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Charge without BMS!!
#1
Now I am sure the title of this thread will get some people shuddering at even the thought of charging a powerwall without some form of BMS but I want to ask this question purely out of curiosity.

If I wanted to build a 12v system (I'm leaning towards 3s) and each pack had it's own CV/CC charging board limited to 4v with each 18650 cell fused would this 1.) be a concept that would work and 2.) how easy would it be to implement?

Your thoughts and views would be greatly appreciated.
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#2
well... once over 4.2V or below 2.8 it will get dangereous, as far as i read myself into this... a BMS is there to prevent fire hazards.... fuzes won't help for to high voltage...
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#3
One function of a BMS is low voltage (not just high voltage).   When you have cells in series, its possible a failing cells (in the series) go all the way to negative voltage as you draw power from the overall battery.       For example  Bat1=3.0v, Bat2=-0.3v, Bat3=3.0v ..   e.g. overall battery is a little less than 6 volts.     A BMS will protect against this. 

I ran my original 7s battery bank without a BMS for about 6 months - thinking "I'll just measure things with a volt meter once a week".   Not a good idea as I found out eventually that I had some cells going down to 2.5v'ish and then bouncing up to 3.2 in the morning - when I thought all were cutting off at 3.3'ish.   If I had continued it 'could' have spiraled out of control. A BMS would have stopped that in it's tracks.
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#4
Like offgridinthecity, I ran mine for 3 months without BMS (when I accidently broke it) and was fine. In fact it didn't even deviate much the entire time, only by 50mV at most. But it was dangerous to say the least. Discharging it low isn't too dangerous, just the risk of ruining your batteries. The problem is if a pack fails and deviates by a lot and you charge it the next day.

So let's say you are putting a 3s pack. The max voltage is 4.2V and let's say you put in a safety by only charging it to 4.1V, that means you have 4.1V + 4.1V + 4.1V = 12.3V. You set your CC/CV to 12.3V. The problem comes if one pack suddenly fails, ie. one of the cells starts to create a self-discharge situation, pulling down the pack. Let's say one pack suddenly develops a short so it's at 0V. The CC/CV doesn't know that one cell failed, it just wants to charge up to 12.3V. Now you try to charge it to 12.3V but since one cell is shorted, you end up with 6.15V + 6.15V + 0V = 12.3V. That's a KABOOM scenario where your cells catch on fire.
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#5
If you run one charger per pack that is basically what a BMS do. It makes sure each pack in the serie have its own controlled voltage and current limits (kind of)

Its cheaper and easier just to add a decent BMS...
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#6
(02-18-2020, 08:45 PM)daromer Wrote: If you run one charger per pack that is basically what a BMS do. It makes sure each pack in the serie have its own controlled voltage and current limits (kind of)

Its cheaper and easier just to add a decent BMS...


Ah I misread it! Though it was one charger for the entire packs. Then yes it would technically work. Then I guess the limitation would be back to the low voltage cutoff again.
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#7
It also depends on the health of the cells. If they are brand new or lightly used and have 95+% of original capacity, then you can get away with not having a bms for awhile. But if they are abused, old, or <90% of original capacity, it's a really good idea to use a bms, even a cheap one, to keep things in line. 3/4s bms units are sooo cheap, it's worth putting one on. You could probably even add a little extra "smarts" to light an led if the bms is triggered so you could at least visually see something is up with that pack.
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#8
(02-18-2020, 07:56 PM)Pietershomeprojects Wrote: well... once over 4.2V or below 2.8 it will get dangereous, as far as i read myself into this... a BMS is there to prevent fire hazards.... fuzes won't help for to high voltage...

Okay so I am aware fuses won't help with this they would be there in case of individual cell shorting and cut out out of the pack. So apologies this wasn't relevant to my core question/curiosity.

(02-18-2020, 10:04 PM)not2bme Wrote:
(02-18-2020, 08:45 PM)daromer Wrote: If you run one charger per pack that is basically what a BMS do. It makes sure each pack in the serie have its own controlled voltage and current limits (kind of)

Its cheaper and easier just to add a decent BMS...


