Low cost BMS

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rhills
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Re: Low cost BMS

Post by rhills »

coulomb wrote:
Mon, 03 Feb 2020, 06:58
rhills wrote:
Sun, 02 Feb 2020, 14:21
the LiPO cells I've installed...
Rob, do you really mean LiPo (as in Lithium Polymer), or LFP (Lithium Iron Phosphate, LiFePO₄)? It makes a big difference.

LFP is nominally 3.2, and should not go much beyond 3.6 V.
Nearly all other lithium chemistries are nominally 3.7 or 3.75 V, and should not go beyond 4.2 V.
You're quite correct @coulomb, I was careless in my nomenclature. I meant LiFeP04 :-(
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Re: Low cost BMS

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T1 Terry wrote:
Mon, 03 Feb 2020, 08:51
They most definitely don't start bypassing till the cell reaches 3.8v and stop bypassing at 3.6v. These units shift 5 amps with a cell voltage of 3.8v or higher
The other cell top units start balancing at 3.5V but not full current and work up to the full bypass current (in my case 700mA) which I guess is done using a zener diode set to pass at 3.5V and only the voltage over 3.5 goes across the balance resistor.
Eg the cell is at 3.8V and it's balancing 700mA.
3.8V - 3.5V = 0.3V
0.3V/0.7A = 0.428 Ohms

So at 3.6V its
3.6V - 3.5V = 0.1V
0.1V/0.428 Ohms = 0.234A

And for the 5A balance at 3.8V
0.3V/5A = 0.06 Ohms
Then 3.6v would be
0.1V/0.06 Ohms = 1.667A

Now it could be the units you have don't start to pass until they avalanche at 3.8V and then burn off until they get back down below 3.5v. So they are really all or nothing rather than a small amount of bypass at first and working up to full current only if the cell is a long way out of balance, and because it doesn't do small balancing it will, when it does balance it has alot of energy to dissipate as heat in a short period of time.
Doing a small amount of bypass will add the same amount of heat energy but spread out over hours not minutes so it won't get as hot.

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weber
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Re: Low cost BMS

Post by weber »

@T1 Terry, It's possible that some old models of the EV Power CMUs started bypassing at a higher voltage, but the present-day specification clearly states that it starts bypassing at 3.5 V.

But I agree that the present-day EV Power spec of not turning on the alarm until 4.0 V leaves much to be desired, as does its use of the cell interconnecting links as a heatsink. As I've said, our CMUs alarm at 3.65 V and don't use the cell or interconnects as a heatsink.

Although I was unwilling to take any of my new aluminium-case cells beyond 3.65 V, I have been holding one of my 10.5-year-old plastic-case 40 Ah Sky Energy cells at 4.0 V for the past 5 hours. Admittedly I haven't thermally insulated it, but I am comparing its temperature to an identical but unconnected cell standing beside it. I charged both cells to 3.8 V last night (it felt very wicked :twisted: ) then I let them rest over night. This morning around 9:00 am I checked that they were both at the same temperature, and connected one of them to a lab supply set to 4.00 V while leaving the other one unconnected.

The current into the connected cell was initially about 0.5 A, but dropped rapidly to near zero within a few minutes.

After 4 hours I measured the current at 1.5 mA ± 0.2 mA and measured the two cell temperatures as identical to within ±0.05 °C (the resolution of my IR thermometer). And at 5 hours the situation was the same.

So you are simply wrong about high voltage alone causing heating of cells. However, that constant 1.5 mA does represent continual slow damage being done to the cell.
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Re: Low cost BMS

Post by brendon_m »

weber wrote:
Mon, 03 Feb 2020, 12:16
But I agree that the present-day EV Power spec of not turning on the alarm until 4.0 V leaves much to be desired
The other big downside is not being able to check an individual cell.

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Re: Low cost BMS

Post by T1 Terry »

weber wrote:
Mon, 03 Feb 2020, 12:16
@T1 Terry, It's possible that some old models of the EV Power CMUs started bypassing at a higher voltage, but the present-day specification clearly states that it starts bypassing at 3.5 V.

But I agree that the present-day EV Power spec of not turning on the alarm until 4.0 V leaves much to be desired, as does its use of the cell interconnecting links as a heatsink. As I've said, our CMUs alarm at 3.65 V and don't use the cell or interconnects as a heatsink.

Although I was unwilling to take any of my new aluminium-case cells beyond 3.65 V, I have been holding one of my 10.5-year-old plastic-case 40 Ah Sky Energy cells at 4.0 V for the past 5 hours. Admittedly I haven't thermally insulated it, but I am comparing its temperature to an identical but unconnected cell standing beside it. I charged both cells to 3.8 V last night (it felt very wicked :twisted: ) then I let them rest over night. This morning around 9:00 am I checked that they were both at the same temperature, and connected one of them to a lab supply set to 4.00 V while leaving the other one unconnected.

The current into the connected cell was initially about 0.5 A, but dropped rapidly to near zero within a few minutes.

After 4 hours I measured the current at 1.5 mA ± 0.2 mA and measured the two cell temperatures as identical to within ±0.05 °C (the resolution of my IR thermometer). And at 5 hours the situation was the same.

