a DC-DC to charge li-ion... GREAT!!
- Thread starter Doctorbass
- Start date
Thanks a lot,
I am sure i 'll read this again,
I am now looking for a good power supply. This one
http://cgi.ebay.com/ws/eBayISAPI.dl...loc=closed_view_item&refwidgettype=osi_widget
is currently the best choice so far for me well 2 of them
or a mastech 50v 20 adjustable $330 versus $200 for two of the above shipped.
I also need something to power my megapower 960 it needs a 12v 20 amp i think or it can use that at least .
I remember seeing a nice multi output ( 3 i think ) that would have been perfect
efreak
I am sure i 'll read this again,
I am now looking for a good power supply. This one
http://cgi.ebay.com/ws/eBayISAPI.dl...loc=closed_view_item&refwidgettype=osi_widget
is currently the best choice so far for me well 2 of them
or a mastech 50v 20 adjustable $330 versus $200 for two of the above shipped.
I also need something to power my megapower 960 it needs a 12v 20 amp i think or it can use that at least .
I remember seeing a nice multi output ( 3 i think ) that would have been perfect
efreak
Doctorbass
100 GW
efreak said:
I also looked for the 500W 48V power supply, but my two 1000VA toroidal will do the job :wink:
How do you appreciate your megapower charger?
Mine is great with the LBC-10 12s balancer that i modified to monitor and 1 to 12s pack while charging or discharging.
Doc
flip_normal
100 W
I just got 6 of those 48V 500W PSUs. One DOA (trips the RCD on the fuse box when I try to power it up) vendor says return it for a replacement, we'll see, postage back to HK with tracking is about the same as a new PSU. :?
I can confirm that they put out 10A @ 48V no problem. After about 10 mins the thick alli case is quite warm to the touch and the fan kicks in intermitently for a few seconds at a time. I've had them delivering 14A using a load resistor made from some old bar fire elements I had lying around. Since they have overload and short curcuit protection I felt OK about pushing them a bit and they seem to hold up well.
I can confirm that they put out 10A @ 48V no problem. After about 10 mins the thick alli case is quite warm to the touch and the fan kicks in intermitently for a few seconds at a time. I've had them delivering 14A using a load resistor made from some old bar fire elements I had lying around. Since they have overload and short curcuit protection I felt OK about pushing them a bit and they seem to hold up well.
Doctorbass
100 GW
great, but about chinesse stuff, i would not recommand to push them over their limit!... their normal spec are already borderline!
Doc
Doc
I also looked for the 500W 48V power supply, but my two 1000VA toroidal will do the job :wink:
How do you appreciate your megapower charger?
Mine is great with the LBC-10 12s balancer that i modified to monitor and 1 to 12s pack while charging or discharging.
Doc[/quote]
They are on backorder. hopefully not for too long . other than it being max 180watt its great for price and i need something that should be good them after i am in need a power suppl y for this as well reviewer saying it can use a 20 amp 12v ps for optimum charge ! Can't wait honestly.
efreak
How do you appreciate your megapower charger?
Mine is great with the LBC-10 12s balancer that i modified to monitor and 1 to 12s pack while charging or discharging.
Doc[/quote]
They are on backorder. hopefully not for too long . other than it being max 180watt its great for price and i need something that should be good them after i am in need a power suppl y for this as well reviewer saying it can use a 20 amp 12v ps for optimum charge ! Can't wait honestly.
efreak
Beagle123
10 kW
Hi Doc:
Thanks for all your great info on the Artesyn DC-DC converter. I bought one and I'm trying to make it work for my LED headlight. I beleive that LEDs work best at around 4v, but I don't understand how to adjust the voltage down on the DC-DC converter. You said to connect a pot between the -IN pin and the Vadj pin, but a pot has 3 connections. Where does each pin go?
Does anyone know how to set the V(adjust) pin?
