Calling All Current Regulation Masters - Unique Driver Needed

[chefla]

Then these ones are ok ..... and for the slow charger, i will suggest you a very easy and efficent solution.

Get a transformer with a 24V 1A secondary (relatively small and common), followed from a rectifier bridge and a 1000uF 50V capacitor ..... then build a voltage regulator, followed from a current regulator (you can use a pair of LM317 in serie, for this, with a pair of small heatsinks) ..... set the first one (voltage regulator) for get 28.5 / 29V (the max nominal voltage of the 8 cells pack plus the dropout of the second IC and the diode), then set the second (current regulator) for 1/8 to 1/10 of the rated current of the cells (like, if the cells are rated as example 2Ah, set it for 200 to 300 mA), and finally a silicon rectifier diode (high current, 3 to 5 A rated) in serie, connected directly to the cells pack.

The diode prevent te discharge of the pack through the regulators (so you don't need a switch for change from use to charge), the regulators acts as CV/CC charger ..... when the cells are undercharged or weak, they become charged in current, until they reach the nominal value and start to "eat" always less current til it become constant voltage maintaining at low current ..... this also cannot overcharge the cells, cause when they are at the full voltage, the current draining will virtually stop.

That is a great idea for a charger. Never thought of that. Do you have a website or something were I can get more info? Does this work for all Li-Ion LiFePo, etc.?

 

[Cyparagon]

It works for any battery. Values change, but the basic design doesn't need to.

 

[Helios]

Thats a little over my head so I will have to do some reading to fully understand the design but it sound great. An integrated charger would be awesome. I keep reading about balance charters which I assume ensure that all the cells are charged the same....is this something I need?

 

[Cyparagon]

Meh... if they're from the same manufacturer and have the same capacity, it should be good enough. Most any battery pack omits this step, and they're okay.

 

[HIMNL9]

I don't have a website, but can draw a schematic and load it on the album, when i finish it.

It's relatively simple ..... give me a bit of time, corel is not the better program, for draw schematics, and i have only it here.

EDIT: here it is:

This is a schematic principle, hope it's clear enough ..... basically, you have a 24V transformer, that passing from a bridge/capacitor will give you at least 32V DC (a little bit more under no load, 24*1.41 multiplication rate) ..... the first 317 must be set for have 29V at its output, then the second one, wired as current limiter, must be set through RSET value for have a current that is from 1/8 to 1/10 of the manufacturer rated current of the cells ..... as example, A123 cells are usually rated at 4 to 4.5 Ah, so a 400mA setting is ok, and this charge the pack in usually a night or few more (12 to 14 hours)

First capacitor can go from 470 to 1000 uF, 50V, the second electrolitic one is enough as 100uF, 50V ..... rectifier bridge must keep at least 50V, or more, and at least 1A, better if 2A.

The LM317 will probably not heat too much with these values, but is always better to heatsink them ..... anyway, also a pair of aluminium strips are enough (just remember to not use a single strip, if the ICs are not insulated from the heatsink ..... if you short the two tabs together, it will not work)

And, thanks to the silicon diode, the circuit can be left permanently connected to the cells, no switches are needed for shift from use to charge.

The main thing to care, anyway, making battery packs, is to always choose all the cells from the same production lot, so they have the more uniform discharge rate, internal resistance and characteristics ..... this will prevent overcharging or undercharging one of the elements, overheating, and made possible to use the whole pack as a single cell both in charge and discharge cycles.

I used this system on a lot of projects from years, for Nichel and Li-Ion packs, and til now none of them gave me problems ..... anyway, i used it for 14.4V cells max, never used for LiFePo (and don't have them for test it myself), and the input voltage is near the limit of the 317 IC in this particular case, so if you decide to use it, remember that is better to test it at least for some hours (or for a complete charging cycle) with the 8 x LiFePo 26V pack, before to enclose it anywhere ..... this for be sure that there is nothing that overheat at this voltage.

 

[chefla]

Thanks for the wiring schematic HIMNL9! I never thought that it was possible to charge them with such an easy circuit. I have done a lot of reading so far and got the impression it was more complicated to build a charger for LiIons.:thanks:

 

[HIMNL9]

Well, is complicated if you want a "smart" charger (so called), that keep in control temperature of cells, current curve in time, number of cells connected, initial conditions, and so on ..... but this is normally used for "fast chargers", that MUST do all this, for avoid to blow up the cells in the charging process ..... usually those chargers uses a microcontroller too.

If, instead, you can accept the normal "slow charging" rate (and if you already know the number of cells and the nominal voltage of the pack), it become much more easy.

EDIT: by the way, i hope it's clear that the circuit i drawed is for a 25/26V LiFePo pack, NOT for single cell, or for less cells, right ?

 

[chefla]

No worries, I understand that this setup needs to be modded for a single cell or two cells. Would you set the voltage to 4.2V for a single cell or maybe go a little lower? On the current side I agree that 0.1C is a good current. But for voltage I might include some headroom for safety and only set it to 4.1 Volt instead of 4.2V/cell.
Have you tried it with unprotected cells? I hope were are not getting too off-topic here...

 

[Cyparagon]

You'd need to set it to 4.2 plus the dropout of the current regulator plus the 0.7-0.8V for the diode. Something like 6-7 volts.

