Charging a Lifepo4 with a power supply and control current with a solid state relay controller by pwm. Good idee?

[Rory1984]

I'm busy designing my own home battery and have a question about charging. I'm planning to make a 24S 105AH Lifepo4 battery with DALY BMS. for charging I want to use an ordinary power supply of around 2000W, set it on 24S (87,6V) and control the current with a solid state relay of 40A using an Arduino with PWM. Charging somewhere between 1A and 22A. Is this possible/a good idee?

 

[Tony Stewart]

You must know how to operate CC then CV and cutoff and know how to prevent miswiring and verify all connections. Just setting it to 87,6V is 3.65 /cell. Increasing the current increases the risk of unbalanced cells so that when the 1st cell reaches Vmax before the 24S string reaches CV then the BMS will be dissipating V*I = watts per BMS cell that reaches Vmax. So rapid charging needs perfectly balanced cells.

 

[Rory1984]

Thanks for your reply, I don't think 22A max charging rate is really rapid charging in a 105Ah battery. Furthermore I will set the parameters in the BMS to keep the battery between 10% and 90% charge. Besides that I also want my Arduino to know the battery voltage for extra safety and further optimalisation purposes. I just spoke with one of my colleagues, the ship electrician. He told me a solid state relay is maybe to slow for PWM and use MOSFET instead. Good idee. Further more I have no experience with (self build) PWM charging in the proposed setup. Im mostly seeking some advice on that topic.

 

[rjenkinsgb]

The charger needs to use high enough frequency PWM and have enough smoothing to give reasonably "clean" DC out; you cannot just switch the power on and off. It needs very fine and very accurate control.

That should really be a fundamental part of the power unit, rather than an add-on, so a fault just stops the whole thing working, rather than a possibility of the add-on failing while the PSU is still giving full output.

I've been designing electronics as a profession for over 40 years and that add-on system is something I would not get involved with!


Lithium battery systems are inherently extremely dangerous and need multiple levels of safety designed in to the charging and control systems.

Your bank of Lifepo4 cells has energy storage capacity equivalent to around 1/4 its weight of TNT.

It must be treated in all respects as if you were working with primed explosive, as it can do as much damage if something goes wrong, just not quite as fast...


Purpose made charger units designed for the job are not expensive, eg. first thing I found via google:

24S 25A 1500W LiPo/LiFe/LiTo Battery Charger

24S 25A 1500W LiPo/LiFe/LiTo Battery Charger

www.chargerystore.com

A bigger one:

4S 8S 10S 12S 13S 14S 15S 16S 20S 24S Li ion Battery or Lifep4 Battery charger with 100A 80A 60A 50A Constant charge current – LLT POWER ELECTRONIC

To add to what Tony Stewart said - it's not the current in proportion to the battery capacity, it's the power in proportion to the BMS capacity!

The total input power with 22A @ 87.6V is near enough 2KW.

If eg. half the cells have reached full voltage and the rest are still in bulk charge, the BMS has to dissipate a significant part of the overall input, bypassing the cells that are at the correct voltage to allow the others to keep charging. The closer the cell capacity matching and the better the cell balance, the less wasted energy and the less work the BMS has to do.


For a serious large battery project I'd suggest adding the Daly active equaliser module to the BMS as well, to keep all cells in balance at all times.

And use high speed fuses rated at eg. double the maximum charge or discharge current, at the battery positive and at a couple of points in the cell series chain, to reduce the chance of a catastrophic failure if anything goes wrong in the battery bank.

Multiple levels of protection with redundancy are essential. You are risking your home and family otherwise.

 

[Rory1984]

Thanks for your reply. My solution sounded a bit too simple for me. That's why I'm seeing advice from experts. I will leave charging current adjustment for a dedicated charger instead of cutting the current myself. For insurance and safety reason, I will put this system in my garage, away from my house.