Re: [electricboats] LifePo4 build questions

I'll do the big question last.

Three 12V-nominal chargers will charge three built-in-BMS modules just fine.

And yes, paralleling the modules during the off season is okay, and is another approach to charging, when you just have one charger.

And now, the big question.

A BMS serves the following functions:

 - prevent overdischarge (too much current)

 - prevent overdischarge (too low voltage)

 - cell-to-cell balancing

 - sometimes, temperature-related cutoffs (throttling charge/discharge based on cell temperatures)

 - prevent overcharge (voltage and current)

In order of the above list:

 - I have a 250A fuse on the output of the entire battery.

 - The motor controller reduces power consumption at 42V and stops completely at 40V.

 - I always start each outing with completely topped-up cells, so there's no cell-to-cell imbalances (each cell is full)

   And, I don't change again until I'm ready to completely top up again.  So there's no partial charging before another

   round of discharge.  So the cells will only get out of balance to the extent that they're not matched capacity-wise.

   Only if I had one obviously-low cell and enough above-average cells to draw from to bring that cell back up would

   any cell-to-cell balancing be worthwhile during a discharge-only use case.  These cells are matched capacity-wise

   just fine, as I get voltages matching to within 0.01V at the end of the day.

 - On temperature, I don't go out in the cold, and I've monitored the air temperature inside the battery case (simple

   indoor/outdoor thermometer), and so far it's stayed under 100F.

 - Charging:  I bought these cells four at a time, and the manufacturer matched them for me.  I've top-balanced all

   sixteen cells a time or two, but mostly I charge them with a current-limited power supply, with output adjusted to

   14.6v.  By removing the three bus-bars you see me install in the video, and instead arranging the four-cell modules

   in parallel (four jumper pairs), I can charge them as one 12.8V 800Ah battery.  The charger's max current is 50A, so

   even with just one module, that's only C/4, with all four modules in parallel, that's just C/16.  And you can parallel

   these power supplies easily to increase charge rate if you like (Mean Well RSP-750-15).

By now you're asking how I can fully charge a four-cell module without cell-to-cell balancing.  First, the cells making

up each module were matched capacity-wise.  But even then, some cells will get to 100% SOC before the others.  But

an LFP cell can tolerate being slightly overcharged: it just turns the excess energy into a little bit of heat.  By charging

until the current drops to a trickle (single-digit watts), I find the cells all reach 100% SOC, and with no apparent harm.

I suspect that even on not-so-well-matched cells this approach will work: by the time voltages get close to 3.65V per

cell, current levels are so low that the current reaching a full-before-the-others cell is easily and harmlessly absorbed.

I do have a ~56V model of the same power supply (RSP-750-48), and I've tried using that to charge the battery in the

16S arrangement, and I got slight voltage variations between modules (but within each module they still matched).

This was mostly due to the 56V charger not quite hitting the full voltage needed to hit 100% SOC (58.4V), combined

with the fact that between modules the cells aren't as closely matched (each module's cells were bought at different

times).

'But Bob' (I hear you saying), you have to reconfigure the battery between charge and discharge!  That's just until I get

four of the 14.6V power supplies, then I can hook them up to each module while they're still in the 16S arrangement.

That will allow me to charge at C/4 (about 3kW from the wall) instead of just C/16.  The addition of one 50A Anderson

connector per module to the battery case will allow this to happen without even opening the battery case.

---

From: electricboats@groups.io <electricboats@groups.io> on behalf of Skray775 <kelly@skraye.com>

Sent: Wednesday, May 11, 2022 7:42 PM
To: electricboats@groups.io <electricboats@groups.io>
Subject: Re: [electricboats] LifePo4 build questions

 

Bob, can you help me understand the no BMS? I know from charging Radio Control airplane batteries that we do not carry a BMS on the batteries (to save weight). The BMS is built into the charger and a set of balancing leads plug into the pack/individual cells.

If I were to go with Amper Time or any 12-volt 3s arrangement that has a built-in BMS, and I am using a 3 bank charger, won't I achieve the same thing?

Could I simply parallel the three 12-volt batteries over the winter months to keep them balanced as a set?