Before I connect these batteries in parallel…

Eastwood · Oct 27, 2021

Well as title says about to connect my batteries in parallel and just want to make sure I’m doing everything right.
Is there anything I need to look out for, besides having equal voltage? I didn’t use any resistors with the parallel connector I made. I just connected the + wires together and then connected the - wires together, pictures below.

Plan to charge both battery packs with the same charger to fully charged and then connect, little nervous and would be lying if I said otherwise lol
I’m a parallel connection virgin :lol: :lol:
Also I’ll double check the voltage with a voltmeter before connecting.

Eastwood · Oct 28, 2021

Dang nobody?
Well I just connected the batteries and It didn’t explode so that’s a good thing :lol:

Manbeer · Oct 28, 2021

Literally did the same exact thing a while back after asking the same question, I generally tried to separate them for charging but it's been quite a few times I didn't remember or just plugged one battery in to charge up a little bit And forgot and they both ended up charging, One through the discharge port. Never had a problem

Comrade · Oct 28, 2021

Eastwood said:
Also I’ll double check the voltage with a voltmeter before connecting.

Why? They should be close enough. They'll even out by themselves.

You have cells in parallel inside your pack. Two packs in parallel is the same thing. It's not as efficient as serial connecting though.

Eastwood · Oct 28, 2021

Manbeer said:
Literally did the same exact thing a while back after asking the same question, I generally tried to separate them for charging but it's been quite a few times I didn't remember or just plugged one battery in to charge up a little bit And forgot and they both ended up charging, One through the discharge port. Never had a problem

Yeah I plan to keep them connected and just charge through one battery and allow the other pack to get charged through the discharge connection.

Maybe for balancing the cells I’ll separate them occasionally and charge them individually.

Eastwood · Oct 28, 2021

Comrade said:
Eastwood said:

Also I’ll double check the voltage with a voltmeter before connecting.

Click to expand...

Why? They should be close enough. They'll even out by themselves.

I wonder how much of a difference in voltage before it actually becomes an issue?

Comrade said:
Eastwood said:

Also I’ll double check the voltage with a voltmeter before connecting.

Click to expand...

You have cells in parallel inside your pack. Two packs in parallel is the same thing.

Yeah I was thinking along the same lines but it being my first time was a little

:lol:

Eastwood · Oct 28, 2021

Just did some test riding in it actually puts out more power, I wasn’t expecting that! The display showed 7.5 kW and it wasn’t even going uphill. Before the second battery it was pushing 6.7 kW under heavy load. It’ll be interesting to see how many watts while climbing a large hill.

Maybe you guys experience this too? . I can understand if I change the settings on the controller but everything is still the same. Happy about that!

4πr^2 · Oct 28, 2021

"Everything" isn't the same. By paralleling, you essentially halved the internal resistance of the overall battery pack. Apparently that was the limiting factor in getting power through the system, so now you have a bit more to play with.

Comrade · Oct 28, 2021

Eastwood said:
I wonder how much of a difference in voltage before it actually becomes an issue?

Think of the battery with the higher voltage as a very powerful charger, and think of the battery with the lower voltage as the battery that is being charged. You don't want them to be too different. Charging them connected in parallel a good idea.

TDB · Oct 28, 2021

Eastwood said:
Manbeer said:

Literally did the same exact thing a while back after asking the same question, I generally tried to separate them for charging but it's been quite a few times I didn't remember or just plugged one battery in to charge up a little bit And forgot and they both ended up charging, One through the discharge port. Never had a problem

Click to expand...

Yeah I plan to keep them connected and just charge through one battery and allow the other pack to get charged through the discharge connection.

Maybe for balancing the cells I’ll separate them occasionally and charge them individually.

Yes, good idea to keep an eye on pack voltages and keep them as close to the same as possible. Current can be very high between grossly mismatched packs.

I do the same Y connection on discharge. But I charge through the BMS and made another Y for the charger. My discharge ports are not live during charging.

I charge them together and discharge them together. If I screw up on or off I run them one or the other until I get back to the charger then bring them back up separately. I think a couple of tenths of a volt mismatch off the charger is okay to start. After that they balance themselves as long as I stick to the regime.

