Charging?

green1

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So I've finally acquired and tested enough 18650 cells to build something, and I have a sudden need to do so (The UPS powering a large portion of my electronics died a couple days ago). But I'm finding a gap in my knowledge.

Everyone talks about the battery and BMS portion of building a pack. Occasionally people talk about the inverter, but rarely does anyone talk about how they get the AC from their wall into the pack. So the question is relatively straight forward:

120v 60hz AC outlet here ------ magic here ------ balancing BMS connected to 18650 cells here (still deciding, but let's say 7s for now)

What replaces the magic? I believe that connecting DC power straight in to the BMS is usually possible, but I've also been told that constantly powering lithium ion batteries is bad and that you should have some form of "charge controller" I'm just not sure what that looks like exactly.

So if I want to build a basic UPS from a bunch of 18650s, what exactly do I need to make this work? (ignore the output side for now, both because it's already been discussed, but also because in the end, I don't really need it as every item plugged into that UPS just converted the AC output right back to DC12v anyway, so I'll probably just use a DC-DC converter instead of an inverter powering a dozen wall-warts for this build)
 

Roland W

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You get a charger, which is built for a 7s Li-Ion chemistry

Example:


https://www.aliexpress.com/item/330...earchweb0_0,searchweb201602_,searchweb201603_

This chargers, if you don't add any extra controlling circuit, will keep you pack full to the 29,4V for Li-Ion 7s.

If you would not like to charge your pack to 100% SOC, but rather stop charging at a lower SOC in a automated way, you would have to add some sort of micro-controlled monitoring and switching device.
I have build that for my AC-coupled Powerwall and basically doing exactly that thing, switching AC chargers....
It could easily be configured to do the 7s pack without the grid metering, etc. Just being used as a universal controller.
 
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green1

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Is it healthy to leave that connected full time? I keep hearing how you shouldn't have the batteries on continuous charge?

Could you provide any more detail about your method for stopping charging at a lower SOC? (I see lots of people talking about 4.1v)
 

Roland W

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Is it healthy to leave that connected full time? I keep hearing how you shouldn't have the batteries on continuous charge?

Could you provide any more detail about your method for stopping charging at a lower SOC? (I see lots of people talking about 4.1v)

For you as a user, letting a battery sit at 4,0V a cell or 4,2V a cell, does indeed make no difference, as the additional energy from 4,0 to 4,2V is miniscule. The battery will surely thank you if you keep it away from limits. Using a system as a UPS, as well means, that the battery will mostly sit there at its fully charged state. So yes, it will be healthier no to always be charged to 100%.

Not allowing a full charge could be achieved in 2 ways.
1. you find a charger which is set, or can be set to only go to 28V max for a 7s, or


This 10s LTO charger would do the trick

2. you monitor the pack with a separate circuit, which will switch off/on the charger at a set voltage
Micro-controller using its on-board or separate ADC (analog to digital converter) over a voltage divider (to reduce the 30V to 5V) and switch a relay to shut down or start the charger, a display to keep you in the loop. All for less then 20 bucks (if you use an Arduino Nano clone or ESP32 developing board plus other components), and anytime upgradeable with extra feature you come up with (like temperature monitoring, SOC calculation, Wifi, etc)
 
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green1

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So if you have a 28v charger, and a 7S system, that would be 4v per cell, that part sounds ok. But even at the lower SOC, is it a good idea to have the charger constantly providing power? I've heard people saying that these batteries shouldn't be constantly charged.

Your second approach would cycle the cells more (in fact you could configure as deep or as shallow a cycle as you want) but I'm not sure which is better for the cells themselves, nor am I sure which is more efficient from a total electricity usage stand point.
 

Roland W

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So if you have a 28v charger, and a 7S system, that would be 4v per cell, that part sounds ok. But even at the lower SOC, is it a good idea to have the charger constantly providing power? I've heard people saying that these batteries shouldn't be constantly charged.