Thanks
Thanks for all your great info on the Artesyn DC-DC converter. I bought one and I'm trying to make it work for my LED headlight. I beleive that LEDs work best at around 4v, but I don't understand how to adjust the voltage down on the DC-DC converter. You said to connect a pot between the -IN pin and the Vadj pin, but a pot has 3 connections. Where does each pin go?
Does anyone know how to set the V(adjust) pin?
Thanks
Yes Thanks to all the knowledgable people and their contributions
From what I understand from the data sheet on the tyco modules
Data Sheet
http://power.tycoelectronics.com/BinaryGet.aspx?ID=02c39fa8-b980-4ed3-ae07-0b21ba259203%20%20
Output Voltage Set-Point Adjustment (Trim)
Output voltage trim allows the user to increase or
decrease the output voltage set point of a module. This
is accomplished by connecting an external resistor
between the TRIM pin and either the SENSE(+) or
SENSE(–) pins. The trim resistor should be positioned
close to the module.
If not using the trim feature, leave the TRIM pin open.
With an external resistor between the TRIM and
SENSE(–) pins (Radj-down), the output voltage set point
(VO, adj) decreases (see Figure 22). The following equation
determines the required external-resistor value to
obtain a percentage output voltage change of ÃŽâ€%.
With an external resistor connected between the TRIM
and SENSE(+) pins (Radj-up), the output voltage set
point (VO, adj) increases (see Figure 23).
The following equation determines the required external-
resistor value to obtain a percentage output voltage
change of ÃŽâ€%.
The voltage between the VO(+) and VO(–) terminals
must not exceed the minimum output overvoltage protection
value shown in the Feature Specifications table.
This limit includes any increase in voltage due to
remote-sense compensation and output voltage setpoint
adjustment (trim). See Figure 21.
I ll try to dig a bit more
efreak
From what I understand from the data sheet on the tyco modules
Data Sheet
http://power.tycoelectronics.com/BinaryGet.aspx?ID=02c39fa8-b980-4ed3-ae07-0b21ba259203%20%20
Output Voltage Set-Point Adjustment (Trim)
Output voltage trim allows the user to increase or
decrease the output voltage set point of a module. This
is accomplished by connecting an external resistor
between the TRIM pin and either the SENSE(+) or
SENSE(–) pins. The trim resistor should be positioned
close to the module.
If not using the trim feature, leave the TRIM pin open.
With an external resistor between the TRIM and
SENSE(–) pins (Radj-down), the output voltage set point
(VO, adj) decreases (see Figure 22). The following equation
determines the required external-resistor value to
obtain a percentage output voltage change of ÃŽâ€%.
With an external resistor connected between the TRIM
and SENSE(+) pins (Radj-up), the output voltage set
point (VO, adj) increases (see Figure 23).
The following equation determines the required external-
resistor value to obtain a percentage output voltage
change of ÃŽâ€%.
The voltage between the VO(+) and VO(–) terminals
must not exceed the minimum output overvoltage protection
value shown in the Feature Specifications table.
This limit includes any increase in voltage due to
remote-sense compensation and output voltage setpoint
adjustment (trim). See Figure 21.
I ll try to dig a bit more
efreak
Doctorbass
100 GW
It's exactly what efreak explained.. maybe i did a mistake about the Vin - pin... it is the Vout- pin that is ok.. noyt the Vin..
the third pin of the pot dont need to be connected.. only ise pin 1 and 2... not 3.
Also, for your led, you NEED to put a current limiting resistor between the led and the DC_DC because the forward voltage of the led will change as its heat increase... so a serie resistor will help prevent damaging the led.
Doc
the third pin of the pot dont need to be connected.. only ise pin 1 and 2... not 3.
Also, for your led, you NEED to put a current limiting resistor between the led and the DC_DC because the forward voltage of the led will change as its heat increase... so a serie resistor will help prevent damaging the led.
Doc
Doctorbass
100 GW
----last update 15fev 2008----
I'm working on the current limiting circuit now.
