LiFePO4 charging and protection circuit

[YoSuk]

Hello, first time posting here
I am currently working on a project involving LiFePO4 batteries which will be charged by a solar panel. The batteries will at the same time provide power to a load circuit. This is the circuit i have made so far:



This circuit, if i have understood the datasheet correctly, will keep a regulated output voltage of around 3.6V, which is the voltage required to charge the LiFePO4 battery used in the application.

What i now need is a circuit to prevent overcharging and prevent undervoltage. The battery datasheet lists the "discharge cutoff" as 1.8V. Is this the lowest recommended voltage the battery can reach without damaging it? The solar panel in the project can not be changed for one with a higher voltage/power output, and the batteries has to be LiFePO4 type batteries. The entire purpose of the solar panel is to help extend the battery life of the batteries. The load in question will on average use around 1mA of current, with a maximum of 500mA for short periods.

I have been searching for battery management ICs, but have been unable to find any that provides overcharging and undervoltage protection for LiFePO4 type batteries. Does anyone have any suggestions as to how i can provide this protection to these batteries?

[Sunking]

Blow your drawing up si my tired ole eyes can see it. Kill the green or make it darker please.

[inetdog]

Blow your drawing up si my tired ole eyes can see it. Kill the green or make it darker please.

Right click on the image in your browser and select something like view in new window. You will then be able to zoom in (CTRL-SHIFT-+) to get it to a readable size.
Then you will see that it is a low resolution image, but the text is still tolerable, even in green.
It would be nicer for the OP to have attached it in higher resolution though.

[YoSuk]

Ahh sorry i was on my phone so i didn't quite see the resolution. Just reuploaded it now, is this one better?

[inetdog]

Ahh sorry i was on my phone so i didn't quite see the resolution. Just reuploaded it now, is this one better?

Lovely! Thank you.

[Sunking]

Here is my two-cents worth. Why reinvent the wheel? Basically what you want is a standard USB charger for a cell phone. There are hundreds of designs out there. Most notable ones built around Texas Instruments TPS54240 and TPS2511. Goggle this search term then click Images: "USB Charger Schematic". You will get thousands of circuits to look at. Anyone of them can easily be modified to work with a solar panel and adjust the voltage for LFP. In fact many need no mods at all.

Good Luck to you.

SK

[PNjunction]

This circuit, if i have understood the datasheet correctly, will keep a regulated output voltage of around 3.6V, which is the voltage required to charge the LiFePO4 battery used in the application.

That would be fine for a Lifepo4, BUT you must ensure that it eventually stops when the battery is no longer demanding any absorb current. Keeping it at 3.6v with no current flowing after the full charge is completed is not good.

What i now need is a circuit to prevent overcharging and prevent undervoltage. The battery datasheet lists the "discharge cutoff" as 1.8V. Is this the lowest recommended voltage the battery can reach without damaging it?

That would be the immediate damage voltage, and is NOT what you'd want as an LVC cutoff. Understand that Lifepo4 has a very flat discharge curve, and when it just starts to fall off the edge, that is the time to stop. This would be around 3.2 or perhaps 3.1v max.

If you go any further down into the discharge curve, you definitely hurt battery life. What is commonly overlooked is that when your battery is below 3.2v, then one must apply NO MORE than 0.1C, sometimes even less when recharging until the battery reaches 3.2v. Only then should you apply the normal bulk current charge. So the best way to avoid this double-whammy of too low an LVC, and subsequent too high current recharge when at these low levels, set your LVC to the point at which the lifepo4 just starts to drop into the deep discharge knee.

I've seen way too many homebrew videos of guys smoking their packs setting their lvc too low, and then applying full recharge current when they are deep into the knee because the manufacturer has spec'ed the discharge voltage level at the one that causes immediate damage, and not the one that you should use under normal circumstances.

