Heat build up at 0.125C discharge rate.

[scott harris]

I am building a 60kWh LiFePO4 battery bank to power the motor of a salt water boat. The normal discharge rate will be 0.125c and never more than 0.2c. I want to enclose the battery in a water tight and air tight polycarbonate case to prevent exposure to salt air. Do i need to worry about heat build up at this discharge rate?

I have built a small 60Ahr battery for a raft and placed it in an air tight ammo can. I have never noticed any heat build up. I'm just wondering if I can expect the same with a much larger battery.

Thanks

 

[snoobler]

Wow. Big battery. If you can really keep those currents that low on a sustained basis, you'll likely be fine. I'm not so confident that I wouldn't want temperature monitoring though and the ability to unseal the container if temps pushed > 35°C.

 

[scott harris]

There will actually be two 60kWh batteries. One for each of the outboards as shown in my avatar. It will be operated mostly in cool weather environments (50-80 deg F) in the Puget Sound and up to Alaska.

If cooling is needed I could attach a fan to the box. But this would mean a bigger box and some spacing between the cells. I just wanted to know what to expect as I design the box. I prefer no fan at all to keep the salt air from interacting with the cells.

 

[snoobler]

Should have asked first:

What configuration of battery?
Cells:
Capacity:
Arrangement:

 

[scott harris]

Eve 280Ah cells. Four strings of 16s connected to a common bus bar. Each string with an Orion bms and temperature sensors on each cell.

 

[snoobler]

Ah... Orion... nice... money is no object then.

12kW @ 48V = 250A/4 = 62.5A/string

Assume .5mΩ resistance per cell, that's about 30W of heating per string @ 0.2C or 120W total. That's also a LOT of mass to heat with that kind of power. Estimate a 5-10°F increase over the duration of the discharge.

Charging will produce more heat than discharging, but not much.

 

[scott harris]

Oh, using internal resistance to calculate heat, I didn't think of that. Excellent.

The manufacturer of the boat, Great Harbor Trawlers, calculated 7000 watts to go 7kts. We shall see. But at that rate, it would be less than 20 watts per string.

Money is an object. That is one reason I am building my own. (My other reason is that I want to know and understand every component). But when your battery system goes out on an RV it means you can't heat your hotdogs until you get to some place to fix it. If the traction batteries go out on a boat it means you aren't going anywhere. So I am trying use the best components and practices.


Thanks for your help.

 

[snoobler]

Happy to help.

 

[Luthj]

I would assume an extra 50-100% over the cells internal heating for worst case. There is the bus bars to start with. Ff you have fuses inside the enclosure that will also dissipate 2-10W depending on the size.

From there its easy to calculate the max internal temp based on insulation and ambient.

I would guess your pack won't be more than 10F above ambient after a complete discharge cycle.

With an air tight enclosure, you will want some desiccant. True air tightness is very uncommon, and temperature/barometric pressure swings could result in come condensation long term.

Personally I would go with a ventilated enclosure that is splash and drip proof. That would make access for inspection easier. In a marine environment I would suggest a yearly visual inspection.

 

[scott harris]

I would assume an extra 50-100% over the cells internal heating for worst case. There is the bus bars to start with. Ff you have fuses inside the enclosure that will also dissipate 2-10W depending on the size.

From there its easy to calculate the max internal temp based on insulation and ambient.

I would guess your pack won't be more than 10F above ambient after a complete discharge cycle.

With an air tight enclosure, you will want some desiccant. True air tightness is very uncommon, and temperature/barometric pressure swings could result in come condensation long term.

Personally I would go with a ventilated enclosure that is splash and drip proof. That would make access for inspection easier. In a marine environment I would suggest a yearly visual inspection.

ok. desiccant sounds like a good idea.

I will have two outboard motors on the boat. But according to the boat builder's calculations I will only need to use one at 7000watts to reach my desired cruise speed of 7kts. So I could switch motors every hour or every couple of hours. Since each motor has its own battery bank that should go a long way to prevent heat build up.

The battery cases will be fabricated out of clear polycarbonate. So inspection from all sides will be easy.

 

[Luthj]

A kinda wacky idea, but in some really harsh environment stuff, I have seen the whole unit submerged in light oil. Obviously you need to make sure all the plastics/insulations are compatible. That also has the added benefit of transferring heat to the case quite effectively. It also effectively stops corrosion, even in harsh conditions.

Obviously not applicable to your application, you you can also easily cool the oil bath with a simple radiator/fan setup.

With regards to desiccant, choose a type which changes color when saturated. You can then confirm air tightness of the case easily, as infiltration will eventually cause the desiccant to saturate.

 

[cinergi]

When I pull ~50 amps from my 48v 280ah battery, I get no discernable temperature increase. I have to get to 100+ ... I've also noticed that charging is creating more heat than discharging.

 

[chrisski]

Assume .5mΩ resistance per cell, that's about 30W of heating per string @ 0.2C or 120W total.

I know you can calculate battery resistance with a known load, but do you think this resistance changes noticeably with heat or charging amps?

 

[Luthj]

Internal resistance changes a bit with SOC, and also with temperature. Its inversely proportional to temperature. However once you reach room temp it stabilizes, as ion mobility no long impacts IR.

Here is the nominal IR curve for a random cell which is stored in my Orion BMS defaults folder.

 

[Maast]

Perhaps an aluminum box with a temperature actuated internal fan to swirl air around inside to help out heat transfer. You wouldnt need a huge CFM - just enough to mix the air around.

 

[scott harris]

When I pull ~50 amps from my 48v 280ah battery, I get no discernable temperature increase. I have to get to 100+ ... I've also noticed that charging is creating more heat than discharging.

Do you get any heat when you charge at 50 amps?

 

[scott harris]

Here is the nominal IR curve for a random cell which is stored in my Orion BMS defaults folder.

What is the difference between the yellow and blue curve?

 

[Luthj]

Here is the nominal IR curve for a random cell which is stored in my Orion BMS defaults folder.

What is the difference between the yellow and blue curve?

Blue is nominal, yellow is when the BMS generates a cell performance code/error.

 

[scott harris]

Oh, this is very interesting. So did the Orion BMS generate this graph because it detected a problem with the cell associated with the yellow curve? Did this cell have this problem when it was new or did it acquire the problem with usage? Also, which model of Orion BMS do you use?

 

[Luthj]

This is from the Orion Jr 2. The Orion JR configuration tool has a number of cells which they have characterized. These are full populated profiles intended for EV or similar usage. So they have quite a few graphs with resistance, open cell voltage, SOC etc.

The purpose of this graph is too allow the BMS to detect when a cell is having a problem. When the IR is above the yellow line a code is set. It could be a weak cell, or a bad bus bar connection. On my pack the IR values are valuable for detecting a couple of bus bars that needed polishing.