The reason is that the 150W modules i are able to output 39A when a cell is low so i dont want to limit my cahrger to only cahrge 4p pack and more..(10A/cell max) i also want to charge 1p 2p and 3p so i need to limit the current. i'm building a circuit that act directly on the trim pin with an amp-op and a reference voltage to set the curent limit.
another reason is that at 39A and 3.6V, it's 140W out and become 175W on the input on the DC-DC if taking account of the efficiency of the dc-dc.
175W * 12s modules =2100W
and i want to built a charger that can be used on the 120V .. not only the 240V... so 1800W for a 15A 120V is my limit...
My last tests circuit results are good and i just need to work on the temperature stability of the curcuit and i will be able to finish that project!
Doc
I'm working on the current limiting circuit now.
The reason is that the 150W modules i are able to output 39A when a cell is low so i dont want to limit my cahrger to only cahrge 4p pack and more..(10A/cell max) i also want to charge 1p 2p and 3p so i need to limit the current. i'm building a circuit that act directly on the trim pin with an amp-op and a reference voltage to set the curent limit.
another reason is that at 39A and 3.6V, it's 140W out and become 175W on the input on the DC-DC if taking account of the efficiency of the dc-dc.
175W * 12s modules =2100W
and i want to built a charger that can be used on the 120V .. not only the 240V... so 1800W for a 15A 120V is my limit...My last tests circuit results are good and i just need to work on the temperature stability of the curcuit and i will be able to finish that project!
Doc
TylerDurden
100 GW
^^^ 
^^^
^^^
safe
1 GW
- Joined
- Dec 22, 2006
- Messages
- 5,681
Me Too 
My ideal design would have to work well with:
(12) Thundersky 40 Ah - 3.2 volt cells - 38.4 volts
Max Charge 3C = 120 amps
Max Power in Total = 12 * 3.2 * 120 = 4608 watts
Which is basically insane... I could live with a lot less than that, but that's the upper limit of the cells. For now I've been devoted to the LVC side where I want the LVC to combine with an Armature Current Limiting circuit so that the end of ride will happen slowly because as a cell approaches it's cutoff voltage due to sag I want to pull down the throttle. Anyway, I've got that going on another thread.
Eventually I'm going to need to figure out the charging side... and I want individual cell balancing like Doctorbass defined at the beginning of the thread. That general outline that he defined sounds right to me.
Any suggestions on my needs are welcomed...
My ideal design would have to work well with:
(12) Thundersky 40 Ah - 3.2 volt cells - 38.4 volts
Max Charge 3C = 120 amps
Max Power in Total = 12 * 3.2 * 120 = 4608 watts
Which is basically insane... I could live with a lot less than that, but that's the upper limit of the cells. For now I've been devoted to the LVC side where I want the LVC to combine with an Armature Current Limiting circuit so that the end of ride will happen slowly because as a cell approaches it's cutoff voltage due to sag I want to pull down the throttle. Anyway, I've got that going on another thread.
Eventually I'm going to need to figure out the charging side... and I want individual cell balancing like Doctorbass defined at the beginning of the thread. That general outline that he defined sounds right to me.
Any suggestions on my needs are welcomed...
Doctorbass
100 GW
efreak said:
Yes, each dc-dc will have their pot, but that will be only to set the precision of the voltage. the 3.6V or the 4.2V seting will be realized with a resistor added to each pot to jump from 3.6V to 4.2V when cahrging Lipo or other 3.7V(4.2Vchr) chemistry. I will use 3x 4pst relay for a total of 12 indsependent contact for the resistor switching.
Doc
so you need one for each module connected at all times?
i need a fuse for voltage in side as recommended and heatsinks
this article about these modules thatoperate without heatsink
http://www.edn.com/index.asp?layout=article&articleid=CA90785
anything else/
thanks
efreak
i need a fuse for voltage in side as recommended and heatsinks
this article about these modules thatoperate without heatsink
http://www.edn.com/index.asp?layout=article&articleid=CA90785
anything else/
thanks
efreak
Hi well I need a lot of help with the design of this charger using Dc Dc converters as single cell chargers.
