To BMS or not...

[snaker]

Yes, PL8 insight sounds not simple

Programing the microcontroller to compute those volts is not too hard. But I admire the guys in Revolectrix who found out this computing. Is the trick of stopping several times a second for measuring a PL8's 'invention'? Or is it a common method in electronics?

Sorry for my bad English, my question in my previous post was unclear. Let me correct it

My question is much simple: how does PL8/BMS read cell votls through balance wires?

I know for a single cell, it might be simple because a microcontroller is able to read a low voltage (e.g < 5v). For reading multiple cells connected in serial, that might require using voltage level shifters to drop volts down to values that the microcontroller can accept. By this way, in the worst case, it must shift a 29V (3.65V x 8) down to say 5V. The distance between 29V and 5V is large, so the read cell volt might not be accurate enough. So I thought PL8/BMS might be using other way to read cell volts.

[Burgerman]

Dont know.

[ex-Gooserider]

I don't know for sure, but my first guess is that they are only reading the difference between adjacent cells, so each sensor is only reading a low value in relation to the cell below it, not in reference to ground...

ex-Gooserider

[Burgerman]

Does this help you figure out how?

[snaker]

I tried to search on google. There is no trivial way to read/monitor cell volts in a pack. This job requires some special chips.

[Burgerman]

Well there is. You can just measure across the 2 wires going to each cell. But you cant do it while charging or discharging. Esp if those things are pulse width loads or have breaks in it every few ms.

[snaker]

You can just measure across the 2 wires going to each cell.

How can I do that programmatically? Actually a microcontroller can directly read an input voltage but it requires using a common GND. So to measure say cell #2, I have to measure a total (cell #1 + cell #2). There is no direct way to measure cell #2 alone.

[Burgerman]

You need a bunch of seperate analog digital conversion chips to read volts, on seperate floating voltage supply chips. Those are not cheap.

Added.
Say, 8 of these powered from the battery full voltage, plus 8 voltage reading analog to digital converters - one per cell to read voltage.
https://uk.rs-online.com/web/p/isolated ... s/2932757/ These are way bigger 5 watt devices, more than needed. Look for much lower powered ones.

[shirley_hkg]

Can't hold back. Will get a set to try it out. Going to retire the following week , and will have plenty of time.

[Burgerman]

Set of what?
What are you actually doing with all that stuff in the picture?

Puxxled!

[snaker]

@shirley_hk: does the manufacturer of this BMS release any documentations in English?

[shirley_hkg]

Not yet, but it's simple to use. Other than ↑&↓, just one button to navigate through.

It's the simplest and viable BMS I want.

Use contactor, not MOS, for output so less trouble.

[snaker]

Does it have any docs that describe the communication to remote control or monitor cells (like the PL8's docs in the attached file below)? It's Ok in Chinese, I will read it through google translate

[stevelawiw]

Hi Shirley, the active balancer you linked to earlier in this thread, could you please post a new link?(none of the existing links work anymore) You mentioned they were developing an English version of it, and I wanted to check if they have done this yet.
Thanks

[11ydy]

Does it have any docs that describe the communication to remote control or monitor cells (like the PL8's docs in the attached file below)? It's Ok in Chinese, I will read it through google translate

In China, there are many smart BMS with Bluetooth or LCD display. You can connect the protection board through mobile app Bluetooth, view the battery pack information on the mobile phone, and customize various protection options, such as battery pack and single cell charging cutoff voltage, discharge cutoff voltage, etc

More people use this model
https://item.taobao.com/item.htm?spm=a2 ... =17#detail

This newly developed, active equalization current 2A
https://item.taobao.com/item.htm?spm=a2 ... =17#detail

[Burgerman]

Can they be configured to only balance above say 3.55V though? Or they are useless for large LiFePO4 cells.

[shirley_hkg]

https://m.tb.cn/h.4ZFfv1u?sm=185037

[shirley_hkg]

Bought one set to try it out.

[stevelawiw]

That's great Shirley I hope your test goes well.
As Burgerman says the big thing is being able to control the voltage at which the balancer kicks in, and then to divert the current to the lowest cell.
And a bluetooth connection to a device running a monitoring app would be nice also.

Oh and an English speaking website Would be nice also

[terry2]

I'm in contact with these guys. https://overkillsolar.com/product/bms-100a-8s-lifepo4/

They have a Github https://github.com/FurTrader/OverkillSolarBMS

Every setting can be done via bluetooth.

https://www.youtube.com/watch?v=h2ftiGI ... larChannel

Going to chat with them a bit more.
But I think I will get one as a backup, if it works with my cells(which it should.

Anyone want new 248ah cells CHEAP?

https://www.alibaba.com/product-detail/ ... 43518PyX5z

[shirley_hkg]

Looks like it is doing what it claims . Can set the balance kick-in volt .

Accurate too. All cells volt are indifferent , as I hooked up PL8 and showed the same .

Balance is kind of slow as it is doing ONE cell at a time , whereas PL8 is doing all 8 cells simultaneously .



Took different Ah out from several cells to make battery pack out of balance and recharge now. Log it with PL8 to see how good it will do.

[Burgerman]

Can you set accurate termination current?
Can you tell it not to end even if termination current is reached unless balance current is low and all cells balanced?
Can you set a CV time limit after balance is achieved at low mA? Say balance achieved plus 20 mins, or termination current reached?
Can you set it to not bounce cells up and down by somehow moderating the chargers current?

On a big pack with 8S with a low cell, doing one cell at a time will take it forever.

Also does it speak english because I cannot understand the little pictures...

[shirley_hkg]

The battery :

It's a 70Ah pouch cells , with loose, messy connections.
It's pre-balanced by PL8 , and is removed 60Ah out if it , and it is also deliberately unbalanced by taken 3 to 5 Ah amongst cells 3 to 8 as well.

It is an every way out of balance pack now .

[shirley_hkg]

I set charge volt @3.48V and balance starts @3.39V deliberately, to see how it performs at low charge volt.

Took 4 hours to rebalance the pack .

No cell experiences higher than 3.48V throughout the charge.

Cell volts are solidly packed together , and need 60 mins to drop to 3.43V for BMS to reset over-volt protection and to start charging again.

[Burgerman]

Its bouncing cells up and down though. Cant you set it up so the overvoltge/release/restart charge is much closer together?

It appears to add a mass of complication!
And large components.


If a mosfet fails shorted, trust me they do! All those balance wires melt their insulation and burn each other which then also short out. The source of many DIY EV cars and bikes on the EV forums. And why at least one of the pro car EV converters, will not allow a BMS equipped vehicle inside his workshop.

So CAREFUL wire routing so any one burning out will not cause it to short out others aound it would seem wise. Maybe safer to run each pvc balance wire (for physical protection) inside a silicone tube for heat protection. Then it wont burn your house down.

[steves1977uk]

That's very artistic Shirley! What is the balance current on that BMS?

Steve

[Burgerman]

Also, that looks better than it really is. Set PL8 to 4 digits, that turns off all the averaging and shows reality.

See here:

[steves1977uk]

You can't set it to 4 digits in normal mode BM.

Steve

[stevelawiw]

If a mosfet fails shorted, trust me they do! All those balance wires melt their insulation and burn each other which then also short out.

Good reason to put a fuse in every balance wire? (I have always done this)
Shirley, excelent work! Are they working on an english version?

With the shortage of PL8's it would be really nice to have something like this that could be (at least) a backup method of charging

[Burgerman]

You can't set it to 4 digits in normal mode BM.
Steve

Shirley can I think.