Charging with a coil (Adaptto)?

There's a little bit of laziness to this post, but I will honestly say that i have very little idea on how to setup bulk charging with a 70a coil

I'll be charging up at 20s8p samsung 25R pack. I guess my options are the bmsbattery bulk chargers or getting a set of meanwells from teslanv and wiring them in series. I get how i'm gonna need three wired in series to manage charging my pack...

What i don't understand is how to gauge what kind of psu is required if you are using the 70a charge coil. I've heard from sources...that I should get a psu lower than my battery voltage after full discharge (in this case 50V). How do you know how much extra voltage the 70a coil will provide? how do you titrate that voltage? Also, how does using a coil help/hurt charging? Faster charge times? Any insight and guidance on what to get what be greatly appreciated.

endless-sphere.com/forums/viewtopic.php?f=2&t=67579 Some folks use power supplies such as the ones found in here. Incredible pricing for what you get.

To my understanding, so long as the PSU voltage is below your packs voltage it'll work out.

Hmmm, i see they're still selling a few in the UK. I'll be honest, I'm not the best at soldering and i'm nervous I'll jack up the connections and get a problem down the line. Are they difficult to solder/set up?

Soldering is easy, very, very easy. https://www.youtube.com/watch?v=J5Sb21qbpEQ Watch some videos, gain a level of understanding, splice some wires, then you'll understand and know a lot of what is necessary. In case you didn't know, there is something known as desoldering wick. It seems like braided copper that has flux on it, maybe. It soaks up most mistakes with great ease.

There are a lot of different types of connections and connectors to make, I am not exactly sure what you are asking. Soldering is always an option, but in cases where you want to make frequent connections and disconnections, something like anderson power poles are fantastic.

What the coil does: the coil combined with the Adaptto controller form a boost converter, which is a type of DC-DC converter that boosts a lower DC voltage to a higher DC voltage. The Adaptto does what I guess you're calling "titrating" - it regulates the output to the battery to create the CC-CV charging profile.

If you charge at 0.2 C, that's about 4 A, so toward the end of the charge at 80 V you'll be at 320 W. A quicker charge of 0.5 C would put you at 800 W. In the latter case, even a 24 V power supply would be putting out only 33 A, so your 70 A coil is oversized, but that's not a problem. Just make sure the PSU is rated for the appropriate power and its output is below 50 V.

Cyborg: thanks for replying and shedding some light on coil charging. I have ordered 3 24V meanwells in case I want to just bulk charge without the coil at 83-84V for 20s. I can use another tap that has an output of up to 57V (basically using 2 meanwells in series). My question is...

Constant Current Limiting (~14-16A) and ~1200W
Voltage adjustable from 21.6V to 28.8V per unit (up to 86.4V)

I didn't think about it, but I suppose in the future I can get a bulk charger that has a higher amp rating...I forget the c-ratings on the Samsung 25Rs...but I think typical charge is 0.5C but you can do rapid charge at 4A.

As for the output of the charger with the coil....should you set it to <50V? The pack fully discharged would be 50V. I guess i'm having confusion as to knowing what to set the PSU voltage to....sorry if i'm being a brickhead.

For charging with the coil+Adaptto, the PSU voltage must be below the pack voltage, so you could use two of your PSUs in series and set them for 24 V each for a total of 48 V.

Do I understand correctly that the 1200 W number is for the three chargers together, not each? If they're nominally 24 V and 15 A, that's 360 W.

If so, then at the lower end of the charge, say 60 V, that means that you could put 12 A into the battery, while at the top end, 80 V, it would be 9 A. So roughly 0.5 C. This would take roughly 1.5 times as long as bulk charging with the 3 PSUs in series. The tradeoff, of course, is that the Adaptto will give you more control over the charging profile and will balance the cells, assuming you have the Adaptto BMS.

Ahhhh...I see why the coil charger is helpful and yes I will have the adaptto BMS. So, bottom line i'm happy that I got the 3 series setup since its capable of both bulk and balance charging. Hopefully I won't have to balance to often.

The 1200W I guess would be from 3 series chargers. The link to meanwells I bought are below...maybe i'm interpreting this wrong. Obviously uncharted territory for me. I've only had bulk chargers from bmsbattery :?

http://endless-sphere.com/forums/viewtopic.php?f=31&t=68195

So I guess my question is how much to set the voltage to during balance charge? I know the pack fully discharged is 50V, so I would set it to 50V or 49V or 48V? I would only be using 2 units so presumably a max of 720W. At 720W and 50V that would be 14.4amps....which is a little above 0.5C for a 20ah pack maybe that's too much.

Also, for coil+adaptto if the psu voltage MUST be below pack voltage...do I have to change the psu voltage if I happened to not discharge the pack fully? or will the adaptto take care of that?

