Unusual behaviour from LiFePo4 battery

[poot_traveller]

I pulled the fuse between the controller and panel so I can discharge the battery from 100% charged down to 30% charged. However, in the first two days it went from 100% to 74%, which is what I was expecting it to do, but then it went from 74% to 40% within one hour and down to 30% in the next half an hour.

That is an extremely quick discharge/percentage drop in a short amount of time and I was only charging a laptop and powering a modem, which is what I was doing before, using a double plug on the inverter.

The controller was indicating low voltage point reached at 12v and it stopped charging my laptop.

[font=Tahoma,Verdana,Arial,Sans-Serif]Any ideas why the rapid drop in percentage in such a short space of time?  [/font]

This is the battery: https://www.trademe.co.nz/motors/ca...tm?rsqid=5ead1b95d4034d49a2e58ad5e3d4cae7-001

 

[jonyjoe303]

how are you measuring the capacity of your lifepo4? Lifepo4 requires a coulombmeter (amp counter) to give you a reliable reading. If the voltage drops quickly it usually means the battery wasn't fully charge. Voltage will only tell you if you have a full battery if you can actually see 14.6 volts at the battery terminals. 

[font=Tahoma, Verdana, Arial, sans-serif]From your description it sounds like your battery was at 50 percent or less, at 50 pe[/font][font=Tahoma, Verdana, Arial, sans-serif]rcent it would read 13.1 volts, and unless you were counting amps you would think it was full or nearly full[/font][font=Tahoma, Verdana, Arial, sans-serif].[/font][font=Tahoma, Verdana, Arial, sans-serif] [/font]It has happened to me before, when I wasn't using a coulombmeter. The battery always reads 13.1 volts when its between 10 percent and 90 percent, once it gets around 12.8 volts the voltage drops quickly and then it stops working.   

A coulombmeter will only give you an accurate reading if you can get your lifepo4 up to 14.6 volts, then it will know the battery is at 100 percent. Thats when it calibrates itself.

 

[poot_traveller]

I'm measuring the capacity of the LiFePo4 battery using this remote meter https://fazcorp.co.nz/collections/m...-1/products/remote-meter-mppt-controller-rm-6

I don't know what a coulombmeter is, but looking at google images it appears you're referring to what we call a multimeter here in NZ.
https://en.wikipedia.org/wiki/Multimeter

I'll buy a multimeter and get the charge up to 14.6v to calibrate it. Then I'll see what it does.

Thank you for your help.

 

[Trebor English]

Lead acid batteries occasionally need an equalizing charge.  That's where th battery charging continues after being filled to make it so that all the cells are filled, equalized.  With one cell a little bit low the whole battery can appear to be full but when it discharges the lowest cell runs out first. 

Lithium batteries don't get equalized.  It is called balanced and is a similar function.  Top balanced means they are all full together.  Bottom balanced means they all are empty together. 

Some battery management systems will apply a load to the most full cell to bring it down to match the others.  Some battery management systems will shut off the current when the top cell is full or the bottom cell is empty without doing any balancing.  Low current lithium batteries tend to stay balanced much better than lead acid batteries. 

According to the web page your battery has "Cell balancer/Protection" so it should be doing the balancing.  If it really does the balancing then each charge/discharge cycle should make it better.  The bottom cell will end up closer to the others as the top cells get lowered. 

According to the web page it is supposed to cut off at 8.4 volts.  If the lowest cell got to the low cell limit long before the other cells that would account for the shutting off way above the 8.4 limit.  Your battery management may be doing exactly what it should.

 

[poot_traveller]

I to believed it was a cell balancing problem and maybe it still is, and like you say it each charge/discharge should make it better. I guess time will tell.

The solar system shutting down at around 12v could also be the MPPT controller set to shut down below 12v (by default), idk, I'm still new to all of this and still trying to figure out how to use the remote display to look at the different settings.

 

[jonyjoe303]

The coulombmeter is different then a multimeter. I use the tk15 coulombmeter (cost about 25 dollars) it can handle 50 amps, they sell ones that can read 300 amps if your power use is larger. I use the tk15 on all my lithium batteries, its very accurate for the price. With this coulombmeter you don't have to guess if your battery is getting a full charge, also you can get an idea if your battery is actually giving you 100ah. The shunt (the part with wires connected) goes connected near the battery terminals, so its reading the battery actual voltage, if your battery isn't getting to 14.6 volts, you'll know there is something wrong somewhere.  