Your second approach would cycle the cells more (in fact you could configure as deep or as shallow a cycle as you want) but I'm not sure which is better for the cells themselves, nor am I sure which is more efficient from a total electricity usage stand point.

I don't see a problem with the 28V constantly provided on the battery, as the charger is not pushing any current when the pack is basically just floating. A Mppt or PWM charge controller would do the same.

From efficiency standpoint, of course, less additional devices, electronics, etc, means less power drain. Just go with the simple charger without
any further stuff.
 

green1

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I don't see a problem with the 28V constantly provided on the battery, as the charger is not pushing any current when the pack is basically just floating. A Mppt or PWM charge controller would do the same.

From efficiency standpoint, of course, less additional devices, electronics, etc, means less power drain. Just go with the simple charger without
any further stuff.
You imply however that you chose the switching option instead. What are the reasons to take that approach?
 

Roland W

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In my project? Well, the complexity there is a bit higher ;)
2 or 3 chargers, switched when excess power available or by timers, parameters can be set totally flexible directly on the controller
WiFi, Modbus, Indicators, relays, switches, 16-bit ADC, current measurement, and so on...

That's why I have said before. If you are planning to upgrade your system, like to do a bit of Micro-controller coding, then going the 2nd option is the far more interesting one :)

The picture is my version 2 board. Version 3 will have additionally Ethernet, connectors for 3 temperature probes and a long interval hardware watchdog (as the onboard watchdog with max 8 second intervals is not suitable for all online situations -> occasional resets)
 

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Cypher618

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If you want a little more go than the 2A charger you can also get something like this which is basically a CC/CV buck converter powered by AC mains:


Has to be one of the versions with both voltage and current adjustment. It will let you set your desired SOC very easily. If you set the output voltage such that you are at 4.0 or 4.1 V/cell then you can leave it connected directly. Current will drop to zero on its own over time provided you have good balance. In any case your BMS will stop charging if one set of cells goes too high.

Set the max charge current based on your pack and bms's capabilities with some safety factor, don't go right up to the limit. Because the knobs look easy to turn I would recommend if you go this route you set it then put a bit of hot glue on the knobs so they don't move.
 

green1

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Well that brings up a bit of a question, what's the difference between just a power supply, and a "charger"?

Is it as simple as applying the right voltage, limited to an appropriate current? or do you need a device that does more than that?
 

OffGridInTheCity

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Well that brings up a bit of a question, what's the difference between just a power supply, and a "charger"?

Is it as simple as applying the right voltage, limited to an appropriate current? or do you need a device that does more than that?
@Cypher618 says "....which is basically a CC/CV buck converter...." - notice the CC/CV part of the comment.

CC is constant current and CV is constant voltage - and this feature makes the difference between a 'straight power supply' and a power supply than can act as a 'charger'. CC (constant current) is where the power supply or charger will raise it's voltage above the battery voltage to maintain a constant current. CV (constant voltage) is when the power supply or charger hits the ceiling voltage and can no longer raise the voltage to maintain current... so it goes into CV mode and maintains the voltage at the ceiling voltage and as the battery get's closer and closer in voltage the current get's less and less until the 2 equalized = e.g. the battery is fully charged. Not all power supplies have CC/CV feature - but some do. A power supply with CC/CV will set you set the top (ceiling) voltage and max current. A charger typically has these preset.

I hope this clarifies a bit :)
 
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green1

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In that case, how do we know that the power supply @Cypher618 linked is in fact CC/CV? The listing doesn't use those terms that I can see.

How does one know if a given supply qualifies?
 

OffGridInTheCity

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In that case, how do we know that the power supply @Cypher618 linked is in fact CC/CV? The listing doesn't use those terms that I can see.

How does one know if a given supply qualifies?
I agree - I don't see it explicitly listed. It might be in the fine print (I don't know) but that's a key question to ask/find-out before making a purchase. I get the impression that it's 'assumed' sometimes? but to me, you should know explicitly before purchasing.

