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I have 4000mAh 2 cell series 8.4V lipo battery

I need make a custom charger I am thinking of using LT3650-8.4 2A charging Ic

if I set 2A how long will it take to charge this fully /if timer is disabled timer?

is it (1st cell 4000mAh + 2nd cell 4000mAh) / 2000mA = 4h ?

if I set 1A how long will it take to charge this fully /if timer is disabled timer?

is it (1st cell 4000mAh + 2nd cell 4000mAh) / 1000mA = 8h ?

enter image description here

What is the maximum capacity of 2 cell battery this IC can charge properly? without a timer?

as I know we have to manage the full charge cycle (5.6-8.4) within 4h at least, so to charge it within 4h with the set current to 2A as in the figure, how long will it take to fully charge a 4000mAH 2 cell battery?

I think everybody has misunderstood the question I am asking. now everybody should be able to understand the question

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  • \$\begingroup\$ this depends on whether your cells are in series or parallel. Considering your voltage is 8.4 V, they are in series, so you're wrong. \$\endgroup\$ Jan 24 '20 at 18:49
  • \$\begingroup\$ @Marcus Müller my battery is series 2 cell lipo battery voltage 8.4V capacity 4000mAh, what is the charging time theoretically if charging current is 2A with above IC? \$\endgroup\$
    – vassidefuk
    Jan 24 '20 at 18:52
  • \$\begingroup\$ it is sereis 2 cell 8.4V,if charge current is 2A , then time to fully charge = 4000mAh / 2000mA = 2h ? \$\endgroup\$
    – vassidefuk
    Jan 24 '20 at 19:07
  • \$\begingroup\$ @vassidefuk It does not charge at a constant current for the complete charge cycle. Read its documentation and look at the charts. \$\endgroup\$ Jan 24 '20 at 19:19
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    \$\begingroup\$ @vassidefuk LOOK at the reference Andrew cited. READ what it says. Graph on 1st page is theoretical and practical result of CCCV LiIon charging. | You are dangerously short on theoretical knowledge to build your own charger. If charging current flows in series through multiple identical cells it will charge all of them at the same rate and in the same time. The voltage needed will increase with the cell count. | You can only charge a cell at CC (constant current) until it reaches Vmax 9typically 4.2V/celll. Then you MUST ho;d Vcell constant at Vmax and allow the current to "taper"(reduce) .. \$\endgroup\$
    – Russell McMahon
    Jan 24 '20 at 19:42
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Summary:

  • Charging 2 x 4Ah cells in series with 2A current will charge them to ABOUT
    time = Ah/Charge_rate x 90% = 4/2 x 0.90 = 1.8 hours

  • At that stage they will be at 8.4V and ABOUT 90% of capacity.

  • Beyond that, if you wish to continue charging you must hold voltage at 4.2V/cell maximum and allow current to decrease under control of battery chemistry.

  • Reduction of current to about C/5 will take around an additional 2 hours. Stopping at V=4.2V/cell without CC taper charging greatly increases battery life in return for a small reduction in per cycle capacity.

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LOOK at the LT3650 data sheet that Andrew cited.
READ what it says.

The graph on the 1st page shows the theoretical and practical result of CCCV LiIon charging. You are dangerously short on theoretical knowledge to build your own charger.

If charging current flows in series through multiple identical cells it will charge all of them at the same rate and in the same time. The voltage needed will increase with the cell count. You can only charge a cell at CC (constant current) until it reaches Vmax (typically 4.2V/celll).
Then you MUST either cease charging or hold Vcell constant at Vmax and allow the current to "taper"(reduce) under control of the cell chemistry. If you hold I at 2A then after a while Vcell will rise above Vmax and the cell will be damaged or destroyed.

IF the cell is rated at 1C = 4A charge max then you can charge at C/2 = 2A until cell reaches Vmax. Cell capacity will be about 90% of max. If there are 2 cells in series the time taken will be the same but the required voltage will be double.

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  • \$\begingroup\$ then what is this ic supposed to do? if it says 8.4V 2A max current where do we need these sort of ic? if it cannot charge 2 cell lipo battery with voltage 8.4? 8.4 means fully charged voltage \$\endgroup\$
    – vassidefuk
    Jan 25 '20 at 4:28
  • \$\begingroup\$ I say "then you must hold Vcell constant and allow the current to taper ..." -> That is what the charger IC does. It implements CCCV charging for you. | Read my answer carefully point by point. It addresses your questions. | Ask clear specific questions with ONE point per question if anything is still unclear. \$\endgroup\$
    – Russell McMahon
    Jan 25 '20 at 11:34

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