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Understanding LIFEPO4 battery terminology and figuring out when its charged...

lucl

New Member
Joined
Jan 6, 2022
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38
Hi guys,

This community has been great at providing valuable tips in the last few days since I've joined. Thank you. I greatly appreciate it. I am watching a video about LIFEPO4 batteries posted by one of the members to learn more about them.

...

Question #1:

I'm a total beginner to LIFEPO4 batteries and I'm having a hard time pinning down definitions for some of the common terms. The specs for my battery are next to the term.

Nominal Voltage - 12.8 = ?
Voltage Window - 9.2 – 14.6V = ?.. my guess is that this is the range of voltage the battery can have. and the voltage is an indicator of energy remaining.
Charge Voltage - 14.4V = ? ... my guess is that this is the voltage at which the battery should be charged... but maybe its the voltage when it's fully charged?
Standby Voltage - 13.8V = ? ... no idea

Max. Continue Discharge Current - 100AH
Max. Permanent Discharge Current - 200A 2 seconds

...

Question #2:

I'm trying to figure out how to determine if my battery is charged. I have it plugged into a Progressive Dynamics PD9145ALV 12V Lithium Ion Battery Converter/Charger - 45 Amp. It charges at 14.6V @ 45 amps. My battery has been sitting plugged in and I've been taking voltage readings for the past few hours (while it's charging).

For the last 2 hours, the battery has been sitting at 13.71V while it's plugged into the charger (and charger is on). There has been NO CHANGE in voltage in about 2 hours. When I unplug the charger, remove loads (no loads anyways) and take readings 1 hour after unplugging from charger, the battery indicates a charge of 13.35V.

From LIFEOPO4 voltage charts (ones on this forum) 13.35V is about 90-95% charged but my battery never achieves 13.6V (with no load and tested using multiple instruments).

The battery is an expertpower 12V 100AH battery, this is all they provide as far as specs: https://cdn.shopify.com/s/files/1/2364/9089/files/EP12100_v2_100A.pdf?v=1610472867
 
Looks like some Chinese to English translation problems in spec.

"Max. Permanent Discharge Current = 200A 2secs" should read "Max Permissible Discharge Current = 200 Amps max for 2 seconds max."

Nominal voltage is 4 cells for 12v self contained battery x 3.3v nominal per cell = 13.2 v. This is about 50% state of charge.

Range per cell is 3.65v to 2.5v per cell X 4 = 14.6v to 10 v for 12v LFP battery. Although a cell at 3.0v is pretty much discharged, x4 = 12v min.

Standby may mean Storage. Preferred to be at 50% state of charge = 3.3v per cell X 4 = 13.2 v.

Charge max of 14.4v /4 = 3.60v per cell. This is fine. Alway stay below 14.6v maximum. Any long term float charger should stay below 3.45v x 4 = 13.8v which may be their term for 'Standby'.

All these voltage numbers are very close to a normal lead acid 12v battery which is why LFP 12v batteries are a compatible with charge profile of lead acid charger. With exception of lead acid equalize which is higher voltage and must not be done on LFP battery.
 
Last edited:
Nominal Voltage - 12.8 = ?
Voltage Window - 9.2 – 14.6V = ?.. my guess is that this is the range of voltage the battery can have. and the voltage is an indicator of energy remaining.
Charge Voltage - 14.4V = ? ... my guess is that this is the voltage at which the battery should be charged... but maybe its the voltage when it's fully charged?
Standby Voltage - 13.8V = ? ... no idea
Nominal Voltage - 12.8 = ?
Nominal voltage is kinda an average voltage. I rarely ever use nominal voltage for anything. Instead I look at what happens at full voltage and low voltage.

