I’m also pretty sure that you need more than just a buck converter to charge Lithium batteries. Li chargers charge with a constant current until about half capacity and then constant voltage after that.
so I suppose your idea would work so long as you never go below 50% charge? not worth the risk IMHO.
You’re right, I don’t think the above one would work. This one however is CC/CV and says it will charge lithium ion batteries ($1.48/ea):
Unfortunately, I think @Pedrodemio is correct, and the power supplies would need to be isolated for each module. Which makes this idea much less attactive
A step down needs higher input voltage than its output. To charge 10S you would need like 50V minimum. The better way is to use a boost converter that can use a laptop style PSU and has variable output. I did exactly that. I charge my 10S battery to 41V with a lenovo 90W 24V PSU and a DROK in a 3D printed case. Possible but cost was ~25€ not factoring in the PSU
The whole idea was to charge each cell individually. 12V PSU to 4.2V output, for each cell. So not 42V, but 4.2V x 10
It would be a cheap balance charger, if you could find a clever way to power 10 different regulators. Otherwise it’s just a dumb idea
I have an adjustable power supply that works great. But it’s not a balance charger. And my BMS sucks at balancing
Sorry - I just read the first sentence on mobile and assumed this is going to be about a 10S charger with variable voltage output. I am still wondering though why people want to balance so badly with all the good experience people had without a BMS and balancer on quality 18650 cells.
$25 for 10x isolated DC outputs. Each output is 100mA at 9V. Assuming a 92% efficiency for the CC/CV step-down regulators, that’s about 200mA at 4.2V (for each cell), which would be the equivalent of 42V 2A overall
I’m not going to go through with this project, but it actually does seem possible. $40-50 for a 10S 2A charger that balances each cell individually. It would be super bulky though
I think it will not work as your grounds are different from each other. This would only work if you have independent power supplies for each step down converter “port”.
Awesome idea! #Tracked 
This will not work without 10 PSUs which will make it more expensive than just a 10S balance charger. To illustrate:
Not when each PSU costs $0.99 
The transformer should isolate the output from the input
http://www.ebay.com/itm/172306524675?var=471125700480
So your balancer charger would be one of each of these, for each cell. $1 for the AC to DC 9v 280mA transformer. $1.50 for the CC/CV adjustable step-down voltage regulator. Comes out to about $2.50 per cell. $25 for 10S / $30 for 12S
Now I’m just curious. I want to try to build a 2S version with a cheap 2S battery. (Because if 2S works, the same concept would work for 12S)
Well at 280mA it will take 10h to charge a normal 18650 battery. As soon as you use a PSu with more current capabilities price will go up.
It will work this way sure, but at the cost of convenience (just think about how many cables you will have to manually connect) and it will be slow.
You could just get a 6S RC balance charger and charge the battery as two times 5S. Will be cheaper, easier and faster. I don’t see the benefit in your method.
This would be for balancing only. I already have an adjustable voltage charger. I just want something you’d use occasionally to trim up all the cell voltages with a single plug
280mA @ 9V with a 92% efficiency is 2.31W (per cell)
2.31W / 4.20V = 550 mA balance current per cell (while my BMS can only do 60mA)
Here’s a 20W PSU for $2.17, bringing the total to $3.65 per cell. Ten of them gets you a 200W balance charger for $37-ish. Still pretty reasonable for 10S @ 4.7A
You don’t seem to know how balancing works. You will be in the CV stage so the current will be dynamic and go down to almost 0
I am still not convinced where the benefit is compared to getting one or two 6S RC chargers. Why even do 10 simultaneously when you just want to do this occasionally.
You don’t seem to know how to engage in respectful discussion.
I already have a dual 6S balance charger. That’s not the point. There’s the potential to build a homemade balance charger for the same price as a standard brick charger
I don’t feel I was direspecting you. I was trying my best to keep you from burning your house down with the initial idea of using one PSU to power your step down converters.
Your answers made me honestly think you might not know (which is not a bad thing - you can always learn). You cannot increase balancing current - that is the whole point of the CV phase.
My 6S RC charger was like 20bucks - how is this more expensive than your proposed system? Especially when you don’t even want to use it for charging but only for balancing. So you’ll need two systems?
I think it makes more sense to just build a single (1S) cell charger. So for example, bulk charge to an average cell voltage of 4.10V. Some cells will be 4.13V, while others might be 4.07V. Individually charge them all to 4.15V. This would manually balance the pack. You’d only need to do this occasionally. Often the pack will stay nearly perfectly balanced with no maintenance
The problem I have is my BMS doesn’t seem to balance very well. Some cells consistently lag behind 0.05V at EOC
0,05V apart is well enough…
