands solar says you’re going backwards. Solar Spotter knows the sun isn’t the only thing filling your battery, and it separates what the sun gave you from what the alternator did.
Your rig can say what else charges it
Alongside panels, battery and camp gear, a rig carries one more answer: what else puts charge in. It lives in Your Gear under Alternate charging, part of Solar Spotter Pro, and it is two switches — DC-DC charger (Tops up on a drive) and Generator. Tick what you’ve got; “nothing set” is a real answer, and plenty of rigs are panels-only.
It belongs to the rig rather than to one panel, so you tick it once and every screen agrees — there is nothing to repeat for each blanket. Each switch takes an optional output in amps, and leaving that blank is a real answer too: owning a charger and never having measured it is fully supported, and the app falls back to a sensible rate instead of a made-up one.
If you do fill it in, one thing is worth getting right: the figure is your charger’s output, not the generator’s. A 2 kW genny feeding a 15 A charger is a 15 A source. The number is never printed back at you, either — it only helps the app work out which hours of a long gap the charge most likely arrived in.
The live row: Net alternate charge
With a Bluetooth battery connected, the app can see when the battery is gaining more than a cloudless sky could possibly deliver through your panels. Cloud only ever reduces solar — so anything above that ceiling provably didn’t come from them. That surplus appears on its own row: Net alternate charge.
The row isn’t always on screen, and that’s deliberate. It wants a paired Bluetooth battery — Pro, and still in BETA while more brands are confirmed — a position fix and at least one panel ticked in use, so the app can state what a cloudless sky could be doing; a reading from the last couple of minutes; and a net figure genuinely above that ceiling. Short of any of those it stays quiet rather than guessing.
Three things about that number, each there on purpose. It’s net — a battery reads one figure at one point, so a rig taking 400 W from the charger while the fridge pulls 60 shows 340. It never names the source — an alternator, a generator and a mains charger are indistinguishable from the battery’s side, so the app won’t pretend to know. And it drops off when the reading goes stale, rather than asserting a charge that stopped ten minutes ago.
It isn’t added to the solar row, either — one figure is modelled, the other measured, and a sum of those two would be a number nobody could stand behind.
One thing it won’t do is mark your charger’s homework. The app never checks what arrived against the amps you typed in, and never reports what your charger actually delivered — one shunt at the battery, netted against whatever the fridge is doing, cannot spec a charger. If you want to know whether your 40 A DC-DC is really a 40 A DC-DC, that’s still a job for a clamp meter.
All of this rides on the battery connection — so if yours won’t pair, don’t let it fail silently. Report a Bluetooth issue in the app opens a short form and then hands you a ready-made email with the diagnostics already in it: you read it and send it, or copy the lot and paste it wherever you like. Nothing leaves your phone on its own. Telling us your setup at solarspotter.app/bt is how batteries get added to the supported list.
How fast, and when it’ll be full
Lower down the same card, in the slot the sunset and overnight figures usually hold, one row prints the pack’s own rate of change. With a live reading it says Battery charging and a figure in %/hr, and the line beneath names where that came from — Live from battery — followed by at this rate, full in ~2h 10m, with the clock time beside it when full lands inside a day. Take the charge away and the same row reads Battery dropping.
It’s the plainest number on the screen: the current your battery is reporting, divided by the capacity you declared for that bank, and nothing modelled into it at all. It needs a bank with a stated capacity, a fresh reading, and a current that isn’t zero — a fridge between cycles reads 0.0 A more often than not — and without those the row falls back to the app’s estimated rate instead.
Read the time as a floor, not a promise. It’s a straight line drawn from this second’s current: 20 A now is taken as 20 A all the way to 100%. Real chargers ease off as the pack fills, and the last part is always the slowest — so full arrives later than the row says, never sooner. Same caution as amp-hours divided by rated amps, and why a charger eases off near the top is a guide of its own.
Drive detection: the alternator gets its own credit
Say yes to one optional permission — Motion & Fitness on iPhone, Physical activity on Android — and the app knows when the rig was being driven. Charge that arrived on the road is then told apart from solar in your recorded history: three hours of highway stops being credited to panels that spent the trip folded in the back.
