So reversing a shunt is almost as good as a BMS?

Dzl

Perpetual Newbie & Unofficial Forum Librarian
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Moderator
She's charging them in parallel, due to lack of BMS.
Then discharging them in series while scooting across town.
I think that completely solves all the technical problems.
What sort of charger is being used? I thought it was a 12v charger
 

SherylinRM

GO GO JUICE BOOMER ZOOMER MAKER
Have you thought about possibly posting some photos of your build and how you carry it on your scooter? I think it's fascinating what you have accomplished and the care you have put forth to protect your cells without a BMS.
Thanks :)
Well the wooden boxes I have them in now is something I threw together and is off by as much as an eighth of an inch in places.
But once I make the second set I might take some pictures. :)

They will drift. Paralleling cells without current flow transfers almost no charge. Current flows when there's a voltage difference. When you parallel them, their voltages almost instantly equalize. There is an initial surge and then it drops to low milli or micro amps. If you do this for weeks, it can be meaningful, but doing it for 2.75 hr is just a placebo.

Once you series them and subject them to a charge/discharge, they will diverge again.
Actually, I think I figured out a secret about paralleling them. That was the second time I did this. And the last time it went pretty fast as well.

What I did was put them in order from least full to most full. This seems to change the voltages of the weaker to mosre and the ones with more to less a lot faster.
For example. the last time I charged them [in series by the way] it looked like this:

At 8 pm [after a 2 hour rest] it is the following.
November 3rd 2020

A 3.338
B 3.605
C 3.351
D 4.04
E 3.338
F 3.343
G 3.337
H 3.343
A to D 14.34
E to H 13.38
Total 27.73

At 9 pm I paralleled the cells in the following order
G, A, E, F, H, C, B, D.

At 11:48 pm all cells are at 3.349 volts.
VERY cool.
At 5:30 am each cell is 3.35 volts after 6 hours of resting.


So as you can see. They all pretty much changed. And even today after a few more hours they were still holding steady at 3.35 volts.

So I think that may be the secret to a fast parallel stabilization of the cells.

Once I sleep and get some rest I will test them again though to see if what you have said is correct.
However, even the last time I did this they remained equal UNTIL I started to charge them again.
But while discharging they remained all equal.
It was only when charging that they diverged.

So later I will see how it went. This is only the second time I tried to charge them to some sort of fullness. :)
 

SherylinRM

GO GO JUICE BOOMER ZOOMER MAKER
Yah. I saw @Hedges suggest parallel, and she agreed that it's a good idea, but I saw now post indicating that's what she was doing.
I had hoped to use a power supply that I have to maybe try to charge them when in parallel, but I found out it does not work so I threw it in the garbage and went with the 24 volt series charging with the lead acid gel charger instead.
 

snoobler

Solar Honey Badger
However, even the last time I did this they remained equal UNTIL I started to charge them again.
But while discharging they remained all equal.
It was only when charging that they diverged.

This is exactly what I'm talking about. You're equalizing voltage, but you're not equalizing charge. That's why they diverge again as you get into the upper leg. They don't diverge when you discharge because you're in the flat part of the curve.

You're not hurting anything, but you're not really accomplishing anything either. It's a placebo effect.

I recommend not paralleling cells that have more than 0.2V different between them. Here's why - connecting 4.04 to 3.605 is a 0.435V difference. Assuming a 0.2mΩ resistance, when you parallel them, you get a 0.435V/.0002Ω = 2160A surge. This occurs for a fraction of a second as the two voltages meet somewhere in the middle. Current flow slows to a tiny trickle as the voltages equalize. Very little capacity is passed between the cells.

Balancing isn't about getting all the voltages the same. It's about getting them all to the same state of charge. As we have established multiple times, voltage is not a reliable way to establish state of charge. When you're top balancing, you're in the upper leg of the charge curve, and voltage is a good indication of similar states of charge near peak voltages well above typical operating voltages.

Technically, the one good thing about paralleling them in your situation is it serves to pull the 4.04V cell down. Please try to avoid pushing cells that high.
 

SherylinRM

GO GO JUICE BOOMER ZOOMER MAKER
Technically, the one good thing about paralleling them in your situation is it serves to pull the 4.04V cell down. Please try to avoid pushing cells that high.
And as I have said in numerous threads that you have read.
I do not have a BMS or relays or even a proper charger.
So I can't "avoid pushing the cells that high" at he moment.
You should know this by now.

It is obvious from the comments that many here never actually read what I say.
It might be wise to just no longer bother.

I mean why bother writing out in detail things if only two or three are going to bother to even read what is written.
 
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snoobler

Solar Honey Badger
And as I have said in numerous threads that you have read.
I do not have a BMS or relays or even a proper charger.
So I can't "avoid pushing the cells that high" at he moment.
You should know this by now.

It is obvious from the comments that many here never actually read what I say.
It might be wise to just no longer bother.

I mean why bother writing out in detail things if only two or three are going to bother to even read what is written.

I do read your posts, and I've commented regularly including both threads you started about your 12V power supply. I even re-read all your posts in this thread to find evidence of you parallel charging when @Hedges indicated that what you were doing because I couldn't believe how I missed that.

Don't be a victim here. You CAN avoid pushing them that high. It's a choice. You simply monitor your cells frequently and stop charging when one exceeds 3.65.

I have a moral obligation to caution someone when they're engaging in bad practices especially when I've cautioned them before. you may not like it, but it is done in the spirit of YOUR best interest, not mine.
 
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Hedges

I See Electromagnetic Fields!
I also noted the lack of a statement SherylinRM was charging while parallel. Most of the effort was reconnecting the cells in parallel.
Top-balancing them during each charge cycle seemed like an easy solution, but she didn't have working equipment to implement that.

I have a variety of lab equipment and electrical/electronic parts so would have tried to find a way. Maybe a dumb 6V battery charger and relay to disconnect when a target voltage is reached (requires electronics.) Or mechanical timer to deliver limited number of Ah, then watch it like a hawk toward the end of charging. But since I do have bench supplies with voltage/current limit, I would use that instead.
 

snoobler

Solar Honey Badger
I do not have, and have never had a 12 volt power supply.

You indicated you had this power supply:


Describing it as a 12V supply isn't outlandish.
 

SherylinRM

GO GO JUICE BOOMER ZOOMER MAKER
You indicated you had this power supply:


Describing it as a 12V supply isn't outlandish.
Ah ok, got it now. I never really considered that as a charger though. And as it is broken I never used it.
Got it now. I understand the confusion. :)
 
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