Thread Rating:
  • 1 Vote(s) - 5 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Top vs bottom balancing?
#1
Could someone please describe top and bottom balancing? I understand what it is to balance packs, but I'm unclear on the meaning and reasons for top or bottom balancing.
Reply
#2
i would like clarification as well my understanding is

bottom balance, bring all cells to lets say 2.75v and connect
top balance, bring all cells to full charge and connect

top balancing gives you the most capacity and is best for powerwalls
bottom balancing lets you run down to closer to 0% without killing cells, more useful in vehicles but you lose capacity

in a bottom balance scenario all cells hit bottom together
top balance they all hit 100% together
Reply
#3
Its just what it says. Its either you make them even in the bottom or the top.

Ie either you start all cells at 4.2v and have them equal at top.

Bottom then you go to your lower setting lets say 3.2v and make them even there.

No matter what you will in theory get the same capacity. For high current systems the bottom type may be better in terms of saving the battery since when you get low all get low at same time and you will stop the application. If you have top balanced them 1 pack or a couple of them will go dead before rest do and you may kill it.

What you use for your home made powerwall all depends on the application. What is worst: Killing a pack due to charging it to high or taking it to low? Neither way you choose i recommend some kind of active monitoring that can disable discharge or charge when that pack that reaches the bottom or top end goes bananas Smile


For passive balancing you almost always top balance since its quite useless to drain energy in the bottom.. You want to do that in the top end when you have excess.

Active balancing where you transfer energy that can be done in booth ends and therefore extend the capacity range slightly....
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
Reply
#4
I suppose technically you could balance at any voltage, my charger will allow a balance charge at storage voltage although I don't remember using the option.

Think of top balance as "equalize all the cells at the highest voltage I wish to allow them" and for bottom balance "equalize all cells at lowest voltage I wish to allow them". It's not any specific voltage it's just the maximum or minimum you wish them to operate to.

When a really well matched and balanced low internal resistance pack goes off the curve it's obvious, there's no there there any more.
Reply
#5
Have a question that relates to top and bottom balancing.  The DIY pack builds that I have seen with 18650s, most are 100 or more cells per pack.  Lets take for example 100P14s that I saw for a DIY powerwall.  For the pack there are 1400 cells, but lets just take one of the 100p that are then wired in series, it has 100 cells.  These are soldered with fuses, battery bars etc. how do you go about discharging each of the 100 cells for a bottom balance? And assuming this is part of a larger 100P14s pack, each 100 in each of the 14 in series would also have to be bottom balanced.

Is the only way to balance before building the 100p pack? And they you don't ever balance again?

Thanks in advance,
Robb
Reply
#6
First of all its 14s100p and not the other way around. Its not 100 strings.. Its 14 packs in series.

1 pack of 100 cells are in parallel = they are balanced together. Cant be anything else.

No matter if you bottom or top you take your 14 pack and paralell at cel voltage. They will then balance to each other and thats where you have your balance point set. If its bottom or top or middle doesnt matter. All cells in parallel will equalize.
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
Reply
#7
And it is the job of a BMS to balance the individual packs in series while in use. (top or bottom if you choose a decent bms)
___________________________________
18x 300Wp solar off grid and 10x 180Wp solar tracker grid-tie
10KW 3phase hybrid inverter. 40Kwh 18650 storage (for now)
My setup: https://secondlifestorage.com/t-Instant-powerwall
Reply
#8
Cells in parallel is like having two (or more) tanks of water that have their bottoms all interconnected. No matter how much water you take out of one tank, the others will flow and balance out. No matter how much you dump into one cell/tank, it will flow out into the other cells/tanks in parallel. They cannot be in any other way.

Cells in series is like having a way way check valve between each cell/tank. Nothing can flow between cells/tanks in this mode
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
Reply
#9
(01-24-2018, 07:33 PM)Robb Melançon Wrote: [...]

Is the only way to balance before building the 100p pack? And they you don't ever balance again?

Great question -- and great thread, thanks OP.

I am currently practicing my 'tuning' skills, in building packs that are capacity matched -- precisely matched.  My theory is, that if I can top balance, then discharge to bottom, and its still balanced then heh ... that would be awesome, eh?

my aim is to run 10-20kWh without any real BMS.  NB I aim to monitor these packs, and top them off at ~ 3.92V (normally).

so YES, to answer your question here, I believe that if you solder/weld up a pack, with mismatched (even 1%) capacities then you will be stuck with a battery that goes out of balance....



Quote:Cells in parallel is like having two (or more) tanks of water that have their bottoms all interconnected. No matter how much water you take out of one tank, the others will flow and balance out. No matter how much you dump into one cell/tank, it will flow out into the other cells/tanks in parallel. They cannot be in any other way.


um ... no, not quite.

in my experience this very rationale expectation is only true when charge/discharge rate is relatively low.  think < 0.1C ... once the current gets up to 0.5C and certainly any more, then the cell's relatively different internal resistances start messing with everything.  IE effective capacity changes, differently for each pack, and therefore battery goes out of balance.

so my practice/idea/tuning thoughts (above) are ONLY valid with loads and charging at relatively low rates.

if the cells were all homogeneous (IE new, from same-ish batch etc) then their internal resistances would generally all be matched, so then one can go higher in the C rate before things get wonky.  but I'm using harvested (used) cells exclusively, so ... tuning practice for me.

I no longer use the cell repacker on this site, as it 'only' (great job though) works with matching capacity.  since I'm only making smallish batteries (1-3kWh) at the moment, which don't have enough cell in each pack (low P count) then the standard deviation on internal resistances in each one is far too high.  hence me developing another way to 'tune' the construction to _try_ to match this as well as pack capacities.


*DCkiwi puts on fireproof jacket, so flame away*  ; P

ps as I noticed on one of Jehu's vids, a smart guy at EV West said the trick is to have it bottom balnced, then charge it, and the battery still balanced at the top.  this is the trick, Im quite convinced.  its not about one, or the other, really.  albeit they both can be useful in their own way..
Wattsup likes this post
--    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----    ----    --
I am an iSDT fanbois.  also a proud owner of two SC608AC's .. totally recommended.
-----------------------------------------------------------------------------------
Reply
#10
I'm currently Running my 4.6kWh bank as a Peak energy offset power source without a BMS. I created it using tested 1s packs of 8 cells that are the same as as close to the same Ah quantity as possible. I have 56 of these in my 7s setup. I top balanced (@4.1v/cell) maually and got them within 8mV per cell and I watch them during the discharge and as long as I keep the pack above 3.2V/cell I stay within 40mV spread. Now if I start to discharge more than that it gets worse pretty quick.

Just food for thought.
DCkiwi likes this post
******Hi My name is Jason and I have SOCD (Solar Obsessive Compulsive Disorder)*******
Current Powerwall - 1400 Cells 7s200p (modular 40p packs) ~ 12kWh of storage     4x 315W Canadian Solar Panels

Working on the next 7s40p packs     ~2.5kWh

Waiting on 2000 Cells of unused Sony vt4 (2000mah 30A) ~ 15kWh      hehehehehe  More Power
Reply


Forum Jump:


Users browsing this thread: 2 Guest(s)