Use Boost converters parallel

[ThaHandy]

Hi,

I'm looking to for a 60V charger for my LiFepo4 battery(with BMS).

The battery charger came with the battery is a "EMC-400" cheap type charger. To speed things up i want a bigger one.

What I has in mind is to get a 1000/2000W 48V power supply, but without current control I want to add these "600W Boost converters".
DX.com saying  (they sell them to and I have my thoughts) these can be connected parallel. If so i only need 3 to get 1500W output.

So, any of you can tell for sure they can be connected parallel and if so, how to connect them properly?

[Whales]

Woah... that's a lot of power.  I'm going to have to make an extended response, sorry 

Also: I don't know what 'BMS' is.  A charge balancing system?

Practicality
(I'm assuming charging methods for LiPo ~= Li-ion here.  Please correct me if I am wrong)

Li-ion batteries require constant current charging for most of their charging process -- constant voltage is only done at the end to eke out a few more % (^wiki ^batuni).  Constant voltage charging the batteries will either shove way too much current through them initially (destroying your charger and/or batteries with copious fireworks) or not charge them enough unless you slowly & manually raise the voltage. 

IE you are better off using or designing (we prefer the latter around here ) something that regulates current and voltage for you, else you will have to sit by the batteries for hours with two multimeters (one measuring current, the other voltage) manually tweaking the PSU output on the batteries.  I have done this before -- but for lead-acids, NOT lithium chemistry -- and although good for meditation, you don't want to do it often, nor on more dangerous chemistries such as the Li* family.

You are also going to need to supply the boost converter with power.  Getting your 48v PSU or a 1.5K to 2KW computer PSU will be well into hundreds of dollars.  Depending on your country, you may not be able to power this rig off normal wall sockets.

Finally putting power supplies in parallel is not as easy as you think -- unless they all have the same output voltage, or a constant load, the slightly higher V ones will try and drive the other chargers.  Generally they don't like this, and they shut off.  Your best bet would be to unify them all to run off one piece of boost controlling circuitry, but this would take a lot of time, testing and reverse-engineering.  Don't trust promises by DX, you might be lucky and they have functionality to aid being put in parallel, but don't rely on it to be there if you are not certain.


Safety
1500 watts = 1.5Kj per sec = 358.5 calories per sec (water heat constant = 4.184)
Average lab mouse = 35g (^yahoo answers)
Degrees C rise needed to take a mouse from room (25C) to boiling = 75C
Time needed to turn a mouse into vapour with this power (assuming 100% water) = 75( 35 / 358.5 ) = 7.32 seconds

Regardless of the fact you are definitely not a mouse (me, on the other hand...) 1.5kW is not something to toy with without either current limiting or with lower power tests first.  I have not the foggiest if your batteries can even charge at this rate -- what is their max charging 'C' (not celcius, in amps)?  If they can't, they will overheat and catch fire/explode.


Recommendations
I'm not an EE, just a hobbyist.  Get what I write here confirmed by other people before taking it for anything resembling objective facts.  No warranty of any kind, etc

(1) Find out the max C (current) charge rate of your batteries.  You cannot go past this per cell -- else they will damage themselves and then possibly damage you.
(2) Look at your current charger.  Is it already charging them near/at their max C already?  In that case, there is not much you can do.
(3) Consider designing your own charging regulation circuit or obtaining one.  Controlling battery charging manually is silly, for safety and sanity.
(4) If you want to go down this route, buy only one of the boost converters and try with just that.  It will still deliver you tsunamis of power.

Neither the stats on the DX boost PSU nor your original charger will probably by true FYI -- expect them to be 10% to 50% overstated on what they actually deliver.  Welcome to the world of power supplies!

Sorry for being so morbid!  Welcome to EEVforums, join us in the discussions!

Regards, Hales

[ThaHandy]

answers between the quotes

Woah... that's a lot of power.  I'm going to have to make an extended response, sorry 

Also: I don't know what 'BMS' is.  A charge balancing system?

It's short for "Battery Management System"

Practicality
(I'm assuming charging methods for LiPo ~= Li-ion here.  Please correct me if I am wrong)

[...]

LiFePO4  is this case. Similar to lipo/li-ion beside the 3.3V/cel compared to 3.7v for lipo/li-ion

Recommendations
I'm not an EE, just a hobbyist.  Get what I write here confirmed by other people before taking it for anything resembling objective facts.  No warranty of any kind, etc

I did some research about this, but did not mentioned this. Should have to

(1) Find out the max C (current) charge rate of your batteries.  You cannot go past this per cell -- else they will damage themselves and then possibly damage you.

