Battery charging

[absrec]

Forgive me if this is not the right place/forum. I primarily build audio electronics but I have a question about charging LiFEPO4 batteries. If it makes any difference, I’m using it to power small speakers and other audio related mobile gear.

I’ve been searching the net for a while and can’t seem to find an answer about how to charge  a 12V Lithium Iron Phosphate battery. It came in a small UPS but the BMS failed so I disassembled the whole thing down to the 18650 cells and purchased a new BMS on Amazon. I now have it fully assembled and would like to make an automatic rechargeable battery bank. The chargers I find online are either USB interface for a single cell or hobby chargers aimed at people that want to charge a bunch of individual cells or charge them in a balanced configuration.

I found some articles and videos where people use a CV CC DC-DC power supply. My question is what else I need and how I would set it up. I see other modules that are supposed to act as charge controllers but they all seem to be aimed at people interfacing with solar cells. Is the BMS supposed to cut off the charging or is that the job of another component?

Basically, I just want to set up a system that I can plug in to the wall when power is available and not have to babysit the charging process.

Again, sorry for the seemingly random question. I imagine with all the smart people on this forum, someone has some experience with batteries.

Thanks,
-Aaron

 

[Bo Deadly]

I don't have any direct experience but just from a quick search I found a page that claims you're supposed to do constant current up to 60% and then constant voltage (3.65V / cell) to get to 100%.

One could probably make a relatively simple discrete circuit that consists of a voltage regulator followed by a current limiter where the current limiter "bottoms out" when the current falls below a certain level at which point the voltage becomes more and more constant. It would literally be a few transistors and resistors and an old laptop brick. When it's fully charged, it would stop current and safely sit indefinitely until you unplug it. Add a comparator w/ LED to indicate when full charge has been reached.

However, I'm sure you realize this is vaguely dangerous. If you get the circuity a little wrong or a solder joint fails, the battery could burst into flames and burn your house down. Put it in the shed if you have one.

 

[L´Andratté]

With LiPO it´s essential to never go below a certain voltage (vastly degrading capacity) and never above (house burning down)...
There are also a bunch of dedicated chips from Maxim (and others) to do just that: turn off power to main circuit below some voltage, turn off charging circuit above some other voltage.

 

[absrec]

Just to clarify… I am working with lithium iron phosphate cells.  A fully constructed battery actually. The battery has a BMS. That is battery management system. The individual cells are managed by this board.  At this point you can pretty much think of it as a single 12V battery. Of course,  keeping in mind the specifics of the chemistry. The rated voltage is 12.8V. Nominal is 13.6V. Saturation charge voltage is 14.4V -14.6V.  I’ve read in more than one place that they can be charged at 1C.  The particular battery I’m dealing with is 2.4AH.  Pretty small as far as these things go. It’ll probably run my portable speakers or a wireless transmitter for a whole day or more on a single charge though.

It will charge with a standard lead acid charger. I’ve already done that.  Lead acid battery specs are slightly different so it never really gets a full charge which isn’t necessarily a bad thing.  Lead acid chargers keep a trickle charge on the battery which I’d rather not have.

As far as building a charger, I’d rather stay with prefab circuits specifically designed for this task. I’m just not exactly sure what to use.  I am also not 100% sure how much responsibility should be left up to the BMS.

 

[Bo Deadly]

Then what you're really asking is how to interface with the "BMS" board so it seems to me that you need to find and post information about that. At least post a pic.

 

[Matador]

If you aren't concerned with time to charge, you can generally charge CV at the nominal terminal voltage, which only requires a simple voltage regulator.  However time to charge will be long as the cells internal impedance rises, but the battery can be left on CV essentially forever without risking overcharging (because the current falls to zero as the battery voltage rises).

 

[absrec]

Here's the BMS board I used. The battery is 4S2P. The BMS has connections for each of the 4 pairs of cells. My understanding is that it is there to prevent each individual parallel pair from overcharging. Thus optimizing the life of the battery and preventing various kinds of damage.

https://www.ebay.com/itm/For-LiFePo4-LiFe-18650-Battery-4S-30A-12V-BMS-PCB-Protection-W-Balance-Board/153285127567?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

So, a CV CC DC-DC supply hooked up to this battery will charge it. My main concern is - when the battery reaches a full charge, what is there to let said DC-DC supply know that it's time to take a break?

