Second (back up) battery on a Bafang mid motor

BrianK

Well-Known Member
Question: Did anyone ever mount a second battery on the rear rack of a Bafang mid drive, with a toggle switch to change over from a dead primary battery to the fully charged back up battery?

Obviously each battery would have to be charged separately.

How hard would it be to add one this way?

It would (obviously) double the mileage of the fully charged e bike and might be a DIY option worth exploring.
 

tomjasz

Well-Known Member
Question: Did anyone ever mount a second battery on the rear rack of a Bafang mid drive, with a toggle switch to change over from a dead primary battery to the fully charged back up battery?

Obviously each battery would have to be charged separately.

How hard would it be to add one this way?

It would (obviously) double the mileage of the fully charged e bike and might be a DIY option worth exploring.
My posts with a toggle, DPDT, switch is here somewhere. There’s only one switch I’ve ever found to meet the specs.
 

Gionnirocket

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Y. O.
Question: Did anyone ever mount a second battery on the rear rack of a Bafang mid drive, with a toggle switch to change over from a dead primary battery to the fully charged back up battery?

Obviously each battery would have to be charged separately.

How hard would it be to add one this way?

It would (obviously) double the mileage of the fully charged e bike and might be a DIY option worth exploring.

Hey Brian

I've never done it but this is common practice in the RV and Marine application world.
Most importantly you want to use the correct switch that is properly rated for 48v DC... NOT the common 120v/277v AC switches available.
One like this 2 Battery Switch would allow you to select between Battery 1/Battery 2/ Both ON/ Both OFF
818UBQADEqL._SL1500_.jpg

You do not want to use a DPDT switch as the chances of a short circuit are greatly increased, especially if you don't use a properly rated one
 
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Gionnirocket

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Y. O.
While having coffee a couple of considerations came to mind that you should be aware of.

1. I would not operate the switch under load, meaning while in motion... and personally I would only operate the switch with the controller powered Off. We don't know how sensitive the electronics are in the display, BMS and controller and there's no need to stress them with voltage variations. At the very least you're extending the life of the switch.
2. Though not essential I would match the second battery to the the first as far as manufacture and amp_hour. Having two that operate and drain similarly is always best practice.
3. And Most importantly is that if you run them in parallel (both batteries at the same time) be very careful not to switch to this mode when the batteries are at different voltages. You will get a serious in_rush of current from the battery at the higher voltage to the battery at the lower voltage as they try to equalize. That said, running them in parallel starting with an equal voltage will be the most efficient use of the dual battery set up as each will drain equally and under half the stress as individually.
 

BrianK

Well-Known Member
And Most importantly is that if you run them in parallel (both batteries at the same time) be very careful not to switch to this mode when the batteries are at different voltages. You will get a serious in_rush of current from the battery at the higher voltage to the battery at the lower voltage as they try to equalize. That said, running them in parallel starting with an equal voltage will be the most efficient use of the dual battery set up as each will drain equally and under half the stress as individually.
As far as your #3 goes, I wouldn’t attempt to use both batteries at once, in parallel. I would simply like to have the back up battery carried securely in its own mount and already wired in, so it can be switched on like a back up gas tank.
 

BrianK

Well-Known Member
One like this 2 Battery Switch would allow you to select between Battery 1/Battery 2/ Both ON/ Both OFF
I hadn’t thought of this application, honestly.

Would that switch - alone - be sufficient to create a reliable two battery parallel application, if your batteries were indeed of equal voltages?

I assumed the e bike companies currently offering two batteries had a bit more electronics involved than just directly wiring them in parallel.
 

Gionnirocket

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USA
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Y. O.
As far as your #3 goes, I wouldn’t attempt to use both batteries at once, in parallel. I would simply like to have the back up battery carried securely in its own mount and already wired in, so it can be switched on like a back up gas tank.