 

[Helios]

No worries, I understand that this setup needs to be modded for a single cell or two cells. Would you set the voltage to 4.2V for a single cell or maybe go a little lower? On the current side I agree that 0.1C is a good current. But for voltage I might include some headroom for safety and only set it to 4.1 Volt instead of 4.2V/cell.
Have you tried it with unprotected cells? I hope were are not getting too off-topic here...

He's was describing a charger for my application so he was on topic.



I think for now I am going to just run it off of a car plug and at batteries later.

What can I do to make the output adjustable?

 

[HIMNL9]

No worries, I understand that this setup needs to be modded for a single cell or two cells. Would you set the voltage to 4.2V for a single cell or maybe go a little lower? On the current side I agree that 0.1C is a good current. But for voltage I might include some headroom for safety and only set it to 4.1 Volt instead of 4.2V/cell.
Have you tried it with unprotected cells? I hope were are not getting too off-topic here...

Yes, used that configuraton for unprotected cells ..... and yes, i was also more cautious than you, setting the first IC for 4V each cell, it was usually enough, cause the charging is slow type, not fast type.

For a single cell, anyway, you have also to reduce the transformer outoput, otherwise you fry the first 317 ..... basically, you can choose it in this way: 7.5 to 9V for a single cell, 10 to 12V for 2 cells (supposing we're speaking about Li-Ion) ..... build the circuit and set the current with the RSET value, then place a 100 to 220 ohm resistor in place of the cells (so the current is lower than the one you've set, and you have it in voltage mode), and regulate the voltage at the resistor til you get 4V for each cell.



EDIT: @ Helios: as i said, if you use the 150W one, that have a max limiter resistor for the current, the better way is to change that resistor (with one of enough power, ofcourse) for have the maximum output current of, say, 3A, then reducing the voltage, you reduce the luminosity, and when you increase it, the limiter stop automatically the increase when the max current is reached.

 

[chefla]

Thanks again. I tought about your circuit and I am still a bit confused. I would put the current limiter after the rectifier and then the voltage regulator. Therefore I get a more precise way to set the voltage? :thinking: ... hm, your way should work also. Since you have the 10k poti in there I can use my voltmeter and do the fine trimming.
What happens when the current drops below 10mA as the batteries are almost fully charged? Will the 317s still be able to regulate correctly? I think I read somewhere that they would need at least 10mA for correct operation?
Maybe I should stop asking questions and just build this charger and test it

 

[HIMNL9]

Well, no, the current regulator must be after the voltage regulator, for work stable ..... and yes, some of the 317 requires a minimum of current for work correctly, but this is easily solved with 2 leds and 2 resistors ..... placing a led at the output of each one of the regulators, and setting them for 15mA current ..... this also give you a visual indication of charge circuit working.

Being "picky" (hope is the right word ), you can also use a led and a resistor for the first 317 ("on when is powered" indication), and a serie made with led - resistor - zener for the second one, with the zener calculated for turn on the second led when the circuit is going in voltage mode ("almost charged" indication)

 

[Helios]

Heres a question that has completely escaped me up until this point. How do I set the current? A testload would be ridiculous for this thing! Since its not as sensitive as a laser diode can I just hook it up with the pot turned low and then increase until I get a 3A reading with the meter in series?

 

[HIMNL9]

Do you mean the current of the led driver ?

If you use the 150W one, you need to know the value of the limiter resistor, and the part number of the IC, then you can calculate that resistor for limit the current to 3A, and then use the trimmer for change the luminosity.

As alternative (less safe, but still usable), You need to check if the trimmer on the driver PCB have two pins connected together (the central one and one of the side ones) ..... if yes, this means that the trimmer is used as variable resistor, so you can easily modify the driver, and substitute the trimmer on the driver with a potentiometer and a trimmer in serie (or just cut one of the tracks, and add the potentiometer in serie with the trimmer on the PCB), and place the potentiometer accessible from the external ..... then you can regulate them for have a preset at the max voltage for the max current with the trimmer), and use the potentiometer for lower the luminosity.

Do you already have the driver in hand ? ..... for know the values and the configuration, you need first to check if the voltage at the output increase when the trimmer value is lowerewd, or when is increased.

 

[Helios]

Do you mean the current of the led driver ?

If you use the 150W one, you need to know the value of the limiter resistor, and the part number of the IC, then you can calculate that resistor for limit the current to 3A, and then use the trimmer for change the luminosity.

As alternative (less safe, but still usable), You need to check if the trimmer on the driver PCB have two pins connected together (the central one and one of the side ones) ..... if yes, this means that the trimmer is used as variable resistor, so you can easily modify the driver, and substitute the trimmer on the driver with a potentiometer and a trimmer in serie (or just cut one of the tracks, and add the potentiometer in serie with the trimmer on the PCB), and place the potentiometer accessible from the external ..... then you can regulate them for have a preset at the max voltage for the max current with the trimmer), and use the potentiometer for lower the luminosity.

Do you already have the driver in hand ? ..... for know the values and the configuration, you need first to check if the voltage at the output increase when the trimmer value is lowerewd, or when is increased.

I do not have it yet unfortunately. I will post back when it gets here.