Eastwood · Oct 28, 2021

4πr^2 said:
"Everything" isn't the same. By paralleling, you essentially halved the internal resistance of the overall battery pack.

Well I understand that the packs are under less resistant now but I guess I still don’t understand why there’s more watts.

So at 84v x 80a = 6.720kw and that’s exactly what the display showed before. Now with the second battery added it’s passing that 6.7 kW? I can understand if the last battery by itself failed to achieve the maximum power but it produce exactly 84 V x 80 A.

But anyways I have a little bit extra power definitely a good thing :mrgreen:

justlooking808 · Oct 28, 2021

Eastwood said:
4πr^2 said:

"Everything" isn't the same. By paralleling, you essentially halved the internal resistance of the overall battery pack.

Click to expand...

Well I understand that the packs are under less resistant now but I guess I still don’t understand why there’s more watts.

So at 84v x 80a = 6.720kw and that’s exactly what the display showed before. Now with the second battery added it’s passing that 6.7 kW? I can understand if the last battery by itself failed to achieve the maximum power but it produce exactly 84 V x 80 A.

But anyways I have a little bit extra power definitely a good thing

from my understanding it goes a little something like this. one pack would equal only what the BMS is capable of, so if BMS is maxed at 80A, it should be able to put out that much continuously. when connecting two battery packs in parallel, we double the capacity and add the discharge amp rating. so if one battery is 80a and the other is 60a, we would have 140a at our disposal. so besides higher capacity, we have higher discharge also. so that's why the battery can supply more power with two battery packs instead of only one. so all this time you power was limited by the battery :shock:

( BTW i live by the 80% rule. in anything electrical, only 80% is useable, the other 20% is lost to heat :twisted:

)

Eastwood · Oct 28, 2021

TDB said:
I do the same Y connection on discharge. But I charge through the BMS and made another Y for the charger. My discharge ports are not live during charging.

Yeah I was wondering if you could do the same and connect the charger ports in parallel. So I believe that’s what you’re saying you’ve connected your charger port in parallel but then you disconnect the two batteries on the discharge side while charging right?

So charging in parallel, could you leave the discharge cables connected? I mean I know the battery system could never completely turn off because of the other battery pack.

Seems like it would be better for the cells to have both charger ports connected in parallel so not all the current is running through the first pack. But I don’t know that’s way more technical than I can get with electrical haha

4πr^2 · Oct 28, 2021

If you were showing 84V / 80A and 6.7kW before, then volts, amps - or both - must have changed to get you the 7.5kW now.

What would typically happen, as you draw large amount of current from a battery, the voltage will sag due to resistance. When you parallel an additional battery, the voltage won't sag as much - hence the extra power. I'm not clear if the 84V/80A are actual readings of V and A, or maybe they are just ratings of the controller?

Eastwood · Oct 28, 2021

4πr^2 said:
If you were showing 84V / 80A and 6.7kW before, then volts, amps - or both - must have changed to get you the 7.5kW now.

Yeah that’s the really weird part nothings changed, same voltage and same exact settings on my Sabvoton controller. Well the only thing that changed is double the amp hours and also the possibility to have double the current output but didn’t adjust anything, just literally added the second battery and now it produces more watts. It’s a good problem to have though right? :lol:

I’m curious to see tomorrow going up a large hill to see how many watts. Would test now but it’s raining

Eastwood · Oct 28, 2021

So what would be considered too much voltage drop and what would be considered good? ha ha I should’ve paid attention what it was before.

Comrade · Oct 28, 2021

justlooking808 said:
when connecting two battery packs in parallel, we double the capacity and add the discharge amp rating.

Yes, somewhat. The packs have to be identical. Like a matched pair of cells in a battery pack. Meaning they came from the same factory batch, and lived identical lives.

justlooking808 said:
so if one battery is 80a and the other is 60a, we would have 140a at our disposal.

