To BMS or not...

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Re: To BMS or not...

Postby Burgerman » 27 Sep 2020, 12:06

The PL8 does not charge continually. Even though it appears to do so. It measures several lots of voltages.

During the breaks several times per second it reads all the cell voltages.
During the charge periods it also reads the (higher incorrect) cell volts - due to cell resistance this is higher. It uses the difference in readings to give cell resistance.

During this time it also reads the difference between unloaded cell volts and charge cable volts, so it can read the charge cable resistance... And so it has:

Peak charge volts (higher than the average because of the gaps in charging) and the pulsewidth used to control voltage. Thats 3 different battery volts.
Averaged charge voltage, measured during charge periods
Battery voltage measured while in the unloaded gaps...
Battery voltage while on charge load, also averaged to remove the gaps... And displayed.
Measure cell voltages that are actual during the gaps that are displayed on screen. So cell volts added together may not = battery voltage displayed.

Theres lots of measurement and corrections and averaging and peak values known. Its not so simple.

At least thats what I was told by the beta developer guy I was emailing...

A bms? God knows.
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Re: To BMS or not...

Postby snaker » 28 Sep 2020, 02:45

Yes, PL8 insight sounds not simple :shock:

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 :oops:

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.
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Re: To BMS or not...

Postby Burgerman » 28 Sep 2020, 05:11

Dont know.
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Re: To BMS or not...

Postby ex-Gooserider » 29 Sep 2020, 01:24

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...

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Re: To BMS or not...

Postby Burgerman » 29 Sep 2020, 02:06

Does this help you figure out how?
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Re: To BMS or not...

Postby snaker » 30 Sep 2020, 02:59

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.
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Re: To BMS or not...

Postby Burgerman » 30 Sep 2020, 03:51

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.
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Re: To BMS or not...

Postby snaker » 30 Sep 2020, 04:18

Burgerman wrote: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.
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Re: To BMS or not...

Postby Burgerman » 30 Sep 2020, 04:58

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.
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Re: To BMS or not...

Postby shirley_hkg » 30 Sep 2020, 08:12


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

Postby Burgerman » 30 Sep 2020, 09:33

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

Puxxled! czy :eh:
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Re: To BMS or not...

Postby snaker » 05 Oct 2020, 01:44

@shirley_hk: does the manufacturer of this BMS release any documentations in English?
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Re: To BMS or not...

Postby shirley_hkg » 05 Oct 2020, 03:20


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.
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Re: To BMS or not...

Postby snaker » 05 Oct 2020, 07:54

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 :mrgreen:
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Re: To BMS or not...

Postby stevelawiw » 05 Oct 2020, 12:29

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
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Re: To BMS or not...

Postby 11ydy » 05 Oct 2020, 15:17

snaker wrote: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 :mrgreen:

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
QQ图片20201005220639.jpg




This newly developed, active equalization current 2A
https://item.taobao.com/item.htm?spm=a2 ... =17#detail
微信图片_20201005221338.jpg
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Re: To BMS or not...

Postby Burgerman » 05 Oct 2020, 16:02

Can they be configured to only balance above say 3.55V though? Or they are useless for large LiFePO4 cells.
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Re: To BMS or not...

Postby shirley_hkg » 05 Oct 2020, 16:11

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Re: To BMS or not...

Postby shirley_hkg » 23 Oct 2020, 02:46


Bought one set to try it out.
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Re: To BMS or not...

Postby stevelawiw » 23 Oct 2020, 15:27

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 :shifty:
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Re: To BMS or not...

Postby terry2 » 05 Nov 2020, 14:22

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
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Re: To BMS or not...

Postby shirley_hkg » 13 Nov 2020, 03:09

shirley_hkg wrote:

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.
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Re: To BMS or not...

Postby Burgerman » 13 Nov 2020, 03:15

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... :oops:
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Re: To BMS or not...

Postby shirley_hkg » 13 Nov 2020, 10:51



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 .
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Re: To BMS or not...

Postby shirley_hkg » 13 Nov 2020, 11:06


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.
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Re: To BMS or not...

Postby Burgerman » 13 Nov 2020, 11:11

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.
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Re: To BMS or not...

Postby steves1977uk » 13 Nov 2020, 11:19

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

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Re: To BMS or not...

Postby Burgerman » 13 Nov 2020, 11:19

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

See here:
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Re: To BMS or not...

Postby steves1977uk » 13 Nov 2020, 11:29

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

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Re: To BMS or not...

Postby stevelawiw » 13 Nov 2020, 12:30

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 :thumbup:
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