On the other extreme, for illustration purposes, here's a description of a bench power supply that explicitly says CC & CV in the main title of the description - "ITECH Bench Power Supply 60V/5A/100W DC Power Supply Regulated CC & CV Mode Adjustable Switching Power Supply for Lab Equipment Research IT6720"
 
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green1

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Further to that, how would one test a supply to know if it's doing the right thing? (I love cheap stuff from China, I don't trust cheap stuff from China)
 

OffGridInTheCity

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Further to that, how would one test a supply to know if it's doing the right thing? (I love cheap stuff from China, I don't trust cheap stuff from China)
Measure voltage and current as it charges your battery. You should see CC and CV in operation :)
 

Cypher618

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The power supply I put forward is indeed CC/CV. Don't get caught up too much in the terminology. If the PSU has voltage and current regulation that you can set then it is CC/CV.

Besides all of these devices power supplies, converters, chargers, are pretty much exclusively made in china. Kinda hard to get away from it...

I would also recommend you get a current meter at the very least. That said the psu i suggested has a voltage and current display, so you can go without it.

If you get a meter the power/energy meters are very useful. Something like this:

Which is also extremely useful to test capacity on your battery packs.
 

green1

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No offence, but I'm not sure how having knobs makes it special here.
Every power supply has a voltage and a current limit. Making them adjustable doesn't necessarily change anything. Hence my question on whether you just need to find a supply with the right numbers, or if there's something different about a cc/cv supply.

I've honestly never seen any power supply that didn't put out its maximum current until the voltage gets up, and then hold at its maximum voltage. Does that mean all power supplies are actually cc/cv and this is all just marketing? I'm honestly not sure.

Or is it really more about the power supply's ability to run at max current for long periods? (Some do get rather hot at max output)
 

Cypher618

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No offence taken. And yes you are hitting upon it with your second paragraph.

All of these switch mode power supplies are based on ICs which create pulses which are passed through an Inductor and some capacitors to generate an output voltage. If the voltage drops the IC creates more pulses to bring it back up. If the current load increases the IC generates more pulses to maintain the voltage and current. There's more to it but that's the gist.

In order to do this these IC's have feedback for output voltage and current which are set with some reference circuitry. This reference circuitry is what actually sets the output voltage and current limits of the supply. In your typical power supply these references are fixed so your 10A rated 5V supply is CC/CV for 10A/5V.

The only difference here is that the reference circuitry is variable through the pots.

An expert can take any common PC PSU and turn it into a variable current and voltage supply with modifications to the reference circuitry. I admit I myself am not at that level of skill. There are videos on youtube of people doing this.

So in conclusion, the knobs do make it special :).
 

green1

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So in conclusion, the knobs do make it special :).
I'm getting the opposite conclusion from your post. It seems like knobs are nice, but really as long as the numbers are right, it doesn't matter if they are just what the supply was built for, or settings you adjusted yourself with the knobs.
 

Cypher618

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If your design calls for a 5V/10A supply for example then by all means yes you don't need the knobs.

The problem is more about flexibility. These supplies let me have 28V-29.4V for a 7S (4.0-4.2v/cell) pack and let you set the current based on your desired max charge rate. Most chargers out there are set for 29.4V. Beyond that this one supply will let me charge packs all the way down to 1S at any amperage I want.

And then there is price. I am paying $100 cad delivered for a CC/CV power supply for my 7S packs which is 1000W and will give me a peak of 27A. If you add another $50-100 you can get a charger that can meet those specs but if your packs cant handle the amperage you will have a bad day. Or maybe you want to vary the charge rate to not be too aggressive...etc.

In my case since some of my setups wont handle the full amperage. I need the ability to vary it based on my needs. I also like to have the ability to vary the peak voltage so I can choose to extend life by lowering voltage if I desire.

To each there own. No problem with a charger, thats probably ok for your application.
 
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