- 1. Intro
- 2. Charger description
- 3. Charger modifications
- 3.1. Anti spark circuit
- 3.2. Permanently stop
- 3.3. LiFePO4 cycle life versus charge current
- 3.4. Adjustable charge current
- 3.5. Voltage adjust
- 3.6. Housing
- 3.7. Heat sinks
- 3.8. Assembly
- 3.9. Vibration protection
- 4. Further weight reduction
- 5. Disclaimer
Because I want to fast recharge the e-bike LiFePO battery during a trip, I need a lightweight mobile charger. Special lightweight battery chargers for ebikes don't exist. So I made one myself. I took a Chinese battery charger (Kingpan) at which I have done modifications to the housing and the electronics. Here are the images of the Chinese battery charger (right) and the new lightweight battery charger (left)
2. Charger description
2.1. Weight and dimensions
- Kingpan battery charger dimensions: 17.2 x 9 x 6.3 cm. Weight = 1400g.
- Lightweight battery charger dimensions: 19 x 11 x 6.1 cm. Weight = 820g.
2.2. Maximum charge current 10A
The maximum current is set by the factory at 8A. But the charger can deliver more current. You can increase the maximum current to at least 10A. I have not yet tried higher currents.
2.3. Advantages of the battery charger
- The weight, inclusive mains cable, is reduced from 1400g to 820g.
- The charge current is adjustable from 1A to 8A (10A) instead of a fixed charge current.
- Integrated anti spark circuit which runs automatically.
- Permanent turn off when the battery is fully charged.
2.4. Charging dead batteries
The charger has no safety timer built in to prevent charging dead batteries for an excessively long time. So we have to keep an eye on the time ourselves.
2.5. The battery charger circuit
By reengineering the E-400 battery charger I have created its schematic:
2.6. Bi-color LED and fan control
During charging, the Bi-color LED is red and the fan is turned on. When the battery is full, U1 turns off charging, the fan is switched off and the LED turns green. All this is controlled by U2. With VR3, the current is set at which the circuit switches over.
2.7. Minimum dead time
At the end of the charge cycle, the TL494 should not be turned off completely. A minimum dead time of 3% ensures that the internal supply voltage of 20V will be maintained. Actually, the internal supply voltage drops to about 10V.
2.8. Battery charger under test
Here we see how the battery charger is tested with a bench of 12V / 100W halogen lamps:
3. Charger modifications
3.1. Anti spark circuit
3.1.1. Connecting sequence
This is a high power charger where we need special attention with connecting batteries; use this connecting sequence:
- Connect the battery first
- Connect the charger to the mains
- Disconnect the charger from the mains and wait three seconds
- Disconnect the battery
Another sequence can create huge sparks which is fatal for the battery connectors; the charger wants to keep the charge current constant.
3.1.2. The need for the anti spark circuit
There will still be created a spark when we connect the battery to the charger, when not powered, because the big capacitor C17 short circuits the battery momentarily. Here we need a solution to avoid sparks.
3.1.3. Explanation of the anti spark circuit
The anti spark circuit is actually a diode at the output. When the battery is connected to the charger, when not powered, the diode is in reverse direction. No sparks can occur during connecting the battery. During charging the diode will be in forward direction.
A normal diode would dissipate about 8W at 8A. Therefore, a power MOSFET is used to create a lossless ideal diode. The IRF1405 drain to source on-resistance is typically 4.6m Ω. This is even lower than the contact resistance of a mechanical switch! See here for the ideal diode circuit explanation.
The transistors Q7, Q8 and Q9 are switching the MOSFET on when the charger is powered. Transistor Q7 is a current source and Q8 and Q9 are together a current mirror. Diode D18 and D19 ensure that the MOSFET gate voltage is always high enough even when the output voltage is 0V.
3.2. Permanently stop
Note: If the battery has a bleeding BMS inside (or any other charge only cell balancer), a permanently stop is not allowed. A bleeding BMS balances the battery pack at the end of the charge period; the charger should not be turned off.
When the battery is fully charged, the charging stops by one of the following reasons:
- The BMS inside the battery switches off the battery pack, this is preferred.
- Without BMS, the battery voltage becomes higher than the charger voltage limit, so the charger turns off the charge current.
However, when the charging is terminated, the battery voltage will decrease slowly so that charging starts again. This process will repeat forever:
If desired, the charger can be permanently turned off; this can be obtained by adding these components:
- Resistor Rm3 creates a Schmitt trigger.
- Cm1 and Rm4 provide a start-up delay. Charging can restart only after unplugging the AC cable.
3.3. LiFePO4 cycle life versus charge current
As we can see, the LiFePO4 battery life time can be enhanced by keeping the battery temperature low:
The battery will be warmed up by the charge current. The internal resistance of my 12s2p battery pack is 0.06Ω. The power dissipated in the battery pack is proportional to the square of the charging current; I² * 0.06Ω. At a charge current of 10A the battery power loss is 6W, which could heat up the battery pack up to the maximum temperature of 60°. At a charge current of 5A the battery power loss is only 1.5W, which is safer. To enhance the battery life time do not use a higher chare current as necessary, to charge the battery within a certain time.
3.4. Adjustable charge current
The A123 ANR26650 LiFePO4 batteries can be fast charged in 15 minutes with 10A. The recommended standard charge method is 3A in 45 minutes. When we have time enough for charging the batteries it is safer to use a lower current of 3A. Therefore the charger current is made adjustable, see here:
Two modifications are necessary:
- Replace VR1 by a potentiometer on the front from 100Ω.
- Remove R8.
3.5. Voltage adjust
With VR2 the output voltage is adjusted at 43.8V by the factory. With 12 LiFePO batteries the maximum voltage is thus 3.65V per cell. But I don’t use the battery charger for the battery voltage monitoring. Here I use the BMS inside the battery package, which is more accurate. So the voltage can be adjusted with VR2 to a higher voltage of for instance 45V.
I use a plastic housing, the Hammond 1591-EGY, instead of the black alloy shell. This reduces the weight. The 1591 ESBK dimensions are 190 x 110 x 61 mm, the weight is 210g. Now the battery charger is isolated, we can use a two core mains cable. The mains cable weight is reduced from 255g to 50g.
3.7. Heat sinks
The E-400 aluminium housing was used as heat sink. Now we use two small heat sinks of 0.5mm thick aluminium instead. By the design of the ventilation openings, the air flow is such a way that both heat sinks are forced cooled by the fan. Despite the small size of the heat sinks, the cooling is sufficient. Note that the distance between the fan and the small heat sink should be 5mm.
Use nylon screw to mount the print to the case. To protect the inductors against vibrations they should be glued with for example PU adhesive.
3.9. Vibration protection
Glue the heavy components onto the board with PU adhesive, see picture:
4. Further weight reduction
The battery charger runs at a frequency of 28kHz. By increasing the frequency, maybe a smaller transformer T2 and inductor L2 can be used which reduces the weight significantly.
It has to found out if a lighter battery charger, for instance the 180W version, can be tuned from 3A to 8A. The weight can so be reduced to about 500g. Here we can change among other D15 by a Schottky diode and Q1 and Q2 by power mosfets. A smaller housing is the Hammond 1591-DGY, 150 x 80 x 50 mm, weight 128g.
So called brick DC-DC converters, that use high switching frequencies, can reach a power-to-weight ratio of 3kW/kg. With this technology it must be possible to build a 36V / 10A charger of just 200g.
Due to the high power of the battery charger use it never unattended. See also the disclaimer.