I looked at this on eBay and it seems like it could be what I'm looking for. However, while I'm not hesitant with building a pedal or a mic preamp, I'm hesitant with anything involving a lithium battery. Even LiFEPO4 which is professed to be the safest of all lithium types.

https://www.ebay.com/itm/18650-Lithium-Battery-Charger-Board-with-Over-Charge-Discharge-Protection-6-40V/132699236160?_trksid=p2485497.m4902.l9144

Maybe I'm just slow but I can't tell if this product still requires the use of the CV CC supply mentioned above or perhaps just a standard AC-DC "wall wart" supply. One of the pictures show a 220VAC connection. (???) I understand it depends on how you are using it. Which is why I'm seeking the advice of others who maybe have some experience with such things.

Thanks again for all the kind and helpful responses.

 

[Bo Deadly]

That board handles the CV / CC stuff for you. CV means constant voltage. CC means constant current. That board handles all of that. Again, I know nothing about battery charging specifically but my guess is that this is just looking at voltages and supplying CC until it reaches a certain voltage and then transitions to CV all the while monitoring the voltage on each cell to adjust the supply to each so that they charge evenly and to implement the various protections.

So in other words, you don't need anything else special to use it. You just need to supply 14.8V DC. Although that is a rather specific voltage. So you will need to find a suitable DC supply. A 15V DC supply would almost cetainly be fine. A regular SMPS like this:

Mean Well LRS-150-15

would be the right type and voltage but it's only 10A and that BMS board is 30A so depending on the battery it might not be enough. I would have to understand what sort of current the BMS fully loaded is drawing. But 10A is pretty beefy. Be careful. Make sure you use a really good thick wire like 18 AWG or thicker and solder the pads well. If your iron is adjustable, crank it up hot, give it a decent amount of solder and tape the wire and board down when you solder the pads so that the wire doesn't move around after you remove the iron. That copper layer is going to require a lot of heat to really get the solder liquid. Use a flux pen if you have one. Then put spades on the ends for the SMPS and maybe solder them as well if you don't have a proper crimper. Then just connect the SMPS to a power strip, squint and throw the switch but keep your finger on it and switch off if you see anything funny. You might also hook a meter up to the output of the BMS to look at what the series voltage is. You should see it slowly rise from whatever the starting voltage is to around 14.4V DC.

It will quit when the threshold voltage is reached. I don't know if it will reach a trickle state. It could be that when the threashold is reached it flips into a no-current state that will only reset when the load / supply is removed.

 

[absrec]

That’s kinda what I was thinking. The 30A rating on that BMS board is for discharge. Since the overall pack is 2.4AH, the max you would want to charge at is 2.4A. That’s what 1C means on the spec sheet for a battery. A lot of folks prefer to go with 0.5C to preserve the life of the cells. In my case, that would be ~1.2A. The use of a Constant Voltage/Constant Current supply is necessary for sure. Most of them have a trim pot to adjust the current to whatever you need.

The main thing I’m concerned about is having a system where once the battery reaches it’s fully charged state, the output of the charging supply will be cut off until the battery is again in need of a charge. Probably after it has been used. I’ve seen some boards on eBay & Amazon that will interrupt any load on the battery when it drops below a certain voltage and reconnect it once the battery has been charged to an acceptable range. Maybe cut off at a lower limit of 11.4V and reconnect at 12.8. These voltages are configured by dip switches usually. This is certainly useful but it doesn’t seem like it would do anything for the charging process.

I wish I could find more info on BMS boards. Rod Elliott did a great article on this very subject but he openly states that “BMS” could stand for battery management system or battery monitoring system. While I’m pretty sure that what I have is the former, the lack of specifics and broken English in the description of the items still leaves me a bit unsure.

 

[Matador]

The use of a Constant Voltage/Constant Current supply is necessary for sure.

It isn't necessary - the only reason to use CC is to speed up charging.  Most batteries look like voltage sources in series with a resistance.  When the battery is empty, the resistance is low, as is the terminal voltage.

Here is an example:  a drained LiPo looks like 3.6V.  It may take a terminal voltage of 4.1V in order to charge at 1A, which means the battery looks like a 0.5 ohm resistor (at that point!).  As the battery charges, the resistance rises, which means in order to keep 1A flowing into the battery you must raise the terminal voltage.  A system that sets the terminal voltage at whatever needed to maintain 1A means it is in CC mode.