So you are simply wrong about high voltage alone causing heating of cells. However, that constant 1.5 mA does represent continual slow damage being done to the cell.
Interesting, no gassing or cell swelling? When the charging is disconnected and a few amp load connected across the cell, how long does it remain above 3.6v? I'm just wondering if the internal resistance is so high that @ 4v there is very little current flowing into the cell that would result in the movement of the lithium ions
or
is there no lithium ions left to move out of the LiFe coating into the electrolyte to cause the over saturation? *

I must admit, I've never tried it on an old cell to test the temperature increase, but I know the charging voltage can go very high before the current starts to flow. Sometimes once the current does start to flow the voltage drops back, but that is part of a different contentious issue :lol:

*As far as I understand it, the voltage only rises (as does the temperature) when the lithium ions become over saturated in the electrolyte because there is no available spots in the graphite to accept them.

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Re: Low cost BMS

Post by weber »

T1 Terry wrote:
Mon, 03 Feb 2020, 15:34
Interesting, no gassing or cell swelling?
Thanks for reminding me. I was interested in this question myself. I deliberately chose a cell that was not already noticeably swollen. I measured its thickness with dial calipers last night before topping it up, as close to the middle of its broad faces as I could get, and wrote it down. 46.8 mm. The nominal cell thickness is 46 mm.

I just went and measured it now. 46.7 mm. No swelling.

I certainly expect that one of the things that 1.5 mA is doing is generating gas, but it is clearly not enough to have a measurable effect in 5 hours.
When the charging is disconnected and a few amp load connected across the cell, how long does it remain above 3.6v?
Only a few [edit: tens of] seconds. But, as expected, after I disconnect the load (a headlight bulb, which drew about 2 A) the voltage slowly rises above 3.6 V again.

I will now try to measure what the final current is after the cell has been held at 3.6 V for many hours.
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Re: Low cost BMS

Post by T1 Terry »


"When the charging is disconnected and a few amp load connected across the cell, how long does it remain above 3.6v?
Only a few [edit: tens of] seconds. But, as expected, after I disconnect the load (a headlight bulb) the voltage slowly rises above 3.6 V again."

Sounds like a high internal resistance cell, possibly has serious crap broken down from the electrolyte coating the graphite plates. The result is a very high differential between charging voltage where current is accepted into the cell compared to the discharge voltage where current is drawn from the cell.
Back to the subject though, it does put into question my first assumption that it was the high voltage in the electrolyte causing the heating but does lean towards the idea of an excess of lithium ions with no path out of the electrolyte causing the problem ... but just a personal theory really because I don't have a clue as to how to prove or disprove the theory. I do know serious over/under voltage of the cell does cause the partial separation of the electrolyte and coating of the plates, the more often this occurs the greater the coating and therefore the greater to resistance to electron flow in or out of the plates.

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Re: Low cost BMS

Post by weber »

Yes. As I tried to explain earlier: Both high voltage and high temperature accelerate the ageing processes, one of which is an increase in the thickness of the otherwise beneficial SEI layer, which causes an increase in internal resistance. High voltage alone doesn't cause heating. Current causes heating. But high voltage accelerates these destructive reactions directly. It doesn't have to do it via heating.

As best I can tell, the final current after holding the nominally 40 Ah cell at 3.6 V for many hours is somewhere between 0 mA and 0.2 mA. That can be compared with the 1.5 mA ±0.2 mA I measured at 4.0 V. [Edit: Corrected "3.8 V" to "4.0 V".]
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Re: Low cost BMS

Post by T1 Terry »

As a matter of interest, what voltage is required for the current to increase to an amount that would be regarded a charging current, say above 1 amp or more? Once this threshold has been reached, if the voltage is again dialled back to 3.8v, is the current still only 1.5mA?

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Re: Low cost BMS

Post by weber »

T1 Terry wrote:
Tue, 04 Feb 2020, 12:48
As a matter of interest, what voltage is required for the current to increase to an amount that would be regarded a charging current, say above 1 amp or more? Once this threshold has been reached, if the voltage is again dialled back to 3.8v, is the current still only 1.5mA?
Sorry Terry, but I've already discharged the cell to about 90% SoC to put it back in storage until it can go into my home energy system. And I need to work on other things. But you reminded me that it was actually 4.0 V at which I measured the 1.5 mA. I have corrected the above post.

At the end of the discharge to approximately 90% SoC, the headlight bulb was still drawing 1.99 A even though the voltage was now 3.269 V (which shows the approximately constant-current nature of incandescent bulbs). On disconnecting the bulb the voltage rose to 3.280 V within two updates of the multimeter (4 updates per second). The third update showed 3.281 V. When rested for a few hours, I expect it will have risen to the magic number of 3.333 V.

That gives an internal resistance of (3.280 - 3.269)/1.99 = 5.5 mΩ. That's about double what it was 7 years ago, as I remember the battery of 218 cells would sag from 720 V to 600 V with a current of 200 A (5C, 120 kW) when we floored the accelerator in the MX-5. (720-600)/200/218 = 2.75 mΩ. Of course internal resistance is highly dependent on temperature. Today the cell was at 26 °C.
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