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Charging a 12v 100Ah LiFePo

Farzad_K

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Jul 27, 2021
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Hello.
I am learning about the LiFePo batteries and am looking for suitable charging specification. I am assuming the charger should be capable of delivering at least as much wattage as the battery can deliver. Is there any guidance anywhere?

Thanks in advance.

Farzad.
 
Here's a good prerequisite video to check out as you inch into LiFePO4 (to help you decide on your charge/discharge voltages):

 
I am assuming the charger should be capable of delivering at least as much wattage as the battery can deliver. Is there any guidance anywhere?
Why are you assuming this?
You can charge and discharge at different rates. The battery has its own manufacturer specified charge and discharge limits which must be followed.
But for “wattage” as you say, you can discharge (for example) at 1000W and charge at 500W.
 
I would never rely on voltage as an indicator for the state of charge for LiFePO4. Especially not using a chart the says you aren't 100% until 3.65v.

Voltage is not accurate until you've let the cells rest for an hour after charge/discharge.

Charging until your cells are 3.65v is a great way to reduce your cell life expectancy.
 
I would never rely on voltage as an indicator for the state of charge for LiFePO4. Especially not using a chart the says you aren't 100% until 3.65v.

Voltage is not accurate until you've let the cells rest for an hour after charge/discharge.

Charging until your cells are 3.65v is a great way to reduce your cell life expectancy.

I agree on letting voltage settle a bit after charging/discharging, but in Will's video, you'd see that he indicates the sweet spot for long life on LiFePO4 is 25%-75% SOC, so we'll never even get close to 100%. Even 20%-80% SOC isn't close. Charging close to 3.65v should only be done for top-balancing anyways (that's why it's red in the chart example)...

Voltage is some kind of indicator if you're in the end legs of the SOC curve. This is how you have to set your charge and discharge limits is by voltage.
 
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I agree on letting voltage settle a bit after charging/discharging, but in Will's video, you'd see that he indicates the sweet spot for long life on LiFePO4 is 25%-75% SOC, so we'll never even get close to 100%. Even 20%-80% SOC isn't close. Charging close to 3.65v should only be done for top-balancing anyways (that's why it's red in the chart example)...

Voltage is some kind of indicator if you're in the end legs of the SOC curve. This is how you have to set your charge and discharge limits is by voltage.
Just pointing out that posting a chart that shows 3.65v as "full" in the beginners section encourages people to trash their cells. 3.4 to 3.45v is full.

I have done more than 100 full charge and discharge cycles with full telemetry to dial in charge and discharge. Notice the manufacturers don't tell you the voltage that corresponds to 80% DOD.
 
Just pointing out that posting a chart that shows 3.65v as "full" in the beginners section encourages people to trash their cells. 3.4 to 3.45v is full.

I have done more than 100 full charge and discharge cycles with full telemetry to dial in charge and discharge. Notice the manufacturers don't tell you the voltage that corresponds to 80% DOD.

I get what you're saying, so then what do we call it when they top balance their cells to 3.6-3.65v, then do we tell them it is actually at SOC like 104%-110% full now?
 
Why are you assuming this?
You can charge and discharge at different rates. The battery has its own manufacturer specified charge and discharge limits which must be followed.
But for “wattage” as you say, you can discharge (for example) at 1000W and charge at 500W.
It was just my layman's instinct. I imagined a 12 volt battery can only be charged with a charger that is 12 volts or higher, for example.

The question was about charging a LiFePo battery which I do not have, but wanted to understand charging and how it works. I currently have a 12V/35Ah (420Wh) AGM battery that I have been charging with a dedicated 12V/7.5Ah (90Wh) smart charger.

So, by "rate" I am assuming you are referring to the "C" rating. My 12V/35Ah (420Wh) AGM battery doesn't indicate any C rating but I always assumed 35Ah means it can provide 12 volts for 35 hours at 1A. I think this is 1C rating, and 0.5C rating might mean discharging at 0.5A?

I gather that the current rating of the charger can be anything, and the battery will draw the maximum that it can, or if it is intelligent, it will draw what is best for it. But what about voltage? Can a 6 volt charger be used to charge a 12v/35Ah battery knowing it might take a long time if we only went by the Watts?

After watching the video I have learned that the less we push and pull on the LiFePo battery's capacity the longer that it can last.
 
