Parallel batteries

Shamcc1985

Finding my (electric) wheels
Jan 21, 2019
10
0
Hi everyone just put a bafang bbs02b 750v 48v on my bike. I have a hailong 48v 12.5 ah battery but also have a 48v 11.5 ah dolphin going spare. What people’s view about putting the dolphin in pannier bag and run both batteries in parallel.
 

vfr400

Esteemed Pedelecer
Jun 12, 2011
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First a warning: A heavy battery in your pannier will soon wear through. It looks safe when you're stationary, but it bounces around when you're riding. If you must put one in there, make sure that it's wrapped in bubbewrap or similar and make sure that it can't bounce.

You can put two batteries in parallel as long as they're of similar actual voltage when you connect them. You must disconnect them for charging otherwise one will charge the other through the output terminals and by-pas the charge controls, which is dangerous.
 
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Shamcc1985

Finding my (electric) wheels
Jan 21, 2019
10
0
I have a up and over pannier rack. My top bag I was going to bolt the battery down to the top of the pannier rack.
 

Nealh

Esteemed Pedelecer
Aug 7, 2014
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Parallel, increase in ah and current discharge so easier on the cells, voltage will remain higher initially for staying power. As vfr mentioned make sure both are quite close voltage wise before connecting, 0.025v or better.
 

Shamcc1985

Finding my (electric) wheels
Jan 21, 2019
10
0
Parallel, increase in ah and current discharge so easier on the cells, voltage will remain higher initially for staying power. As vfr mentioned make sure both are quite close voltage wise before connecting, 0.025v or better.
Is it best to put multimeter on them before I connect them up
 

WheezyRider

Esteemed Pedelecer
Apr 20, 2020
483
198
Hi everyone just put a bafang bbs02b 750v 48v on my bike. I have a hailong 48v 12.5 ah battery but also have a 48v 11.5 ah dolphin going spare. What people’s view about putting the dolphin in pannier bag and run both batteries in parallel.
What you can do to connect two batteries with different states of charge safely is to make a Y cable, where the single end goes to your controller, then in each of the arms, put a high power Schottky diode. This will prevent current flowing from one pack to the other, but allows power to go to the controller from both packs. But make sure you use Schottky diodes, so the voltage drop is only about 0.1V rather than normal diodes with a drop of typically 0.5V. Having a small voltage drop prevents a lot of power being wasted.
 
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Shamcc1985

Finding my (electric) wheels
Jan 21, 2019
10
0
What you can do to connect two batteries with different states of charge safely is to make a Y cable, where the single end goes to your controller, then in each of the arms, put a high power Schottky diode. This will prevent current flowing from one pack to the other, but allows power to go to the controller from both packs. But make sure you use Schottky diodes, so the voltage drop is only about 0.1V rather than normal diodes with a drop of typically 0.5V. Having a small voltage drop prevents a lot of power being wasted.
Ok thank you. I may just run a spare cable from motor to rear battery then just swap over when needed. Trying to find like a waterproof box where I can fit 3 xt60 connector
 

Bikes4two

Pedelecer
Feb 21, 2020
97
55
What you can do to connect two batteries with different states of charge safely is to make a Y cable, where the single end goes to your controller, then in each of the arms, put a high power Schottky diode. This will prevent current flowing from one pack to the other, but allows power to go to the controller from both packs. But make sure you use Schottky diodes, so the voltage drop is only about 0.1V rather than normal diodes with a drop of typically 0.5V. Having a small voltage drop prevents a lot of power being wasted.
A nice idea but given the situation about the multimeter, I suspect the OP might find determining Anodes and Cathode (of diodes) a bit of a challenge?
 

WheezyRider

Esteemed Pedelecer
Apr 20, 2020
483
198
A nice idea but given the situation about the multimeter, I suspect the OP might find determining Anodes and Cathode (of diodes) a bit of a challenge?
Maybe :D but we are here to help...and even if it doesn't help the OP, someone else may find it useful to know.
 
