It is very useful to have an alternate means to charge 48V batteries.
Batteries are required to ship at low states of charge and may be slightly below turn on voltages, depending how long they have been warehoused by distribution.
Batteries may also dip below their turn on voltages if discharged down to low states of charge and not recharged after a period of time.
For example, an offgrid cabin without a generator autostart where solar charging has been disabled by snow and left unattended is a common deep discharge scenario.

Some inverter electronics cannot power up without battery voltage present.
In these cases, it can be difficult to recover from a deep discharge event.
Fortress batteries are programmed with a reserve capacity to help recover from deep discharge events - but programming can only go so far to mitigate the impact of energy fundamentals.

A trickle charger should not charge between 51-56V and charge at 10A  or less.
Caution - many trickle chargers with alligator clamps found online charge at >58V.

Most electric bike chargers work but require modification (the addition of alligator clamps).
Many electric bike chargers work fine: https://www.amazon.com/Charger-Electric-Battery-Scooter-Motorcycle/dp/B07PN7LN92

Cut off the end of the charger and add some "mid sized" alligator clamps: https://www.amazon.com/Fielect-Insulated-Alligator-Crocodile-Extender/dp/B0831HSXWW/

Such chargers can be found at local electric bike shops can confirm charging voltage and amperage.
Simply ask for an electric bike charger - most are 56V but please confirm prior to purchase.

Caution: If the battery is below 10V, charge with a 12V trickle charger first.
Transition to  24V trickle charger before charging at 51-56V.
Charge at <5A when charging in these voltage ranges.

Do not charge the battery in freezing conditions.

One option that we have not yet tested is a 12V->56V boost converter. Keep your car running while using these or you could end up with a dead car battery! 

When recharging a deep discharged battery, it may be necessary to force open the relay contactors of the battery.
This can be easily accomplished on the eVault MAX but eFlex and LFP10 MAX may require a firmwar eupdate tool or battery disassembly.
Please fill out a support ticket at https://support.fortresspower.com prior to recovering a deep discharged battery.

Last but not least, here is an alternate trickle charger that can be powered by a 12V battery (such as a car battery) to recover a 48V battery deep discharged battery: 
https://smachpower.com/products/89-12V-to-56V-3A-BOOST-DC-DC-CONVERTER-12V-Lithium-Lead-acid-Battery-Charger-for-Dual-battery-System.html