LiFePO4 charging voltage

krell313

New Member
Hello, I'm new to LiFePO4 battery charging and have a question. I am converting a 36 volt golf cart to Lithium and have decided on the Fortune cells. They say that the max charge voltage per cell is 3.65v but my charger will only get to 3.6 volts. What effect with this have on the battery as SOC and longevity or should I just get a charger the will go to 3.65v?
 

snoobler

Solar Honey Badger
Welcome to the forum.

It will take slightly longer for your battery to hit 100%. I have demonstrated that extremely high states of charge can be attained at voltages as low as 3.40V:

 

curiouscarbon

Science Penguin
hello and welcome

the 0.05 volt drop is not a problem for how much range you will get from the battery pack. it means the difference of less than 0.1% capacity

as @snoobler mentioned, any final charge voltage above 3.4 volt per cell will result in >99% charge and probably result in some more cycle life long term.

here are some general reference for LFP charge and voltage relationship (prepared the graph but not the data, revisions welcome)

1618379334010.jpeg
1618379384385.jpeg
 

krell313

New Member
So you are saying this is ok for me to charge to 3.6v. I can live with that. The Daly bms I have will cut out at 3.75 volts so I guess that would never be reached with the charger I have. Just for fun, would 3.75v damage the battery if occasionally charged to the level?
 

curiouscarbon

Science Penguin
this is ok for you to charge to 3.6 volts per cell instead of 3.65 volts per cell

regarding occasionally charging to 3.75 volts per cell i think it will probably not explode but it will maybe cause slight gradual loss of capacity over time from being held at over 3.65 volts per cell.

honestly i think it’s a good thing that the charger only can go up to 3.6 volts per cell because that will provide a margin of protection of 0.05 from the maximum safe voltage for LFP cell to be held at, which is 3.65 volts per cell. at the same time this 0.05 volt per cell difference represents maybe at most 0.3% capacity which for a 100 mile range would be about 1500 feet
 

krell313

New Member
Thank you both very much for this information, I have been using FLA forever and I am used to their charge characteristics but Lithium is new to me.
 

Just John

Photon Sorcerer
3.4v per cell is about 95% capacity. Seriously, buy a better BMS. Your cells will last longer if you don't try to get 100% out of them (above 3.4 and below 3.0).
 

krell313

New Member
I was worried about the Daly voltage setpoints too. I cancelled the order for the Daly and am now looking at an Orion Jr 2 BMS. Thanks for the tip.
 

Ampster

Renewable Energy Hobbyist
I was worried about the Daly voltage setpoints too. I cancelled the order for the Daly and am now looking at an Orion Jr 2 BMS. Thanks for the tip.
I have been using an Orion for the past six years and it is very good. If I can answer and questions let me know?
 

Brett V

Solar Addict
So if I understand correctly, a multi stage charger like is required for lead acid batteries is optional for Lifepo4? The only benefit to the higher voltage bulk stage is it gets there sooner?
 

Ampster

Renewable Energy Hobbyist
So if I understand correctly, a multi stage charger like is required for lead acid batteries is optional for Lifepo4? The only benefit to the higher voltage bulk stage is it gets there sooner?
Actually the two most important stages for charging lead acid and Lithium chemistries are Constant Current (often called Bulk) and Constant Voltage (called Absorb} The way it works is that the Power Supply or charger charges at the current the charger is set for until the voltage at the battery reaches the Constant Voltage point and then the current automatically tapers as the voltage stays the same. So there is no such thing as a "higher voltage bulk stage". Actually Lithium batteries can be charged faster and at higher current and they are much more efficient so they charge faster than Lead Acid batteries of the same capacity using the same charger or charge settings.
 
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Diysolar123

Solar Addict
So you are saying this is ok for me to charge to 3.6v. I can live with that. The Daly bms I have will cut out at 3.75 volts so I guess that would never be reached with the charger I have. Just for fun, would 3.75v damage the battery if occasionally charged to the level?

thats the trap you can fall into, thinking that with a 3.6v per cell total charge voltage you cannot reach a cell overvoltage condition...
Definitely not true, which is why a bms is needed for any series build. When, not if, the cells become slightly unbalanced one cell can certainly reach a higher voltage than all the others, and therefore, over the rainbow it goes.
shooting for low 90% charge, under 3.4 is just fine and much safer (lots of folks live by the 80-20 cycle rule for max life).
 

Brett V

Solar Addict
I don't mean to hijack the OP's post here I'm trying to switch my thought process from AGM batteries to Lithium so some details mess with my few still working brain cells. For AGM batteries you have to put in a little more than you take out...hence the adsorb phase. With Lithium it seems a single stage fixed voltage constant current charger which will taper to 0 charge current as the target voltage is achieved will be fine.
 

