Battery charging ? LiFePo4

[rgray107918]

I've done a bit of searching online but I still have some questions.

For LiFePo4 battery charging steps (and all batteries if applicable) during the CC and CV steps, I don't understand the difference between a purpose made charger and a bench power supply with CC and CV settings.  I think I am not understanding more than just the difference, but I'll continue.

I'll use my own experience because it's all I've done.  I have a 4S LiFePo4 12v 12Ah battery.  I set up a bench supply to limit the current at 4 amps and to limit the voltage at 14.4 volts.  While charging, the supply chugged along at 4 amps, and the voltage steadily rose to 14.4 volts.  At this point the amperage began to steadily fall, down to the mA range.

Am I correct in thinking that the beginning step, where amperage was 4 amps and voltage rose, is the CC step, and then when the voltage was steady and the amperage fell, that was the CV step?

What is different between a power supply set up like above, and a purpose made battery charger?

I am asking because I was all set to buy a buck/boost module with CC and CV settings and a power supply to make my own battery charger, but I chickened out, because I don't fully understand battery charging, and do not want to do something dangerous....well, not anything extremely dangerous.

[Algoma]

When first putting power Into a discharged battery, that empty battery is more than willing to absorb energy at a rate far faster than the battery can safely handle. So you want to have a power supply that can keep the maximum charge rate safe for the battery. This is a behaviour a dedicated charger will do, but you would need to configure yourself with a power supply's constant current setting.

As the charge in the battery approaches being full, the voltage in the battery becomes closer to the set voltage of the power supply, The rate at witch the energy is absorbed into the battery naturally slows down with the smaller difference. The current slowly tapers off to only a tiny trickle as the battery voltage becomes more equal to the supply.

A dedicated charger knows when to stop charging, it often has other programming about how best to handle a specific type of battery, within its various safe limits.

You can use a bench supply configured for a constant current anywhere within a safe current limits for the battery. and the voltage set to the nominal charge voltage.

Simply remember Lithium batteries don't handle being over charged, A dedicated charger has the smarts to stop charging automatically, where you'll need to stop a bench power supply yourself once the rate of charge is full, indicated by only a small current transfer.

A dedicated charger may also then monitor the battery for top up, and other issues. Some unhealthy batteries leak power internally.. if a lithium battery self discharges too much, it needs to be safely discharged and disposed of.

[Siwastaja]

Lab supply CC-CV is no different from "purpose-made" charger's CC-CV, except maybe the lab supply may be more accurate.

CC and CVs are strictly not "steps", just two separate limits (one for current, one for voltage), both of which are always satisfied by regulating either one of them to the exact value, keeping the other below the setpoint. Although normally with a non-broken battery it happens that CC always comes first and then comes CV, and there is not much oscillation back and worth. A purpose-made charger may prevent hopping from CV back to CC, considering them as "steps" as you say, while a lab supply definitely can happily alternate between the two limits - but a purpose-made charger may work that way as well. Normally this does not matter, but this distinction offers one more way to detect abnormal battery behavior. (And also offers one more way to fail implementing that check.)

Purpose-made chargers may have additional protection features, like preventing charging (or reducing current) at temperatures too low (generally below 0degC), checking the initial cell voltage before starting charging (so that if the cell is dead due to earlier overdischarging, the charger reports "broken battery" instead of forcing charge in), and maybe a timeout.

Finally the obvious, a dedicated charger automates the task of "stop charging when current has reached below C/20 (or something)" you need to do manually with the lab supply.

[rgray107918]

Thanks for the explanations.  I think I'll move forward with the idea of building a supply.  I have a few supplies, but most are either in use for amateur  radio and limited to 13.8v or variable but only supply 1 or 2 amps.  I considered the supply route, because I figured I could get a lot more use out of a bench supply versus a dedicated charger.  I realize there is more risk going the supply route.

[Algoma]

A Lithium battery actually lives FAR longer if kept within a margin of safety of its limits.