Adaptto will take care of everything so long as your PSU voltage is safely below the lowest voltage your pack will ever get to. So if that's 50 V, set the PSUs to 45-48 and just leave them there regardless of your battery state of charge or anything else. The Adaptto will boost the voltage as needed depending on your state of charge and the charging current you program into it.

Keep in mind that when the voltage is boosted, the current is reduced by the same proportion. So if the supply is rated for 15 A, then toward the end of the charge at 80 V, the current to the battery can't be more than 15 * 48 / 80 = 9 A, so this is probably the highest charging current you'd want to program into the Adaptto, otherwise the PSUs will start limiting current. You can always set it lower if you want.

cycborg said:

Keep in mind that when the voltage is boosted, the current is reduced by the same proportion. So if the supply is rated for 15 A, then toward the end of the charge at 80 V, the current to the battery can't be more than 15 * 48 / 80 = 9 A, so this is probably the highest charging current you'd want to program into the Adaptto, otherwise the PSUs will start limiting current. You can always set it lower if you want.

Click to expand...

In the Adaptto charge menu you can limit the charging current and the PSU current independently and also the max voltage drop admissible for the PSU.

Hmm...i think i have a handle on coil charging. I'll have to set the voltage for 2 units to be 50V and the 3 unit (series) to be 83V. I still don't know why the PSU MUST be less than the lowest voltage for the pack. What would happen if it wasn't?

On another note, although my 3 meanwells in series is a relatively good solution for bulk charging...20s8p (83V, 20Ah, 1660Wh) so about 1.5 hours until full charge. What is the most OPTIMAL/fastest way to charge?

moonshine said:

Hmm...i think i have a handle on coil charging. I'll have to set the voltage for 2 units to be 50V and the 3 unit (series) to be 83V. I still don't know why the PSU MUST be less than the lowest voltage for the pack. What would happen if it wasn't?

Click to expand...

As stated previously, the coil+Adaptto form a boost converter, so if you want the gory details on how that works you can start with the Wikipedia page. But basically the Adaptto switches the current flowing through the coil with a duty cycle D. The voltage relationship is

Vout = Vin/(1-D)

Since D is between 0 and 1 the smallest possible value of Vout is Vin (for D=0 or switch always open), and in theory Vout can become arbitrarily large for D near 1.

I don't know how smart the Adaptto is but hopefully it's able to detect the VPSU>Vbatt condition and prevent charging. If not, it will attempt to limit current to the set CC value but will be unable to do so because it can't go lower than 0% duty cycle, so current will flow at the current limit of the PSU.

On another note, although my 3 meanwells in series is a relatively good solution for bulk charging...20s8p (83V, 20Ah, 1660Wh) so about 1.5 hours until full charge. What is the most OPTIMAL/fastest way to charge?

Click to expand...

"Optimal" would be more complex than the single-step CC-CV profile the PSUs are capable of. The amount of current you can push into a battery without damaging it is related to its state of charge (SOC); large for low SOC and decreasing as SOC increases. For example, the Tesla supercharger uses three steps - a high CC at low SOC, lower CC in the upper part of the SOC range, and then CV at the terminal voltage. "Optimal" would probably be a continuous tapering of current as a function of SOC.

For your PSUs, really the only adjustment you can make is the terminal voltage. Unless you're sure that your cells are well matched and balanced, you should probably set that at some safe level that's something like 0.2 V/cell lower than the specified charging voltage for those cells. This will prevent you from getting that last few percent of charge but you'll be able to get a lot of charge into the pack quickly without worrying about damaging cells that are out of balance on the high side.

Maybe a more "optimal" strategy, but one which would require more interaction on your part, would be to bulk charge to an even safer level to get most of the charge in, then switch over to the coil+Adaptto arrangement to finsh the charge to make sure you get to the full charge level safely.

cycborg said:

"Optimal" would be more complex than the single-step CC-CV profile the PSUs are capable of. The amount of current you can push into a battery without damaging it is related to its state of charge (SOC); large for low SOC and decreasing as SOC increases. For example, the Tesla supercharger uses three steps - a high CC at low SOC, lower CC in the upper part of the SOC range, and then CV at the terminal voltage. "Optimal" would probably be a continuous tapering of current as a function of SOC.

Click to expand...

Sorry if this is too zombie.

I have felt intuitively that the above is true, in general for most chemistries, but especially for the various LI variants.

I am specifically interested in LiFePO4, and by the term "damage", am highly motivated to get maximum longevity, as in decades. In projects I have been working on, my colleagues have been very skeptical of the above, and

I would really like to gather some convincing evidence it is true. Ideally links to cites, any academic or industry research studies, etc.

Or just informal experience based statements from people with credible qualifications.

Thanks in advance

In your case, your best bet would be to limit the SOC to 20-80% to allow for maximum cycle life along with fast charging capability.

Thanks, but this not a practical howto question.

I am specifically asking for any supporting evidence available for the hypothesis I quoted above.