To BMS or not...

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To BMS or not...

Postby thebassman » 11 Apr 2020, 18:21

Building a new add on battery pack to run my ventilator + other things (camera, phone etc)

Currently I have a 4S2P configuration of 15AH Headway LiFe cells (30AH >12V). I just balance charge at 4s with my ISDT charger and run without any sort of BMS.

I'm getting a new ventilator soon, but this new vent requires closer to 24V DC in.

I ordered 8 more cells and am wanting to make a 30AH 8S2P. Should I get a new charger that can charge 8S or should I get a BMS?
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Re: To BMS or not...

Postby thebassman » 11 Apr 2020, 18:24

Forgot pictures.
Screen Shot 2020-04-11 at 10.07.44 AM.jpg

IMG_20190704_141536.jpg
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Re: To BMS or not...

Postby Irving » 11 Apr 2020, 18:36

get a PL8. BMS is the last thing you need, and the PL8 will do other chemistries inc lead better too.
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Re: To BMS or not...

Postby Burgerman » 11 Apr 2020, 19:02

24V in sounds useful. It means 2 things.
Your battery pack could now be plugged into your chair, in an emergency if the chair runs out of steam.
And it could be left connected all day as a long range system too if you want. That way the chair and vent run off a common large supply.

And your vent can be powered fron the chair in an emergency as well as the new pack. 2 options for breathing. Options are always good.

So adding a 24V out anderson to the chair, and the 30A pack may be a good idea.
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Re: To BMS or not...

Postby Burgerman » 11 Apr 2020, 19:33

You do realise that at 24V it will draw around half the current?

So you dont need more cells. You could just reconfigure your pack and keep the weight down. And have the same runtime as before.
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Re: To BMS or not...

Postby thebassman » 11 Apr 2020, 19:47

I know- i want even more energy! It will also power my hand warmer (glorified hair dryer) which unfortunately pulls lots of current.
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Re: To BMS or not...

Postby thebassman » 11 Apr 2020, 19:49

Definitely going to do this..

Burgerman wrote:24V in sounds useful. It means 2 things.
Your battery pack could now be plugged into your chair, in an emergency if the chair runs out of steam.
And it could be left connected all day as a long range system too if you want. That way the chair and vent run off a common large supply.

And your vent can be powered fron the chair in an emergency as well as the new pack. 2 options for breathing. Options are always good.

So adding a 24V out anderson to the chair, and the 30A pack may be a good idea.
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Re: To BMS or not...

Postby Burgerman » 11 Apr 2020, 22:12

When I was forced out in the cold, by my dog, I was considering similar. But as you say you need 10A to get a miserable 250 watts. And that will half the chairs range even if you don't stop every lampost. I used heated gloves instead. 12V motorcycle ones. Running on 24V they got a little toasty. So I ran them in series. Very effective. Cant feel legs or feet. But frozen fingers mean you cant actually drive the thing! How many Amps and is it enough? Whats the LiPo for in the cell photo?
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Re: To BMS or not...

Postby thebassman » 12 Apr 2020, 02:38

Used https://www.amazon.com/gp/product/B07GB ... UTF8&psc=1

Bit of thermal paste to a little heatsink and a fan in a 3d printed housing and voila. My fingers are pretty weak so I can’t use more efficient gloves. This works awesome but is still underpowered. Indoors it burns me but in colder weather its lukewarm/bordering on cold. Next iteration will use a temp sensor and maybe 2 ceramic heater blocks + Arduino with transistor/relay

Cells are headway 15ah lifepo4
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Re: To BMS or not...

Postby Burgerman » 12 Apr 2020, 06:51

I would have thought you need much much more than that. But if it works!

There looks to be a lipo pack behind the headways!
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Re: To BMS or not...

Postby Adverse Effects » 12 Apr 2020, 12:38

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Re: To BMS or not...

Postby Burgerman » 12 Apr 2020, 13:21

I dont know where to start!
OK what will you use that for?
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Re: To BMS or not...

Postby shirley_hkg » 12 Apr 2020, 15:07


Two points I noted.

① It's very wrong to use bare cells without holders. Short circuit if insulation worn out.

② With 30Ah , a decent BMS @100ma balance power will do, provided it can control charge volt ≤ 3.65V .
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Re: To BMS or not...

Postby Irving » 12 Apr 2020, 16:52


You can only use this if you have an accurate and well regulated CC/CV charger or power supply, eg the ZXD one or similar. It must generate a constant charge current up to n * 3.6v then switch to a constant n * 3.6v for the remaining time, where n is the number of cells.

