Quick Answer
A bench charger is a stand-alone, AC-powered DC charger or power supply that charges one battery on its own, disconnected from the inverter, charge controller, and other batteries. It is called a "bench" charger because the work usually happens on a workbench. Technicians use one to wake a battery whose BMS has shut down, bring a deeply discharged battery back into range, or charge and test a single unit before it goes back into the bank. In practice it is either an adjustable DC power supply with voltage and current limits, or a fixed-output converter/charger matched to the battery voltage.
In This Guide
Why You Would Use a Bench Charger
In a working solar system, batteries are charged by the charge controller or the inverter/charger. Those chargers expect the battery to be awake and talking to them. Many cannot start charging a battery that reads 0V at its terminals. A bench charger has no such requirement. It puts a controlled DC voltage on the terminals and lets you watch what the battery does.
Recovering a battery in protection or hibernation
When a lithium battery's BMS trips on low voltage, it disconnects the cells from the terminals. Many BMS boards will not reconnect until they sense an outside charging voltage. A bench charger supplies that voltage on its own, without involving the rest of the system. Our KiloVault HAB troubleshooting guide uses exactly this step to tell a battery that needs a reset from one that needs a new BMS board.
Isolating a suspect battery
In a parallel bank, a healthy battery can hide a weak one, and a communication fault can look like a dead BMS. Charging the suspect unit alone shows whether the fault follows that battery.
Commissioning and matching batteries
Before you add a battery to an existing bank, bring it to the same voltage as the batteries already in service. Then the new unit does not take a large inrush current when it is connected.
Testing and maintenance
You can charge a battery on the bench, then run a controlled discharge to check its real capacity. A bench charger can also keep spare or stored batteries topped up.
The Three Types of Bench Charger
"Bench charger" is a description of how a charger is used, not a product category with a fixed spec. Three kinds of equipment get called a bench charger:
| Type | How it works | Best for |
|---|---|---|
| Adjustable DC power supply | You set the output voltage (CV) and a current limit (CC). It holds whichever limit it reaches first. | Diagnostics and BMS recovery, on any battery voltage the supply can reach |
| Fixed-output converter/charger | Produces a fixed DC voltage matched to a 12V, 24V, or 48V battery and supplies its full rated current until the battery comes up to that voltage. | Charging and keeping batteries topped up from shore, grid, or generator power |
| Multi-stage "smart" charger | Runs bulk, absorb, and float stages from a chemistry profile (lead-acid, AGM, LiFePO4). | Routine charging when the profile matches the battery |
One caution about smart chargers: many will not start on a battery that reads near 0V, because they cannot detect it. That is often the very battery you are trying to recover. An adjustable supply or a fixed-output converter just puts out voltage, so it works in that situation.
How to Choose One: Voltage, Current, Chemistry
1. Match the voltage to the battery, not the system
The output must reach the battery's charge voltage without exceeding its high-voltage limit. Take both numbers from the battery's datasheet. Typical ranges:
| Nominal | Typical LiFePO4 charge voltage | Typical lead-acid / AGM absorb |
|---|---|---|
| 12V | 14.2–14.6V | 14.4–14.7V |
| 24V | 28.4–29.2V | 28.8–29.4V |
| 48V (16-cell LiFePO4, 51.2V) | 56.4–58.4V | 57.6–58.8V |
These are typical figures, not a substitute for the manufacturer's spec. Some 48V lithium batteries use 15 cells and charge lower.
2. Size the current for the job
To wake a BMS or recover a deeply discharged battery, a few amps is enough, and a low current is gentler on the cells. To charge a battery fully on the bench in a reasonable time, aim for roughly 0.1C to 0.2C. That is 10–20A for a 100Ah battery. Stay within the charge-current limit on the battery's datasheet.
3. Check the input power
Most bench chargers run on 120VAC from a wall outlet or generator. Higher-current 48V units draw serious AC power. 13A at 54V is about 700W of DC output, before conversion losses.
