There are situations where a battery charger may need to function as a power supply, as well as maintain batteries. This can be when designed into a UPS (Uninterruptible Power Supply), or when testing or operating a DC system powered by batteries. Not all chargers are suitable for power supply operation. Most 'smart' battery chargers have a 'switching point', where they drop into the final stage (usually 3rd stage) float mode. If the charger never reaches this point because of current passing through to a load, it will stay in the absorption stage, which can damage batteries, and eventually, the charger.
The point of keeping the current draw under the switching point, is keeping the charger out of the second,'boost stage', which has a higher voltage than float, and will overcharge if left in this mode very long. One of the features of the Samlex units we carry, is a dip switch settable mode called 'battery with load'. This setting deactivates the boost stage, allowing the charger to pass up to its rated current load without overcharging batteries connected to it. A 30 amp charger can supply up to 30 amps continuously if the load demands it.
An application like a medical cart with sealed lead acid batteries that need charging, but has a small current drain from some device that is always on, may be a candidate for a switchmode charger like a Samlex unit of appropriate voltage and amps. The charger would charge at up to its current rating, and drop to the current level of the load, without being in the boost (absorption) mode, safe for long term battery management until unplugged/disconnected.
For larger current drains, we always use the Samlex units. We have supplied these for aircraft applications and R&D, where technicians or engineers want to test the avionics without draining the battery system. We have supplied OEM auto manufacturers and equipment manufacturers (such as John Deere) with Samlex units to incorporate into a trade show, auto expo, or even SEMA show display, where vehicle or equipment electrical systems are run for hours on end for demonstration purposes. We also have supplied robotics engineers/students with these units for testing and refining a DC powered robotic device.
There are electronic/radio applications as well. A radio repeating station (smaller DC powered units) can be set up with a battery bank feeding through a charger. When power goes out, the batteries feed the system. When power returns, the charger recharges the batteries while supplying the system load. We have used these setups for wireless internet repeaters, ham radio repeaters, scientific monitoring stations, and others. Email us with your specific application, and we'll see what is appropriate to solve the requirements.