Dual battery

webcurl

Active Member
Hi all, i'm trying to find info on dual battery bafang setup's but to no avail.
On a thread here it mentions the motor alternates between the two, is this automatic or manual via a 2 position switch?
I know the Bosch system uses a parallel cable and each battery's BMS via CAN bus comms has the ability to switch itself off allowing for complete automation in regards to alternating between 2 or more batteries.
Or does the Bafang motor have 2 separate battery inputs and can alternate automatically that way?
And is all of this power wiring simply 2 conductors?
 

pushkar

Well-Known Member
This is a custom part I developed last year. Works very similar to the Bosch system though probably not as advanced. The part is automatic - no switches.. just wire both batteries to this circuit , and the single unified output goes to the motor. The setup can be used with any motor (ultra or otherwise), and same sized batteries (e.g. 2 48V or 2 52V).
 

webcurl

Active Member
This is a custom part I developed last year. Works very similar to the Bosch system though probably not as advanced. The part is automatic - no switches.. just wire both batteries to this circuit , and the single unified output goes to the motor. The setup can be used with any motor (ultra or otherwise), and same sized batteries (e.g. 2 48V or 2 52V).
Thanks, care to elaborate a little?
I assume because it requires same voltage rated batteries it switches based on voltage alone, eg. what happens when one battery is charged to 20% & the other 90%?
Are you using ultra low RDSon FET's or MOSFET's to do the switching?
I'm right into stationary lightweight Solar charging, spent thousands so far on panels, inverters, charge controllers & buffer batteries, for example i have 2 of these very expensive panels: https://p3solar.com/portfolio/p3-125/
With your system i assume i'd have to charge 1 battery at a time.
At the moment the system is DC (Solar) -> MPPT Step Down to 14.4VDC (Charge controller with small LifePo4 buffer battery) -> Invert to 240AC (Inverter) -> Invert & Step Down to 41.8?VDC (Bosch charger) -> Bike batteries.
I'd prefer DC (Solar) -> MPPT Step Down to 14.4VDC (Charge controller with small LifePo4 buffer battery) -> Step Up DC -> Bike batteries.
 
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pushkar

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I’m not sure if there’s a ton of interest in this but I’d be happy to make it available as a stand alone part if there’s demand. Right now it’s a limited run for WW bikes.
 

pushkar

Well-Known Member
Thanks, care to elaborate a little?
I assume because it requires same voltage rated batteries it switches based on voltage alone, eg. what happens when one battery is charged to 20% & the other 90%?

it shows the max of the capacity available


Are you using ultra low RDSon FET's or MOSFET's to do the switching?
I'm right into stationary lightweight Solar charging, spent thousands so far on panels, inverters, charge controllers & buffer batteries, for example i have 2 of these: https://p3solar.com/portfolio/p3-125/
With your system i assume i'd have to charge 1 battery at a time.
I can’t share too much of the design but suffice to say this is not super intricate. You know enough of the underlying components and can probably build one fairly quickly if you wanted to. The biggest difference in is a little bit of Fab, scaling, waterproofing, insulation etc etc. that’s really the hard part IMO.

I’m not sure what you mean by charge one battery at a time ?
 
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pushkar

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With your system i assume i'd have to charge 1 battery at a time.
At the moment the system is DC (Solar) -> MPPT Step Down DC (Charge controller with small LifePo4 buffer battery) -> Invert to 240AC (Inverter) -> Invert & Step Down to 41VDC (Bosch charger) -> Bike batteries.

I'd prefer DC (Solar) -> MPPT Step Down DC (Charge controller with small LifePo4 buffer battery) -> Step Up DC -> Bike batteries.

I haven’t tried with solar and my knowledge on that is limited. However your preferred setup should work. We just need the DC output and MPPT+step up DC should technically work. I don’t know why Bosch needs the additional DC/AC/DC step.
 
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webcurl

Active Member
it shows the max of the capacity available
Hopefully it would use much more, if not all of the 90% charged battery before using the 20% battery.
I can’t share too much of the design but suffice to say this is not super intricate. You know enough of the underlying components and can probably build one fairly quickly if you wanted to. The biggest difference in is a little bit of Fab, scaling, waterproofing, insulation etc etc. that’s really the hard part IMO.
Hopefully it's all solid state (no mechanical relays).
I’m not sure what you mean by charge one battery at a time ?
How do you normally charge a dual battery WW?
 

pushkar

Well-Known Member
Hopefully it would use much more, if not all of the 90% charged battery before using the 20% battery.
Yes. That’s correct. It will ramp down the higher capacity battery till the difference is within the tolerance for the second battery to be tapped.

Hopefully it's all solid state (no mechanical relays).

Yes !



How do you normally charge a dual battery WW?

Ooh I get your question now. Good one. Yes each battery is charged separately.
 

pushkar

Well-Known Member
So here’s a cool thing- and I haven’t verified this but will. The BMS from our vendor allows charge and discharge at the same time. So technically your solar setup can always be hooked in to both batteries (individually ) and be charging when idle or running.

Definitely useful if you are riding the whole day, and want to capture as much power as you can with the panels.
 
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webcurl

Active Member
Ooh I get your question now. Good one. Yes each battery is charged separately.
I ask because for stationary solar charging it would be nice if you could disconnect the motor and feed solar power into the output of your device and have it running in reverse :) That way there's no wasted hours of solar if one battery becomes fuller that the other, etc.
 
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webcurl

Active Member
So here’s a cool thing- and I haven’t verified this but will. The BMS from our vendor allows charge and discharge at the same time. So technically your solar setup can always be hooked in to both batteries (individually ) and be charging when idle or running.

Definitely useful if you are riding the whole day, and want to capture as much power as you can with the panels.
As long as no power can trickled back into the solar setup or damage said setup. And one battery would not be charging the other, etc.
 
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webcurl

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I’ll check this and get back to you.
I'm not interested in charging on the go (unless Solar panels become 4 times more efficient :), i don't want to tow a trailer and anything less would not provide enough Solar surface area to be worth it.
My use would be: ride for a day or so & find a good camp site, take it easy for a day or so and let the solar do it's work, ride for a day or so and either repeat the process or charge at a park/whatever.
 

webcurl

Active Member
This is a custom part I developed last year. Works very similar to the Bosch system though probably not as advanced. The part is automatic - no switches.. just wire both batteries to this circuit , and the single unified output goes to the motor. The setup can be used with any motor (ultra or otherwise), and same sized batteries (e.g. 2 48V or 2 52V).
Looks like DOST bikes have re-invented the wheel :)
 

pushkar

Well-Known Member

pushkar

Well-Known Member
I'm not interested in charging on the go (unless Solar panels become 4 times more efficient :), i don't want to tow a trailer and anything less would not provide enough Solar surface area to be worth it.
My use would be: ride for a day or so & find a good camp site, take it easy for a day or so and let the solar do it's work, ride for a day or so and either repeat the process or charge at a park/whatever.
I looked into it. Technically the circuit works in reverse, however the charger will only detect the voltage of the lowest charged battery - so it will keep feeding the fully charged battery a lot of current till both batteries show the same voltage . This has the potential to damage the battery. Definitely a feature that can be added in the future. Thanks for bringing this up.

For the moment, i would recommend charging each battery separately.
 

Thomas Jaszewski

Well-Known Member
I use a simple switch with an added voltmeter.
 

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