4. Prefer adjustable and current-limited for diagnostics
If the charger is mainly for troubleshooting, choose one with adjustable voltage and a current limit you can set. Then one tool can serve 12V, 24V, and 48V batteries, and you control exactly what the battery sees.
Using a Bench Charger to Wake a Lithium BMS
The exact procedure depends on the manufacturer, so follow your battery's service manual. The general sequence is:
- Isolate the battery. Disconnect it from the inverter, the charge controller, and any parallel batteries.
- Measure the terminal voltage with a multimeter and write it down.
- Set the charger first. If it is adjustable, set the voltage to the battery's charge voltage and the current limit low (a few amps) before you connect anything.
- Connect positive to positive, negative to negative. Use leads and a fuse sized for the charger's current.
- Run the manufacturer's reset with the charge voltage applied. For example, the KiloVault HAB procedure is to hold the reset button for 5–10 seconds.
- Watch for current flow. If the pack starts accepting current, the BMS is working. If it still has not reset after a few hours, the board has probably failed.
Never bypass the BMS
Never connect a charger directly to the raw cells to force a recovery. With the BMS out of the circuit, nothing stops a single cell from being overcharged into thermal runaway. Cell-level charging is a last-resort diagnostic for trained technicians. It is not a repair.
Safety Rules
- Wear insulated gloves and safety glasses, and remove metal jewelry. A 48V battery can deliver thousands of amps into a short circuit.
- Inspect the battery before charging it. Do not charge a battery that is swollen, leaking, cracked, or smells burnt.
- Do not leave the charger set above the battery's high-voltage protection limit. The BMS is a backstop, not a charge controller.
- Charge lithium batteries only above freezing (32°F / 0°C) unless the battery has built-in heating.
- Charge on a non-flammable surface, and stay close by during a recovery charge.
Bench Chargers at Solamp
Our DC power supplies and bench chargers are Iota Engineering DLS converter/chargers. They run on 120VAC and come in 12V, 24V, and 48V models from 13A to 90A. Each one supplies its full rated current until the battery comes up to its output voltage, and then holds it there. That makes them useful both as bench chargers and as permanent AC-to-DC chargers for RV, marine, and off-grid battery banks.
| Battery | Iota DLS models |
|---|---|
| 12V | 15A, 30A, 45A, 55A, 75A, 90A |
| 24V | 15A, 25A, 40A |
| 48V | 13A (54V output) |
Using the 48V model on a 48V lithium battery: its fixed 54V output is well above a typical low-voltage cutoff, so it can provide the outside voltage a BMS needs to wake up. It is below a 16-cell LiFePO4 pack's full charge voltage, though. Once the battery is back in service, let the system charger finish the charge. If you are not sure which unit fits your battery, call 978-451-6890 and we'll check the datasheet with you.
Find the Right Charger for Your Battery
Browse the 12V, 24V, and 48V Iota DLS converter/chargers, or work out how much battery your system needs before you buy.
Frequently Asked Questions
Is a bench charger the same as a bench power supply?
Often, yes. An adjustable bench power supply with a current limit is the most common bench charger in a service shop. "Bench charger" describes the job, which is charging a single battery on its own. A fixed-output converter/charger matched to the battery's voltage can do the same job.
Can I use my inverter/charger instead?
Sometimes. Many inverter/chargers need to see battery voltage before they will start, so they cannot wake a battery whose BMS has disconnected the terminals. They also cannot isolate one battery from the rest of the bank. That is why troubleshooting procedures call for a separate bench charger.
What size bench charger do I need for a 48V lithium battery?
To wake a BMS, any supply that safely reaches the battery's charge voltage at a few amps is enough. To fully charge the battery on the bench, choose about 0.1C to 0.2C, which is 10–20A for a 100Ah pack, at the charge voltage on the manufacturer's datasheet.
How long should a BMS recovery charge take?
A healthy BMS usually responds within minutes of the charge voltage being applied and the reset being run. If the battery has not started accepting current after a few hours, and the reset has been repeated, the BMS board has most likely failed.