# BMS for LiFePO4- Its purpose, function and the difference btwn the 'good' and 'great'



## dexion (Aug 22, 2009)

You have an advantage with a static application we dont have with cars. 

Weight doesnt matter and size doesnt matter as much.

So we need more information. 

Generally with solar you use a charge controller that has some adjustments as to top end of the charge and has safetys built in to not overcharge the cels.

Since size/weight/heat doesnt matter as much. You could simplify things by building/adding larger resistors to solve various problems on the bms.

So you need to tell us the maximum charging rate of the panels best case.

Also the type of lifepo4 you are using (some max at 3.6 others at 4.1.)

Using a kiss method I could just see something like:

say your max charging rate is 24 amps at 15V

You have 6 banks of 4 SE cells in parrallel.

Each 4 cell battery can get a maximum of 4 amps of charge but at 15V.

So 15/4 = 3.75V a bit too high for SE cells so you would need a bms that could either:

Shunt 4.1 amps of charge current at say 3.55V a cell (making much heat so it would have to have heat sinks and fans but space and weight arent a problem) or turn off the charge controller (which may not be ideal since other cells could be low in the 4 cell battery.)

The times you would hit maximum wouldnt be all the time so the bms will shunt only part of the time each day with sunlight. You could have a relay disable the charge controller if any cell gets over 3.7 as a safety (and blare a horn/flash a light etc to let you know.) 

Your other question any thing lower than the 4 cells voltage would not charge the cells (ie 11.5V) and the battery is at 12.4V so you need to get the charge voltage above the battery voltage.

Now 17V is interesting because that is 3.4V a cell with 5 cells in a battery so you could make 5 cell batterys with lifepo4 and store a lot of the charge (it seems 3.5 is a sweat spot for stored energy but 3.4 could be as high as 80% of the cells capacity.) 

LVC could also be done with the bms as well using a relay if any cell gets below 2.5V (or whatever you pick) it could open/close a relay and cut off the output side to whatever (an inverter?) you are using. However most good inverters cut off at 10.5V this isnt enough to protect an individual cell from going below 2.5 if its not ballanced.

I could see a modified mini-bms (look to the right of the forum screen for D's add) doing the job with larger resistors it has an led thats green for good and off for bad (visual clue the cell is happy.) 

Nothing seems like much of a problem really you just need to plan.


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## dimitri (May 16, 2008)

> But do BMS also need to regulate variable charging voltages (e.g. solar panel)? For example, if I had x4 3.2V wired in series for a 12.8V LiFePO4 battery pack, and my solar changes anywhere from between 10 - 17V (Even though it's rated at 12V), will that be a problem (will it damage my cells)?


Theoretically, yes, but in reality LFP cells can suck in all the current you can throw at them, until they are full. So, in real life, your solar panels will always be at the same voltage as the battery and charge regulator is not neccessary if you have a BMS which can cut off the charge source when full. Essentially BMS becomes the charge regulator itself.

Another benefit of LFP, like Dexion pointed out, is that you can construct the bank in 3.2V steps, so you can make 5 cell bank instead of 4. Just make sure your inverter can handle higher input voltage without burning up. In fact higher voltage ( within rated range ) is better for inverter, since it means reduced current, i.e. less heat generated.

On the flipside, if you make 5 cell bank, then you will not be charging it unless solar panel produces more voltage than battery voltage, so you may not be capturing as much solar energy as you could with lower voltage bank.

The more I think about it, I believe 4 cell bank will be better to capture most energy from the panel.

I would advice against increased shunting capacity. It just wastes energy and creates heat which is potential fire risk if shunting is not calculated properly. In reality, well balanced LFP pack will not need much shunting at all, as long as BMS can control the charging source, by disconnecting it upon HVC event.

I have customers using MiniBMS for solar and housebank applications just fine, its a very simple setup and it works well.


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