Re: [electricboats] Do your LiFePO4 batteries ever get too warm?

I don't see anything about conformal coating of BMS boards in this long response.  Heavy conformal coat is the #1 protection mechanism for most automotive, and particularly EV BMS boards for EVs.  Of course, so is also the enclosing of boards in enclosures that dissuade moisture intrusion or provide a means for moisture to escape.  On my THINK EV, the battery enclosure (which gets splashed constantly and is not fully sealed) includes eight 3kwh batteries (each with 2 BMS cards and 24 pairs of Enerdel lithium pouches) and at least 3 contactors.  The batteries are metal cases with plastic tops.  The BMS cards are heavily conformal coated and located underneath the top, but the cases are not at all sealed.  People drive these in humid, wet environments and at the coast.  I live in Portland, Oregon.  Short of driving thru water, these batteries are sealed well enough.  The cells themselves would drain off any moisture if they got flooded however the metal cases might entrap water---guess they thought it best to handle "some" water intrusion into the pack, but not enough to flood into the batteries themselves.  When people have had issues with these BMS' due to moisture, it has been due to "pooling" of moisture on the conformal coat.  It is probably not a good design decision to orient PCB assemblies horizontally---moisture is just happy to sit there.  On a boat, there might be more sloshing factor to get the water off horizontal boards.  Best if they're oriented vertically.

 

While completely sealing a module sounds good, moisture 'entrapment' can be worse than just letting it breathe.  There are filter breathers that can be used that allow sealing while providing ability of modules to breathe.  Works for automobiles…

 

-mt

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of cpcanoesailor via Groups.Io
Sent: Tuesday, February 18, 2020 6:59 AM
To: electricboats@groups.io
Subject: Re: [electricboats] Do your LiFePO4 batteries ever get too warm?

 

Short answer - no.

 

Long answer - I built a 1200Wh LiFePO4 pack for my 6m proa (multihull) sailboat. I was very concerned about water intrusion so I put the battery cells, BMS, a 10A solar charge controller and originally a motor controller into a large dry bag with a clear vinyl window so I could see the displays and find the various toggle switches inside (I operate them by pinching the bag around the switch). The plug for the motor, a 12V aux plug, and wires to the solar panel are mounted on a plate that is sealed to the bag near the top. I can open the bag at the top for maintenance, and to add a dessicant pot when the boat sits over the winter. When operating, I use dry dessicant packs to control moisture in the bag.

 

The battery is physically supported inside the bag using a plastic battery case, and straps and plates to constrain the plastic encased prismatic cells. Outside the bag, there are struts to hold it in place should the boat capsize (I have once before). It's in an open locker at one end of an open cockpit, so sheltered from rain and most spray.

 

Regarding heat management - the motor controller that I first used would shut down because it didn't have sufficient airflow, so I removed it from the pack. Now, I use a brushless motor with its own integrated controller. I did melt an under-rated main switch, but aside from the switch failing, there was no other damage. Last year, I pushed the pack, and found I could draw 25-30A continuously for at least two hours. I could also draw up to 40A for some time, but eventually, the BMS would shut down the pack, and then re-energize it again after a few minutes. I think this must be due to its own thermal shutdown, so it probably isn't because of cell temperatures. The pack output is protected with a 40A circuit breaker, which I've only tripped once when operating above 30A and passing through some nasty chop that made the propeller cavitate over and over for about 10s. As is, I'd say the pack is adequate for my boating needs, so I don't plan to experiment further. If I plan to travel any distance, I also bring a small gas outboard, but the pack works well for 90% of situations I have encountered. Range has been tested at more than 20km, and solar recharge in 2 sunny days.

 

My previous capsize was with a smaller flooded lead acid battery, which stayed put, but made a horrible mess in the locker. The only parts of that propulsion system that survived were the motor (usually submerged anyway), and amazingly the solar panel, which survived two hours underwater while the boat was inverted and being towed. I still use it.

 

While experimenting with CALB prismatic cells I accidentally over-discharged one, causing the cell to vent. It made a small 'puff' sound, and emitted a slight, unusual smell. That was it. No drama. I didn't try to revive the cell, but just recycled it.

 

Curtis