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Top Balancing "How to"

I am going to charge the 4s battery to a maximum of 14.08V to maximize cycle life. Is it worth making 3.52v or 3.6v as the top balance?

What about BMS's overcharge setting, does it matter it is 14.08v or 14.60v?

You want to top balance once to a higher voltage. 3.65V is reasonable.
You want SCC to stop charging at a voltage below where BMS disconnects, and below what causes any cell to run up too high.
Maybe 3.6V per cell?
With SCC set to 14.08V, average 3.52 per cell.

Charging comes through the victron dc/dc converter where the set 14.08v.

But what about that top balance does it really matter if it's made of 3.52v or 3.6v?

If you top-balance to 3.52, then charge to (average) 3.52, i.e. 14.08V, some cells will diverge and run.
 
Thanks for the answers Mr.Sandals and Hedges. So I make top Balance 3.65v, I set typical bms settings like 14.6v overcharge disconnect, but I charge the package with 14.08v.
 
Thanks for the answers Mr.Sandals and Hedges. So I make top Balance 3.65v, I set typical bms settings like 14.6v overcharge disconnect, but I charge the package with 14.08v.

You will figure it out eventually, but the lower your charge voltage, the more likely you are to charge with lower current when your battery gets nearly full. That isn't a bad thing, it does give the cells a chance to get saturated more, but if you have a limited amount of daylight hours (assuming solar charging) you can run into problems. The closer your battery voltage and charge voltage is, the lower the current.
 
You will figure it out eventually, but the lower your charge voltage, the more likely you are to charge with lower current when your battery gets nearly full. That isn't a bad thing, it does give the cells a chance to get saturated more, but if you have a limited amount of daylight hours (assuming solar charging) you can run into problems. The closer your battery voltage and charge voltage is, the lower the current.
For now, I only charge with the boat's outboard motor and shore power, later I may connect solar power.
 
Thanks for the answers Mr.Sandals and Hedges. So I make top Balance 3.65v, I set typical bms settings like 14.6v overcharge disconnect, but I charge the package with 14.08v.
Yes. Although if you like 14.4-14.5 is fine to tighten it up a bit but leave some headroom.
 
Remember. Always verify your power supply voltage.

So I have 16 more of these to balance up... Talk me out of laying my buss bar material across the studs with all 16 in parallel and charging them up. I'd put some weight on top of each section of aluminum for good contact.
... As I was typing, I'm thinking my buss bar stock is going to be to thick and a pain to assemble everything in pack form :unsure:
 

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Remember. Always verify your power supply voltage.

So I have 16 more of these to balance up... Talk me out of laying my buss bar material across the studs with all 16 in parallel and charging them up. I'd put some weight on top of each section of aluminum for good contact.
... As I was typing, I'm thinking my buss bar stock is going to be to thick and a pain to assemble everything in pack form :unsure:
might work, don’t bump it ? not sure the contact will be consistent from cell to cell

connecting them all in series with BMS and then charging at a higher voltage is my favored way.

then, if any cell goes out of wack, the BMS will just disconnect and save the cells from damage.

after the pack is fully charged, individually top off any cells that are behind

food for thought :) good luck with the build
 
For reasonably low-current top balancing, the connection between the bar stock and the pads is not critical. It only needs to be good enough to take on the current you are top balancing with. If one of the connections is slightly higher resistance than the others that cell will charge slower, but as the others reach full charge they will stop taking current. Eventually, the cell with the higher resistance will get full and as the current drops toward zero, the voltage across the resistive connection will drop toward zero as well. When they all stop taking current, they will all be at the same voltage.

Having said that, I would never do what is being suggested. Murphy is alive and well and I am confident he would cause me, my wife, or maybe even a stray cat to bump the bar stock and cause a short.
 
Thanks everyone. I'll throw a BMS on them and bring them up around 3.55 before finishing off in parallel, like the last pack. I always have to try and complicate things.
Murphy is my best friend and worst enemy ?
 
How do you know if the busbars are factory or heavy-duty? Mine are two metals stacked with green shrinking plastic sock, 0,8 mm thick x 20 mm wide apiece, totals 1,6 mm x 20 mm.
 
How do you know if the busbars are factory or heavy-duty? Mine are two metals stacked with green shrinking plastic sock, 0,8 mm thick x 20 mm wide apiece, totals 1,6 mm x 20 mm.
Those are certainly not what I would call heavy duty. With the shrink wrap on them, they probably top out at around 150A (maybe less).
This chart shows ampacity for un-covered busbars:

If you are doing a single 48V 6KW inverter running at 90% efficiency you would have a top sustained current of (6000W/.9)/48V=138.8A.
This would work with your busbars, but I would not do anything more than that.

Note: If you are doing more than one battery and putting them in parallel the current is shared. As an example, if you had two batteries, then the 48V 6KW battery would pull 139A/2=69.4A from each battery.
 
I am working with 16 pcs of LiFePo4 batteries which will go to 2P8S config in a 24V system. The cells are Eve 230Ah models. Which kind of busbars I do need? I have to install the set probably to two times 8 cell rows in order to fit it in the vehicle.
 
I am working with 16 pcs of LiFePo4 batteries which will go to 2P8S config in a 24V system. The cells are Eve 230Ah models. Which kind of busbars I do need? I have to install the set probably to two times 8 cell rows in order to fit it in the vehicle.
What is the total draw you will have from the battery? (Current for the inverter + current for a DC-DC converter if you have one)

As an example, if you have a 3000W 24V inverter and a 24-12V DC-DC converter that outputs 30A the draw on the battery could be:
Inverter: 3000/24 = 125A
DC-DC converter: 30A/2 =15A (divide by two because the voltage step down)

If we assume everything runs at 90% efficientcy we get (125A + 15A)/.9 = 155.5A.

I would consider the 1,6 mm x 20 mm bus bars you described above to be marginally adequate for this example. The only reason I say they might be OK is that it is very unlikely you would ever drive both the inverter and the converter at their full rating at the same time. Even if you did drive them at the full rating, it is very unlikely you would be doing it for a sustained time.
 
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Indeed, I have a 3000W 24V solar MPPT inverter and 1850W of solar panels on the roof. I have also 1800 W 24V ordinary inverter to run some devices separately which I was planning to install to the same LiFePo4. Would this total out as too big set-up?
 
Indeed, I have a 3000W 24V solar MPPT inverter and 1850W of solar panels on the roof. I have also 1800 W 24V ordinary inverter to run some devices separately which I was planning to install to the same LiFePo4. Would this total out as too big set-up?
Do you have any 24V DC loads or a DC-DC converter to 12V?

Assuming you don't have DC loads and assuming the inverters run at 90% efficiency, your total battery load when running both inverters all out would be: ((3000W * 1800W)/.9)/24V = 222A. That would be pushing those thin busbars harder than I am comfortable with.

You have a couple of options:
1) Get thicker busbars or more busbars to stack.
2) Get another BMS and split the cells into two batteries. (8S2P) By having two batteries the load on each would max out at ~111A
 
The busbars would be nice, although they seem to be hard to find here in Northern Europe. Is there any way to make them by myself? Does ordinary copper or steel do?
 
Does ordinary ... steel do?

:ROFLMAO:
Definitely not! 10x the resistance of copper, would likely glow red hot unless 1/2" thick.

Copper or aluminum. Solid or braid, straight or bent to allow expansion. Various benefits of each.
Native aluminum oxide causes problems, wish it was tinned, but same goes for terminals. Various treatments recommended.
Copper reacts with aluminum, so tinned copper is better.
 
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