I needed a charger for a LiFePo4 battery with 13.2V rated voltage.
It should have a wide range entrance and a Europlug, so that I can use it as much as possible outside Germany.
The batteries I use all have an internal Battery Management System (BMS) and therefore only the usual 2 terminals. So I was looking for a charger that doesn't require a balancer connection.
Since no suitable charger was available, I converted a plug-in power supply Voltcraft SPS24-24W-A. It was available from Conrad Electronic (order no. 513003, 23.99 €). The technical data are:
- Operating voltage: 100 bis 240 V, Europlug
- Output voltage: adjustable via 7 voltage jumper from 9 to 24 V
- Output current: up to 1 A (at 24 V output voltage?)
- Output power: 24 W
- Dimensions: 51 × 73 × 98 mm
- Weight: 195 g
Different resistors are included in the original voltage jumpers (1). However, a 2-pin strip (2) also fits into the lateral opening of the plug-in power supply, see above. It is therefore relatively easy to build yourself jumpers (3) for other voltages.
For a LiFePo4 battery with a rated voltage of 13.2 V, the charge limit voltage is 14.4 V. In order to achieve this voltage, the resistance of the voltage jumper must be 4.55 kΩ. This value can be realized by the parallel connection two resistors with 9.1 kΩ each. Alternatively, 4.7 kΩ and 150 kΩ can also be used. The 3.1 mm long SMD resistors of size 1206 fit exactly to the pins of the connector strip (3), which have a distance of 2.54 mm.
The plug-in power supply regulates the voltage down when the output current exceeds 1 A - according to the operating manual. Conversely, the maximum charging current may therefore be greater than the output current specified by the manufacturer. When used as a charger, the plug-in power supply is not overloaded. It only needs to apply power of the differential voltage between 14.4 V and the voltage at the terminals of the battery multiplied with the current as heat. For example, a battery discharged to 10 V has a charge current of 2.33 A. This results in a power of
Heat → P = (14.4 V - 10 V) · 2.33 A = 10,25 W
This value can emitted by the power supply without any problems as heat. The converted power supply charges the battery with
Battery → P = 10 V · 2.33 A = 23.3 W
This value is just below the 24 W indicated in the data sheet. When the charge limit voltage of 14.4 V was reached, the charging current was reduced to almost 0 A, because then the driving potential difference is missing.