When you buy one you program the battery amp hour , for your battery 100ah, then you program the battery full voltage 14.6 volts, also the low voltage. It goes connected to the battery and counts all the amps in/out of the battery.  It won't give you an accurate percentage until the battery voltage reaches 14.6 volt, when it sees that voltage the screen automatically shows 100 percent, now you know you got an accurate reading of the battery capacity. Now as the battery discharges it subtracts the amps. If you use 20 amps today, you know tomorrow you need to add 20 amps back.

A lifepo4 bms will usually shutdown at 12 volts (3 volts per cell) , so more or less your lifepo4 is probably at 0 percent. With my 240 watt panel when my lifepo4 got to "0" percent it took almost a week to get it back to 100 percent. 

picture of the tk15 (50 amp version) official name "DC8-80V 50A Battery Coulometer TK15 Professional Precision LiFePO Battery Tester"

 

[poot_traveller]

11.5v is 20% charged for a LiFePo4 and 12v is somewhere around 35% charged. The remote display showed it at 11.7v and 30% charged but it had shutdown at around 12v, probably the controller shut it down I'm guessing.

It doesn't look like we have coulombmeter in NZ, or maybe these are under another name. I'll keep searching. Thank you for your help.

 

[poot_traveller]

Well this is interesting. Just got home from work and the remote display is saying 13.1v but it's also saying the battery is 96% charged.

13.1v means it is 50% charged. Why is the remote display saying 96% charged?

 

[poot_traveller]

11.5v is 20% charged for a LiFePo4 and 12v is somewhere around 35% charged. The remote display showed it at 11.7v and 30% charged but it had shutdown at around 12v, probably the controller shut it down I'm guessing.

It doesn't look like we have coulombmeter in NZ, or maybe these are under another name. I'll keep searching. Thank you for your help.

I stand corrected.

12.9 volts means the battery is down to 20% charge according to this site: https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/

So according to the above site, at 11.7v I had it down to 5%.

 

[poot_traveller]

Sorry, I've been in a bit of a panic wondering if I've wrecked my LiFePo4 by discharging it too much.

According to the LiFePo4 battery manual, I'm not to discharge below 10 volt under load (which is 11.2 resting voltage). Luckily I only got it down to 11.7 volts.

 

[poot_traveller]

[font=Tahoma, Verdana, Arial, sans-serif]From your description it sounds like your battery was at 50 percent or less, at 50 pe[/font][font=Tahoma, Verdana, Arial, sans-serif]rcent it would read 13.1 volts, and unless you were counting amps you would think it was full or nearly full[/font][font=Tahoma, Verdana, Arial, sans-serif].[/font][font=Tahoma, Verdana, Arial, sans-serif] [/font]It has happened to me before, when I wasn't using a coulombmeter. The battery always reads 13.1 volts when its between 10 percent and 90 percent, once it gets around 12.8 volts the voltage drops quickly and then it stops working.   

According to the LiFePo4 battery manual; 12 volts - 12.6 volts is 40%-50% charged.

 

[jonyjoe303]

This is the battery gauge used by battleborn and should be the same for all lifepo4. But I wouldn't rely on it due to the battery usually sits at 13.1 to 13.3 volt range. When you put a load voltage drops but usually comes back up to 13.1 volts. Don't worry you can drain the battery all the way to zero percent numerous times, it won't hurt a lifepo4. At 10 volts (zero percent) each cell voltage would be 2.5 volts, lifepo4 is design to go that low without harm. As you can see on the gauge once the resting voltage is 12.8 volts, your running on empty. 