It was just my layman's instinct. I imagined a 12 volt battery can only be charged with a charger that is 12 volts or higher, for example.

Yes, the charger voltage must be higher than the battery voltage for any current to flow and charge the battery.

So, by "rate" I am assuming you are referring to the "C" rating. My 12V/35Ah (420Wh) AGM battery doesn't indicate any C rating but I always assumed 35Ah means it can provide 12 volts for 35 hours at 1A. I think this is 1C rating, and 0.5C rating might mean discharging at 0.5A?

For LiFePO4 the C rating refers to charge/discharge rate and is usually (not always) the same as the capacity rating. A 100 amp hour battery thus (usually) has a 100 amp charge/discharge rate (1C). Most manufacturers recommend a 0.5C rate, it usually gives a few percent more capacity. Most can also exceed 1C for short periods of time (it will cause the cells to heat up). Individual cells usually have a detailed spec sheet that tells you rates, and for some, they recommend a .03C or a 0.2C charge and discharge rate for the maximum capacity.
This is the datasheet for the Eve cells I have:

I gather that the current rating of the charger can be anything, and the battery will draw the maximum that it can, or if it is intelligent, it will draw what is best for it. But what about voltage? Can a 6 volt charger be used to charge a 12v/35Ah battery knowing it might take a long time if we only went by the Watts?

A LiFePO4 battery really looks like a dead short to a power supply or charger, so a charger will have to limit itself. That is normal for chargers, so don't worry about that. A 6v charger can not be used to charge a 12v battery, individual cells, maybe, but generally speaking, the charger must put out a higher voltage than the battery or no current will flow.

After watching the video I have learned that the less we push and pull on the LiFePo battery's capacity the longer that it can last.

Really, it depends on a lot of factors.
 
Yes, the charger voltage must be higher than the battery voltage for any current to flow and charge the battery.



For LiFePO4 the C rating refers to charge/discharge rate and is usually (not always) the same as the capacity rating. A 100 amp hour battery thus (usually) has a 100 amp charge/discharge rate (1C). Most manufacturers recommend a 0.5C rate, it usually gives a few percent more capacity. Most can also exceed 1C for short periods of time (it will cause the cells to heat up). Individual cells usually have a detailed spec sheet that tells you rates, and for some, they recommend a .03C or a 0.2C charge and discharge rate for the maximum capacity.
This is the datasheet for the Eve cells I have:



A LiFePO4 battery really looks like a dead short to a power supply or charger, so a charger will have to limit itself. That is normal for chargers, so don't worry about that. A 6v charger can not be used to charge a 12v battery, individual cells, maybe, but generally speaking, the charger must put out a higher voltage than the battery or no current will flow.



Really, it depends on a lot of factors.
Thanks.
 
Is there any guidance anywhere?
The following notes are based on my experience over the last few years with small battery packs ( less than 500Ah) in sailboat and RV instalations.
For a DIY 4 cell battery obtain a charger that can be configured for the main charge voltage, called absorption or boost voltage, the 'target volts', set to a value of 13.8 to 14 volts. Any absorption time, the constant voltage phase, needs to be zero or 30 minutes maximum.
The float voltage set will depend on application, if its a standby application select the float voltage to be 13.3 volts, if its required to power loads from solar and battery then select 13.5 volts.
The re boost setting where the charger re enters the boost stage should be set to 0.1 volts lower than the float voltage.

If the application requires the battery to be left 'idle' for long periods with no loading then it should be discharged to around 13.1 to 13.2 volts with loads and chargers disconnected. If in this 'idle' period there are small system loads, then changing the charge settings to 13.2 volts will maintain the battery at a mid state of charge. The battery should be fully charged to 13.8 volts before normal use.

The maximum charge current output of the charger for long service life should be around 0.25C, where C is the battery capacity. Any current can be used up to 1C for many cell types, but the higher the current the higher the stress on the cells. Very roughly the battery capacity divided by the charge current will indicate the charge time for a fully depleted battery, for example a 100Ah battery with a 20 amp charger will need 5 hours. If the battery was discharged to 50 % capacity, with a 20 amp charger it will take 2.5 hours.
There is no minimum charge current, so if you have a 1 amp charger set at a charge voltage of 13.8 volts it will take around 100 hours to charge the depleted 100Ah battery.

The charge voltages of 13.8 volts and short absorption time will charge the cells up to 98% or higher.

