Charge phones and small batteries without damaging them

The one rule: never connect a panel straight to a device

A solar panel is not a power supply. It is a current source whose output collapses the moment a cloud passes, and recovers the moment it goes.

A phone plugged into a bare panel sees the voltage sag, disconnect, come back, sag again. Over an afternoon of broken cloud that is hundreds of connect and disconnect cycles. Best case, the phone charges barely at all because each interruption restarts the negotiation. Worst case, the unregulated voltage exceeds what the charging circuit tolerates, and you have destroyed a device you cannot replace.

Always put something between the panel and the device. A regulator, a charge controller, or better, a battery.

A panel with a USB socket moulded into it usually has regulation built in, and those are fine. What is not fine is a bare panel with wires, or a panel intended for a 12V system, connected directly to anything with a lithium cell in it. If you cannot establish that regulation exists between the panel and the socket, assume it does not.

Why the buffer battery fixes it

The intermediate battery smooths everything, and it is worth understanding why rather than just doing it.

• The battery absorbs the variation. Clouds affect how fast it fills, not whether the phone is charging.
• The panel can work all day into a battery that is always willing to accept charge, instead of into a phone that stops at 100% and wastes the rest of the afternoon.
• The device sees a proper regulated 5V, exactly as it would from a wall socket.
• You can charge at night, which is when people actually want to charge things.
• The phone's own charging circuit is never asked to cope with anything unusual, which is the entire point.

Panel charges battery. Battery charges device. Never panel to device. That sentence is the page.

What to buy, in order

1. A decent USB power bank, 20,000 mAh or so. The single most useful item in this list. It charges from anything, holds enough for several phone charges, and is the buffer.
2. A folding solar panel with a built-in regulated USB output, 20 to 40 W. Enough to fill that power bank in a day of reasonable weather.
3. A 12V to USB adapter, if you have a 12V system, so the household bank can charge small devices directly. This is far more efficient than going through an inverter.
4. A good multi-bay AA and AAA charger, ideally one that runs from 12V as well as mains.
5. A stock of low self-discharge NiMH cells in the sizes your torches and radios take.

Standardise ruthlessly. One connector, one cell size, one charger. In a household under stress, the item nobody can find the right lead for is the item that does not get used.

Getting a panel to actually charge a phone

Small solar panels routinely disappoint, and it is nearly always one of these.

• It is not aimed. A panel flat on the ground in Britain collects a fraction of what an aimed one does. See angling for the season. Aim it, and re-aim it when you walk past.
• Part of it is shaded. A single shaded cell can cut a panel's output enormously, because the cells are in series and the weakest one limits the string. A leaf, a washing line, or the shadow of a chimney across one corner is enough. This surprises people more than any other fact about solar.
• It is dirty. Dust, pollen, salt and bird mess. Wipe it.
• It is hot. Panels lose output as they warm. Airflow behind the panel is worth real percentage points, so do not lay it flat on a dark roof.
• The lead is too long or too thin. Loss over the lead at 5V is significant, and this is a common failing of cheap kits.
• The weather is genuinely against you. December in Britain gives roughly 0.8 peak sun hours against June's 5.5. A panel that fills a power bank in a day in July may take a week in December, and that is not a fault.

Expect less than the rating. A 20 W panel in real British conditions typically delivers well under its rated figure for most of the day. Size accordingly, and prefer a bigger panel to a cleverer one.

Hand cranks, honestly

Hand-crank chargers are prominent in emergency kits and are the most oversold item in this whole domain. The arithmetic is worth knowing before you rely on one.

Cranking is hard work. Twenty five watts sounds trivial written down and is genuinely tiring within a couple of minutes, in the arm and shoulder rather than the lungs. An hour of it is not something a tired, cold, underfed person will do.

So what is a hand crank for?

• Enough charge for one call or one message, which is exactly the emergency case it is sold for and is a real capability.
• A radio, which draws a few watts and is the honest use case. A wind-up radio is genuinely excellent. A wind-up phone charger is marginal.
• A torch, same reasoning.
• Absolute last resort when everything else has failed.

It is not a power supply and should never be anyone's plan. Buy the panel and the power bank first. Buy the crank as the fourth or fifth item, if at all, and buy it built into a radio rather than as a separate device.

A bicycle-driven generator is a different proposition entirely. Legs sustain far more than arms, so a trained person on a bike can hold 75 to 150 W comfortably for a long time, which is a genuinely useful amount of power. If you are serious about human-generated electricity, it is pedals rather than a handle.

Rechargeable cells: which chemistry and how to treat it

Low self-discharge NiMH is the right answer for AA and AAA.

• They hold most of their charge for years on the shelf, unlike older NiMH which went flat in a month and was therefore useless for anything kept in a drawer.
• They tolerate cold better than lithium.
• They can be charged from almost anything, including improvised sources, in a way lithium cannot.
• Perhaps 500 to 1,000 cycles.
• Buy a good charger, one that charges each cell independently and stops when each is done. Cheap chargers treat all cells as one and overcharge the ones that finish first, which is what kills them.

Treating them well:

• Charge at a moderate rate, not fast, if you have the choice.
• Do not leave them on a charger that never terminates.
• Do not mix old and new cells in the same device. The weak one is driven into reverse by the strong ones, which destroys it and can make it leak.
• Store them charged, cool, out of the device.
• Label them with the date they entered service. A pack of cells with no history is a pack you cannot trust.

Keep some ordinary alkaline cells too. They store for a decade, work straight from the packet, and are the right answer for something that sits unused for years like a smoke alarm or an emergency torch. Do not store alkalines inside a device you are not using, because they leak and destroy the contacts, and the item you find in three years will be ruined.

Lithium primary AA cells are worth a small stock for the things that must work: they last well over a decade, weigh less, and perform far better in the cold than either alkaline or NiMH. They are expensive, so buy them for the torch that matters rather than for everything.

Making a phone battery last

The device is likely irreplaceable, so treat it as a long-term asset rather than a consumable.

• Aeroplane mode is the single biggest saving. With no signal, a phone burns its battery hunting for a network continuously, and that search is one of the most power-hungry things it does. With no cellular service at all, aeroplane mode is not a saving, it is the difference between days and hours.
• Screen brightness down, and screen timeout short.
• Keep it between roughly 20% and 80% where you can. Both extremes age a lithium cell fastest.
• Do not leave it plugged in permanently at 100%.
• Keep it warm in cold weather. Lithium capacity falls steeply in the cold, and a phone that dies at minus five is usually not flat, just cold. Inside a coat, next to the body.
• Do not charge it while it is very cold, for the same reason as the LiFePO4 rule: charging a cold lithium cell plates lithium onto the anode and permanently costs capacity. Let it warm up first.
• Turn it off rather than leaving it idle. A phone that is only switched on twice a day for the reference library it holds will last weeks.
• Store the offline content you need before you need it. A phone with no signal is still a torch, a camera, a clock, a compass and an entire library, and all of that is worthless if the material was never downloaded.

Standardise on one connector

Small, dull, and it saves more grief than any other decision here.

• USB-C for everything, if you can. It is now the common standard and it is reversible, which matters in the dark.
• Keep two or three spare leads, and buy decent ones. Cheap leads with thin conductors charge slowly and fail at the plug.
• A short lead loses less than a long one.
• Keep one lead of each older type you still need, taped together and labelled, in the kit.
• A powered multi-port hub running off 12V lets everything charge from the household bank at once without an inverter, and that is the arrangement to aim for.