I took a chance on the CEF50 because I wanted a very light eMTB. Because of that I chose a light battery and planned to get an external battery to supplement the internal on long rides. Two things got in the way, Bafang is way behind on delivering internal batteries let alone their promised external.
Adding an an external battery is not a straight forward job.
First when you connect two batteries in series or end to end, the voltage adds and the current is the same. That is how most of our internal batteries are built. The basic cell in a lithium ion battery build is 3.7v. So if you need a 36v battery you need to connect 10 of them in series yielding a 37v battery. If you need more current you need to connect more than one 10s (10 cells in series) group to form multiple parallel connected batteries. In a parallel connected system the current adds and the voltage is constant.
When we have multiple batteries made up of series and parallel cells we still can connect those battery packs together in series or parallel.
Since we need to match the required battery specification of the eMTB we are going to connect the battery packs together in parallel so that the current increases but the voltage stays at the specified level.
Now when we connect two battery packs together the current will flow from the battery with the greater voltage to the one with the lower voltage. If they are nearly same voltage they will equalize a little and no problem. But if one battery is very low and the other fully charge there can be a rush of current from the charged battery to the discharged battery causing high current to flow with great heat. This can damage the batteries and cause a fire.
I have found a couple of ways to stop this from happening. There are electronic switches available from Aliexpress that monitor the voltage of two or more batteries and when one battery is lower in voltage than the other it will shut off the low battery. When the active batteries voltage drops to within a safe level the switch turns on the low battery and they work in parallel until one battery drops too low again. If you get a switch with a charge port it works the opposite when charging. It will turn off the higher voltage battery until the lower is within a safe level then if will charge both batteries at the same time.
The problem with the switch is that it only works with batteries that charge and discharge on the same connection. If the battery has a separate charge and discharge connection the switch will not work.
The external battery I bought early on has a separate charge and discharge port so it will not work with a switch.
So plan B was to use ideal diodes. A diode acts like a gate and allows current flow one way but not the other way.
If I put a diode in the output of each battery, on the way to the motor, it will allow current to flow to the motor but not the other way. So no current can flow from one battery to the other. Since both my internal and external batteries have separate charge ports this will not stop the batteries from being charged. The one problem with a normal diode is that there is a 1.7v drop across the diode when current is flowing. We really don't want the voltage to drop because the display determines the remaining battery power by measuring the voltage and it will think the battery is down a little due to the diode.
So to solve this I ordered ideal diodes from Aliexpress. These are circuit boards that function like a diode without the voltage drop.
Now I need a new connector for the bike frame because I need a connection for the external charger and I need a connection for the external battery. I found a connector on Aliexpress which waterproof and is the right size for the original hole in the frame. The connector is a right angle SD20. I use two plugs and one socket for the frame. One plug, the right angle, connects the frame connector to the external battery and the other is used to connect the battery charger to the internal battery and the old battery charger connector must be swapped with a new one or and adapter cable if you prefer not to change the original charger. The bottle battery has it's own charger.
There are two variations of the frame connector a square one and a diamond shaped. The diamond shape is close enough to be a direct replacement for the original connector. No cutting the frame needed and you can make the old frame connector bolts work on the new connector.
It is a four pin connector so two pins will be used for the external battery and two pins for the external charger for the internal battery.
I will use two different external connectors, one for the bafang charger to replace it's connector and one for the external battery.
I made a cable with the connector with two pins going to the motor connected via a Y cable so both the external battery and the internal battery could go to the motor but with ideal diodes to stop the batteries from charging each other.
The other cable from the connector goes to the charge port on the internal battery so it can be charged.
Whatever connectors you decide to use remember they need to handle at least 20amps.
This image shows the external right angle connector going to the external battery. I hope it is going to clear the crank. It works and the external battery is powering the bike.
I will attach a cable guide to the battery holder plate to stop the cable from flopping around.
The battery I used was purchased from Aliexpress
It is the S002 36V7Ah Sanyo, China and you must be careful to buy one small enough to fit the frame. You do have to get a battery to match the voltage of your motor, of course. There is very little room on the frame of a large and barely enough room on a medium for the smallest battery from this vendor the S002 36v 7ah.
You also need a side entry water bottle cage with and opening in the bottom so the cable can exit the battery. Most water bottles have sold lip on the bottom and won't work.
