Batteries and Charging


Li-Ion batteries must not be charged in freezing conditions!


RM Frobisher (1986) Ltd’s products contain 'Li-Ion' battery cells.
  • 10Wh Capacity (TAR-3) and 27Wh (DFG-1)
  • Nominal Voltage 4.2V (4V2).
  • 50g per cell
  • A five Volt, 500mA USB power supply required.
  • Power supply connection 'Micro USB (type B)'.
  • Built in LI-ION battery charger with three stage charging.
RM Frobisher (1986) Ltd’s products are supplied with
  • Mains powered USB charger. (240V, 230V, 220V or 110V 50 and 60Hz).
  • UK or Euro plug depending on shipping address.
  • Power supplies for other countries are purchased and supplied direct locally.
  • USB 'Type A' to 'Type B micro' plug cable.
  • A 12V car socket to 5V USB adaptor.

Battery Charging Cycle

If the Li-Ion batteries are fully discharged (or close to) they are 'Trickle' charged to precondition them. When the batteries voltage rises the charging enters the next phase.
Fast Charge Constant-Current Mode
The batteries are charged at a constant current (480mA) until the voltage rises to 4.2V when the next phase is started.
Constant-Voltage Mode
The charging voltage remains constant at 4.2V Volts and the charging current falls asymptotically to zero. During this time the charging current falls to a very low value. Switching the unit on will consume more current than is being used to charge the unit and delay the end of charging by a significant time.
Depending on how 'flat' the batteries are step one may be skipped. The third stage can take a long time if you switch the unit on to check the battery voltage frequently. During the final tens of minutes the charging rate is less than 40mA.

Other Information

Can I take my unit on an aircraft?
Yes. See

The TAR-3 contains two 9.5Wh Li-Ion batteries that are fixed in position in a tough plastic case. Net weight 50g each. A TAR-3 may be carried in hand luggage or in “check-in” (hold) luggage.

The DFG-1 contains One 27Wh Li-Ion twin cell battery that is fixed in position in a tough plastic case. Net weight 100g. A DFG-1 may be carried in hand luggage or in “check-in” (hold) luggage.

WARNING Do not crush, heat, incinerate, short circuit, dismantle or immerse in any liquid that may vent or rupture the batteries.
Can I over charge my unit?
No. The built in charger stops charging when the batteries are fully charged. Leaving the charger on charges the batteries and then enters maintenance mode. During the following weeks or months the charging circuit monitors the voltage of the batteries and will charge them as they self discharge. It is not recommended, or environmentally friendly, to charge the unit for long periods (weeks) as the external USB supply will consume energy and the TAR-3 unit consumes around 150mW when in standby mode during charging.
Note: DFG-1 Magnetic Gradiometers and TAR-3 Resistance Meters can be charged using any USB power supply (5V, 500mA) including mobile phone chargers.


Li-Ion batteries must not be charged in freezing conditions!

Battery Life

The battery cells contained in RM Frobisher products are guaranteed for two years and should last over five years. The batteries are sealed inside the meters and the connections soldered to the printed circuit board to ensure reliability. Meters should be returned to RM Frobisher (1986) Ltd for replacement.

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