However at some point you will surpass the maximum safe terminal voltage (4.2V for LiPo), which means once this happens you need to switch to CV mode, otherwise the terminal voltage will need to exceed 4.2V in order to maintain the 1A.  Once you are in CV mode, the terminal voltage will be held at 4.2V, which means as the resistance of the battery continues to rise, the current will start to fall.

You can connect a LiPo to a 4.2V source forever, because the battery will only accept enough charge to bring it up to said 4.2V terminal voltage, which is the maximum safe level.  Because there is no CC mode, the charging is very slow, because the resistance rises quickly which means charging current also falls off quickly.  But you don't need CC mode unless you want to charge faster (all other things being equal).  You can made a 4.2V charging source with about 5 or 6 parts (a standard LM317 circuit), and it is flawless charging (provided it's built correctly).

 

[Bo Deadly]

Because there is no CC mode, the charging is very slow, because the resistance rises quickly which means charging current also falls off quickly.  But you don't need CC mode unless you want to charge faster (all other things being equal).

I don't understand this. Charging would be fastest with CV because holding the voltage at 4.2V (for LiPo in your example) would yield the maximum amount of current.

My understanding is that CC is used to limit the amount of current to prevent damage to the battery. Not to charge faster.

Also, it is not crystal clear that you understand that the BMS board that absrec is using is doing important things between the power supply and the battery.

Regarding "The use of a Constant Voltage/Constant Current supply is necessary for sure", I don't understand that either since SMPS power supplies are either CV or CC but not both. However, a CV supply is constant voltage up to the rated current at which point the voltage starts to drop in which case it does become CC in a way.

My best guess at this point is still that you just supply a constant voltage to the BMS and it does the rest. It seems perfectly reasonable that a board with MOSFETs and resistors like the one cited would monitor the terminal voltage and adjust the current as necessary to maintain a safe voltage.

So, the short answer is that you just need to connect a CV SMPS with a current rating sufficient to handle the maximum charging current. So if that is 2.4A, you might use something like a Mean Well RS-50-15.

Then again, I could be wr-wr-wrong.

 

[abbey road d enfer]

Maybe I'm just slow but I can't tell if this product still requires the use of the CV CC supply mentioned above or perhaps just a standard AC-DC "wall wart" supply.

The text is quite clear; the board requires a DC source of 6-40V. Indeed the actual value must be higher than the nominal battery voltage, so for a 12V battery, I would think a minimum of 16V is safe. No need for regulation here, it's done by the charger.
The illustration with a 220Vac actually shows an AC/DC converter inserted.

 

[Bo Deadly]

The text is quite clear; the board requires a DC source of 6-40V. Indeed the actual value must be higher than the nominal battery voltage, so for a 12V battery, I would think a minimum of 16V is safe. No need for regulation here, it's done by the charger.
The illustration with a 220Vac actually shows an AC/DC converter inserted.

I think that board is just a voltage regulator, cut-off relay and volt meter (which what the 4-60V input is for). If you hooked that up to a 4 pack of 18650, I think bad things could happen. The previous link is the BMS board that handles the CC / CV work and makes sure that the batteries charge evenly.

But they could be used together. So you could do:

Constant Voltage SMPS 15V 2.5A+ >>> "charger" board >>> BMS board >>> 4 pack of 18650 LiFEPO4

 

[Gold]

When I was reading up on solar battery banks it always  was recommended to have the batteries in series.  Charging in parallel requires more sophisticated monitoring and is more dangerous because the batteries can charge unevenly.

 

[Matador]

I don't understand this. Charging would be fastest with CV because holding the voltage at 4.2V (for LiPo in your example) would yield the maximum amount of current.

My understanding is that CC is used to limit the amount of current to prevent damage to the battery. Not to charge faster.

In the abstract, yes, however in practical terms the impedance rises quickly during the first 5-10% of charging, so you won't be 'current limiting' for very long.  Yes, the charge current may be initially high (in the 5C to 10C range), but it drops quickly enough (like in the matter of minutes) that it isn't the limiting factor in charging safety.

Also, it is not crystal clear that you understand that the BMS board that absrec is using is doing important things between the power supply and the battery.

