diy solar

diy solar

Top Balancing using a PSU: is this exactly the other way around?

You lost me on that one. How is it faster?
Firstly you do most of the charge at system voltage.
Secondly its faster because you don't have to break the pack.
Its been a while but here is what I did.
I charged the pack until the bms tripped at 3.55 volts.
I bled the high cell down to 3.4ish(with a capacity tester) which was in range with the others.
Then I charged until the bms tripped again.
Then I topped the cells indvidually to 3.625 volts with a charge voltage of 3.65 volts.
The longest cell took under <1 hour and the shortest cell took <15 minutes.

It probably took me longer to do all the joinery on the battery.
 
Firstly you do most of the charge at system voltage.
Secondly its faster because you don't have to break the pack.
Its been a while but here is what I did.
I charged the pack until the bms tripped at 3.55 volts.
I bled the high cell down to 3.4ish(with a capacity tester) which was in range with the others.
Then I charged until the bms tripped again.
Then I topped the cells indvidually to 3.625 volts with a charge voltage of 3.65 volts.
The longest cell took under <1 hour and the shortest cell took <15 minutes.

It probably took me longer to do all the joinery on the battery.
Seems to me all that wriggling around chargers for each individual cell is keeping your hand close to those terminals more often than just arranging a parallel pack once, and a series pack once. Anyway, to each his own, I suppose. I like the idea of paralleling them for the last mile. I’ll give the terminals a light scuff with 600 grit and wipe them with isopropyl first. I’ll hold on oxguard until the final assembly.
 
Seems to me all that wriggling around chargers for each individual cell is keeping your hand close to those terminals more often than just arranging a parallel pack once, and a series pack once.
I used 10 awg leads off the terminals back wago 221-613 lever nuts which cross connect to the corresponding balance leads.
Because of the 3 position lever nuts I have the option to add an active balancer easily but I doubt it will ever be necessary.
 
I used 10 awg leads off the terminals back wago 221-613 lever nuts which cross connect to the corresponding balance leads.
Because of the 3 position lever nuts I have the option to add an active balancer easily but I doubt it will ever be necessary.
If I am understanding you correctly, you buy a wago connector per cell, attach it once, then you rewire the cells how you want and when you want quickly and easily? Sounds like a nice system when you’re shuffling batteries around a lot.
 
If I am understanding you correctly, you buy a wago connector per cell, attach it once, then you rewire the cells how you want and when you want quickly and easily? Sounds like a nice system when you’re shuffling batteries around a lot.
The idea is to not shuffle things.
IMG_20210312_172951.jpg
 
This is the approach I might take:


... especially since I have one in my rack. Limited to 1A, but could automate everything with a series-connected batch of cells.
 
This is the approach I might take:


... especially since I have one in my rack. Limited to 1A, but could automate everything with a series-connected batch of cells.
Nice but expensive.
 
Its what I described 1 post earlier in this thread.
That is the negative end of an 8s 280ah pack.


Nope.
I take it expandability is the reasoning? (like you said, active balancer) how long is the 10 awg lead? How much vdrop? Not a concern? Is the planning for an active balancer foresight into them aging?
 
I take it expandability is the reasoning? (like you said, active balancer) how long is the 10 awg lead? How much vdrop? Not a concern? Is the planning for an active balancer foresight into them aging?
The 10 awg and wago connectors are negligible but not 0 impact on the sense leads.
If I were to do it again I would use 12 awg thhn and 221-413s.

I did it that way to allow me to charge/discahrge the cells individually but also with the idea that my cells might need an active balancer.
Turned out the cells exceeded my expectations.
 
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Nice but expensive.
Yeah.
Cost me a grand on eBay for the chassis with DMM built in and two cards (mux and multifunction). Picked up a coax switch for another hundred.
I was spending my own money to get work done for my employer. I get to keep what I bought and what I learned.

I also got a Keithley chassis with 10x cards of 10 channels each for $50 at HalTed Specialties.


But I used the HP one for all my automation work.

Rather than rewiring inside for different projects I connected 3' 28 awg ribbon cables, 40 and 50 pin for the two cards.
At 1A that would be 0.3V drop so would have to use 4-wire through the relays and remote-sense of supply.

Having it at home helped me debug code I was going to use for remote (due to COVID) characterization of a project related to HoloLens, using similar equipment:



The 10 awg and wago connectors are negligible but not 0 impact on the sense leads.

I did it that way to allow me to charge/discahrge the cells individually but also with the idea that my cells might need an active balancer.
Turned out the cells exceeded my expectations.

Ideally BMS would use 4-wire kelvin sense.
But if smart enough it could toggle between balance and sense, include estimate of sense wire IR drop.
 
For the next iteration, I could use a bms to drive an ssr to terminate the charge programmatically.
Should work fine.
 
The 10 awg and wago connectors are negligible but not 0 impact on the sense leads.
If I were to do it again I would use 12 awg thhn and 221-413s.
i am not so familiar with this stuff that i know what the wago numbers translate to in model or rating nor do i have full knowledge of different insulation types.

why the differences you mention? what would you gain?

I did it that way to allow me to charge/discahrge the cells individually but also with the idea that my cells might need an active balancer.
Turned out the cells exceeded my expectations.
i like the idea of that. ease of maintenance is something i can get on board with.
 
why the differences you mention? what would you gain?
221-413s are smaller, easier to source and cheaper.
10 awg was overkill.
12 awg would be fine.

the 4 in 413 is 4mm2 or roughly 12 awg
the 6 in 613 is 6mm2 or roughly 10 awg
 
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I'm charging 16 280AH cells in parallel. The PSU is in CC mode, do I need to change it to CV mode some how? It came with instructions but they are difficult to understand.
 

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The PSU is in CC mode, do I need to change it to CV mode some how?

No, the supply will be in CC mode until the cell voltage catches up. Whatever you do, don't adjust the voltage while connected to the cells. In fact, make sure you're not over 3.65V with the supply disconnected from the cells.

Technically, I also recommend doing one cell at a time. There are some inherent issues with putting cells in parallel to top balance, even though for the majority of people it shouldn't be a problem. One advantage of doing one cell at a time for people new to this is that if you screw it up, you lose one cell, not potentially all of them.
 
These threads pop up constantly. It never ceases to amaze me how people take a simple concept and make it complicated.

Parallel top balancing is for people that are clearly not forward thinking.

If the self discharge variance between your cells is smaller than the balancing ability of your BMS, and your pack capacity is acceptable, a top balance is unnecessary.

If the self discharge variance of your cells is greater than the balancing ability of your BMS, a top balance will still not help you for long.

Nobody has ever given a good reason to parallel top balance. It is a quite recent fad bought about by a few youtubers that obviously had no long-term experience with LiFePO4 packs.

Of course you can get the SOC close with a parallel top balance, but that is pointless anyway because:
1. that is not going to be the exact SOC that the BMS invokes balancing / disconnect.
2. the instant any balancing takes place your “perfect” balance has been altered.
 
Most BMSs don't have the balance current capability to get 16 280Ah cells 'in sync' at the top when they're potentially 100Ah or so apart between them. An initial top balance alleviates this so that the BMS balancer can deal with it. It's not so much to get the cells at the same state of charge per se, more to have them close enough that the BMS can deal with the remaining imbalance without taking months to do so.

Also, I personally prefer to do this on a per cell basis instead of putting them all in parallel, since that can cause other issues.
 
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