Why programmable controller for prismatic cell battery?

[Sverige]

From 9:28 - 10:00 in this video, Will explains that for systems using a custom LiFePO4 prismatic cell battery, a programmable MPPT controller is required, because you will need to set a custom upper voltage limit instead of using 14.4V which the “out of the box” lithium compatible controllers will offer.

I don’t understand why this is the case, can anyone explain please? Surely the BMS will adequately protect a battery built with 4S prismatic cells and so why would you need to program your charge controller for any top voltage other than 14.4V / 3.6V per cell?

 

[gnubie]

Tip: When linking to a youtube clip and querying part of it, mention the time that you are referring to, or post a link that includes the timecode. Right click the clip where the area of interest is and select 'copy video url at current time' and it will include the timecode

Having listened to the clip now Will does not actually say that you must set a custom voltage. He merely advises that if you do need to set a custom voltage charger brand X is the better choice. You might also consider that someone might build a 5s or 6s battery for what ever reason and they would need to be able to set custom voltages.

 

[Sverige]

Thanks. I’m not using a PC, so right clicking is not an option, however I did state at the beginning of my question the time section which I was referring to (9:28-10:00). Maybe I can learn how to embed timecodes into links I generate from the YouTube mobile app, but it doesn’t seem to be designed to make this easy.

I‘ve just watched the video again and he does indeed state what I enquired about in my OP - that if you’re building a custom LiFe battery from prismatic cells, you need to use a programmable controller. I understand your point that if someone were to make a 5S or 6S battery they would need to set custom voltages, however Will does not qualify his point as such, so although you may be right, I’m not clear it’s what he meant.

 

[gnubie]

"...but if you want to set the upper voltage limit..."

So yes, if you carefully manipulate what is actually being said by truncating the clip, he says exactly what you claim. If on the other hand you listen for a few more seconds it become entirely clear that he is saying something quite different. It's quite clear what he meant.

 

[Dzl]

First watch I agreed with Gnubie's takeaway, second I agreed with yours, third I think it falls somewhere in between.
I see how you can arrive at that conclusion (Will sorta muddles his point in this part, and starts out by saying with a DIY LFP battery you need to use a programmable controller without qualifying it, and then gets a little off track before qualifying that is only if the default LFP profile won't work for you--or at least implying it). How he phrases it does make it seem a little like its non-negotiable, but I suspect that is not what he meant.

It may be because early on, he often bypassed the BMS on the charge side making the ability to reduce the upper voltage limit on the charger pretty much non-negotiable. Maybe that is what he is thinking or maybe he just wouldn't consider a non programmable charge controller for LFP, I really can't say for certain.

Whatever the case, LFP doesn't need a customizable profile so long as it has an acceptable profile for LFP, but personally I wouldn't consider a controller that couldn't be customized in most situations. But if the standard LFP profile works for you and you don't foresee the need to tweak anything ever, than I don't see a reason you couldnt use a non-programmable controller.

 

[Sverige]

Thanks for clarifying DZL, very helpful. ?

I’m a beginner with solar and it’s not clear to me what he meant, or why he said it, which is why I asked.

 

[MisterSandals]

Surely the BMS will adequately protect a battery built with 4S prismatic cells and so why would you need to program your charge controller for any top voltage other than 14.4V / 3.6V per cell?

In regards to this, the BMS is not for routine charge controlling but rather a last ditch safety disconnect.

And as others have stated/eluded, being able to charge the way YOU want to charge is a little more than a nicety. For me, its extremely important.

 

[Jeremiah]

Something to keep in mind, if your BMS has the low voltage disconnect you like, and the inverter wants to go lower, there's a good chance this is going to result in annoying beeping/alerts from said inverter every single time it senses that the battery is "gone."
You may also decide that you want your LiFePO4 cells to last a LOOOONG time and go for a charge profile that does not dip bellow the bottom 15% or so of your battery and no higher than the top 15% (as an example). You can't really make the BMS be in charge of this; it gets extremely annoying when anything other than the inverter / charger is handling the cutoffs (except as @MisterSandals put it, the last ditch save from the BMS).