>Voltage Window - 9.2 – 14.6V = ?.. my guess is that this is the range of voltage the battery can have.
Correct, but I never run the batteries to as low as 9.2V I usually set things to shut off at 11 or 12V.
>and the voltage is an indicator of energy remaining.
On liFePO4, voltage is a very poor indicator of energy remaining. For most of the time, the voltage will sit in a very narrow range of voltage. Only when the battery is nearly full or nearly empty will there be any significant voltage swing.
> Charge Voltage - 14.4V = ? ... my guess is that this is the voltage at which the battery should be charged...
Correct. However, when charging the battery will reach the target voltage and still take current for a while. The battery is not fully charged to 14.4V till the current drops to zero. Notice that they tell you to charge to a lower voltage than the voltage range. This is to maximize the life of the battery.
> Standby Voltage - 13.8V = ? ... no idea
Many chargers will drop to a 'float' or 'maintenence' voltage after the charge is complete. This is the best voltage for that. However, If the charger allowed it, I prefer to turn off the charger all together.
 
I'm trying to figure out how to determine if my battery is charged. I have it plugged into a Progressive Dynamics PD9145ALV 12V Lithium Ion Battery Converter/Charger - 45 Amp. It charges at 14.6V @ 45 amps. My battery has been sitting plugged in and I've been taking voltage readings for the past few hours (while it's charging).

For the last 2 hours, the battery has been sitting at 13.71V while it's plugged into the charger (and charger is on). There has been NO CHANGE in voltage in about 2 hours. When I unplug the charger, remove loads (no loads anyways) and take readings 1 hour after unplugging from charger, the battery indicates a charge of 13.35V.
The most likely scenario is that the battery is still charging. As I mentioned in my previous post, the voltage will stay the same till the very end of the charge cycle.

You can check to see if it is still charging by checking the amperage. My guess is that you will find that there is a small current still going through the batteries. When the batteries finally get charged you will see the voltage go to 14.6V fairly quickly and soon after that the current will go to zero.

From LIFEOPO4 voltage charts (ones on this forum) 13.35V is about 90-95% charged but my battery never achieves 13.6V (with no load and tested using multiple instruments).
I ignore those charts. The only time voltage tells you anything meaningful about state of charge is when the batteries are nearly empty or nearly full.
 
OP: Do you have Clamp-on DC Amp meter? If not, you should get one, it will be very helpful.
 
When I unplug the charger, remove loads (no loads anyways) and take readings 1 hour after unplugging from charger, the battery indicates a charge of 13.35V.
This is normal. After the charger is removed from the battery, the voltage will drop to a resting voltage.
 
Awesome. Thanks for the help guys. As far as the meters I am using, I have both a clamp one and a regular one.
Nominal Voltage - 12.8 = ?
Nominal voltage is kinda an average voltage. I rarely ever use nominal voltage for anything. Instead I look at what happens at full voltage and low voltage.

>Voltage Window - 9.2 – 14.6V = ?.. my guess is that this is the range of voltage the battery can have.
Correct, but I never run the batteries to as low as 9.2V I usually set things to shut off at 11 or 12V.
>and the voltage is an indicator of energy remaining.
On liFePO4, voltage is a very poor indicator of energy remaining. For most of the time, the voltage will sit in a very narrow range of voltage. Only when the battery is nearly full or nearly empty will there be any significant voltage swing.
> Charge Voltage - 14.4V = ? ... my guess is that this is the voltage at which the battery should be charged...
Correct. However, when charging the battery will reach the target voltage and still take current for a while. The battery is not fully charged to 14.4V till the current drops to zero. Notice that they tell you to charge to a lower voltage than the voltage range. This is to maximize the life of the battery.
> Standby Voltage - 13.8V = ? ... no idea
Many chargers will drop to a 'float' or 'maintenence' voltage after the charge is complete. This is the best voltage for that. However, If the charger allowed it, I prefer to turn off the charger all together.
Thanks for the great info. So a follow up question for you...

How do YOU check remaining energy level of a LIFEPO4 battery? I'm just curious how experienced users do it. Let's say you got a battery and you are trying to establish it's energy level and you know nothing about it other than its a 12V 100AH LIFEPO4 battery.

Also... correct me if I'm wrong... but the "proper" way of keeping track of energy capacity is if you know that you are starting off with 100% (or 0%) you plug the battery bank into a shunt + charge controller + inverter (if discharging) and you keep track of charge/discharge using an automated logger. And essentially what's happening is the shunt (say viktron smartshunt) is "guessing" the battery levels based on energy pulled/push into the battery bank? Is this the "by the book" way of keeping track of battery energy levels?

