lithium charging limitations?

[Hummina Shadeeba]

can I charge all my lithium cells in parallel by hooking them up to a transformer secondary at 4 volts? Are there limitations because of the lithium chemistry or is due to the charging method and this old fashioned voltage supply is different than a typical bulk charger and complexity with pwm?

 

[Chalo]

As long as you don't exceed the charging C rate specification, there's no reason that won't work. The easiest way to limit the charge rate is to use a transformer that is inherently limited to a tolerable maximum current.

 

[Hummina Shadeeba]

great. do you have any advice or direction? as it is i'm hunting for electronics people on craigslist. there aren't many. I'd like to make it charge at 400 watts 12s all in parallel. this seems a huge simplification and possibly a safety improvement over bms which I don't trust and balance chargers which are a pain and I suspect them in fires too. KISS.

theres math for sure but the basic components would be the two coils on a core and a rectifier and a cap to smooth it out and maybe resistors somewhere...what am I missing?

I'd love to hook this up to all my balance plugs. one 13 wire plug. Or maybe it would have to be 24 wires.?

 

[Chalo]

What's the maximum charging amps or C rate of your cells, and how many cells do you want to charge at a time?

If you have cells that can charge at 1C, and you have 400Wh worth of cells, then 400W will charge them at 1C. If you have 1000Wh of cells, then 400W will charge them at 0.4C.

Note that the charge rate will drop greatly as the cell voltage approaches the charging voltage.

 

[Hummina Shadeeba]

charge current up to 4 amps a cell. so doing that at 4 volts and 12 cells...16 x12...only 200 watt charging ..but I'll have 4 in parallel so 800 watts possible.

 

[danielrlee]

This sounds like a disaster waiting to happen. To parallel the cells you'll first have to remove the series connections between them. IMO you're much more likely to experience user error while doing this, than a failure occurring through the use of a BMS or RC balance charger. Hell, you're probably even less likely to experience problems acting as a 'human BMS' and bulk charging the entire pack as a series string.

 

[Hummina Shadeeba]

of course you're right and all the cells would need to use their own transformer secondary coil and then there wouldn't be any parallel connections that would need to be broken. doing multiple secondary coils isn't a problem, infact it's a simplification maybe. fixing that what's next?

i still think this could be a safer more foolproof method for balancing.

 

[danielrlee]

of course you're right and all the cells would need to use their own transformer secondary coil and then there wouldn't be any parallel connections that would need to be broken. doing multiple secondary coils isn't a problem, infact it's a simplification maybe. fixing that what's next?

i still think this could be a safer more foolproof method for balancing.

Oh, I misunderstood. With a separate transformer for each cell, the pack could be left in series. Still though, increasing the number of components makes for a system with more failure points.

I still think that you'd be better off with a tried and tested BMS if it's a 'set and forget' solution that you're after. As well as balancing duties, you also get LVC protection. I can almost see the point for a pack that sees infrequent usage, but for a pack with a more frequent charge cycle, traditional BMS works just fine.

 

[Chalo]

Really, you'd only need to get the connection right once-- one "charge harness" with a cluster of Powerpoles hooking up to the charger, and one "series harness" with the same cluster, but different wiring behind it. Shape the gang of Powerpoles so that they can only be plugged in one way.

 

[bnoooogers]

all the cells would need to use their own transformer secondary coil and then there wouldn't be any parallel connections that would need to be broken

I recognize that you're looking for a DIY solution, but in case you (or others) don't know, there is a commercial product that does this, and is a programmable balance charger that charges through the balance wires with no need to disconnect series wiring. Up to 8A per cell. It is called the UN-A6 and costs ~$60 + 12V power supply.

Apparently the downside is that it uses the crappy JST connectors (on LiPo balance leads) that aren't meant for lots of plugging and unplugging, so if this is meant to be your primary charger you might want to replace the connectors.

Here is a big thread about its predecessor the BC168, with pages towards the end about the UN-A6
https://endless-sphere.com/forums/viewtopic.php?f=31&t=41107&hilit=Hyena+168+balance&start=300

 

[Hummina Shadeeba]

I'll be charging 12 cells with this. the esc has two low voltage cut-offs that I feel safe using and have been using in place of a bms. This method, using a transformer with a bunch of secondary coils, to me is a simplification and possibly safer and definitely an easier way to balance charge.

I'm simply trying to make a transformer with a smooth output of 4.15 volts at 4 amps max for each of 12 different cells in parallel. I guess the output will change as the charge changes but its looking to be about a total of..about 200 watts max. If anyone can give me a good argument why this is a bad idea I'm willing to listen but as it is now I'm not discouraged with this project.

it will be more inefficient than a typical charger, and bigger, but I don't mind and the benefits of a single plug that connects to all my balance wires (13 or 24 wires?) is worth it.

 

[Hillhater]

So you really think you can build a 12 channel , current and voltage limiting , isolated, charger, that will be more reliable, safer, and cheaper than a commercial unit ?......good luck .!
Why not just use 12 of the single cell "wall wart" type chargers as others have done here before ?

 

[Hummina Shadeeba]

could use wall warts but dont like it. I use one to bulk charge to 48volts at 2 amps.


" build a 12 channel , current and voltage limiting , isolated, charger, that will be more reliable, safer, and cheaper than a commercial unit "....i dont know. I'm waiting to see an hoping for the best.

you all missed my earlier bad math and with 48 cells at 4 amp charge rate each cell. 48 x 4 volts x 4 amps. 16x 48 =...about 700 or 800 i guess. anyway point is I"m going for high power, really 400 watts would be good enough. I'm sure the cost of parts will go up paralleling wattage ability. maybe I can get right in the low spot of the curve somewhere but ...what's next. anyone? waiting for my craiglist "stereo fixer" to figure the circuit and the cost of the parts and go from there.

 

[Hillhater]

A $30, 400W meanwell PS, and a bunch of those $5 dc/dc adjustable voltage/current regulator modules from Ebay, would probably be better than anything you can build yourself.
....but still a messy solution.