Electrical – Charging LiFePO4 at low voltage – how do you know when to stop

LiFePO4 vs LiIon vs LiPo

Some people have commented on LiIon batteries, but the question and this answer are about Lithium Ferro Phosphate batteries / LiFePO4 which I'll abbreviate in places as LFP4. .
These are related to LiIon and LiPo batteries but have major differences.
Notable, compared to LiIon and LiPo (which are chemically similar) LFP4 has an added internal inactive matrix which the Lithium resides within. This reduces mass and volume energy densities but also makes them free from the destructive melt down modes, gives them a much longer cycle life and a much longer calendar life, lower capacity, lower maximum voltage, better temperature operating and storage range, better overall energy efficiency - and a whole of life cost below to far below that of any lead acid battery.

LFP4 (LiFePO4) 4 cell batteries do make excellent replacements for normal 12V lead acid car batteries BUT proper attention to care and feeding is necessary to achieve the very long cycle and calendar lifetimes that they are capable of. They are far more "whole of life cost effective" than any lead acid cells in (probably) any application when properly used. Wrongly used they will die an early and expensive death.

A few manufacturers of cells or batteries suited to this use are

Thundersky, Winston & SinoPoly

There are others which I can list but a search on those names will get you into the right area. All those brands started out as Thundersky but there have been partings of the way and expensive litigation is happening.

Winston make 12V 4 cell batteries with no access to the inter-battery connections. Their 40 Ah and 60 Ah sizes are claimed to be well suited to automotive use. Sinopoly an Winston both make single-cell batteries in sizes from around 10 Ah up to vastly-huge_Ah and you can strap these together as required. I am currently experimenting with 2 x SinoPoly 12v, 40Ah batteries each comprising 4 x 40 Ah single cells with strapping to produce a physical pack. A 12V, 40Ah 4 cell battery is about the size of a Ford-Prefect battery and less weight but is equal to about the largest LA battery you will see in a say 4 cylinder car.

HOWEVER

Claims from various battery makers overlap but are not identical, charging specs are suspect, claimed lifetimes vary depending on sales path and even reputable sellers disagree. The Winston battery charging specs use suspiciously high end point voltages - higher than I'd expect for LFP4 but below LiIon- almost like LiIon being run super conservatively to get good cycle life. Sellers warrant most brands of 10 Ah + cells for 5 years or 10 years use subject to various conditions and probably subject to use of charge/discharge management electronics. Criteria for adequate monitoring varies - many claim that Vmax and Vmin are enough as long as C rates of charge and discharge are within spec BUT a local supplier demands a gas-gauge type monitor and a low voltage cutoff. (I'd consider the gas-gauge excessive and the cutoff or similar protection essential)

Charging voltage. Since most alternators produce around 14V volts when charging it should be fine with LiFePO4 battery since maximum voltage for those is 3.6V which gives us little headroom up to 14.4V (4*3.6V=14.4V).

Some claim Winston LFP4 needs higher V than car systems provide. Others use them regardless.

SinoPoly OK on car voltages BUT ...

Individual cell balancing can be done with simple dissipative BMS which dumps excessive charge to resistor.

Winston cannot balance as sealed but claim superbly balanced cells are OK. Numerous dealers sell them and say this but at least one large site says not to use for deep discharge due to this inability. I'd be very wary. I made a Winston/SinoPoly initial choice for experience gaining based on this and strange Winston voltages and bought SinoPoly - despite apparently superior Winston specs.

But we don't have possibility to disconnect alternator when battery is charged,

Don't start. You must be able to manage your energy source - AND it is easy enough to do - but if your system insists on violating battery specs (which may not be the case) do not use LFP4.

so here is first Question: is it OK to hold LiFePO4 battery at floating charge with voltage which is close to maximum battery voltage? Or this will significantly decrease battery life?

Opinions vary
Small LFP4 - say < 1 Ah to few Ah MUST NOT be floated or they DO die early.
Some LFP4 large battery makers claim floating is OK.
Thou shalt not float LiIon or LiPo or else ... and while LFP4 has differences I'm uneasy.

Charging current. Some LiFePO4 cells can be configured so they can easily take all charge current produced by alternator. (assuming 70A output and 3 parallel 40152S cells) So second question: Can it overload alternator or modern alternators are smart enough to lower voltage to avoid being overloaded?

This deep-ends entirely on battery and alternator specs. Batteries have clear specs. Exceed them not. If your battery has NOT got good specs available do not buy it.

No over discharge protection. Currently have no idea how to deal with it besides not letting it to discharge completely.

