Battery cooling question.

[Mouse]

I'm about to make a casing for 5.6kWh, 2 X 48V 60Ah of 18650 cells as part of a motorbike conversion.
What I don't have is any kind of experience of is how hot (if at all) they will get under heavy use or charging.
I'm not expecting the maximum discharge to be much over 20kW or 3C and the maximum charging will be 4kW or about 0.7C
Probably worth saying I'm in the UK so there'll be no extremes of ambient temperature, but plenty of wet hence an enclosure

They are installed in a strong OEM casing that will allow air to pass through so I could install ducting ports to put a fan on it if needed for forced air cooling but would rather not, and most definitely don't want to build it all with no air cooling then have to rework it all later on after it's finished.

TLDR: Do DIY motorbike batteries (bigger than ebike) get hot enough to need cooling under heavy use?

Pic for attention, the current mockup.

 

[pwd]

It depends on the cells/BMS being used in the modals and how long you are pushing them to your controller's max current draw. On my ebike build for example, I'm using Samsung 40T cells that see maximum peaks of 20A per cell for periods of less than 30 seconds. Even on the hottest summer days here; I haven't seen the BMS or cells go above 40 Celsius (according to the temp sensors). My battery pack (with rubber and heatshrink) is enclosed in a steel/aluminum box and has no ventilation and based on the temp readings I am not concerned with cooling/venting the cells. If I increase the duration of peak current draw then I might consider adding vents etc...

 

[Mouse]

Ah Thanks,
That's the sort of real-life feedback I'm after.
Mine won't see much more than about 10-11A per cell so I guess it will be even less of a problem than what you're experiencing.
I'm not sure of the actual cell model but they're from a server instillation so I expect they are not knock-off brands and I'm not exceeding the spec on the glued casing that I can not dismantle.

I'm making aluminium casings to keep the water and debris out as I intend using this all weathers. Making a shielded air intake + ducting wasn't on my list of easy things to achieve

 

[pwd]

No problem. Be careful though, 10-11A per cell *may* be more of an issue with your 18650 cells (unknown cells) if the internal resistance is higher (and likely is). The Samsung 40T is a larger 21700 cell.

 

[Mouse]

Thanks,
That's something to watch out for.

 

[amberwolf]

If you don't know what the cells actually are or what they were intended to be capable of (or what they are *actually* capable of now), then you'll need to test them anyway, before you spend all that time/etc bulding a case for them, if they turn out to be incapable of running the system the way you need them to.

So you could setup the batteries as they are, connected as they'll be be wired for the system, then connect a static load (heaters, etc) to them that would pull the max current from them that you expect to do for any length of time (more than a few seconds at a time).

Then use a handheld IR thermometer pointed at the cells during this test, and see what temperatures you get all over the packs. Core cells will get hotter than edge cells, though the difference between them will be less the more enclosed the whole setup is (less airflow cooling things off on the "surfaces" of the packs). You can repeat this test inside the container later on, with wired temperature sensors, if you like.

Also measure voltage sag at the pack *and* the cell level while doing this, so you'll know if any pack or cell group has a problem already, and if they will hold voltage under the loads you'll be using them at. If they won't, if they sag a lot, then you won't get as much power out of them (V x A = W) and your system won't work as well as you'd like. And more of the power gets wasted as heat inside the cells, too.

 

[Mouse]

So you could setup the batteries as they are, connected as they'll be be wired for the system, then connect a static load (heaters, etc) to them that would pull the max current from them that you expect to do for any length of time (more than a few seconds at a time).

Amberwolf, That might just be the voice of reason.
I've been pondering actually load testing the battery packs properly and have finally admitted this is an essential part of the process.
I've got a Seek thermal imaging camera module for my phone so will be able to keep track of temperatures and and hot cells etc.
I'll probably be posting photos of glowing cooker elements some time soon. :lol:

I'm off to the scrap yard where I know they sort heating elements into a separate storage area....

 

[amberwolf]

Just remember that things like stovetop and oven elements and room heaters and such in the UK (where your profile says you are at) are generally made for 220-250vac, so they wont' make nearly as much of a load for a "48v" battery. (or even 96v, two in series).

So you will need to parallel some of the elements for higher loads. Measure the current with just one hooked up, and then start adding them in parallel until it reaches the load you expect your system to put on the batteries.

 

[Mouse]

...generally made for 220-250vac...

All in hand thanks. I've got a bunch of old cooker elements that I'll cut into half (maybe thirds) so they will consume suitable power at 100V which is the full battery pack voltage if I decide to test both battery packs at the same in parallel.