Ah I misread it! Though it was one charger for the entire packs. Then yes it would technically work. Then I guess the limitation would be back to the low voltage cutoff again.

Correct I was thinking essentially a charger per pack rather than one charger for the bank. How could you see this working i.e. configuration/wiring?

(02-18-2020, 08:01 PM)OffGridInTheCity Wrote: One function of a BMS is low voltage (not just high voltage).   When you have cells in series, its possible a failing cells (in the series) go all the way to negative voltage as you draw power from the overall battery.       For example  Bat1=3.0v, Bat2=-0.3v, Bat3=3.0v ..   e.g. overall battery is a little less than 6 volts.     A BMS will protect against this. 

I ran my original 7s battery bank without a BMS for about 6 months - thinking "I'll just measure things with a volt meter once a week".   Not a good idea as I found out eventually that I had some cells going down to 2.5v'ish and then bouncing up to 3.2 in the morning - when I thought all were cutting off at 3.3'ish.   If I had continued it 'could' have spiraled out of control. A BMS would have stopped that in it's tracks.

The question I'm asking is assuming I have a solution for the low voltage therefore my logic was if each pack has its own charger then there max voltage is limited by the 4v setting of the CCCV board.

(02-18-2020, 08:35 PM)not2bme Wrote: Like offgridinthecity, I ran mine for 3 months without BMS (when I accidently broke it) and was fine. In fact it didn't even deviate much the entire time, only by 50mV at most. But it was dangerous to say the least. Discharging it low isn't too dangerous, just the risk of ruining your batteries. The problem is if a pack fails and deviates by a lot and you charge it the next day.

So let's say you are putting a 3s pack. The max voltage is 4.2V and let's say you put in a safety by only charging it to 4.1V, that means you have 4.1V + 4.1V + 4.1V = 12.3V. You set your CC/CV to 12.3V. The problem comes if one pack suddenly fails, ie. one of the cells starts to create a self-discharge situation, pulling down the pack. Let's say one pack suddenly develops a short so it's at 0V. The CC/CV doesn't know that one cell failed, it just wants to charge up to 12.3V. Now you try to charge it to 12.3V but since one cell is shorted, you end up with 6.15V + 6.15V + 0V = 12.3V. That's a KABOOM scenario where your cells catch on fire.

Pardon my ignorance but how would a BMS avoid this situation? I'm talking about each pack having its own charger so how would this push the other packs above 4v?

(02-18-2020, 08:45 PM)daromer Wrote: If you run one charger per pack that is basically what a BMS do. It makes sure each pack in the serie have its own controlled voltage and current limits (kind of)

Its cheaper and easier just to add a decent BMS...

What BMS would you recommend?

(02-18-2020, 10:06 PM)Korishan Wrote: It also depends on the health of the cells. If they are brand new or lightly used and have 95+% of original capacity, then you can get away with not having a bms for awhile. But if they are abused, old, or <90% of original capacity, it's a really good idea to use a bms, even a cheap one, to keep things in line. 3/4s bms units are sooo cheap, it's worth putting one on. You could probably even add a little extra "smarts" to light an led if the bms is triggered so you could at least visually see something is up with that pack.

What BMS would you recommend?
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#9
Bms depends on application. You need to dig into that.

Cant recommend any unless you tell us about what the usecase is and how much you want to spend.
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#10
(02-18-2020, 11:24 PM)daromer Wrote: Bms depends on application. You need to dig into that.

Cant recommend any unless you tell us about what the usecase is and how much you want to spend.

Using to power shed with some led lighting, Chinese diesel heater (they run at 12v and 12Wh (1Ah) per hour and some electric garden tools for example grass strimmer (650w max) so not massive draw.

Charged with solar PV via MPPT charge controller and 4 X 280w panels.

Haven't decided on pack size but was thinking 60 to 80p pack.

Does that provide you with enough detail or have I missed the mark?
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