# charging 3v LiFePO4 and miniBMS



## PStechPaul (May 1, 2012)

It seems like your pack is not well balanced, and there should not be any cells reading as high as 3.9V. The BMS will try to shunt some of the charge current to keep the voltage under about 3.6V but if the charger is still in fast charge mode at 1C (180A) the BMS will probably be grossly overloaded trying to shunt 100A at 3.6V or 360W per cell.

The charger should enter a low current (C/20) charge mode when the highest cell reaches 3.6V, and then the BMS shunts would only need to work at about 10A and 3.6V or 36W, which is still a lot. Eventually the low cells should reach 3.6V and the pack will be top balanced, at which point the charger should shut off, or after a certain time of low current charging, perhaps an hour.

You might have a charger made for 24 Li-Ion cells which are normally 3.7V and charge at up to 4.2 Vpc, for 100.8V maximum. If the charger is programmable, you might be able to set it for 20 cells and it should turn off at about 84V or 3.5 Vpc for 24 LiFePO4.


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## prefetch (Oct 17, 2015)

thanks pstechpaul. i think you might be right about the charger being mismatched.

things i learned today:

* miniBMS units will fry if the cell is > ~3.65 and be permanently busted

* you can pry off the little chips on the miniBMS units and use them directly as a shunt to drain voltage on a cell that is too high

* shunt is the same thing as a 'vampire' chip which drains off excess charge

* my miniBMS is set to shunt until the charge is about 3.45v

i guess i have to hook up my "not very smart" smart charger to the miniBMS system and put it in charge of turning it off for me.

and, yeah, i gotta order a bunch more miniBMS units because i torched several..


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## Moltenmetal (Mar 20, 2014)

Use any charger you like, and have the miniBMS trip the charger when the voltage in any one cell exceeds 3.7 volts.

You have the miniBMS cell top boards- presume you also have the master/head board. It has several outputs that will allow you to use a relay to shut off the charger.

If some cells are low, you can charge them individually to get them closer to the others. If dome are high, you can use a resistor such as a headlight bulb yo shunt charge around them while the others catch up. The shunt resistors on the miniBMS boards are meant only to balance a bit- the cells have to be close for the BMS balancing to be useful. And honestly, top balancing only gets you an Ah or two of useful charge as long as the cells are reasonably close in capacity yo start with.

The miniBMS website has a few instruction sheets you should download and read.

Never let any LiFePO4 cell go above 3.8 V- you're doing permanent damage to them at that point. Time at high voltage and high temperature are both bad for the electrolyte, and together are doubly bad.

Test everything carefully- open the BMS loop and make sure the charger trips, and don't charge unattended until you know your high voltage interlock works reliably.


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## prefetch (Oct 17, 2015)

thanks moltenmetal. the vendor that sold me my charger, the battery cells and the miniBMS system assured me that the charger would cut off by itself and that it wasn't necessary for my to hook it up to the miniBMS system.

turned out he was wrong. it really sucks when someone sells you a bunch of stuff and claims to be an expert but then really doesn't know their products well enough to give you accurate guidance.

hopefully i didn't permanently damage some of my cells. lesson learned.


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## Moltenmetal (Mar 20, 2014)

You can have a charger trip itself at a particular voltage, that is only safe if the voltage you end the charge at will not take any one cell over its high voltage limit. That requires some initial balancing- you can't just plug in the charger and expect it to safely charge the pack.

Think of the following analogy: we're trickling the exact same amount of water into 24 wine bottles in parallel. Some of them have a little wine in them, and some of them are only 730, 735 or 740 instead of 750 mL. When any bottle spills over, you've ruined the carpet. What is the safer method to use: filling all the bottles with 730 mL each, or stopping the tap when any one bottle fills to the neck, by monitoring the level in each bottle? What if one or two of the 730 mL ones has a little wine in them already? If you don't balance them, either at the top or the bottom (ie by emptying them all completely before you start to fill), you're going to be in trouble relying on filling them all to the same level (tripping the charger at a fixed voltage), even if that voltage per cell is pretty conservative. Set it too low of course and you'll stop well before the pack is truly full. The level detector method (the BMS) makes it safe even if you are a little out of balance- in fact, even if you're a fair bit out if balance. 

If any of the cells that hit 3.9 V is bloated, they may be cooked. But if you implement the miniBMS interlock, you won't cook any more cells- as long as you don't over-discharge either. The miniBMS can be used to protect against that too.


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## dimitri (May 16, 2008)

prefetch said:


> thanks moltenmetal. the vendor that sold me my charger, the battery cells and the miniBMS system assured me that the charger would cut off by itself and that it wasn't necessary for my to hook it up to the miniBMS system.
> 
> turned out he was wrong. it really sucks when someone sells you a bunch of stuff and claims to be an expert but then really doesn't know their products well enough to give you accurate guidance.
> 
> hopefully i didn't permanently damage some of my cells. lesson learned.


Please tell me the name of the vendor who made such claim and I will make sure it never happens again. Use PM if you don't feel like putting names on public forum.

If MiniBMS ( or any other BMS for that matter ) does not have ability to control/stop the charger, then you wasted your money and put your cells at risk.

If you buy a bunch of circuit boards and don't inquire a detailed documentation describing operation and containing wiring diagrams, then you have learned a good lesson, sorry for being blunt.

MiniBMS user guide, available for download at our Web store, clearly explains how critical it is for BMS to be in control of the charger, regardless of how intelligent the charger is. Charger's intelligence is utterly useless when it only sees a pack as a whole and does not see individual cells.

You should also read the PDF paper posted at our Web store ( Product Support page ) regarding initial pack balancing, which is the process you are going thru right now.


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## prefetch (Oct 17, 2015)

dimitri said:


> If you buy a bunch of circuit boards and don't inquire a detailed documentation describing operation and containing wiring diagrams, then you have learned a good lesson, sorry for being blunt.


i read through the documentation of the miniBMS, and of course i saw the wiring diagrams, but like i said, the vendor specifically told me that i did not have to connect the charger shutoff because it was "smart" so i went ahead and took his word for it.

one thing that i wish the documentation said was that you can fry a board if a cell is a little overcharged. eg. at 3.5v the board is fine, but if you attach the board to a cell at 3.7v it will fry it. i did not know that the boards were so extremely sensitive.

i'm glad there are lots of miniBMS documents - they are confusing for a beginner like me, but at least there is documentation which is nice.


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## dimitri (May 16, 2008)

prefetch said:


> one thing that i wish the documentation said was that you can fry a board if a cell is a little overcharged. eg. at 3.5v the board is fine, but if you attach the board to a cell at 3.7v it will fry it. i did not know that the boards were so extremely sensitive.


this is not true, boards are safe up to 6V

most common way of damaging one is described in the manual, so it can be easily avoided


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## prefetch (Oct 17, 2015)

dimitri said:


> this is not true, boards are safe up to 6V
> 
> most common way of damaging one is described in the manual, so it can be easily avoided


hm. strange. this has not been my experience.

i had a cell at 3.8v and it instantly fried the board when i attached it. i did it again, and it fried the next board.

i then shunted it "by hand" to 3.5 and attached a board and it worked fine.

so...what do you think was going on there?


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## dimitri (May 16, 2008)

when attaching a board to a cell, what was connected to most positive and most negative battery cables? were there any devices at all in the battery circuit? or was at least one end of the battery physically disconnected from everything?

its more likely that the difference between 2 events was due to sequence of making connections between cell terminal, BMS ring terminal and buss bar or cable, rather than difference in cell voltages


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