I am planning to use 80 M1 cells basically arranging them four packs of dewalt batteries
and then running 14 gaug wire from the balancing tabs to make 10s 4 p pack so to speak.
build two of these . the charging set up is going to be the bigger challenge for me these tyco 3.3 modules are capable of performing higher than rated according to their datasheethttp://power.tycoelectronics.com/BinaryGet.aspx?ID=02c39fa8-b980-4ed3-ae07-0b21ba259203
so i am learning that using a 10 k potentiometer the output could be adjusted to produce 3.6 volt for example to charge each cell to that voltage.
( not sure but i think i need a 10k pot for each module 20 exactlly)
http://endless-sphere.com/forums/viewtopic.php?f=14&t=2824
there is where i read about this idea and i here i am giving it a shot i trust Stephane the doc to know . he is currently working on his set up .
the innput for these modules i think is optimized at 48 volt so i would need somthing along these lines
the balancer wire since they are isolated they can be wire like Gary Goodrum's single charger set up thanks to everyone for their input BTW
and this for the opposite end where the charger hooks up to the balancing wires
there there is what i am trying to do i first of all i have to charge each pack to its max these are supposedly new packs once i receive my mega power 960 i will slow charge initially perhaps some test cycles and then start assembling the packs
any ideas for the balancing harness plug types? anderson 30 amps there is gonna be 11 wires?
that enough for now please chime in thanks a lot
efreak
I am planning to use 80 M1 cells basically arranging them four packs of dewalt batteries
build two of these . the charging set up is going to be the bigger challenge for me these tyco 3.3 modules are capable of performing higher than rated according to their datasheethttp://power.tycoelectronics.com/BinaryGet.aspx?ID=02c39fa8-b980-4ed3-ae07-0b21ba259203
so i am learning that using a 10 k potentiometer the output could be adjusted to produce 3.6 volt for example to charge each cell to that voltage.
( not sure but i think i need a 10k pot for each module 20 exactlly)
http://endless-sphere.com/forums/viewtopic.php?f=14&t=2824
there is where i read about this idea and i here i am giving it a shot i trust Stephane the doc to know . he is currently working on his set up .
the innput for these modules i think is optimized at 48 volt so i would need somthing along these lines
the balancer wire since they are isolated they can be wire like Gary Goodrum's single charger set up thanks to everyone for their input BTW
and this for the opposite end where the charger hooks up to the balancing wires
there there is what i am trying to do i first of all i have to charge each pack to its max these are supposedly new packs once i receive my mega power 960 i will slow charge initially perhaps some test cycles and then start assembling the packs
any ideas for the balancing harness plug types? anderson 30 amps there is gonna be 11 wires?
that enough for now please chime in thanks a lot
efreak
Doctorbass
100 GW
I will use amphenol DIN industrial style connector (green army style)
(the 97 series)
the contact pin size n 8 should be able to hold 46A... the bxb150 can pull 39A...
http://www.newark.com/pdfs/techarticles/amphenolRF/97Guide.pdf
Doc
(the 97 series)
the contact pin size n 8 should be able to hold 46A... the bxb150 can pull 39A...
http://www.newark.com/pdfs/techarticles/amphenolRF/97Guide.pdf
Doc
The two main pos. and neg. terminals on the pack will take the full current and need big pins, but all the taps between cells will normally see very little current, so I think you could get away with smaller wires and pins there. On a big amphenol connector, you can use multiple pins tied together for higher current rating.
Don't the dc-dc converters have adjustable output. Do you need voltage monitors for each cell?
Don't the dc-dc converters have adjustable output. Do you need voltage monitors for each cell?
