How to get the most accurate reading on LiFePo4s?

[UFO]

Originally Posted by CaptainRivet

An active balancer is connected paralell to the voltage sense cables, sonif you have a 4S BMS you also get a 4S balancer for Lifepo4, if 8S an 8S balancer. Number of cells parallel don't matter but the higher they are the higher the blancing current of the balancer should be.
But balancing should only be happening when above 3.4V and not.permanent. so if your BMS can steer an active balancer you can take the cheap heltec capacitor based ones. If your BMS can't you need the neey active balancer, its the only that properly switches on above 3.4V or an adjustable threshold you set and off if all is balanced.

Thanks


My next question is.....


Do I really need an active balancer?


My cells are never more than 15Mv's out from one another (the system has been used live-aboard for a few years now) and the BMS has an individual cell High/Low Voltage cut off as well as an overall voltage cut off

[markcouz]

Originally Posted by sailingharry

Do you have real life experience to back that up (in other words, have you used passive balancing and found that the cells get too far out of balance)? Some have indicated that if you routinely hit full charge and get perhaps an hour of day of balancing, passive balancing is sufficient. Then I hear others, such as yourself, indicating that much more balancing than that is needed even with good cells, and recommend active balancing.
It is sometimes very difficult to pick out the difference between theoretical benefits and real life benefits. And as an engineer, I am sometimes too eager to fall into the trap of theoretical perfection.

This seems like 1 question but requires 2 answers...

To the 1st part - Yes, I do have experience of that situation.

My yacht sat in a marina berth in Cape Town for several months while refitting, during that time I floated my 400ah house bank (originally well top balanced) at 13.2v (3.3v per cell) on shore power.

As part of final prep I shut off shore power and switched to solar, I found it impossible to set 13.8v bulk/float without running into balance issues - as the bank approached 13.5v the balance went nuts with the high cell climbing into dangerous territory and there was no way my active balancer could keep up (rated at 5A but in reality only balancing at 150-300mA).

In the end I built an external device that could discharge the high cell at 5amps while at the same time charging the low cell at 5A. I monitored all 4 cell voltages and flipped switches for hours until I eventually managed to get all cells with 10mV at 13.8v

To the 2nd part...
Since doing the above my batteries reach float for a few hours more often than not, allowing the pathetic balancer to maintain balance.

So in my opinion if your cells are very well balanced and you are getting your bank to 13.8v or higher for a few hours very regularly you will probably be OK.

[donradcliffe]

After waiting for a month for my BMS to balance, I still had one cell which was 150-200 mv high. I sprang for a $10 1 amp active balancer on fleabay, and it took 24 hours to bring the cells within 10 mv. I disconnected the active balancer and three years later, things are still i balancce. I do not have high loads on my battery.

[CaptainRivet]

Originally Posted by UFO

Thanks


My next question is.....


Do I really need an active balancer?


My cells are never more than 15Mv's out from one another (the system has been used live-aboard for a few years now) and the BMS has an individual cell High/Low Voltage cut off as well as an overall voltage cut off

In this case a clear NO.
Mine the same as long as i stay below 150A but if i pull 500A i see 50-70mV somerimes 100mV deviation but all over the cells not one specific cell pack. This is normal as cells are never 100% identical in internal resistance and connection.

[billdomb]

Take a look at

https://www.youtube.com/watch?v=qTwJDrdXAeQ

for instance.

Clark has a battery control/monitoring system that seems extremely sensible and low in cost. He has a number of videos on the subject.

[Wandering]

Originally Posted by UFO

I have not looked at active balancers... Do you attach to the Parallel sets or to all the individual cells - So for me that would either be 4P Sets or 16 Individual cells

Just buy a BMS with active balancing built in. Something like a JK 200A with 2A active balance