 

[MotorVation]

jonyjoe303This is the battery gauge used by battleborn and should be the same for all lifepo4. But I wouldn't rely on it due to the battery usually sits at 13.1 to 13.3 volt range. When you put a load voltage drops but usually comes back up to 13.1 volts. Don't worry you can drain the battery all the way to zero percent numerous times, it won't hurt a lifepo4. At 10 volts (zero percent) each cell voltage would be 2.5 volts, lifepo4 is design to go that low without harm. As you can see on the gauge once the resting voltage is 12.8 volts, your running on empty. 
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My Battleborn's hang right between 13.1 to 13.3 just like you mentioned. There's a very light load on them.
I have a voltage and capacity meter on them but I don't feel the state of charge reading is correct with that particular meter as it doesn't keep track of amps in and out. I just bought this one off Amazon. I haven't hooked it up yet. Here's the link if you want to check it out.  I guess you could call it a coulombmeter. It's rated for 200AH


https://www.amazon.com/gp/product/B07B4CWKRJ/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

Edited to add: There is a PDF manual somewhere in the description with more details.

 

[poot_traveller]

This is the battery gauge used by battleborn and should be the same for all lifepo4. But I wouldn't rely on it due to the battery usually sits at 13.1 to 13.3 volt range. When you put a load voltage drops but usually comes back up to 13.1 volts. Don't worry you can drain the battery all the way to zero percent numerous times, it won't hurt a lifepo4. At 10 volts (zero percent) each cell voltage would be 2.5 volts, lifepo4 is design to go that low without harm. As you can see on the gauge once the resting voltage is 12.8 volts, your running on empty. 

Yeah it was sitting on 14.3v today and the MPPT controller was indicating overvoltage (I'm guessing the controllers over-voltage protection is set at 14.3v by default). As soon as I put a load on it, the voltage dropped into the 13.1v - 13.3v range. So I still have no idea how charged up the battery is at any point until it's almost empty.

It gives me peace of mind to know I can discharge down to 10v/empty, although I don't intend to purposely take it that low. Still good to know that information so I don't stress out.

Thank you

 

[poot_traveller]

Edited to add: There is a PDF manual somewhere in the description with more details.

Yeah I got a similar remote display, this one: https://fazcorp.co.nz/collections/m...-1/products/remote-meter-mppt-controller-rm-6

Remote displays don't tell the full story about what's going on with the battery charge, but apparently with these LiFePo4's it's hard to get a accurate charge reading anyway due to the battery discharging in a flat line.

 

[Markk60]

Glad I found this thread! I have a similar situation. I just installed a BigBattery 12v 202 ah battery in my trailer. I'm monitoring battery function with a Victron SmartShunt. After fully charging the battery it reads 13.6, but within 3 hours it reads 13.3, with just 1.0 amp of discharge. I reset the BMS and attempted to recharge the battery again, but it was still full. Once again the battery was at 13.6, and within 3 hours was back at 13.3 with about the same amount of discharge. The battery is 99.95% full but its reading 13.3. Any idea what is going on with this thing?

 

[MotorVation]

I don't know the science behind it but my Battleborn's charge at 14.4 in bulk (or mppt if you prefer) then 14.4 for 30 minutes for each battery in absorption (one hour), then they float at 13.6 volts till the sun goes down. From there, they drop down to 13.3 and hang there for hours. If I stay up late enough the voltage will drop to 13.2 or even 13.1 if I'm using a lot of power.

The above settings are what Battleborn recommended for my charge controller. Your mileage may differ but it sounds normal to me

 

[wayne49]

"in bulk (or mppt if you prefer)"
bulk has to do with battery side charging.
mppt has to do with solar panel side controlling.

 

[Markk60]

I don't know the science behind it but my Battleborn's charge at 14.4 in bulk (or mppt if you prefer) then 14.4 for 30 minutes for each battery in absorption (one hour), then they float at 13.6 volts till the sun goes down. From there, they drop down to 13.3 and hang there for hours. If I stay up late enough the voltage will drop to 13.2 or even 13.1 if I'm using a lot of power.

The above settings are what Battleborn recommended for my charge controller. Your mileage may differ but it sounds normal to me

Thanks for the reply. My voltage reading goes down without any load on it, even though the battery if fully charged.

 

[gtinseth]

Definitely get a shunt based meter to monitor state of charge/depth of discharge. LiFePO4 has such a flat discharge curve that voltage alone is not a good way to keep track.

Many folks who have LiFePO4 banks charge up to 14 volts and discharge to 12.5 volts. Above and below these values and you are in the "knees" of the curve. There is not much charge left on either end and you will only decrease your lifetime by going there. Once you are in the knee, voltage will change very quickly as the curve becomes steep.

Hope your battery is OK.