The analogy here would be like seeing a spec for 12v lead acid battery having an LVC cutoff of 10V, which WILL cause some damage, but under normal circumstances, nobody in their right mind would go down that low.

[Sunny Solar]

2.2v is ABSOLUTE ROCK BOTTOM FOR lV CUTOFF.. 12.4 is the prefered voltage.. 3.9v is the upper limit for long life.

[Sunking]

2.2v is ABSOLUTE ROCK BOTTOM FOR lV CUTOFF.. 12.4 is the prefered voltage.. 3.9v is the upper limit for long life.

Huh?

[mattrix]

... you must ensure that it eventually stops when the battery is no longer demanding any absorb current. Keeping it at 3.6v with no current flowing after the full charge is completed is not good.

Can you explain why?
I mean, if there is no current flowing and never goes above 3.6v?

I've seen this said before, but never understood it.
As far as I know, these things are normally charged with a constant current initially, and to get enough current they are often charged from a higher voltage. The charge has to be interrupted occasionally to check that the battery hasn't reached 3.6v, and if it has STOP.

But if the voltage is never over 3.6v, surely the tiny current is not a concern.

[billvon]

But if the voltage is never over 3.6v, surely the tiny current is not a concern.

No, but the voltage is. High voltages are bad for LiFePO4 life. By stopping the charge you allow it to slowly settle back down to 3.4 volts or so, where it will experience less degradation.

[Sunking]

Can you explain why?
I mean, if there is no current flowing and never goes above 3.6v?

Your analogy is incorrect. The current does not stop with a Constant Voltage, it tapers off, but does not stop. Lead acid and all the other chemistry except lithium are chemical reactions. Lithium is a Lithium Ion Exchange between Anode and Cathode. Once you reach the prescribed full voltage a continuous trickle charge would cause plating of metallic lithium and compromise the battery causing it to heat up to the point it can explode.

Unlike lead acid batteries, lithium does not need to be charged to 100% capacity. In fact every battery manufacture does not recommend charging to 100% as is significantly reduces cycle life. You get more capacity or run time at 100%, but you sacrifice cycle life doing so. Some manufactures underrate their capacity and specify a lower charge voltage set point to 80% SOC. This way the customer gets the expected capacity and long cycle life.

Motto to the story is only charge your lithium to 80% SOV voltage using a Constant Voltage Current Taper at .5C until the charge current tapers off to 3% of rated C. Never discharge below 20% SOC voltage. Do that and you maximize cycle life. For some lithium batteries can be as much as 2000 to 3000 cycles. Take them up to 100% SOC and you reduce cycle life to less than 1000 cycles. Pick your poison carefully.

[Sunny Solar]

2.2v is ABSOLUTE ROCK BOTTOM FOR lV CUTOFF.. 12.4 is the prefered voltage.. 3.9v is the upper limit for long life.

An obvious mistype.. it should read.. 2.4v is the prefered voltage..

But engineers sometimes dont have to ability to see the obvious mistake and mentally make the correction..

[mattrix]

Thanks for that information Sunking.

A lot to digest. I guess the question is, "what conditions do lead to plating?"

billvon seems to suggest holding them at 3.6v is the problem, so how about holding them at a lower voltage, say 3.4v?
or is the problem that they are charging too SLOWLY. As you say, the current wont be 0, as the battery will never reach the charger voltage, but the current will become tiny.

[jon_r]

I believe that you can hold them at a constant voltage of some level (depends on the battery) without damage and that current will effectively stop. Most maintenance chargers don't do this because it isn't "full charge". So they wait for voltage to drop to some level and then charge it back up (using a higher voltage and a more complicated algorithm).


I would check out this kickstarter project which will soon be open source:


Mod Note - no direct links to electrodacus's Li_xxxx projects allowed. We
are not pleased with the obfuscation and poor information he uses to promote his scheme.
We caution anyone constructing projects with the charge and discharge levels he is suggesting, to be aware of the damage it could cause.