Doctorbass
100 GW
fechter, I will use the total main power supply power consumption to monitor the SOC of the pack.
I will only need to power up the complete charger with no battery connected on it (i will get the standby current)
and after that i will connect a pack with all cells fully charged.. and i will get current consumption of the complete charger with battery pack plugged to it... so that will become the indication of a battery pack charged...
then i will know how much currentthe charger is consumming to indicate me to disconnect the battery pack..
exemple:
-standby mode (120V 0.67A)
-full empty pack (119V 14.2A)
-fully charged pack(120V 1.02A) (around 50mA per cell)
Doc
I will only need to power up the complete charger with no battery connected on it (i will get the standby current)
and after that i will connect a pack with all cells fully charged.. and i will get current consumption of the complete charger with battery pack plugged to it... so that will become the indication of a battery pack charged...
then i will know how much currentthe charger is consumming to indicate me to disconnect the battery pack..
exemple:
-standby mode (120V 0.67A)
-full empty pack (119V 14.2A)
-fully charged pack(120V 1.02A) (around 50mA per cell)
Doc
An update from my slow progress, I have a question after
I am still looking for a power supply 48 volts 20 amps or 2 10 amp ones that are reasonably priced for toronto shipping
the dc cnverters are quarter brick very small compact .
i read maybe more stable to use a resistor rathar than a 10 k potentiometr for the voltage adjustmen purpose
3.3volt to 3.6
i have made a power supply out of a pc PS and works good that is to power my mega power infinity 960 a great balancer tester unit more than a charger stilll learning it nice unit ( thanks doc )easy to operate and best of all FOOL proof lol.
next step is to get the packs assembled Ny thoughts on slow charging the new cell packs for first time using infinty charger or dewalt charger?
like Gary suggested at 0.3 amps trickle charge for long period .
efreak
I am still looking for a power supply 48 volts 20 amps or 2 10 amp ones that are reasonably priced for toronto shipping
the dc cnverters are quarter brick very small compact .
i read maybe more stable to use a resistor rathar than a 10 k potentiometr for the voltage adjustmen purpose
3.3volt to 3.6
i have made a power supply out of a pc PS and works good that is to power my mega power infinity 960 a great balancer tester unit more than a charger stilll learning it nice unit ( thanks doc )easy to operate and best of all FOOL proof lol.
next step is to get the packs assembled Ny thoughts on slow charging the new cell packs for first time using infinty charger or dewalt charger?
like Gary suggested at 0.3 amps trickle charge for long period .
efreak
flip_normal
100 W
I don't think it'll make much difference if you use a sub-min pot or fixed resistors or a combination of both. They can be soldered directly to the converter pins. In fact the pot is probably better because then you can adjust the voltage for different batteries, for example Thundersky LiFePO4 recomends a charging voltage of 4.25V, or a lower charge voltage of say 3.5V for M1s or 40138s may help increase lifecycles, I think I read somewhere.
Have you done a current limiting circuit on the output side of the converters efreak? The 500W 48V supplies I got seem quite robust and only weigh 1166g and can be paralleled for more power but if you load them too much the output voltage drops to keep the power at ~500W. If it drops too much (below 36V according to spec) my converters may not work. I can't test much of this tho' till I get some more batts.
The dewalt chargers will charge in one hour, 1C i.e. 2.3A and don't seem to do the cells much harm.
Have you done a current limiting circuit on the output side of the converters efreak? The 500W 48V supplies I got seem quite robust and only weigh 1166g and can be paralleled for more power but if you load them too much the output voltage drops to keep the power at ~500W. If it drops too much (below 36V according to spec) my converters may not work. I can't test much of this tho' till I get some more batts.
The dewalt chargers will charge in one hour, 1C i.e. 2.3A and don't seem to do the cells much harm.
Thanks flip,
That is the same power supply that I was planning on purchasing but now I can find them anymore @(ebay.com) i look at one at a e- store but i can't find it either!
but anyhow that is good info on the parallel set up that is what i was pallning on as well but now ....
i suppose should I look for a 48 volt at 20amp supply ??
