CEF50 External battery build

mike_kelly

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Aug 11, 2022
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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.

diode.png

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.

NewConnector.png

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.
Connecotr.png


Cable.png



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.
EBwithCable.jpg


I will attach a cable guide to the battery holder plate to stop the cable from flopping around.

Soo2.png


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.

***************************************************************************
New wiring hardness and connector. New connecter (M25) is just as chunky but gives 5mm more clearance for crank and is a bayonet mount. External power coming n can not go to the battery because the diode blocks it.

Wiringharness.JPG



*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. New harness gives 5mm more clearance.
 
Last edited:

patdam

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Jan 17, 2019
889
598
france
Thanks to sharing your experiency.

One question abourt the "electronic swithes available from Aliexpress". why you don't use it only to couple the batteries when use the two and charge on separately each battery (diode will not be necessary) ? or i have bad understanding ?


Another interogations for those who have used several various batteries and additional batteries.

Are you sure that assembly of 2 battery will be as efficient that only one with same capacity ? i have 2 M510, one with an 15Ah battery and one with the 19 Ah, in use with the 19 Ah i can do +-2 more uphill than the 15Ah.
 

Lussy

New Member
Jul 18, 2023
24
41
New Zealand
I came to the same conclusions as you Mike for the two extender battery connection options. Did you connect the ground to the negative battery pole on the diodes? Also can you share the links to the Aliexpress 4 pole connectors. Thanks
 

mike_kelly

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@patdam When using the ideal diodes the two batteries are effectively separated. So having two batteries of different capacities are not as problematic. As far as using the switches, I bought the ones with the charge port. So just too little information about what is circuit diagram. They are also bigger and will not fit as easily in the space around the motor than the diodes. But it should work if you charge separately.
@Lussy I have not received my diodes yet. My internal battery was broken in shipment so I don't actually have the internal battery connected yet. But at least I can use the bike until the new internal battery arrives.
Here are the SD20 connectors
Note that the current capacity changes in the SD20 under 5 pins they support 20amps and over 5 pins it drops.
 
Last edited:

mike_kelly

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Aug 11, 2022
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I came to the same conclusions as you Mike for the two extender battery connection options. Did you connect the ground to the negative battery pole on the diodes? Also can you share the links to the Aliexpress 4 pole connectors. Thanks
If you are talking about the little ground pads, yes you need those to power the circuit. Even tho the "diode" itself is only in the positive leg of the connectiion the negative is also needed to make the ideal diode work. The circuit taps off a little of the voltage to run the "ideal diode" so it needs a ground reference.
 

Bengy22

Member
Aug 25, 2022
124
80
USA
Does that right angle connector when you're using the range extender get in the way while pedaling? Seems very large and like it sticks out a lot. Can't help but think it would get in the way and that it could be a lot lower profile. Very interesting though how you setup and wired in the range extender though, looks good and very thought out!
 

mike_kelly

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It is large but clears just fine. Nice to have the external battery since I am waiting for replacement battery shells.

Note: It may make a difference what the "Q" of your crank is as far as clearance is concerned.
 
Last edited:

scm007

Member
Mar 23, 2022
49
3
Seattle
I want to do the same but use 100wH subpacks for my external frame battery. Reconnecting them together into a 6P system. This way I will be able to fly with it (100wH batteries you can take as many as you want carry-on).

You seem knowledgable! Any ideas? Also would you suppose this system would be compatible with shimano, specialized, etc systems? I'm currently deciding which system to go with and I like that Bafang we can actually purchase ourselves and don't have to go through a stupid dealer network. Why did you decide to go with bafang vs some of the lightweight ebikes?
 

mike_kelly

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The bottom line was price. The M820 was a new entry and with nobody having any experience with it but I am very pleased. I did not want an electric motorcycle feel. I wanted an MTB feel with a "tailwind" like my Orbea Rise. The M820 have exceeded my expectations. Zhaosheng has been good to deal with and my total cost with budget build was $3800 and 39lbs.
Now you do have to be careful with adding external batteries to an unknown system. How the external batteries are connected can cause problems and fires which I have documented. SO you need to know the system and how it works before you try and add an external battery.
 

scm007

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Mar 23, 2022
49
3
Seattle
How do you like it vs the rise? I owned a 2021 rise m20 for reference. Also anybody selling assembled bikes or having to order thru lightcarbon and shipping from china?
 
