paralleling 2 18650 packs

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joby1
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paralleling 2 18650 packs

Post by joby1 » Jan 17, 2017 2:21 am

I have a 48v 20ah pack, that is basically fine, no issues....looking at other batteries, possibly higher voltage.

I am sure this question has been asked a million times, but I see different answers.

The question is this, if I just get another 48v pack, to parallel to the pack I have, can I get a larger AH pack, like a 30AH, or even 35ah pack and parallel that to the 20ah pack? I think thats ok but I did see where people were recommending matching capacities somewhere..

Just getting the 48v might be the best option for me, as I am using this bike for transportation, not fun..well its fun, but with too much fun, things start burning out or breaking :)

I need more range I think, will mostly be around 20mph or so, and I am guessing putting 2 48's together will lighten the load on both...as opposed to running 2 different batteries, each independently.. I am shooting for 50 AH or so..

I know this is a dead horse, just beat on it one more time please :)
Joby

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Sunder
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Re: paralleling 2 18650 packs

Post by Sunder » Jan 17, 2017 2:27 am

What you describe is fine, but never connect the batteries when there is more than say a volt or so of difference.

I have a 43S LiFePo4 together with a 36S LiPo. As long as the pack never goes over the lower of the two limits and never drops under the higher of the two limits, its fine.

Just to throw a spanner in the works though, if you choose a different chemistry from your first, and they have very different discharge curves, there is a possibility that you will cross charge the battery, with the worse case scenario being a fire. Unlikely though.
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After 5 builds, the best advice I can give, is start with high quality products. I prefer http://www.ebikes.ca

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dogman dan
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Re: paralleling 2 18650 packs

Post by dogman dan » Jan 17, 2017 7:37 am

Match the voltages,, charge to full,, which is the same voltage for both,, then connect. Discharge, disconnect, charge separately,,

They do have to be similar chemistry. Or better that way at least. Lifepo4 mixed with li-ion works poor, because the end of discharge voltages are different. Can still work though, if both packs have a bms to cut off when the pack is empty.

Capacities don't matter for parallel. Capacity matters for series connections.

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Marin
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Re: paralleling 2 18650 packs

Post by Marin » Jan 17, 2017 2:42 pm

dogman dan wrote:Match the voltages,, charge to full,, which is the same voltage for both,, then connect. Discharge, disconnect, charge separately,,

They do have to be similar chemistry. Or better that way at least. Lifepo4 mixed with li-ion works poor, because the end of discharge voltages are different. Can still work though, if both packs have a bms to cut off when the pack is empty.

Capacities don't matter for parallel. Capacity matters for series connections.
X2 on this.

Your 20ah is pretty big now compared to lots of bikes and if it's a triangle the new one goes on a rear rack. You should strap a weight (concrete block) on the rack to see if you like the ride with so much weight before you spend.

I had to learn this the hard way.
Marin ex-rental hybrid with 48v bionx...sold
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Lunacycle with l/r mid drive coming soon
Kona Caldera with QS 30H (maybe), and phase runner

joby1
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Re: paralleling 2 18650 packs

Post by joby1 » Jan 17, 2017 7:04 pm

Thanks again for the relplies, makes a lot of sense..good stuff.
Marin wrote:
dogman dan wrote:Match the voltages,, charge to full,, which is the same voltage for both,, then connect. Discharge, disconnect, charge separately,,

They do have to be similar chemistry. Or better that way at least. Lifepo4 mixed with li-ion works poor, because the end of discharge voltages are different. Can still work though, if both packs have a bms to cut off when the pack is empty.

Capacities don't matter for parallel. Capacity matters for series connections.
X2 on this.

Your 20ah is pretty big now compared to lots of bikes and if it's a triangle the new one goes on a rear rack. You should strap a weight (concrete block) on the rack to see if you like the ride with so much weight before you spend.

I had to learn this the hard way.
I was planning on shifting my current bag over and mounting the other one side by side with it. My top tube is 1.5" my bag is about 3.25" wide, if other battery is comparable width, which it should be close, both together should be about 6.5 inches, which only puts the batteries something like 2.5" out on either side..I think I can deal with that fairly well, and weight will be centered on the bike. I am thinking of trying a setback seatpost, so dont want much more weight on the back, my frame is smaller and I have seat low..might try moving seat back 3-4 inches..

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dogman dan
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Re: paralleling 2 18650 packs

Post by dogman dan » Jan 18, 2017 7:02 am

Up to about 6" wide is possible to pedal around a fat battery. You just pedal a bit bowlegged, by pointing your toes out a bit on the pedals. More than that,, you are looking at a bike you don't really pedal.

If you did carry on a rear rack,, a 10 ah would do er without too much added weight. Carrying 10 ah on a rear rack is hardly noticeable. but above 10 pounds,, you feel it.

30 ah is a pretty big battery, with long range at 20 mph speeds. I carry 33 ah a lot, plenty to do a 40 mile round trip around town. Much more than that would get awkward, but the weight won't be a problem if its placed in the center. Just the size.

Sidotian
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Re: paralleling 2 18650 packs

Post by Sidotian » Jan 03, 2018 2:37 am

I have the same question here but I'm building my own pack (12s8p) with recycled cells and was curious if I could combine different individual 18650 cells in parallel mah ranges from 1500mah per cell up to 3300mah. obviously I would make sure to balance the mah between series packs. I use two venom duo chargers for testing 4.2volts down to 3.0 however when tested with a multi meter they read 3.6 fresh off the discharge cycle. I'm useing a buss bar with 4amp wire Fuse as my bike only draws 20amps.

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