Hi,
I'm new to this forum, from Germany and thinking about building an electric bike. I study electrical engineering and am an electronics hobbyist, so I would find it pretty interesting to build the controller and BMS myself.
The bike should have about 2 kW of power; and first I started looking around for a suitable battery. It should have about 1000 Wh of capacity and shouldn't be too expensive. I would prefer LiFePO4 over LiIon or LiPo since they tend to live longer and are safer.
One of the first batteries I found are the 40Ah LiFePO4 blocks; they are available vrom various manufacturers. Sometimes one can get such a block for about €50; a pack of eight in series has 1024Wh (25,6V/40Ah). These batteries are rated at 120 amps discharge current; this would be more than enough for a 2 kW bike.
The only real downside is that the batteries are heavy; a 8S1P pack would weigh a good 10 kg.
So I came up with the idea of using a step up converter which converts the 25V from the batteries to 25 to 100 volts for the controller. A viable design would be a boost converter with a synchronous rectifier; I have simulated one in LTSpice:
Once I already made such a converter, but this is only a small one with 20 watts of output power and it is about 92 percent efficient.
To increase power, I would use three converters in parallel. The switching points of the MOSFETs will be apart by a third of a period; this will vastly decrease input and output current ripple compared to one single large boost converter:
There are some ATmel microcontrollers (for example ATTiny261) which would be very suitable to control the six FETs.
The output load is modelled by a resistor. In the simulation the 3 phase converter delivers 2181W and draws 2263W from the battery; this would mean a 96.3% efficiency in the worst case scenario of 100V output voltage. So the general statement "Don't use a boost converter, it is inefficient" doesn't seem to be true. The part costs should be about €25 for the power parts (FETs and inductors) plus maybe €15 for the controlling parts. I would build it on an aluminium core PCB and mount it in a heat-sinking case, together with the controller.
At low speeds, the converter can be bypassed by continouosly turning the high side MOSFETs on and leaving the lowside FETs off. Then the converter would have an internal resistance of about 6.5 mOhm, which would still mean 0.65V drop at 100A. I don't really like this point
. Of course one could add some low Rds MOSFETs to bypass the converter entirely, lowering the resistance further.
Another advantage of the boost converter is that it can be used as a step down converter for regerative braking. The controller bus voltage may rise up to 100 volts, and the converter could still charge the battery with a variable current.
It would even be possible to use the converter for charging the battery, one would just need to connect a 30V to 100V power supply in parallel to the motor controller.
But since i have found some pretty impressive Li-Ion packs from a German Ebay seller (so much about LiFePO4...) i don't think I'm going to use the boost converter concept in my bike. I'd rather use maybe a 20S4P pack of 18650 cells; this would cost about €500 and weigh like 4 kg . In fact, it may take a long time befire I even start making my bike.
But still I think some people might be interested in the idea of using a boost converter, so I decided to share this.
I'm new to this forum, from Germany and thinking about building an electric bike. I study electrical engineering and am an electronics hobbyist, so I would find it pretty interesting to build the controller and BMS myself.
The bike should have about 2 kW of power; and first I started looking around for a suitable battery. It should have about 1000 Wh of capacity and shouldn't be too expensive. I would prefer LiFePO4 over LiIon or LiPo since they tend to live longer and are safer.
One of the first batteries I found are the 40Ah LiFePO4 blocks; they are available vrom various manufacturers. Sometimes one can get such a block for about €50; a pack of eight in series has 1024Wh (25,6V/40Ah). These batteries are rated at 120 amps discharge current; this would be more than enough for a 2 kW bike.
The only real downside is that the batteries are heavy; a 8S1P pack would weigh a good 10 kg.
So I came up with the idea of using a step up converter which converts the 25V from the batteries to 25 to 100 volts for the controller. A viable design would be a boost converter with a synchronous rectifier; I have simulated one in LTSpice:
Once I already made such a converter, but this is only a small one with 20 watts of output power and it is about 92 percent efficient.
To increase power, I would use three converters in parallel. The switching points of the MOSFETs will be apart by a third of a period; this will vastly decrease input and output current ripple compared to one single large boost converter:
There are some ATmel microcontrollers (for example ATTiny261) which would be very suitable to control the six FETs.
The output load is modelled by a resistor. In the simulation the 3 phase converter delivers 2181W and draws 2263W from the battery; this would mean a 96.3% efficiency in the worst case scenario of 100V output voltage. So the general statement "Don't use a boost converter, it is inefficient" doesn't seem to be true. The part costs should be about €25 for the power parts (FETs and inductors) plus maybe €15 for the controlling parts. I would build it on an aluminium core PCB and mount it in a heat-sinking case, together with the controller.
At low speeds, the converter can be bypassed by continouosly turning the high side MOSFETs on and leaving the lowside FETs off. Then the converter would have an internal resistance of about 6.5 mOhm, which would still mean 0.65V drop at 100A. I don't really like this point
. Of course one could add some low Rds MOSFETs to bypass the converter entirely, lowering the resistance further.Another advantage of the boost converter is that it can be used as a step down converter for regerative braking. The controller bus voltage may rise up to 100 volts, and the converter could still charge the battery with a variable current.
It would even be possible to use the converter for charging the battery, one would just need to connect a 30V to 100V power supply in parallel to the motor controller.
But since i have found some pretty impressive Li-Ion packs from a German Ebay seller (so much about LiFePO4...) i don't think I'm going to use the boost converter concept in my bike. I'd rather use maybe a 20S4P pack of 18650 cells; this would cost about €500 and weigh like 4 kg
. In fact, it may take a long time befire I even start making my bike.But still I think some people might be interested in the idea of using a boost converter, so I decided to share this.
