Re: [electricboats] Another LifePo4 build question

Thanks Curtis---great explanation.

When you say "4s pack" --- normally I'd think this means 4 cells in series, which might be just 17v fully charged.  Is that about what you have or are you talking about 4 modules in series of, say, nom. 13v lithium batteries that have internal BMS themselves also?

 

Bottom, Top and Continuous balance techniques certainly have their distinct benefits and tradeoffs.  Ideally some sort of continuous balance would be optimal---balancing during operation throughout the operational voltage.  Then, the cells could conceivably better track throughout the range.  I think the main thing that this is good for is to address "self discharge or leakage" that causes one or a few cells to droop while the pack is just sitting there at intermediate voltages.  I see this on my boat---over months of time about 6 pairs of cells drift.  I don't know for sure what the cause is but self-leakage is possible.  With the BMS running always or periodically while at the dock, the rest of the pack can be made to track the drifters.

 

Bottom balancing certainly is good if you're hyper-miling your pack usage and trying to get that last bit of energy out while not reversing a cell.

In my case, as a rule I never take my pack below 33v (and usually 36v) with 30v as the spec'd lowest voltage for a 2.5v/cell minimum voltage.  At 33v, the average cell voltage would be 2.75v/cell.  Even if I had 4 cells in the string that at that point drooped to 2.5v/cell, the remaining cells would on average be at 2.875v/cell.  This represents 375mv/cell of imbalance for those 4 cells.

I have had a single cell pair get almost about this far out of balance and since have been careful to stop discharge at around 36v.

Better still, my cells are now all balanced to within about 50mv. J

 

Top balancing is very useful if you're more concerned about cells exceeding the max voltage rating during charging.  This is the default balancing mode for the RLECs I have from Enerdel.  However, they don't start bypassing until something like 4.1vpc and I have only been charging my pack to 47.9v or 3.99vpc, so if I powered the RLECs during charging, for the most part they would likely not do anything unless I boosted my charge voltage another volt or so.  I might go that direction…after all, an extra volt would be nice J.

 

So for the most part, I am neither Top-, nor Bottom-balancing mine.  I use a CI/CV power supply for charging, never discharge below an average of 2.75vpc and take a glimpse at the cells every few months to confirm no cell pairs more than 100mv from the rest.

 

Thanks.

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of cpcanoesailor via groups.io
Sent: Wednesday, June 1, 2022 7:39 AM
To: electricboats@groups.io
Subject: Re: [electricboats] Another LifePo4 build question

 

Myles - I bottom balance, but I have a little 4s pack. Some DIY EVers were very keen on it, and it made sense to me for a simple traction pack.

My BMS is not balancing, and it uses very little power. I have left it connected since assembling the battery 3 years ago, even when storing for 4 months with no charging. In fact, I have only used an AC charger once, after bottom balancing the cells. Everything else has been solar charging on the boat.

Certainly, in a larger, more complex battery, there will be more variation between cells, and balancing and BMS activity will be much more complex.

Interesting discussion.
Curtis Patzer