I have not check the modules as yet and as soon as i secure the power supply again i am gonna do some testing . not sure exactly 1 pot per modules and dont know what else but i am excited about the concept as long as the cells in the pack don't get imbalanced hopefully
efreak
That is the same power supply that I was planning on purchasing but now I can find them anymore @(ebay.com) i look at one at a e- store but i can't find it either!
but anyhow that is good info on the parallel set up that is what i was pallning on as well but now ....
i suppose should I look for a 48 volt at 20amp supply ??
I have not check the modules as yet and as soon as i secure the power supply again i am gonna do some testing . not sure exactly 1 pot per modules and dont know what else but i am excited about the concept as long as the cells in the pack don't get imbalanced hopefully
efreak
Doctorbass
100 GW
Hey guys,
I absolutly need to advice you about a technical problem that may occur if they are not current limited!
Ex:
Your LiFe cell is completly Empty and present a floating voltage of 2.8V(no load) (2.5V under normal load)
When you will apply current on it to recharge it let say 5A, the voltage will keep under 3V for a couple of second
in this case, the voltage difference between your dc-dc (3.6V) and the cell will start to around 0.8V... 0.6V and will stabilyze to around 3.3V during 80% of the cahrging process.
What is important to unserstant is:
AS THE VOLTAGE OF THE CELL HAVE NOT REACH THE VOLTAGE OF THE OUTPUT OF THE DC-DC THE CURRENT WILL BE MAX !!!
The DC-DC used with no current limit but that is just CV controlled will put the max current availlable.. and for exemple, the BXB150 can put 39A !! that is around 7A at input with 36V and around 5A with 48V input.
YOU NEED TO PUT A RESISTOR BETWEEN THE DC_DC AND THE CELL to limit the current.
the problem that can occur is that if you use many DC-DC to charge 12s.. and the cells are empty or not full, the voltage difference between the dc-dc and the cell will make it to control the CV to stabilyse to 3.6V (preseted by the resistor) and to do that it will put ALL current availlable if it is not limited. Then you could need to supply the 12 DC-DC with 5A each at 48V that is 60A... and using only 2 or 3 500W 48V power supply will never be able to handle that and their voltage will drop too low and will SHUT UP THE DC_DC because their input will go below the 32.4V low limit.
Without limiting the current, a single cell A123 would be recharged to 39A.. AND THAT IS BAD
... You need to limit the current! oir buy the brands new Vicor DC DC 150W that have a I limit but they are $$$... the BXB150 was 10$ so finding a way to limit their output current seems more adequate.
You need to understand that as far as the cell voltage and the dc dc are not the same, the current will be max. If you add a serie resistor,, that will be ok... and solve this problem.. but two disadvantages will occur: first, you will dissipate and loose energy in heat, and finally, you will not be able to preserve the ideal CC-CV mode because that the current will decrease as the cell voltage increase (why? .. just because the less the voltage difference between the cell and the dc-dc is, the less the current will be high thru the resistor.... and the cell will recharge slower as his voltage increase...
CC-CV is the most efficient method for getting a short charging time. With a resistor in serie, the CC charging time can't exist.. that increase the charge time.
I'M working on a current limiter that affect the V trim input to control current.. be patient, i will post something about my final desing soon..
Doc
to avoit drawing full power on the dc-dc with
I absolutly need to advice you about a technical problem that may occur if they are not current limited!
Ex:
Your LiFe cell is completly Empty and present a floating voltage of 2.8V(no load) (2.5V under normal load)
When you will apply current on it to recharge it let say 5A, the voltage will keep under 3V for a couple of second
in this case, the voltage difference between your dc-dc (3.6V) and the cell will start to around 0.8V... 0.6V and will stabilyze to around 3.3V during 80% of the cahrging process.