16 of these in serie (~48V)
40152S 15Ah 10C LiFePO4 Cylindrical Battery Cell with Screws Specification:
1. Nominal Voltage:3.2Volts.
2. Dimension:Diameter(mm) X High(mm) = 40±1mm X 165±1mm(with screw)
3. Capacity: >15Ah.
4. Maximal Charge Current: 3C(45Amps). (BMS capped @ 30Amp)
5. Maximal Continuous Discharge C-Rate: 10C(150Amps).
6. Maximal Discharge C-Rate: 15C(225Amps).
7. Overvoltage Protection: 3.65±0.05Volts.
8. Undervoltage Protection: 2.0Volts.
9. Lifecycle: 2000Cycles.
10. Weight: 480Grams.
11. Impedance: 8MOhm.
12. Chemical: LiFePO4.

(2) Look at your current charger.  Is it already charging them near/at their max C already?  In that case, there is not much you can do.

yes, its a 400W charger

(3) Consider designing your own charging regulation circuit or obtaining one.  Controlling battery charging manually is silly, for safety and sanity.

thats where the BMS comes in, no need to worry about that.

(4) If you want to go down this route, buy only one of the boost converters and try with just that.  It will still deliver you tsunamis of power.

Neither the stats on the DX boost PSU nor your original charger will probably by true FYI -- expect them to be 10% to 50% overstated on what they actually deliver.  Welcome to the world of power supplies!

Thats more or less the reason I ask here 

How to use more of these booster in parallel. 48V -> 58v and use 3 of these boosters @ 8Amp (=475Watt). So the target will be ~25Amp @ 58v = ~1450Watt (not mentioned the power loss in conversion)

Unless there is a 2000W booster out there (modify the 600W one?) I'll go with this setup
These boosters is the only way I can think of to get the Charge voltage and limit the current

Sorry for being so morbid!  Welcome to EEVforums, join us in the discussions!

Regards, Hales

Love it tnx 

[Whales]

answers between the quotes
It's short for "Battery Management System"

Sounds a bit more effective than my bodged protection circuits

16 of these in serie (~48V)
40152S 15Ah 10C LiFePO4 Cylindrical Battery Cell with Screws Specification:
1. Nominal Voltage:3.2Volts.
2. Dimension:Diameter(mm) X High(mm) = 40±1mm X 165±1mm(with screw)
3. Capacity: >15Ah.
4. Maximal Charge Current: 3C(45Amps). (BMS capped @ 30Amp)
5. Maximal Continuous Discharge C-Rate: 10C(150Amps).
6. Maximal Discharge C-Rate: 15C(225Amps).
7. Overvoltage Protection: 3.65±0.05Volts.
8. Undervoltage Protection: 2.0Volts.
9. Lifecycle: 2000Cycles.
10. Weight: 480Grams.
11. Impedance: 8MOhm.
12. Chemical: LiFePO4.

Wow.  I need an excuse to play with these. 

*Imagines a battery powered arc welder without voltage bucking circuitry*
*Looks at the price*
*Goes back to dreaming*

If your BMS does indeed cut out @30A it should save you from the particular problem of over-current charging.  If it does not regulate power however, only shut it off if a dangerous condition is detected, you will still need to have some sort of automatic regulator/controller for charging.

Does your BMS do charge balancing for each cell?  If not, you will find the charge levels of each cell drifting from one-another over time.  Ideally you would be able to disassemble your battery pack every time you charge it, so each cell is charged independently.  This brings a number of advantages:

• Lower voltages -> safer.  Although you are working in DC, ~48V is still pretty high -- it will happily arc and set fire to debris making its way between contacts.
• You can identify single dud cells, rather than have them bring down the performance of the whole pack.  Very useful -- especially after the first few months/years of use.
• Each cell gets individually charged, so the charge levels between them do not drift.  Variance leads to improper charging OR inability to charge to full capacity, depending on the charger design.

Unless there is a 2000W booster out there (modify the 600W one?) I'll go with this setup
These boosters is the only way I can think of to get the Charge voltage and limit the current

You would be better off looking for a PSU that converts mains power directly into the charging voltage range you desire.  Perhaps some lab power supplies can do this -- I'm unsure where too look for this sort of thing.  Other members here would be better aligned to help you.

Modifying the booster for a higher power level would require re-designing and replacing its most expensive components.  It would also have to be physically bigger to accommodate better heat dissipation.  It will probably be much easier (and possibly cheaper, I'm not sure) to design and make your own (less efficient) power supply yourself than try and modify one like this, although these might be good feedstock for parts, depending on how things price out for you.

This writing is going to make me seem like an old man, but I must reinforce how much energy these things have.  I've melted plastic encasings and lost wire insulation using mere AA alkaline batteries before, without noticing until I wake up and find them in my bedroom.  Your BMS system WILL NOT protect you from everything, only SOME COMMON failure conditions for the batteries. 

[ThaHandy]

[...]

You would be better off looking for a PSU that converts mains power directly into the charging voltage range you desire.  Perhaps some lab power supplies can do this -- I'm unsure where too look for this sort of thing.  Other members here would be better aligned to help you.

[....]

I'm aware there are a few ways to get what i need which also increasing the cost big time.

atm. my question is:

How to use boost converters parallel