That said, it is one of the better balancer designs and its not a BMS - it doesn't control charge or discharge rate, just aims to balance voltages across a pair of cells while the charger manages the charging profile. It uses a 'lossless' inductive technique, using energy from the higher voltage cell to charge the lower voltage cell in the pair - which then ripples energy up and down the chain, rather than just throwing energy away from high cells by heating up a balance resistor. It therefore doesn't require control of the charging current like a passive balancer would, as by pumping charge from higher to lower cells it modifies the overall charge current (through Kirchoff laws) as long as the external supply maintains an accurate and constant overall voltage. The balanced voltage of each cell will be the average (charge voltage/num of cells) +/- 30mV, but balancing only starts when there is at least 0.1v between adjacent cell voltages. This means a pack with reasonably balanced cells may never actively balance.

A review of the 3S version here...

Note that the commercial versions as sold on AliExpress will continue to balance if cells get out of balance during discharge. For a well designed/configured pack this shouldn't be a problem. If a cell drops more than 0.1v compared to its neighbour(s) the balancer will attempt to pull it up by draining the higher cell above or below it. While this uses energy from the higher cell, it's still returning 92%+ of this to the pack overall. There is a more sophisticated version of the chip that can enable/disable the balancer and also signals if balancing is taking place - this would allow a microcontroller to monitor cell voltages and control balancing more accurately but I've nt seen anything based on that version.
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Re: To BMS or not...

Postby Burgerman » 12 Apr 2020, 18:59

Plus you would have to charge at below 1A or the charger will allow the high cell to keep climbing as if no balancer exists. And ruin the cell. And 30mv isnt balanced! And if it attempts to balance below 3.50V it will unbalance tha pack by around 1Ah per hour... Based on its 30mA accuracy. It might be OK in addition to a BMS or balance charger, on a lithium ion pack, or lithium polymer, but not on LiFePO4.
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Re: To BMS or not...

Postby stevelawiw » 21 Jun 2020, 11:25

I'm currently trying to find some @80ah cells that gives me a little more vertical space in the battery box to fit Lenny's CAN system into my chair, and one of the suppliers I'm talking too asked me if there was any thing else he could help with, so I asked if he could supply an active balancer that I could charge 16s LIFEP04 at 5.5x volts and he came up with this

https://www.aliexpress.com/item/4000575904733.html

Manual
https://preview-static.clewm.net/cli/view-doc/view.html?url=https%3A%2F%2Fncstatic.clewm.net%2Frsrc%2F2020%2F0226%2F09%2F067c648caa02281d6203a33c34130b68.pdf&filename=Active%20balancer%20Specification%20and%20operation%20manual(JK-B1A24S)(JK-B2A24S)V4.1.pdf&download=1&code=qj6Mvz9&open=62dc79dd8f404c9ba181b67b317552f2&_=1592677764018&frompc=false

What do you all think, teamed up with a ZXD would this work? There is also a 2a version
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Re: To BMS or not...

Postby steves1977uk » 21 Jun 2020, 11:42

stevelawiw wrote:so I asked if he could supply an active balancer that I could charge 16s LIFEP04 at 5.5x volts and he came up with this


I hope you mean 3.3x volts! :shock: :lol:

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Re: To BMS or not...

Postby stevelawiw » 21 Jun 2020, 11:52

I hope you mean 3.3x volts! :shock: :lol:


Ha ha! Ooops yeah
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Re: To BMS or not...

Postby shirley_hkg » 21 Jun 2020, 15:54


Never use active balancer on LiFePO4. :fencing
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Re: To BMS or not...

Postby terry2 » 21 Jun 2020, 16:25

I would like a BMS with bluetooth.
Not to charge them or anything.

I can keep an eye on the power as I go along.
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Re: To BMS or not...

Postby LROBBINS » 21 Jun 2020, 20:09

Shirley,

Shirley,

I think that the intent is to use the balancer (NOT a BMS) only during charge. This one is "active" in the sense that it transfers charge from the highest cell to the lowest rather than just dumping charge from the high cell into a resistor. Doesn't the PL8 also do that? I would certainly like it if you, and Burgerman and Irving could read through the pdf for this one to see if mightn't be suitable for us. Ciao, Lenny
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Re: To BMS or not...