I'm speaking about how to charge LiPo cells, not about a board implementation.  That board looks like a standard MOSFET shunt balancer which limits charging current to the maximum that can be handled by the most-charged cell.  In practice, with a sealed multi-cell pack, these generally aren't necessary (the exception to this is when you mix 18650 cells of various manufacturers and various charge states - I not sure the OP is trying to do this however). 

 

[abbey road d enfer]

I think that board is just a voltage regulator, cut-off relay and volt meter (which what the 4-60V input is for). If you hooked that up to a 4 pack of 18650, I think bad things could happen. The previous link is the BMS board that handles the CC / CV work and makes sure that the batteries charge evenly.

But they could be used together. So you could do:

Constant Voltage SMPS 15V 2.5A+ >>> "charger" board >>> BMS board >>> 4 pack of 18650 LiFEPO4

What about the illustration that shows the battery connected to the board and the board connected to a basic DC source? And the description "18650 Lithium Battery Charger Board with Over Charge Discharge Protection "? What more is needed than "Over Charge Discharge Protection"?
What is not clear is how it manages series-connected cells (if ir does).
The BMS does clearly manage series-connected cells, with balancing.
I have several chargers that do not have balancing connections for series connected cells; so far they work quite well and I have not noticed any premature aging of the batteries.

 

[Bo Deadly]

What about the illustration that shows the battery connected to the board and the board connected to a basic DC source?

Actually if you look closely at those illustrations, it's actually not a "Battery Charger Board" at all. There is no current going through the board. It looks like it just monitors the terminal voltage and opens the relay when the target voltage is reached. And it acts as an indicator so that you can see what the voltage is.

Also, even though "18650 Lithium" is in the name, there is no such 12V battery. 18650 cells are 3.6V. Four cells in series is apparently a common configuration (as evidenced by the BMS boards supporting 4 cells in series) which yields 14.4V. If you own a power tools you know that is a common battery voltage.

So that board and it's illustrations are somewhat general approximations of how it might be used. I don't know how important the BMS is but it seems like a good idea. Maybe it's only really important if you're paralleling cells to make 14.4V packs of 8, 12,  16, 20 and so on.

 

[Matador]

As I said above, those boards are useful if you are going to use 18650 cells in a manner analogous to regular AA batteries: e.g., you are going to pop them in and out individually, and charge cells from different manufacturers / time periods together.

Say you try and charge one full cell in series with 3 empty cells.  The circuit is going to recognize this condition, and shunt the charge current from "around" the full cell into the balancing resistor(s).  This is a terrible way to balance because it bleeds the excess charging current out of the circuit until the other cells "catch up".

Better circuits are called "active" balancers, as they essentially insert themselves in parallel with the cell and steer charge current "around" the charged cell, which means you can charge the empty cells at high current even with the full cell in series.  They require quite a few more parts though.

If you are dealing with "packs" made of known cells that are charged and discharged as a unit, then this board likely doesn't do anything 95% of the time.

 

[Bo Deadly]

As I said above, those boards are useful if you are going to use 18650 cells in a manner analogous to regular AA batteries: e.g., you are going to pop them in and out individually, and charge cells from different manufacturers / time periods together.

No. That BMS board cannot be used for that. If you look at the description of other 4S boards just like it (nearly identical) the charging current is 30A but the balancing current is only 58mA. So it will only work on packs of similar cells.

 

[abbey road d enfer]

Say you try and charge one full cell in series with 3 empty cells.  The circuit is going to recognize this condition, and shunt the charge current from "around" the full cell into the balancing resistor(s).  This is a terrible way to balance because it bleeds the excess charging current out of the circuit until the other cells "catch up".

Better circuits are called "active" balancers, as they essentially insert themselves in parallel with the cell and steer charge current "around" the charged cell, which means you can charge the empty cells at high current even with the full cell in series.  They require quite a few more parts though.

I don't see much difference between the two schemes you propose; both steer charging current from the cells that don't need it. That works only if there is individual access to the cells.

If you are dealing with "packs" made of known cells that are charged and discharged as a unit, then this board likely doesn't do anything 95% of the time.

There are two types of boards that are being discussed. One does not have access to the individual cells, the other (BMS) has. I would think the latter caters to the needs you express, the former being adequate for packs that are always connected in series, and thus supposedly in teh same state of charge/discharge.
The one you say "doesn't do anything 95% of the time." actually charges the battery!