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Bikes4two

Pedelecer
Feb 21, 2020
97
55
Agreed - using Schotkys as steering diodes is as I said, a neat idea and something I shall gladly store away for a future project.
 

Woosh

Trade Member
May 19, 2012
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Southend on Sea
wooshbikes.co.uk
you'll need this double battery converter.
It's an automatic switch.
Battery A and battery B are alternately connected to the controller.
You adjust the voltage points with two small buttons where the switch over happens.
It works with both 36v battery and 48V battery.
I'll get them in September.

 

Benjahmin

Esteemed Pedelecer
Nov 10, 2014
1,741
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But the advantage of running two in parrallel is less 'wear and tear' on the cells because the maximum current is nominally 50% from each battery, and charging cycles are not so deep.
I've been running two batteries for about 18 months now. Voltage is measured every time before connecting. The difference is generally 0.05-0.1v. at connection, I use a home made Y lead. The second battery does ride in a pannier but it is wrapped in a double layer of 12mm high density foam. The pannier itself is starting to show signs of weight stress, but it's about 5 years old and has done a lot of shopping.
I get way less voltage sag around the welsh hills and, with a nominal 28Ah, more range than I can use ( my legs run out first).
 

Woosh

Trade Member
May 19, 2012
14,487
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wooshbikes.co.uk
the trouble with home made Y connector is the second battery's connector remains live if you don't have the battery in its cradle.
That's why we couldn't supply the Faro with internal and external battery. The KM849 fixes that problem.
 
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Atlav4

Pedelecer
Feb 16, 2020
130
49
So with that converter fitted then the Faro can be ran as a duel battery option? After purchasing a down tube fitted battery of course. Also with the option of running with just the internal battery when longer range isn’t required. Sounds very appealing for the price of a battery and converter.
 

WheezyRider

Esteemed Pedelecer
Apr 20, 2020
483
198
you'll need this double battery converter.
It's an automatic switch.
Battery A and battery B are alternately connected to the controller.
You adjust the voltage points with two small buttons where the switch over happens.
It works with both 36v battery and 48V battery.
I'll get them in September.

Looks interesting Woosh. Couple of questions, 1) how much power does it consume and 2) if battery A hits LVC under load and then recovers once load is removed, is it constantly switching from A to B and back again until A is at LVC rested condition? Or does it trip out A completely as soon as A hits LVC under load and then you have to reset the box to get it to look at A again?

Oh, and Q2a) can you supply it with XT60 connectors, to save cutting the bullet ones off and a soldering job? :)
 
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WheezyRider

Esteemed Pedelecer
Apr 20, 2020
483
198
the trouble with home made Y connector is the second battery's connector remains live if you don't have the battery in its cradle.
That's why we couldn't supply the Faro with internal and external battery. The KM849 fixes that problem.
This is where the Schottky diodes come in...:)
 
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Sturmey

Pedelecer
Jan 26, 2018
175
84
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Ireland
This is where the Schottky diodes come in...:)
I have used a dual 20 + 20 amp (MBR4045, total 40 amp) as below bolted to a flat piece of aluminium for heat sink (about 2 square inches area) and placed in a small plastic box. Works great at 36 volts. For a 48volt battery, you need something like the MBR 4060 (60 volt) if you want full isolation.

 
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WheezyRider

Esteemed Pedelecer
Apr 20, 2020
483
198
I have used a dual 20 + 20 amp (MBR4045, total 40 amp) as below bolted to a flat piece of aluminium for heat sink (about 2 square inches area) and placed in a small plastic box. Works great.

Nice, I've got the same, but the 60A versions. They only cost a few pounds.
 
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awol

Esteemed Pedelecer
Sep 4, 2013
1,192
365
I have used a dual 20 + 20 amp (MBR4045, total 40 amp) as below bolted to a flat piece of aluminium for heat sink (about 2 square inches area) and placed in a small plastic box. Works great at 36 volts. For a 48volt battery, you need something like the MBR 4060 (60 volt) if you want full isolation.
How close do the 2xbatteries volts need to be when coonnecting up and using something like this?