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Help Determining Absorption, Float, and Equalization Voltage for 18V 5S Li-ion battery pack.

I've created a 5S2P Li-ion battery pack using the LG 3.6V INR18650 cells. I think the chemistry is Lithium Manganese Nickel LiNiMnCoO2. A single cell is fully charged at 4.2V, so the 5S pack would be fully charged at 21V. I'm creating a preset since it doesn't fit into any of the 12/24V presets available on my 75/10 BlueSolar MPPT. I have no idea what to set my absorption, float, or equalization voltages to. Could someone help me find these voltage values or explain to me how I could calculate them? Thanks in advance!

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Murray van Graan avatar image
Murray van Graan answered ·

For the 5 cells your absorption voltage would be 21V, after a bit of googling your float would hover around 18.5 to 19 or even 20V. For a lithium you don’t need an equalization charge but you do need a proper cell balancer. Therefore set equalization to the normal absorption voltage, and disable automatic equalization.

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debug avatar image debug commented ·

Thank you very much! How did you determine the float voltage? I wan't sure what to do with the equalization voltage, creating a new preset wouldn't allow me to set it to 0V but on the LiFe preset it had 0.0V for equalization so I just editted that one for my 5s pack. Should I have the equalization at 0 or at absorption, or does it not matter?

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Murray van Graan avatar image Murray van Graan debug commented ·

Float voltage is to sustain the battery and prevent self discharge after the absorption charge that brings the battery to 100%, therefore it’s normally set just above the battery voltage rating. In your case, 3.6V x 5 would be 18V, so I would try it in the range 18.5 to 20V. Equalization voltage doesn’t readily matter as you would never use that function.

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debug avatar image debug Murray van Graan commented ·

Thanks again! I am a little confused with the float. Assuming absorption voltage at 21V, the batteries would become fully charged and have a voltage of 21. Then it switched to float mode, where it supplies a voltage of 20V....wouldn't applying a float voltage of 20V to a fully charged 21V pack start to drain the battery? I don't understand why applying a lower float voltage would keep it trickle / fully charged.

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Murray van Graan avatar image Murray van Graan debug commented ·

At an absorption voltage of 21v, we are forcing current into the battery. If you had to remove the charger after reaching 21v, the battery voltage would drop to it’s rated 3,6V per cell. The float charge at a voltage slightly higher than that will ensure the battery stays charged. The battery bank will not stay at 21V after dropping to float.

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boekel avatar image boekel ♦ Murray van Graan commented ·

Li-ion doesn't drop back after charging like LiFe(Y)PO4

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svtti123 avatar image
svtti123 answered ·

Most batteries i known of need an absorption charge to reach full capacity the lithium battery included.

Ending the charge at the end of bulk charge will not give you a full 100% charge.

The down side is charging a lithium battery to max reduces cycle life. So i would base your charge level decision on your capacity needs. In my case i only charge to 4 volts per cell and only discharge to 3.5 per cell. I chose to greatly increase battery cycle life.

As far as battery balance goes keeping the discharge rate under 1c per battery bank greatly reduces balance maintenance. I monitor each 4.2 volt battery bank and have to balance monthly. I prefer to manually balance my batteries. Most checks there's only 50 mv difference between battery banks. I only charge to 4 volts so i have plenty of fudge room.


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debug avatar image debug commented ·

Yes I will need to find that balance between capacity and battery life. Thanks for the input. Assuming your 4V per cell absorption voltage (which in my case my 5S pack would be 20V), what would you set your float voltage to?

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boekel avatar image boekel ♦ debug commented ·

at 4V/cell you can set float also at 4V
But it all depends on how you are using the setup (constantly charging or only to charge and recharge when empty)

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debug avatar image debug boekel ♦ commented ·

The setup will always be active day and night. The batteries may or may not become full during that day depending on light and energy usage. I'm a little confused on float, keeping it the same as the absorption makes sense to me. For example the absorption at 20V will get the pack to 20V, then a float of 20V would keep it fully charged. But most of the presets I've seen always have a lower float than absorption. Would a say 19V float applied to a fully charged pack of 20V absorption cause it to drain the battery? How would a 19V float keep a 20V pack fully charged? I don't get it.

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boekel avatar image boekel ♦ debug commented ·

Just out of curiosity: what are you running on 18 volts? not a very usual voltage.

about the voltages: please try to learn about li-ion batteries and general electronics before you use them, especially without a BMS.

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svtti123 avatar image svtti123 debug commented ·
  • Because you maintaining a lower float voltage yon can match the float voltage with absorption in a perfect world this would keep the battery fully charged.. a new bulk charge would start when the votage drops 200mv below the float voltage set point.for example a heavy load. The only current you will see is any small load that appear and any small current the battery needs to maintain set float voltage by the battery. As long as the float setting can keep the battery at its setpoint your fine if it drops to much as i said it will start a new bulk float absorption cycle. If you look at the charge table for lithium battery you would see. You could set absorbtion lets say 4 volts float at 4 volts but set absorption time to 0 so you would go right to float after bulk float would keep the battery at 4 volts less stress on the battery. If the loads disappear battery is at selected voltage you will see little amps going to battery.
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boekel avatar image
boekel answered ·

What kind of BMS are you using?

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debug avatar image debug commented ·

No BMS yet, just a 5S pack with +/- leaders and balancing wires. Planning on getting a bms a litter later

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boekel avatar image boekel ♦ debug commented ·

Please be safe! a wrong setting and things are on fire...

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