Boot-Full of Electrons: Field Charging That Actually Works on a UK Club Site

Electric flight has a lovely, smug quiet about it, right up until you are stood on a windy strip with three depleted 6S packs and a charging lead that goes nowhere. Mains-free club sites are brilliant for noise and neighbours, less brilliant for electrons. The usual answer is a leisure battery in a box. The trick is building one that matches your actual flying day rather than the one you imagine in the shed.

Do the field maths before you buy the box

Start with the load, not the battery. A 6S 5000mAh pack is about 111Wh nominal. If you land at storage voltage and put back roughly 70 per cent, that is around 75Wh per pack. Six packs is about 450Wh. Add charger losses of 15 per cent and you are pulling perhaps 520Wh from the battery. A lead-acid leisure battery should not be run below about 50 per cent depth of discharge if you want it to last, so you need roughly 1040Wh available, which means a 100Ah unit at 12V. Cold weather and voltage sag will nibble at that, so a 110Ah AGM is a sensible minimum. A 100Ah LiFePO4 will do the same job with less weight and more usable capacity, but at a price.

If you typically fly four packs, not six, say so. Buying for the fantasy weekend is how people end up lugging 30kg of lead around for a 20-minute session.

Twelve volts is a polite fiction

A nominal 12V battery is not 12V under load. Pull 30A and an AGM can drop to 11.5V or lower, and many chargers will throttle or cut out on low input. Keep the run from battery to charger short and use proper cable, not the thin stuff from a car accessory shop. A 16mm2 cable and an Anderson SB50 connector will handle a sensible field charger without cooking. Long, thin leads add voltage drop and heat, which is a poor use of your stored energy.

The 6S elephant in the box

Most modern chargers need more than 12V to push a 6S pack properly, because 6S is around 25V at full charge. Check your charger input range before you build anything. Some will boost from 12V but at reduced power, which can turn a quick field top-up into a long wait. Others want 24V input, which means either a 24V battery bank or an inverter running a mains charger. The inverter route works, but you lose perhaps 15 per cent in the inverter and another 15 per cent in the charger. Plan for 30 per cent more battery than the simple maths suggests, and accept the fan noise.

A DC-DC charger that accepts a wide input and outputs what your charger needs is cleaner if your kit supports it. Do not assume. Read the label on the back of your charger and do the sum.

Fuse at the battery, not in your imagination

The single most important part of a field box is the fuse. Fit a proper fuse or MRBF block on the positive terminal, within about 15cm of the battery. A 60A or 80A rating is sensible for a 12V field setup, depending on cable and charger. Add an isolation switch so the box is dead when not in use. Use a small busbar for positive and negative rather than a nest of ring terminals. Cover every exposed terminal. If you are using a lead-acid battery, vent the box and do not seal it like a lunchbox. If you are using LiFePO4, check that the BMS has low-temperature cut-off, because charging a cold lithium pack is not a clever winter habit.

Solar is a top-up, not a weekend plan

A 100W panel on a UK club site in October might give you 20 to 40Wh on a decent day, and less under cloud. That is maintenance, not flying. It will keep a stored battery topped up between visits and slowly recover a small deficit, but it will not replace a 500Wh flying day. A simple 10A solar controller is fine for a maintenance panel. Do not let the marketing wattage convince you that a folding panel is a field charger.

Transport, storage and club manners

A leisure battery in the boot needs a proper box with a strap, not a loose lump sliding into the wheel arch. Lead-acid is heavy, so lift with your legs and keep it upright. Lithium is lighter but still wants secure mounting and protected terminals. Do not charge frozen LiPo packs; let them warm to ambient first. Store the field battery cool and dry, and leave lead-acid charged rather than flat. Check your club rules and insurance before you turn up with a home-built box, and be ready to show the fuse and isolation switch to the safety officer. Most clubs are fine with a sensible setup, less fine with crocodile clips and a bare battery on the grass.

A realistic cost breakdown

A 100Ah AGM leisure battery is around £90 to £130. A 100Ah LiFePO4 is more like £250 to £350. A sturdy box, fuse block, isolation switch, cable, connectors and a small busbar will add £60 to £90. A 100W solar panel and controller is £80 to £120 if you want the maintenance top-up. If you need a new charger that works from 12V, budget another £60 to £100. A practical AGM field box lands around £250 to £350; a lithium version is closer to £400 to £500. That is not nothing, but it is a one-off that replaces a lot of mains-site frustration.

Build for the day you actually have

Count your packs, check your charger input, fuse the positive, and size the battery for the depth of discharge you are willing to live with. A field box is not a gadget; it is a small power station with a job. Build it around your real flying day, test it at home, and it will earn its space in the boot. Build it around a fantasy, and you will be the person at the club site asking if anyone has a spare 12V lead and a prayer.