CC keeps the charge rate safe for the battery.
CV sets the maximum charge voltage level.

You could rapid charge with a higher voltage power supply, provided you control / cut off charge when the battery's limits are reached.. That's why many cheap 3.7v 18650 lithium chargers are nearly straight 5V USB supplies, they simply cut off the battery/supply when its reaches its 4.2V charge limit. And a USB supply is often unable to supply above the 1A limit of most cheap 18650 batteries.  Its when those charger's protection MOSFETs fail short, that things get spectacularly fiery with those cheap chargers.

[rgray107918]

I still haven't nailed down the specific charge time for my batteries (12v 12Ah) but I figure I'll just be limited to charging them while I can keep an eye on them.  I thought about putting a timer on the charging supply outlet, so it would completely cut off after a certain time, to eliminate the chance of over charging.  Then, I can check it later and top off the battery if it didn't quite take all it could.
Of course, my fear is that I'll get lazy and trust the timer, setting myself up for a bad time.

Thoughts on a timer on supply?  Good idea, overkill, too likely to fail?

[Algoma]

You can get / build very low cost battery protection boards, basically a voltage monitoring chip and a pair of mosfet gates, low cost, dead simple and effective protection from overcharge and over-discharge. China sells them for very low cost.

Provided you're not moving large currents into or out of the battery

[rgray107918]

Nah, the buck/boost I'm looking at only accepts 7 amps, and I won't put more than 4 into the battery.

Thanks for the board suggestion.  I didn't know about those.

[rgray107918]

One last question - these battery charge boards I'm looking at seem similar to a BMS, if not actual BMS boards.  If my battery has a BMS board already, does my charger need one as well?  Does having a charge control board on the charger lessen the likelihood of damage to the battery BMS pcb?

Also, if down the road my battery BMS starts acting weird, it seems I could replace it with one of these?  I doubt the board I have now is of any higher quality.

[Algoma]

If you have a BMS on the battery, then it likely has the overcharge protection. No need to add any more.

A dedicated charger should be smart enough to stop charging long before the battery's management system board shuts off it's charge gate as a last resort to prevent battery overcharge abuse. If you can see the BMS board, Looks for a pair of fairly large (usually square) MOSFETs for each cell in the battery. Those are the protection MOSFETs that get shut off as needed to protect the battery.

[rgray107918]

This is the board.  The battery was DOA.  Not actually doa, but it would not power any loads that required moderate in rush current.  It would discharge through resistor loads, and power an led, but not my radio.

I cracked it open and found the capacitor popped off.  Lucky I found it.  I soldered it back, and it seems to be working.

The charging dilemma I'm facing is that the battery description says it can be charged via lead acid charger, and after the fact I realized I'm losing the last bit of juice of a LiFePo4 battery only charging at 13.8v.  So, I'm trying to cobble together something.

[Siwastaja]

One last question - these battery charge boards I'm looking at seem similar to a BMS, if not actual BMS boards.  If my battery has a BMS board already, does my charger need one as well?  Does having a charge control board on the charger lessen the likelihood of damage to the battery BMS pcb?

Unable to answer.

BMS is completely meaningless vomit of letters. In real world, it means anything.

You need to know what the cells BMS boards actually implement, have the specifications, and understand the whole system. Then you have the answer to your question as well.

Often some cell level boards just implement safety cutouts beyond the normal operating range for last resort protection.

Also take note if the BMS boards have means to actually switch the current off, and how the switches are rated. Switching DC in general-purpose application (i.e., where large amounts of inductance and/or capacitance, and short circuits can be present) is non-trivial. The switches may not survive the series pack total voltage, or the inrush current of a capacitive bank. I would trust only a top brand product with guarantee (so probably price tag in thousands ea., custom ordered through a 6-month long process); a mass market product by the known consumer brands used for its intended purpose only; or one designed by myself.

[Algoma]

That Battery management system will protect the battery from overcharge, and should be trying to balance the charge across the cells. (provided there are no failed components.)