That's incorrect. The paralleled battery packs should be very close in their capacity (and in all other ways, really) for this method of paralleling packs to work.

justlooking808 · Oct 28, 2021

Comrade said:
justlooking808 said:

when connecting two battery packs in parallel, we double the capacity and add the discharge amp rating.

Click to expand...

Yes, somewhat. The packs have to be identical. Like a matched pair of cells in a battery pack. Meaning they came from the same factory batch, and lived identical lives.

justlooking808 said:

so if one battery is 80a and the other is 60a, we would have 140a at our disposal.

Click to expand...

That's incorrect. The paralleled battery packs should be very close in their capacity (and in all other ways, really) for this method of paralleling packs to work.

to answer both of these questions, i have two mismatched packs that i have been running for a year without any issues. one is a 10 ah 30a bms and the other is a 21ah 60a bms. both have the common port for charge and discharge. i keep them both connected with it's charging and in use and till this day i still have good pack life and little voltage sag (compared to running each one by it's sself.) but my current pull on the bike is only 20a, so maybe that's why it's lasting. i guess we will see when i change to controller to a 30a one :twisted:

i think the matched pair is more of a selling point and maybe good practice, but for me, doesn't seem to matter.

justlooking808 · Oct 28, 2021

Eastwood said:
So what would be considered too much voltage drop and what would be considered good? ha ha I should’ve paid attention what it was before.

most i have read is 3v is good, 2v is great, and 1v is outstanding. anything more that 3 volts( 3.1-5) is acceptable and 6v+ is a very bad match for the ebike. on tow of my oem bikes, v drop was like 3-5v, but once i added a second battery, it's more like 1-2v and the bike get's up to speed a little faster.

Comrade · Oct 28, 2021

justlooking808 said:
one is a 10 ah 30a bms and the other is a 21ah 60a bms

justlooking808 said:
but for me, doesn't seem to matter.

I'm pretty sure it does matter. You can't just ignore Ohm's law. :mrgreen:

Eastwood · Oct 28, 2021

justlooking808 said:
most i have read is 3v is good, 2v is great, and 1v is outstanding. anything more that 3 volts( 3.1-5) is acceptable and 6v+ is a very bad match for the ebike. on tow of my oem bikes, v drop was like 3-5v, but once i added a second battery, it's more like 1-2v and the bike get's up to speed a little faster.

Good to know, i’ll do some testing tomorrow to see what kind of voltage drop I get. Being that I have budget batteries I’m sure the voltage drop is not that great.

I’ll report back tomorrow.

justlooking808 · Oct 28, 2021

Comrade said:
I'm pretty sure it does matter. You can't just ignore Ohm's law.

i'm not too sure what or how this relates to batteries, but i'm willing to learn?

Comrade · Oct 29, 2021

justlooking808 said:
i'm not too sure what or how this relates to batteries, but i'm willing to learn?

In short, two dissimilar battery packs connected in parallel to a load do not work as efficiently as a single pack would. They are always interacting with each other, and not just with the load. How inefficiently? It depends on a lot of factors, and the easiest way to measure would be to measure power "in" while charging, adjust for charge-discharge inefficiency of lithium-ion, and measure power "out".

There is a 100% chance that you will get more range (more energy out of the 2 packs) if you drain one battery pack, disconnect it, and then connect the second pack and drain that.

DogDipstick · Oct 29, 2021

Paralleled batteries are only as strong as the weakest c-rate cell in the paralleled group.

You do not parallelize a "80A" and a "40A" to get a "120A"...

It is still only a 40A continuous battery stack.

All resistors in parallel draw frocking identical current per node.

frocking Kirchoffs laws. Learn em. I am tired explaining things to people and its not very complicated. Its VERY clear cut.

Yall have so much mixed info in this thread. Kirchoffs laws erase every question here.

As for the increase in power, yes capacity directly relates to power. Good batteries you can fee the power. Of course you have a power increased with the same controller, load and output, with more capacity.

Kinda why its calledd.... a " C-Rate.. ( capacity rate)." . cause it takes capacity into account.

spinningmagnets · Oct 29, 2021

I was under the impression that C-rate was named that way in relation to Current.

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