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mike_kelly

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How do you like it vs the rise? I owned a 2021 rise m20 for reference. Also anybody selling assembled bikes or having to order thru lightcarbon and shipping from china?
The RIse EP8 was a little quieter in high output. The M820 has more power. THe application of power was a little bit smoother but the M820 has been better than I expected.
 

mike_kelly

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For the same price point I would choose the Rise because my 2022 had a more "trail" geometry. I don't like LLS geometry. That said they are not the same price point at all and I can ride my CEF50 just fine.
 

brbr

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Dec 28, 2022
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France
I’ve seen this connector for the external battery, looks great and slimer than the one you have, not sure if it’s as waterproof as yours

Je viens juste de trouver cet incroyable article sur AliExpress. Jetez-y un coup d'œil ! 4,74€ | Connecteur de batterie étanche pour vélo électrique, port de charge, câble, prise, scooter, vélo électrique, 30-50A, IP67, 2 + 5, 2 + 0, 24.com
 

mike_kelly

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I’ve seen this connector for the external battery, looks great and slimer than the one you have, not sure if it’s as waterproof as yours

Je viens juste de trouver cet incroyable article sur AliExpress. Jetez-y un coup d'œil ! 4,74€ | Connecteur de batterie étanche pour vélo électrique, port de charge, câble, prise, scooter, vélo électrique, 30-50A, IP67, 2 + 5, 2 + 0, 24.com
Does not work for me. I need four power pins so I can have charging for the internal battery and power input from the external battery.
 

un..inc

Active member
Jun 13, 2023
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Germany
Now that I've read the whole thread and thought about it, I have a question.
Wouldn't it be possible to keep the standard charging port and install a 2-pin connector on the battery holder?
The battery is secured with 4 / 6 screws. If we use one of them (the bottom one) to integrate a connector, would that be possible? Yes, you would have to machine the aluminium rail and the adapter plate of the two bolts, but that would feel like the most visually appealing solution.
And no, I don't have a suitable connector for it yet, but maybe we can think about it together. :)

1704980664863.png
 

un..inc

Active member
Jun 13, 2023
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Germany
Maybe something like this:

Oh and maybe we can merge these two threads:
 
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mike_kelly

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I am not quite sure of your idea. The battery support aluminum square rod goes down the down tube and is affixed by the screws you note. The rod extends down beyond those two screws. It supports the battery so it can't be cut short. I would not be inclined to cut a hole in the downtube for fear of losing strength. The XT60 connector is not waterproof. It seems like a lot of trouble for no gain to me.
 

un..inc

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Jun 13, 2023
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I understand what you mean.
Personally, I find the right-angled outlet near my foot quite dangerous. I would be afraid of snagging my foot on it, hence my search for alternatives.
I don't want to remove the entire bar either, just clear a path for two cables.
1705042238781.png

Or maybe one could go back to @Myalteregohamish 's approach.
He has routed a cable directly into the bottom of the engine panelling.
Here I have the alternative idea of routing the cable out of the UPPER mounting and stowing it nicely on the bottle cage until you use the range extender.
1705042327284.png

The choice of connector would be free and could also be IP65 compliant.
However... I haven't even got my frame yet and I'm already thinking about a range extender that I might not even need. :D
 

mike_kelly

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I would not want to make any more holes in the frame. That is why I took the route I did. Also the original mounting hole has a metal bracket behind it. We have never had any problem snagging the cable as it is but I would not mind a smaller connector. But I have not been able to find any smaller waterproof connector that can handle the power.
 

mike_kelly

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One possibility might be to use a top tube bag for the battery and run the cable to a connector to that unused old power-on switch hole in the top tube. Not sure how much battery you could get in a typical top tube bag.
 

Myalteregohamish

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Jan 4, 2023
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Vancouver, WA
One possibility might be to use a top tube bag for the battery and run the cable to a connector to that unused old power-on switch hole in the top tube. Not sure how much battery you could get in a typical top tube bag.
Do you know if top tube batteries are DIY or are there versions you’ve seen manufactured that you like the look of?
 

un..inc

Active member
Jun 13, 2023
127
109
Germany
Well, they are definately not invisible... :D
1705322274667.png

1705322293623.png

I am wondering where he got the wire into the frame... I recall him saying, he built the batteries himself.
 

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