What is important to unserstant is:
AS THE VOLTAGE OF THE CELL HAVE NOT REACH THE VOLTAGE OF THE OUTPUT OF THE DC-DC THE CURRENT WILL BE MAX !!!
The DC-DC used with no current limit but that is just CV controlled will put the max current availlable.. and for exemple, the BXB150 can put 39A !! that is around 7A at input with 36V and around 5A with 48V input.
YOU NEED TO PUT A RESISTOR BETWEEN THE DC_DC AND THE CELL to limit the current.
the problem that can occur is that if you use many DC-DC to charge 12s.. and the cells are empty or not full, the voltage difference between the dc-dc and the cell will make it to control the CV to stabilyse to 3.6V (preseted by the resistor) and to do that it will put ALL current availlable if it is not limited. Then you could need to supply the 12 DC-DC with 5A each at 48V that is 60A... and using only 2 or 3 500W 48V power supply will never be able to handle that and their voltage will drop too low and will SHUT UP THE DC_DC because their input will go below the 32.4V low limit.
Without limiting the current, a single cell A123 would be recharged to 39A.. AND THAT IS BAD
... You need to limit the current! oir buy the brands new Vicor DC DC 150W that have a I limit but they are $$$... the BXB150 was 10$ so finding a way to limit their output current seems more adequate.You need to understand that as far as the cell voltage and the dc dc are not the same, the current will be max. If you add a serie resistor,, that will be ok... and solve this problem.. but two disadvantages will occur: first, you will dissipate and loose energy in heat, and finally, you will not be able to preserve the ideal CC-CV mode because that the current will decrease as the cell voltage increase (why? .. just because the less the voltage difference between the cell and the dc-dc is, the less the current will be high thru the resistor.... and the cell will recharge slower as his voltage increase...
CC-CV is the most efficient method for getting a short charging time. With a resistor in serie, the CC charging time can't exist.. that increase the charge time.
I'M working on a current limiter that affect the V trim input to control current.. be patient, i will post something about my final desing soon..
Doc
to avoit drawing full power on the dc-dc with
Attachments
What size resistor would you recommend?
It would be nice if the the dc-dc units had current limiting built in.
Do they make smaller ones?
It would be nice if the the dc-dc units had current limiting built in.
Do they make smaller ones?
Doctorbass
100 GW
fechter said:
Yes they make smaller one. like 100W 75W or 50W. On ebay it is not rare to see many of those under 10$ each
To calculate the resistor it is simple: let's supose you have 2x 10A 48V power supply to supply all 12 DC-DC.
you have 20A at 48V
they are efficient to at least 75%
you have 20 x 48V=960W.
960 x .75= 720W .. so you will have 720W max output from the total 12 dc-dc.
that's 60W each.. and at 3.6V it's 16.6A
thay mean you will need to always keep the current to max 16A or lower to ensure that the 48V supply does'nt shutdown or the dc-dc due to the voltage drop at their input.
If you see the graph above, you will note that the initial voltage of a single cell is around 3.25V for 10A current and 3.1V for 6A.. so at 16A we can suppose that the cell will begin its charge to around 3.35V
The DC-DC is set to 3.6V so the difference of voltage need to be absorbed by something to keep that current lower than 16A max
3.6V - 3.35V= 0.25V
0.25V at 16A is a corresponding resistor of R=V/I = 0.015 ohm and it will dissipate 4Watts. I would recommand a 5W or a 7W resistor or multiple parallel 2W.
BUT.. the wire between the cells and the dc-dc will ALSO have a resistance... and.. that could match perfectly and replace the resistor if you have the right lengh or size..
see: http://www.powerstream.com/Wire_Size.htm and look for the TRANSMISSION value.
So with 3 ft of dual wire no awg 14 that could be ok.. but dont join all the 12 wire pair together if you choose that cause 4W x 12 = 48W and it is a bit high for a 3ft cable....
Hope this help to understand
Doc
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