You can use any charger you want (as long as it’s compliant with USB specs), but how much power you’ll be able to get from it will vary.
To make things clear, let me describe the way maximum input current gets determined on the Librem 5.
- If the charger supports Power Delivery (EDIT: or it’s a non-PD USB-C source that signals 1.5A/3A capability at 5V via resistors on CC lines according to USB-C specs), Librem 5 requests voltage to be 5V and uses whatever current gets advertised for 5V by the power supply. You can check the negotiated value in
/sys/class/power_supply/tps6598x-source-psy-0-003f/current_max. The charger included in the box provides 3A at 5V. - If the charger doesn’t support PD, Librem 5 uses BC1.2 to detect whether it’s a dedicated charger. If it detects DCP or CDP port, 1.5A is used as input limit. Note that this is a recent feature, added around two months ago in a kernel update - older kernels would go straight to point 3.
- Otherwise, Librem 5 fallbacks to 500mA as input current limit, which may be enough to slowly charge when idle with screen off, but will likely make it slowly discharge while in use, since it won’t be enough to power the phone itself. No other protocols than PD and BC1.2 are being considered there.
PD negotiation can happen early in the bootloader, but BC1.2 happens only after Linux is already up and running, so if you’re trying to boot with completely depleted battery or without a battery at all, using a powerful enough PD supply is strongly advised.
When the red LED starts blinking, it means that the charging has stopped because of battery’s temperature. This is nothing to be afraid of, fast charging can produce plenty of heat and charging will automatically resume once the battery cools down. The cutoff temperature with charging already in progress is a bit higher than when you start charging, so unplugging and plugging the charger back in can make it start blinking - again, that’s nothing unusual, just let it cool down a bit. You can keep using the phone, it will just take longer for it to reach 100% (it won’t discharge).