It reads one thing only: whether the phone was in a vehicle. No location, no route, no speed, and that reading never leaves your phone — and refusing it breaks nothing beyond that one distinction.
Where you see the result is Where your charge came from, the strip at the top of Your Energy History: a three-way split reading something like 62% sun, 31% driving, 7% other charging. Driving is the only one of the three the app can prove, which is exactly why the permission matters. Without it the alternator’s work still shows up — it just lands under other charging.
It takes a while to appear. Your Energy History needs seven recorded days before it opens at all, and the split needs three days the app could test against a clear-sky ceiling, hedged in its own wording until there are seven. The Energy History guide walks through the whole screen.
Never projected forward
The app will not promise a charger keeps running. It can’t know how much longer you’ll drive or when the genny gets switched off, so alternate charge is never walked forward: real while it’s arriving, banked in the measured battery level once it has, and absent from tonight’s figures. The sunset and sunrise rows are honest given you stop now — start the engine and watch them improve in real time instead.
The live rate row is the one that does follow the charger, and it isn’t a forecast: it describes this second and says so, at this rate. Everything with a horizon on it — by sunset, through to sunrise — still assumes the charging has already stopped.
There’s no daylight gate on any of this, deliberately. Solar can only arrive while the sun is up; a drive or a generator can happen any time, and the row follows the charge, not the clock.
The few minutes after you switch off
One moment worth knowing about so it doesn’t read as a fault: a DC-DC charger keeps delivering for a few minutes after the engine stops. The battery really is still filling, so Net alternate charge stays up and the rate row keeps saying Battery charging. Both are reading the same live current, and both are right — it is genuine charge, and it stays in the pack.
What doesn’t follow it is tonight’s outlook, which assumes the charging has already stopped. So for those few minutes the live rows and the overnight rows are describing different moments — one now, one after it fades. Nothing needs fixing; it settles itself within minutes.
Where it flows
This is the crediting link in the chain: the deployment declaration decides when solar can be credited at all, anything above the cloudless ceiling lands here, a detected drive marks the alternator’s windows, and the whole day settles into Energy History. The full chain is laid out in the main-screen guide.
Common questions
Do I have to set anything up?
Very little. Your rig can say what else charges it — charging while you drive, a generator — as part of Your Gear, and “nothing set” is a perfectly good answer; plenty of rigs are panels-only. The live rows ask a bit more: a paired Bluetooth battery, a stated bank capacity, a position fix and a panel ticked in use. Drive detection is one optional permission.
Why does the time to full keep moving?
Because it is a straight line drawn from this second’s current, and that current changes. A charger eases off as the pack fills and the fridge cycles on and off underneath it, so the figure shifts every reading. Treat it as a floor — the earliest full could arrive — rather than an appointment.
Why doesn’t it say which source the charge came from?
Because it can’t know, and it won’t pretend to. A battery reads one net figure at one point — an alternator, a generator and a mains charger all look identical from there. The row tells you the charge is real and isn’t solar; which wire it came down is something only you can see.
Why did the row disappear?
The reading went stale. The row describes what is happening now, so when fresh readings stop arriving — the phone moved out of range, the screen was off for a while — it drops off rather than asserting a charge that may have stopped. Reconnect and it returns.
Why doesn’t tonight’s forecast include my generator?
Because the app cannot know how long you’ll run it, and promising charge that depends on a decision you haven’t made yet is the over-promise direction. Everything a charger has already delivered is banked in the measured battery level — the overnight figures are honest given you stop now.
Does drive detection track where I go?
No. It reads one thing: whether the phone was in a vehicle. No location, no coordinates, no route, and that reading never leaves your phone. Refusing the permission breaks nothing; the app just can’t separate alternator charge from solar in your history.
Every source, credited honestly
Alternate charging and drive detection are part of Solar Spotter Pro — a one-off purchase, no subscription, and your data stays on your phone.
