Smoke wrote: ↑
May 24 2020 1:18pm
With typical heat shrink insulation on a battery pack, what you can feel on the outside is not indicative of the temperature on the inside.
My packs are not heatshrunk.
They are bare EIG NMC C020 cells, with (thin) aluminum heatspreaders on each cell across it's entire surface of one side, so what is on the side of the cells should be similar to, though lower than, the rest of the cell, and brass busbars from cell to cell on the top (under a flip-up acrylic lid to prevent shorting the bars but allow easy access if necessary) that also conduct heat from the cell tabs directly underneath them.
I'll see if I can find the pics I've posted and link them here (I'll edit the post when I do).
Also, there are not multiple definitions of heat. There just is not.
I'm not implying there are multiple definitions of heat, itself. But what is expected of a particular pack may be different from one application to the next. For me, any noticeable warmth generated by a pack would be "heat", and isn't acceptable--it means something is wrong with the pack itself (or it's sized too small), or the system it's powering, to create excessive current draw for what the pack itself is designed to handle.
From what I read here on ES and elsewhere, for many people using 18650-type packs, or 21700s, etc., that's not only often acceptable, it's expected.
I am currently testing my cells so I can sort them and place them in my pack for the best thermal performance. They are in a fully loaded Opus charger doing 1A charge and discharge rate, inside a .30 call ammo can with the lid completely open (for some fire safety). That's a 0.2C rate, not really insulated at all and you can feel a noticeable rise in temperature on the body of the cells. I noticed the same thing at 0.1C rate also.
That sounds like cells with what I would call a higher internal resistance. Not atypical for many 18650 types and some others, but is not the only type of cell out there.
The EIG cells I'm using are 20Ah, with 5C continous discharge capability, 10C burst for 10seconds, and the pack on the trike at present is 2p (1p gives the same "hand check" temperature results, though an actual instrumented measurement would show some difference). Total 14s2p pack resistance is estimated by the Cycle Analyst in the mid-30s milliohms (34-35). So that's about 5milliohm per cell.
Normally I'm using them at about 100A or so bursts, for a few seconds at a time, which is 2.5c. Assuming I can math today (not guaranteed), and the CA's estimate is correct, that would be about 14w. (20A * 0.035ohms = 0.7v ; 0.7v * 20A = 14w). 3.5watts of heat within the pack during the acceleration from a stop to 20MPH.
Cruising is a lot lower current, around 20A depending on conditions, which is 0.5C. Assuming I can math today (not guaranteed), and the CA's estimate is correct, that would be about 14w. (20A * 0.035ohms = 0.7v ; 0.7v * 20A = 14w).
So yes, there is heat created, but it is not noticeable (by hand) even if running the pack from full to empty (around 30 miles or so of riding; I've only done this a couple of times).
Charging is done at 0.25c, 12A; the charger (Meanwell HLG-600H-54A) is mounted directly under the pack on the bottom (outside) of the wooden cargo/seatbox (the pack is on the top of the same wooden panel, inside the box).
I think most electronics are rated for 80°C but I would like my batteries to stay under 60°C if possible.
I would prefer to keep batteries under 40C, myself.
Mine are typically close to ambient (which can be as high as 40-50C here; even higher if parked in the sun in a parking lot or riding in traffic, in late spring thru summer).
There was more heating when I was using just 1p, though the current was a little lower (a little more than 80A vs 100A peaks, and less than 20A continous). For instance, what I recall of some results a few years ago when testing in cold (for Phoenix, AZ) temperatures:
After sitting for 8-10 hours in the breakroom at work in high 60s F (warming the pack up to equalize to that), and then riding home to park in the shed at night. By morning I would see about 50F pre-ride temperatures at the very core, with the outer edges cooling to about 40F (ambient).
This had significant voltage sag, decreasing accleration; so I used an incandescent lamp under the seatbox for overnight storage to keep the pack warmer. This kept the pack in the 60s F for it's outer edges, and a few degrees warmer at the core, about the same as when I'd leave work. During the commute of around 2.5 miles, with the repeated hard acceleration and then 20MPH cruising between stops, there wasn't enough self-heating to register on the modified BBQ sensor used to test with at the time. (which is not as sensitive to small changes as typical thermometers as it has a much higher range).
As noted, though, the 18650 pack (13s4p) tested under warmer ambient conditions *did* heat quite noticeably, but despite the ratings claimed for it by the seller, it was not designed for anything like the loads I was putting on it. IIRC it was rated at 10Ah, so I was putting it thru up to 8C peaks, and almost 2C continuous. So I wasn't exactly shocked by the heat, even if I was disappointed.