*NOTE * if you have a low "Q" crank arms it might be possible the arms could hit the connector. They clear fine on mine with FSA CK cranks.
Adding an an external battery is not a straight forward job.
First when you connect two batteries in series or end to end, the voltage adds and the current is the same. That is how most of our internal batteries are built. The basic cell in a lithium ion battery build is 3.7v. So if you need a 36v battery you need to connect 10 of them in series yielding a 37v battery. If you need more current you need to connect more than one 10s (10 cells in series) group to form multiple parallel connected batteries. In a parallel connected system the current adds and the voltage is constant.
When we have multiple batteries made up of series and parallel cells we still can connect those battery packs together in series or parallel.
Since we need to match the required battery specification of the eMTB we are going to connect the battery packs together in parallel so that the current increases but the voltage stays at the specified level.
Now when we connect two battery packs together the current will flow from the battery with the greater voltage to the one with the lower voltage. If they are nearly same voltage they will equalize a little and no problem. But if one battery is very low and the other fully charge there can be a rush of current from the charged battery to the discharged battery causing high current to flow with great heat. This can damage the batteries and cause a fire.
I have found a couple of ways to stop this from happening. There are electronic switches available from Aliexpress that monitor the voltage of two or more batteries and when one battery is lower in voltage than the other it will shut off the low battery. When the active batteries voltage drops to within a safe level the switch turns on the low battery and they work in parallel until one battery drops too low again. If you get a switch with a charge port it works the opposite when charging. It will turn off the higher voltage battery until the lower is within a safe level then if will charge both batteries at the same time.
The problem with the switch is that it only works with batteries that charge and discharge on the same connection. If the battery has a separate charge and discharge connection the switch will not work.
The external battery I bought early on has a separate charge and discharge port so it will not work with a switch.
So plan B was to use ideal diodes. A diode acts like a gate and allows current flow one way but not the other way.
If I put a diode in the output of each battery, on the way to the motor, it will allow current to flow to the motor but not the other way. So no current can flow from one battery to the other. Since both my internal and external batteries have separate charge ports this will not stop the batteries from being charged. The one problem with a normal diode is that there is a 1.7v drop across the diode when current is flowing. We really don't want the voltage to drop because the display determines the remaining battery power by measuring the voltage and it will think the battery is down a little due to the diode.
So to solve this I ordered ideal diodes from Aliexpress. These are circuit boards that function like a diode without the voltage drop.
Now I need a new connector for the bike frame because I need a connection for the external charger and I need a connection for the external battery. I found a connector on Aliexpress which waterproof and is the right size for the original hole in the frame. The connector is a right angle SD20. I use two plugs and one socket for the frame. One plug, the right angle, connects the frame connector to the external battery and the other is used to connect the battery charger to the internal battery and the old battery charger connector must be swapped with a new one or and adapter cable if you prefer not to change the original charger. The bottle battery has it's own charger.
There are two variations of the frame connector a square one and a diamond shaped. The diamond shape is close enough to be a direct replacement for the original connector. No cutting the frame needed and you can make the old frame connector bolts work on the new connector.
It is a four pin connector so two pins will be used for the external battery and two pins for the external charger for the internal battery.
I will use two different external connectors, one for the bafang charger to replace it's connector and one for the external battery.
I made a cable with the connector with two pins going to the motor connected via a Y cable so both the external battery and the internal battery could go to the motor but with ideal diodes to stop the batteries from charging each other.
The other cable from the connector goes to the charge port on the internal battery so it can be charged.
Whatever connectors you decide to use remember they need to handle at least 20amps.
This image shows the external right angle connector going to the external battery. I hope it is going to clear the crank. It works and the external battery is powering the bike.
I will attach a cable guide to the battery holder plate to stop the cable from flopping around.
The battery I used was purchased from Aliexpress
It is the S002 36V7Ah Sanyo, China and you must be careful to buy one small enough to fit the frame. You do have to get a battery to match the voltage of your motor, of course. There is very little room on the frame of a large and barely enough room on a medium for the smallest battery from this vendor the S002 36v 7ah.
You also need a side entry water bottle cage with and opening in the bottom so the cable can exit the battery. Most water bottles have sold lip on the bottom and won't work.
*NOTE * if you have a low "Q" crank arms it might be possible the arms could hit the connector. They clear fine on mine with FSA CK cranks.
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