Confused about lifepo4 balancing?

[Offroader]

I've only used Lipo and lithium ion which are easily balanced and SOC determined by voltage.

I was looking to build a large lifepo4 pack for my hybrid car and when testing a couple of lifepo4 cells I am learning that they do not behave like your typical lipo cell. I'm am concerned about using this chemistry for a pack.

The problem is that the voltage stays almost exactly the same through the discharge.

Here are some of my questions.

1) Will lifepo4 cells in parallel balance between each other to the same exact SOC?

2) Can a BMS balance the pack (the different parallel groups) to the same SOC? or do they only prevent a cell from going under 2.5 volts and over 3.65 volts?

3) I plan on building a large 480 cell pack, will I have nothing but problems with balancing and charging the pack because I do not know the SOC?

4) Will I always have to charge the pack to full or close to full voltage because there is no other way to charge it to 90% because you can not determine SOC?

5) Should I avoid using a Lifepo4 pack because of these issues? Mainly, not being able to balance the cells and not being able to determine the SOC.

Thanks.

 

[dogman dan]

It is a dilemma, especially for a car which might run at so many varying discharge rates, that measuring with a watt meter can still end up not as accurate as you'd like. But I still see no reason you cannot use lifepo4, for it's longer potential lifespan for one.

Paralleled anything will equalize in voltage.

A bms can balance your pack, but it will work better to build so that the pack does not get driven out of balance by high discharge rates, or discharging 100% often. The BMS will take a long time to balance, and even balancing manually, it's best to charge 100% for any balancing.

You can guess close enough where 90% is by voltmeter. guessing where 50% is nearly impossible, without a watt meter. So definitely include a watt meter along with the very accurate volt meter. No problem to charge to 90% most of the time.

Once you know your capacity in watt hours, you can then make a very close guess where 50% is. Capacity will change in winter,, so test capacity to 100%,, or 100% of what you plan to use,, once winter comes.