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7s config balancing question
#1
I'm probably not asking this correctly, but how would I go about making sure each cell pack in a 7s configuration is bottom balanced?  Right now, the 1500W APC UPS I am trying to use has a float charge voltage of 27V, making it practically impossible to balance.  Any ideas?

Thanks to everyone!

Matt
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#2
You can't top AND bottom balance cells. You have to choose one over the other. That is, if you plan on using a large portion of the cell voltage range (3.4V - 4.1V). If you plan on using a shorter range (3.6V - 4.0V) then it is "possible", but difficult.

With your numbers, 27V would be correct, 13.5V * 2 (being lead acid 12V batteries). This is top end of the cells, not bottom. So using LiCo cells, voltage ranges with 7s is ~3.85V top voltage, and ~3.29V for bottom voltage. This is a very healthy range for your cells. Altho, you are loosing some capacity at the top end, but not much.

However, to answer your question, you need a bms to do it. And one that is configurable as the default for the cheap chinesiums are 4.2V/cell. So any bms that has bluetooth or network connectivity should be smart enough to be able to be programmed on the voltage ranges you want.
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#3
(07-15-2019, 06:09 PM)MattBarnesJCKS Wrote: ..... the 1500W APC UPS I am trying to use has a float charge voltage of 27V

You might find doing a little reading around the topic of replacing SLA batteries within UPSs with Li chemistry cells, whilst retaining the existing charging system somewhat illuminating.
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#4
I have several APC UPS 1500(s) and started with these for my first 18650 battery work. I discovered that mine top charge at 27.8v (3.97v/cell) and cut-off at 21.3v (3.00v/cell).
I actually use 7s7p 18650 batteries now instead of the lead-acids - and for the most part, they simply stay charged at 27.8v (3.97v/cell).

I use an externally mounted ISDT BatteryGO BG-8s to keep them in balance and provide an audible alarm for alerts and as a quick visual check of cell voltages. This is a cheap BMS solution and what I like about them is that they do not get hot when balancing... and actually do a great job of balancing

I believe this forum would recommend using a BMS that can cut-off voltage in under/over conditions rather than just providing an audible beep - but its adequate for me since I monitor manually / in position to hear the beeping easily.

An APC UPS 1500 has top continuous power delivery of about 1000watts - so 1000/24v = 42amp (e.g. 50+ amp) BMS would be needed.

And yes - I have done full drain tests etc and the 18650s work. Of course discharging to 3.0v is not recommended as a regular practice as this is hard on the cells, but for a power outage (rare) situation, a 7s7p or even a dual stack of 7s7p (for 7s14p) fits nicely in place of the of the lead batteries.
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#5
What is the exact make of the UPS? Some can be modded to charge to a slightly higher voltage. Others will accept a slightly higher battery voltage if you charge it externally.
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#6
(07-16-2019, 12:10 AM)Geek Wrote: What is the exact make of the UPS? Some can be modded to charge to a slightly higher voltage. Others will accept a slightly higher battery voltage if you charge it externally.

(07-15-2019, 06:09 PM)MattBarnesJCKS Wrote: ... 1500W APC UPS ...

Wink

I doubt it's a rack mount unit, probably one of those cheapo ones for computer power saves. I would guess it has the coms port on the back.

Here's a command line interface pdf:
https://iportal2.schneider-electric.com/..._R1_EN.PDF
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#7
(07-16-2019, 12:33 AM)Korishan Wrote:
(07-16-2019, 12:10 AM)Geek Wrote: What is the exact make of the UPS? Some can be modded to charge to a slightly higher voltage. Others will accept a slightly higher battery voltage if you charge it externally.

(07-15-2019, 06:09 PM)MattBarnesJCKS Wrote: ... 1500W APC UPS ...

Wink

I doubt it's a rack mount unit, probably one of those cheapo ones for computer power saves. I would guess it has the coms port on the back.

Here's a command line interface pdf:
https://iportal2.schneider-electric.com/..._R1_EN.PDF

It most likely will handle 28v. Even possibly as high as 29v. Although attempting this would be at your own risk.

Using an external charger may be troublesome. Just be careful, ground loops may cause undesired results.
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#8
Thank you all I have an APC 1500BS (not sure, at work). It was the largest I could find at the time from Newegg that was not a rack mount unit. It has room for a 7s9p pack. My problem stems from 27V charging my batteries led to the first cell at 2V and the other 6 from 3.8 to 4.0.
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#9
You end up with that because you dont have a BMS that do balancing as you need it to do.

Thats also the reason why you need a BMS with proper balancing functionality Smile
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#10
(07-16-2019, 11:56 AM)daromer Wrote: You end up with that because you dont have a BMS that do balancing as you need it to do.

Thats also the reason why you need a BMS with proper balancing functionality Smile

Correct.  That's my issue.  How to keep it bottom balanced...
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