I know these are probably stupid questions but I'm just trying to understand things a bit clearer. I was under the impression that LIFEPO4 batteries had some type of a communication protocol to share with LIFEPO4 chargers (since the tech is new) and other devices so that there is less guessing involved, but I don't think that's the case.
 
How do YOU check remaining energy level of a LIFEPO4 battery? I'm just curious how experienced users do it. Let's say you got a battery and you are trying to establish it's energy level and you know nothing about it other than its a 12V 100AH LIFEPO4 battery.
I don't check. As far as I know, there is no good way to check. In the scenario you describe, I give it a full charge, and then I know it is at 100%

Also... correct me if I'm wrong... but the "proper" way of keeping track of energy capacity is if you know that you are starting off with 100% (or 0%) you plug the battery bank into a shunt + charge controller + inverter (if discharging) and you keep track of charge/discharge using an automated logger.
Yes.
And essentially what's happening is the shunt (say viktron smartshunt) is "guessing" the battery levels based on energy pulled/push into the battery bank? Is this the "by the book" way of keeping track of battery energy levels?
Yes... but I would not call it 'guessing'. It is measuring current in/out and calculates the energy remaining in the battery. LiFePO4 is efficient enough that the measurement is fairly accurate. Lead-acid batteries are not nearly as efficient, so the calculations have to adjust for that. A good SOC meeter will adjust based on battery type.

A shunt is nothing more than a precision resistor of a known value. The meter measures the voltage across the shunt. From that, the current through the shunt is calculated. The current is then integrated over time to determine Amp-hours in/out. The system will typically also measure the battery voltage. With current and voltage, it can calculate Wattage and Watt-hour in/out

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Thank you! Excellent information and all of my questions are answered now.
 
I know these are probably stupid questions but I'm just trying to understand things a bit clearer.
There are no stupid questions. We all started out knowing nothing and we all learned by reading and asking questions.
I was under the impression that LIFEPO4 batteries had some type of a communication protocol to share with LIFEPO4 chargers (since the tech is new) and other devices so that there is less guessing involved, but I don't think that's the case.
In most cases, the battery does not communicate to the rest of the system. However, some of the more advanced batteries do have communication methods that allow them to communicate with the rest of the system. Furthermore, all BMSs have a built-in shunt that allows them to calculate the current in order to do over-current protection. Smart BMSs (with blue-tooth or some other communication path) will almost always use the shunt to track state-of-charge and present that via the communication protocol.

Right now, the com ports are used mostly to communicate data, but not to coordinate among components.....but some advanced systems do coordinate.

The challenge with tieing the BMS/Battery into the rest of the system over the com port is that there is no standard protocol. Therefore, even if everything has something like a CAN bus interface, the chances are good that they can not communicate and coordinate. Right now, systems that integrate and communicate well are systems where everything comes from the same manufacturer. As an example, Victron has ways to get their chargers, inverters, batteries, etc to all communicate and coordinate. We are starting to see some equipment that is designed to mimic the protocols of other manufacturers, but that is still the exception, not the rule. It would be nice if there was a standard for this, but I am not aware of anyone working on a standard.
 
Filter Guy: thank you for being kind and patient to this person new to the world of LifePO4 and answering his/her questions. You represent what I think is the spirit of this forum. Unfortunately, I lately experienced much different treatment by a member while essentially trying (just as luci was doing) to understand a term used in regard to LifePO4 charging.
 
I lately experienced much different treatment by a member while essentially trying (just as luci was doing) to understand a term used in regard to LifePO4 charging.
Sorry to hear that. As the forum grows there will be a few rude folks.... but hopefully they remain the exception.
 
The fact you may not be seeing any change in charge voltage may be:
you battery is charged and the BMS has disabled the charge path, or the charger has completed charging and dropped the float volts , nominally 13.6 for your charger.

The resting volts of 13.35 is full charged for many small Ah cells.

A battery monitor would be useful to establish state of charge.

Mike
 
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