Ampster

Renewable Energy Hobbyist
For AGM batteries you have to put in a little more than you take out...hence the adsorb phase.
They both can use an absorb phase but to be healthy a Lead Acid battery must be kept at the Absorb phase longer. Yes Lead Acid is less efficient because you have to put in more than you took out. A Lithium battery does not require an Absorb phase and could be terminated at any voltage and be left there with no damage. A lead acid battery has to see a prolonged absorb phase to stay healthy. That extra time does not result in much increased capacity hence the less efficient charge characteristic

With Lithium it seems a single stage fixed voltage constant current charger which will taper to 0 charge current as the target voltage is achieved will be fine.

Any charger is a fixed voltage charger as soon as you set the constant voltage setting. However what you set it to is not necessarily what it puts out as soon as you connect it to a battery. For example if my target voltage is 13.6 volts but the voltage of the battery is 13 volts, as soon as you connect them the battery charger or power supply will try to pump as much current as it can to get the voltage to 13.6 but the charger or power supply has a current limit which is why it is called the constant current stage. As the battery is charged, the voltage at the charger/battery increases until finally the constant voltage setting is reached.

Some chargers can be set for an absorb time of zero so in that case that setting would be a single stage of constant current until the voltage setting is reached. Other settings for absorb (Constant Voltage} can be to terminate Absorb after so many minutes or until a minimum number Amps is reached. A power supply typically will not have those choices so it will keep going at the fixed voltage until the current drops to zero whether it is attached to a Lead Acid battery or Lithium battery.
 
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Ampster

Renewable Energy Hobbyist
Thank you both very much for this information, I have been using FLA forever and I am used to their charge characteristics but Lithium is new to me.
Just to pick up on your earlier comment about a 36 volt golf cart, I am confused by the 3.6 volt charger you mentioned. Most Golf carts used lead acid voltages and since a normal 12 volt battery is charged to 14.4 volts that would be 43.2 volts and if using typical LFP cells that would mean 12 of those. Conveniently 43.2 volts divided by 12 is 3.6 volts per cell. Perhaps that is what you are doing. I just did not want you or another reader to think that to convert a golf cart would only take 10 cells at 3.6 volts. That would actually be an underpowered golf cart and if the same charger was left in the ten cells would be way over charged.
 

krell313

New Member
Ampster, I think I meant charge to 3.6 volts per cell and yes, I'm planning a 2p12s configuration. I will be looking for a LiFePO4 charger when all is said and done. Actually I just unboxed my new Fortune 60ah cells and measured the voltage, each cell is 3.289v except 1 and that was 3.288v. From what I gather on this forum, top balancing should be pretty easy. I also may take you up on your offer about the Orion BMS.

Thank you
 

Just John

Photon Sorcerer
They both can use an absorb phase but to be healthy a Lead Acid battery must be kept at the Absorb phase longer. Yes Lead Acid is less efficient because you have to put in more than you took out. A Lithium battery does not require an Absorb phase and could be terminated at any voltage and be left there with no damage. A lead acid battery has to see a prolonged absorb phase to stay healthy. That extra time does not result in much increased capacity hence the less efficient charge characteristic



Any charger is a fixed voltage charger as soon as you set the constant voltage setting. However what you set it to is not necessarily what it puts out as soon as you connect it to a battery. For example if my target voltage is 13.6 volts but the voltage of the battery is 13 volts, as soon as you connect them the battery charger or power supply will try to pump as much current as it can to get the voltage to 13.6 but the charger or power supply has a current limit which is why it is called the constant current stage. As the battery is charged, the voltage at the charger/battery increases until finally the constant voltage setting is reached.

Some chargers can be set for an absorb time of zero so in that case that setting would be a single stage of constant current until the voltage setting is reached. Other settings for absorb (Constant Voltage} can be to terminate Absorb after so many minutes or until a minimum number Amps is reached. A power supply typically will not have those choices so it will keep going at the fixed voltage until the current drops to zero whether it is attached to a Lead Acid battery or Lithium battery.
Ampster is giving good advice here. I have done extensive testing, and the claim of 98% efficient might even be an underestimate. I consistently measured input versus output, and 98% is on the money. No need for anything fancy with adsorption, just make sure to use a good connection (no alligator clips). Don't try to get that last little bit in, 3.4v per cell is more than enough, maybe even less. As long as your connection is low resistance during charging, you can push plenty of amps in like that.
 

Ampster

Renewable Energy Hobbyist
I will be looking for a LiFePO4 charger when all is said and done.
If your golf cart charges to 43.2 volts you should be fine unless you want to go to a lower voltage in which case a programmable charger might be better. There are some that interface with the Orion and that would give you some options. The Orion heritage is motive power like EVs, golf carts and mining vehicles so they do a great job of giving you an accurate SOC.
Here is a link to the Orion App Notes section of their site. They interface with a bunch of chargers that I have seen on other golf carts and some displays.
 
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