Postby Burgerman » 21 Jun 2020, 21:34

I think he is refering to the fact that it attempts to balance not only at the very top, but during charge, during the day etc. Which means it will unbalance the pack severely in a misguided attempt to balance by voltage. It assumes that the highest voltage cell is the fullest. Well thats not true with LiFePO4 even if the balancer is accurate to 0.0001V per cell. Frequently theres 5 or 10mV difference. And the cell that is the lowest may in fact have the exact same charge in Ah as the cell which is the highest. Balance should only occure above 3.50V per cell. Above the natural cell full voltage.

One of the things I had to get them to change before the PL8 was any good with large LiFe packs was to add BALANCE ONLY AT CV, and BALANCE ONLY ABOVE 3.4 and ABOVE 3.5V into the system firmware. Previously it only had start balance at 3.1, 3.2, 3.3V which put our packs maybe 1 to 2 Ah OUT of balance. Requiring a few extra hours to rebalance once full.

So its useless for Lithium ion phosphate. It will be trying to balance by voltage, all day long... And taking many Ah out of parfectly balanced cells because it THINkS they have a higher voltage... I wish these electronics wizards understood batteries or physics. Theres millions of such things out there.

From the manual:
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Re: To BMS or not...

Postby Burgerman » 21 Jun 2020, 21:50

I would like a BMS with bluetooth.
Not to charge them or anything.

I can keep an eye on the power as I go along.


The purpose of a BMS is twofold.
1. To cut off power at every inopertune incorrect moment such as when trying to climb a curb off a busy road. In an attempt to protect a too small battery back from damage. You dont want that...

2. To balance and try to prevent a dumb charger from destroying your battery (which thay are crap at).

What you are looking for is a battery monitor. Which you have already in your joystick. And if you fit the right sized pack wont ever move from full before YOU run out of steam. When it shows half the battery is done, ots fully done and you must stop. Or just fit a voltmeter Dont go past 3.1v per cell. So 24V. ever.
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Re: To BMS or not...

Postby shirley_hkg » 22 Jun 2020, 03:03

Burgerman wrote:. Balance should only occure above 3.50V per cell. Above the natural cell full voltage.



Lenny , John is right again.

However , this balancer lacks the most important control of { only balance @≥3.5V}

My experience is BMS will unbalance a pack to some extent, as it drains the cells to a presumed volt and stop.
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Re: To BMS or not...

Postby shirley_hkg » 22 Jun 2020, 03:36


Reliable BMS with programmerability and Bluetooth are common now. However , they all balance at tiny 100-200 mA. That's ok with most users , because they have options to go (high voltage-low amp hours) .

Our packs are ultra large @ ampereage.

Balance chips claims @360 -- 1500mA . It starts @3.6V. Certainly needs forced ventilation.

A programmable BMS plus this chips may work.

Cells won't be over charged, with additional balance power.

Chips @€2.00.

With a carefully fine tuned ZXD2400 of course. :lol:
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Re: To BMS or not...

Postby snaker » 22 Jun 2020, 03:50

I see the way PL8 balances the pack is different from BMS. PL8 adjusts charging currents going into each cell. E.g 95% of actual charging current to cell1, 100% to cell2, 80% to cell3 ... And it does this job smoothly in real time. While BMS has 2 ways: Using higher volt cells to charge lower volt cells Or/And Discharging too high volt cells (>3.58V) to dump loads. Am I wrong in anything?
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Re: To BMS or not...

Postby shirley_hkg » 22 Jun 2020, 04:03

PL8 balances by dumping loads too,
just it can do 1,000mA max. Can't you hear its fans running ?
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Re: To BMS or not...

Postby snaker » 22 Jun 2020, 04:04

@Shirley: Can you recommend a good programmable and BT BMS? A friend is investing a solar system. He is using a Dialy BMS to charge a lifepo4 24V pack. I worry for him. That BMS cuts off at 3.75V, is it too high? The cells might be dead or burn.
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Re: To BMS or not...

Postby snaker » 22 Jun 2020, 04:13

shirley_hkg wrote:PL8 balances by dumping loads too,
just it can do 1,000mA max. Can't you hear its fans running ?

No. During balancing time, it is quite, fans do not run.
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Re: To BMS or not...

Postby shirley_hkg » 22 Jun 2020, 04:24


3.75 is over volt .

No experience but this seller is the pioneer in programmable bms.

https://m.tb.cn/h.VpOTu6M?sm=83e4bb

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