I understand.. But IMHO... if you are going to have the added expense of the second battery, it would be best for both batteries to have equal yet less current draw by paralleling them.
So if you charged both batteries before use, you can easily match voltage. I would think under 0.5v is more than acceptable difference and you can double check this using the switch... toggle between the two and reading your display before paralleling them. You can also even them up by riding a slightly higher voltage one for a bit. But if you are uncomfortable paralleling it's not a problem. You can probably even find a 2 or 3 position switch that doesn't give you the option.

And yes the switch should be all that is needed. I personally would add fuses to each battery... again Properly rated fuses
I'm sure an eBike manufacturer could implement some safety into a controller that wouldn't allow paralleling if voltages were out of acceptable range.
It might be a good idea to contact the switch manufacturer as well... as you're probably not the first to do this and they might have some information already on hand.
Email: customerservice@nilight.com
 

tomjasz

Well-Known Member
I'm not clear on why there is always reticence by "experts" for using a properly specified DPDT switch. I've been using them for over 4 years now.
BE993B61-47C5-46DF-916E-6E2B7FF1DFDB_4_5005_c.jpeg
 

Gionnirocket

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I'm not clear on why there is always reticence by "experts" for using a properly specified DPDT switch. I've been using them for over 4 years now.


Why thank you good man 😍

Do you have the specs' for that switch and can you explain how it is wired or post a diagram?
 

Gionnirocket

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Y. O.
NKK series S High Capacity Toggles will get you there.

Good choice my friend 👍
They seem like very good switches. That said their amperage rating drops to 10a @48v DC for inductive loads so I would give you the same advice as above and not switch it under load... Which is always good practice for many reasons anyway.
Also I ask about using DP... are you switching both the +/- ?
I don't feel that is necessary and can be real trouble especially with a switch with less than adequate specifications as many are tempted to use.
 

BrianK

Well-Known Member
Turns out there’s a simple solution from Bafang:
 

Gionnirocket

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What they don't specify is that the batteries MUST Be at the same SOC.
And though they hint at connecting two batteries of the same voltage and capacity... I would say that is an absolute must as well.
If it were me, I would still do the selector switch for the added versatility and additional safety of battery isolation.

Pretty much what I said before.
Just because they sell something doesn't mean it's good practice. Also their putting the burden on you for understanding battery safety.
 

BrianK

Well-Known Member
What they don't specify is that the batteries MUST Be at the same SOC.
And though they hint at connecting two batteries of the same voltage and capacity... I would say that is an absolute must as well.
If it were me, I would still do the selector switch for the added versatility and additional safety of battery isolation.

Pretty much what I said before.
Just because they sell something doesn't mean it's good practice. Also their putting the burden on you for understanding battery safety.
What’s the quickest/easiest method to ascertain and obtain an equal SOC between two 48v rated batteries?
 

Gionnirocket

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Y. O.
I guess the simplest method is to use two of the same batteries and use the same charger. Then verify after charging. Again why I like the selector... If you want to parallel as the cables do, you can achieve equal SOC by riding one battery until the voltage matches the other.
I don't think I'd go for the pre-made cables. As mentioned, I would also incorporate fuses on each battery output. Might be over kill but with the reputation of Lithium batteries and the high cost of a quality battery, I feel it's worth the extra $20
 

harryS

Well-Known Member
What’s the quickest/easiest method to ascertain and obtain an equal SOC between two 48v rated batteries?

Verify your chargers are equal/close in voltage and charge them up. That's what I do. I only parallel my 36 V packs. I have four or five chargers that go from 41.7 to 41.9V on my voltmeter. In your case, full charge is 54.6V. It doesn't matter to me if one pack is 0.2 volt different. Not much current flows at those differentials. I've checked.

If you're one of these guys that change to 80% or 90%, you probably shouldn't parallel your packs. Reason is that at 80-90% SOC, the cells are still quite willing to take a lot of charge current. So any error on your part could result in high current. As they get close to 100% SOC, the natural tendency is for the cells to take limited current. so I believe I can be off a little bit and be OK.

I'm still carefull. I do check (with a meter) that my packs are close to full charge if I'm going to parallel them.
 
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