This is essential. There are various ways of doing this but a LFP4 large Ah cell MUST NOT be taken under around 2.75V. If you cannot be SURE of that, do not use them. They cost far too much and are too light per volume to make good boat anchors.

Although we have already spent some time chatting about some details of your implementation, I'll try to take you through the steps I take in designing a long-life LiFePO/Solar project and leave you to fill in the specifics.

First thing to do, with regards to all your power conversions and intermediary steps, is find the losses. If you have a microcontroller that you put to sleep a lot and uses only 1mA on average, you are not going to care about one, two or three conversions in between.

But if you have a module that uses 50mA with 400mA peaks, you may want to pay very close attention to that module: Will it run off the battery as well, or can you cheat it out of the equations by powering it directly from the Solar, for example if it reports the amount of energy generated wirelessly once an hour. In that case you may even want to control its converter with the microcontroller, to save energy for charging and other stuff the 59 minutes each hour you don't need the converter's 3 ~ 10mA quiescent current, if that's a factor.

The next thing you could consider is: Does my MCU and application need a very smooth 3.3V? LiFePO4 is a very good choice for your application for various reasons. One of them is its minimum voltage of 2V (2.5V advised), which you can even safeguard with a 2.7V brown-out setting. Most 3.3V MCU's can also handle 3.6V, which happens to be the LiFePO4 peak voltage. So you may not necessarily need anything between the battery and the application, which saves a lot of waste as well.

For the reference, LiFePO4 in this case is a very good choice for many reasons:

• Their voltage curve is very flat compared to Li-Ion or LiPo. About 80% of its power is delivered between 3.4V and 3.2V, so they offer very easy to dimension conversion settings. (The buck or boost margin to account for remains small over most of the battery energy content).
• Their internal chemistry is very robust, allowing a much wider temperature range of current drain. Be aware, though, they can still not be charged below freezing though, so you need to account for that.
• They don't easily outgas, so they don't inflate as weirdly as LiPo's.
• Damage to a cell is still extremely unlikely to cause explosions or in many cases even fire.
• Their self discharge over wide temperature range is usually marginally lower even than other Lithium chemistries.

As a point of interest: The protected Q&A posted by Russel that you link to for info about LiIon and LiFePO4 is not very useful, there's many assumptions made there that are not even correct for LiIon, let alone LiFePO4. To start with the assumption of linearity of the chemical charge process. Best to forget about that post.

When it comes to charging and discharging LiFePO4 the currents are quite limited compared to modern LiPoly cells, but they are much more permissive toward over-tension, since the Iron Phosphate structure is more resistant to pure lithium plating. But I'd still advise you to use a dedicated protection chip or ready-bought circuit (for sub-1A applications I buy them in bulk for nearly no money at all). They drain micro-ampere's, take a load of testing and risk off your hands, and the best thing is, they feature analogue circuitry that reacts quickly and efficiently to over-current situations caused by damaged wiring.

This will allow you to focus on power-management of all your modules in your MCU without the risk of overloading the interrupt window in your code, and then skipping a beat in detecting over-current, over-coltage, etc.

When charging a LiFePO4 at about 0.75C, you can usually keep the fixed current even up to 3.9V without damage (given the cell is between 5 and 50 degrees Celcius), so if you charge with a fixed current, you can just let the protection switch it off (they are often set to 3.7V and might allow a 10ms peak of 3.8V). So if you have a system (MCU or dedicated) that makes 0.75C current with a 4V or 4.5V limit, or depending on the protection, even just 5V, the protection chip or circuit will take care of it all.

If you assume you have Device 1 that needs 200mA, but not always, at 2.7V to 5V (this is a broad assumption, but many devices like wireless modules have allowances like that). And that you have a uC that uses about 1mA active, and you let it sleep at 25μA as much as possible that also handles 2.7V to 5V, you could do something like this:

^{simulate this circuit – Schematic created using CircuitLab}

D2, D1 R1 and R2 are meant to let the uC know when the solar cell is operational enough for the buck converter to operate. You can then use this information to control charging of the battery, along with the temperature of the cell and you can use that to turn on the high power module when there is enough power.

M2 allows you to actively turn on charging of the battery. M1 allows you to control the extra device.

I added D3 to indicate the presence of the MOST body diode. It's still better to turn on the MOST when you start using higher currents from the battery, to have less waste in the MOST's body diode, or an extra diode you place.

When charging is complete, the Cell protection will release and the power rail will float away from 3.8V up to 4.25V, you could even use that to detect that happening. (Compare VCC versus internal reference, for example). You can then monitor how often you reach the maximum charge state. You can also then disable charging for a while, to prevent continually peaking it off at its limit voltage. It's better to have them relax/drain a bit before you re-charge.