Likely your output issue is due to a bad cell in the pack.. You'll need to test and replace the bad cell that will have the odd voltage.

[uer166]

That Battery management system will protect the battery from overcharge, and should be trying to balance the charge across the cells. (provided there are no failed components.)

Likely your output issue is due to a bad cell in the pack.. You'll need to test and replace the bad cell that will have the odd voltage.

As was said over and over by Siwastaja (but keeps falling on deaf ears somehow), you can't possibly KNOW that the integrated BMS has any balancing. There are plenty of perfectly safe and long life BMS/pack systems with no balancing implemented.

Also, just because the charger charges the pack correctly to the right voltage, doesn't mean individual cells within the pack can't be overcharged.

[Algoma]

Not deaf, just optimistic that some things may work as described, at least keeping the intended use simple to understand.

Yes, its well understood that most of those units are cheap, and made to fail. Always assume everything is fake and useless, until tested and proven reliable.

[rgray107918]

I appreciate everyone's answers.  I am trying to learn about these LiFePo4 batteries, and useful comments with helpful information are appreciated.

I went the route of buying  a buck/boost and I am using an old AT PC power supply.  I plan to use the 12V line which supplies 6 amps.  I am hoping I can get 14.2-14.6 volts and up to 4 amps using this setup.  There is a 5V line that supplies 18 amps, but I'm not going to do anything with it for now, maybe down the road if the 12 volt line doesn't cut it.

I am using a total of 3 batteries, one battery is by itself for the most portable/lightweight option, and the other two I plan to run in parallel inside a slim style ammo box.  The parallel batteries will have a battery monitor unit which can also act as a charge/discharge controller.  The single battery I will just have to keep an eye on while I use the variable bench supply using the buck/boost.

This is a hobby for me, and I buy cheap for my hobbies.  All of my parts are either from my junk bin or from Amazon.

Thanks again for all the help.  For anyone down the road searching similar info, here are the parts I purchased and preliminary schematics (actual layout may change when I get to building these).  I can already tell I want my parallel system different.  I would like to be able to use the controller to charge both batteries in parallel and also view the health of the system but also be able to easily view the health of each battery, and also have the option to charge each battery independently.....so, I've got to work out how to make that possible. 

https://www.amazon.com/dp/B01MS3NEAE/ref=cm_sw_r_cp_apa_glc_i_KPW1K9HCF73NZBEW8DRR?_encoding=UTF8&psc=1

https://www.amazon.com/dp/B089SR5D7P/ref=cm_sw_r_cp_apa_glc_i_Y0651TK2RRYWV2C4WAYD?_encoding=UTF8&psc=1

[Siwastaja]

Implementing balancing is completely optional as it has only secondary safety implications and it's purpose is not related to safety. Balancing is one of the last items in the BMS design "musts", "shoulds" and "coulds".

It's completely normal not to implement balancing at all. The only risk this decision carries is earlier diminish of pack capacity in case of severely mismatched self-discharge of cells. In that case, some of the cells hit OV limit, some others UV limit early. No other harm than reducing usable capacity is done.

Those OV and UV limits are the core of BMS, though, and provide the safety. Balancing may or may not be added to try to maximize energy storage capability after years of operation.

Balancing has a secondary safety aspect in the odd corner case that the cell-level OV checks fails, but the independent charger (pack level) OV (or well adjusted CV mode) continues to be operational. In that case, having the pack already in balance improves safety.

On the other hand, implementing balancing requires responsibility of doing that right. Excessive balancing power combined to poor thermal design may cause overheating.

Seeing an unbalanced system going out of balance is surprisingly rare. Seeing a balanced system go out of balance due to a design failure in the balancer is commonplace.

[rgray107918]

I ended up building the parallel box.  The controller has a relay option, but I haven't worked that out yet.  For now, it relies on me watching the meter and only being cautious with the load my radio draws.

I really appreciate all the comments and help.  This was a fun project and outside my comfort zone as far as design, but I learned a lot.