# How to calculate right current to charge 2 cell series 8.4V 4000mAh battery lipo?

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 ?

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

• 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. Commented Jan 24, 2020 at 18:49
• @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? Commented Jan 24, 2020 at 18:52
• it is sereis 2 cell 8.4V,if charge current is 2A , then time to fully charge = 4000mAh / 2000mA = 2h ? Commented Jan 24, 2020 at 19:07
• @vassidefuk It does not charge at a constant current for the complete charge cycle. Read its documentation and look at the charts. Commented Jan 24, 2020 at 19:19
• @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) .. Commented Jan 24, 2020 at 19:42

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.