Basic BMS charge/load question

[Kennetht]

Hi, I'm looking for a clear answer on how to connect a BMS to a load and charger?
This forum seems to have a lot of knowledge about rechargeable power cells.

I'm not building a large power supply, but using 18650's for multiple electronics projects.
The question is on how to connect the cheaper BMS boards, that only have P- & P+ terminals.
Example: http://www.ebay.nl/itm/NEW-4A-6A-PC...attery-Cell-/201421276209?hash=item2ee5a4c431


I would like to charge and use small series/parallel setups of 18650 batteries as a power source for my projects.
Being able to recharge them without disconnecting the battery would be the goal, so the device would continue to operate.
If that is not possible, then the question would be, how to wire the P- & P+ terminals for Load end Charge.
Does a simple toggle switch suffice to change from charge to load?

Then an extra question about the charger:
I have a CC/CV PSU.
Does it need to be set to CV with "the number of 18650 * 4,2 volt" ?
And the max current set to Number of 18650 in parallel * 1C?
So if the 18650 is 2200mA/H and there are two in parallel and 3 in series (2P3S) then voltage should be 3 x 4,2V = 12,6V with a max current of 4400mA?

Thanks to anyone who can clear this up for me.

 

[Kennetht]

Isn't there anyone that can give me an answer or point me to an information source about this subject?
Please?

 

[kenwood96]

Most of the time a diagram is provided for the board.

You should be able to use the device while charging.

Constant voltage is the way to go. I wouldn't charge any faster then 1c, giving the board a chance to balance correctly.

Your calculations are correct.

 

[dnmun]

Most of the time a diagram is provided for the board.

You should be able to use the device while charging.

Constant voltage is the way to go. I wouldn't charge any faster then 1c, giving the board a chance to balance correctly.

Your calculations are correct.

the charger maximum current rate does not determine the balancing current needed.

if you set your charger to a fixed output voltage, as the charger increases the battery voltage close to the charger final voltage, the difference in voltage between the charger max and the battery currently is small and as the battery increases it's resistance to further charging the current must go down, by ohm's law. so the charging current always decreases as you reach the final voltage. hyperbolic-ally. so you want the balancing current to match the BMS shunt resistor capability at the final voltage, full charge. i always put my charger a little higher +.1-.2V

 

[Kennetht]

So, if I understand correctly, I need to set the chargers fixed output voltage to +.1V-+.2V Above the in series top charge level. (Does that mean that I can set the desired top voltage that the balanced batteries should have by this charger voltage?)

And I can both connect the charger and load to the same P- & P+ rails, as long as the chargers maximum current is high enough to feed both charging and load (if nessescary)
The cheap BMS boards don't control the charge current, so the CC part of the charger should be set at max 1C*P? When used in simultaneous charge+load, the charge current can be less, but never more than 1C*P.

Finally I understand this part of the process. Thank you all.

 

[dnmun]

for lifepo4 the balancing voltage is not the full charge voltage as it is for lipo. use 3.60V/cell for most of the modern lifepo4 BMS unlessyou can find that the BMS still uses 3.65V for the balancing voltage. adjusting the charger voltage above that level forces the charger to continue pushing the balancing current at final voltage.

 

[999zip999]

Maybe you can tell a little more about what you will be using them for so to give proper advice.

 

[Kennetht]

I'm building battery power supplies for arduino, raspberry pi projects. Led lights and other tinkering.
The power supplies are build from reclaimed laptopbatteries 18650 cells.
I'm tying to match the cells as good as I'm able to.

I've bought good battery holders for 4 en 2 cells : http://www.ebay.nl/itm/221760303136?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
I'm going to use a range of CV and CV/CC PCB's depending on max voltage and current needs.
From this very flexible CC/CV: http://www.ebay.nl/itm/NEW-LM2596S-DC-DC-Step-Down-LED-Digital-Display-6V-40V-to-0-36V-/251326848182?hash=item3a843f5cb6
or this internediate CC/CV: http://www.ebay.nl/itm/5A-Adjustable-CC-CV-Display-Step-Down-charge-Module-LED-Panel-Voltmeter-Ammeter-/381374424481?hash=item58cbafe5a1
Or this low end CV: http://www.ebay.nl/itm/25W-3-15V-to-0-5-30V-Auto-DC-DC-Boost-Buck-Converter-Solar-Voltage-Regulator-/261097665760?hash=item3ccaa234e0

Whith one of these, and a BMS I could connect 2S to 4S packs as a power kit to a laptop pSU to charge my projects while they are in use.
I just need to set the CV to +.2V over the xS voltage. The inputs of the CV DC-DC converters all can handle the laptop 19V input voltage.