A final point, if the battery is a drop in type, like Battleborn, then the charge voltage levels should be as recommended, keeping to the lower end of the values.

Mike
 
So, for a 100Ah battery, what charge rate do you recommend?

Assuming that battery has a max discharge rate of 100 amps (different from 100 Ah capacity) then 0.2C is 20 amps.

My LiFePO4 batteries are always charged from an undersized solar panel system and are routinely charged at less than 0.1 C.
 
Yes, the charger voltage must be higher than the battery voltage for any current to flow and charge the battery.



For LiFePO4 the C rating refers to charge/discharge rate and is usually (not always) the same as the capacity rating. A 100 amp hour battery thus (usually) has a 100 amp charge/discharge rate (1C). Most manufacturers recommend a 0.5C rate, it usually gives a few percent more capacity. Most can also exceed 1C for short periods of time (it will cause the cells to heat up). Individual cells usually have a detailed spec sheet that tells you rates, and for some, they recommend a .03C or a 0.2C charge and discharge rate for the maximum capacity.
This is the datasheet for the Eve cells I have:



A LiFePO4 battery really looks like a dead short to a power supply or charger, so a charger will have to limit itself. That is normal for chargers, so don't worry about that. A 6v charger can not be used to charge a 12v battery, individual cells, maybe, but generally speaking, the charger must put out a higher voltage than the battery or no current will flow

The following notes are based on my experience over the last few years with small battery packs ( less than 500Ah) in sailboat and RV instalations.
For a DIY 4 cell battery obtain a charger that can be configured for the main charge voltage, called absorption or boost voltage, the 'target volts', set to a value of 13.8 to 14 volts. Any absorption time, the constant voltage phase, needs to be zero or 30 minutes maximum.
The float voltage set will depend on application, if its a standby application select the float voltage to be 13.3 volts, if its required to power loads from solar and battery then select 13.5 volts.
The re boost setting where the charger re enters the boost stage should be set to 0.1 volts lower than the float voltage.

If the application requires the battery to be left 'idle' for long periods with no loading then it should be discharged to around 13.1 to 13.2 volts with loads and chargers disconnected. If in this 'idle' period there are small system loads, then changing the charge settings to 13.2 volts will maintain the battery at a mid state of charge. The battery should be fully charged to 13.8 volts before normal use.

The maximum charge current output of the charger for long service life should be around 0.25C, where C is the battery capacity. Any current can be used up to 1C for many cell types, but the higher the current the higher the stress on the cells. Very roughly the battery capacity divided by the charge current will indicate the charge time for a fully depleted battery, for example a 100Ah battery with a 20 amp charger will need 5 hours. If the battery was discharged to 50 % capacity, with a 20 amp charger it will take 2.5 hours.
There is no minimum charge current, so if you have a 1 amp charger set at a charge voltage of 13.8 volts it will take around 100 hours to charge the depleted 100Ah battery.

The charge voltages of 13.8 volts and short absorption time will charge the cells up to 98% or higher.

A final point, if the battery is a drop in type, like Battleborn, then the charge voltage levels should be as recommended, keeping to the lower end of the values.

Mike

Thanks for the note and important information on the DYI LiFePO4 batteries. What charger do you recommend that has these features built in?
 
A 100 amp hour battery thus (usually) has a 100 amp charge/discharge rate (1C)
Both SOK and Dakota state a 50A max charge rate. I think BattleBorn is 100A. I'm curious if this is a limitation in the cells, BMS or other. I can't imagine charging them with anymore amperes in my RV with shore power, generator or solar.
 
Both SOK and Dakota state a 50A max charge rate. I think BattleBorn is 100A. I'm curious if this is a limitation in the cells, BMS or other. I can't imagine charging them with anymore amperes in my RV with shore power, generator or solar.
Charge rate under 1C is usually a limitation of the BMS rather than the cells.
 
Thanks for the note and important information on the DYI LiFePO4 batteries. What charger do you recommend that has these features built in?
Personally, I use a Victron charger, more expensive, but also more configurable.


I needed something pretty much "plug it in, and it works" for my daughter (currently in the path of Ida). I just set it properly using the app, and sent it with the battery, inverter, etc.

Anyone interested, the Lion competition for Battleborn is on sale (still expensive) and in stock at Costco.

 
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