Looking for recommendation: 20s 40-60A smart BMS with 5 or more NTC inputs

GalFisk

100 mW
Joined
Aug 30, 2014
Messages
41
Location
Sweden
I'm building a 20s Li-ion moped battery divided into 5 IP67 waterproof boxes of 4s each, and am looking for a BMS that can measure at least one temperature in each box. I have 4 unused pins in my chosen balance connector to each box that can be wired for temperature sensing. I've looked at Daly, Lithiumbatterypcb, Vruzend and a few random google hits, but haven't been able to find what I'm looking for (partially because of bad photos/lacking documentation in the web shops).

The BMS will have its own sealed box, so IP rating for the BMS is unimportant. I'd like PC cable and BT mobile app support, but if I have to pick one, I'd pick temperature monitoring. Any recommendations? Preferred cost <110 $/100 €. The size is preferrably <170mm on the longest edge, though a longer one could fit diagonally. I'm in the EU.
 
Temperature cutoff could be a separate device, just to open options up for you.

Are you cutting off based on too cold, too hot, or both?
 
I've considered a separate temp cutoff, and it's still an option, but if I can have it integrated it's better. If you have a separate one to recommend, I'm interested. One of the BMSes I'm considering has a on/off input, so a separate cutoff could signal to that instead of being capable of cutting off 40-60A by itself.

High temp cutoff is important. Low temp is nice but not necessary.
 
If you don't actually need to monitor the temperature, just shut off if any box reaches a specified temperature, you can use a series-wired array of thermal switches in place of the single one that's often used on small BMS boards.

In theory, you could wire as many as necessary in series to even monitor multiple spots within each box. As soon as any of them opens, the entire string is an open switch, as far as the BMS is concerned, and so it shuts off. (or does whatever it's supposed to do in the case of that switch going open, if not shut the FETs off). Since they don't carry any notable current or high voltage they don't have to be much of a switch. ;)

If you have any existing battery or BMS with this kind of switch on it, you can try it with several in series with teh kind of wiring type and lengths between them that you will need to use in the actual system, then heat various ones to be sure the BMS responds as expected.

Typically those come on the BMS as 70C prewired; they come in other variations. They generlaly look like this, with either ceramic or metal housings:
s-l1600[1].jpg

There are a bunch of them that show up in this image search
https://www.google.com/search?q=70c+micro+thermal+switch&tbm=isch
though not all of the results are that type of item.
 
Thanks for the tip; I didn't realize bimetal switches came in such small size and cost. Those should work. I could save on wiring by connecting them in series with the balance wires, so that if any one of them trips, the BMS thinks the battery is off kilter and shuts down. That'd give me four sensing points per box. I had considered doing this with non-resettable thermal fuses as a backup, but having the only overheat detection being non-resettable wasn't an option.
Knowing the actual temperature would still be nice as a way to characterize battery performance, but since the boxes will be the same, I could get away with actually measuring just one or two of them.

What would be a good cutoff temperature? It's a long range pack for a moped, with a max load of ~0.7C. I don't know the average load yet. The cells (used, ~90% capacity, 10A per cell) will be in sealed plastic electrical boxes, which are not very thermally conductive, but I'm thinking that the low load will limit the heat. I also live in Scandinavia and seldom get >+30°C weather.
 
GalFisk said:
Thanks for the tip; I didn't realize bimetal switches came in such small size and cost. Those should work. I could save on wiring by connecting them in series with the balance wires, so that if any one of them trips, the BMS thinks the battery is off kilter and shuts down. That'd give me four sensing points per box. I had considered doing this with non-resettable thermal fuses as a backup, but having the only overheat detection being non-resettable wasn't an
If you put them in the balance wires, their resistance may affect both cell voltage readings and balancing.

Plus if the BMS isn't the kind that bothers to check if the cell is actually attached (if voltage outside a certain range doesn't trigger shutdown), it will not do anything about an overtemp.

Some BMSs still use the "stack" of individual-cell-style balancer detection / etc chips, which aren't designed for greater than single-cell voltages. SOme of these get damaged by being disconnected/reconnected outside of a specific sequence, whcih may include being disconnected in this kind of situation. (there's a thread with details on this around here someplace).

Myself, I would only use the switches on the BMS wires intended to be switched. ;) Either the thermal switch input or the case switch input.
 
Useful information, thanks.
The voltage drop at 100mA balancing current would be <.005V, so measurements shouldn't be too affected, and I'm going to use a smart BMS (not that this guarantees which situations it can handle), but I understand it's safer overall to keep this fusing separate, so I will.
I'll fuse the balance wires against shorts, because I don't want a fire if anything bad should happen to them.
 
GalFisk said:
The voltage drop at 100mA balancing current would be <.005V, so measurements shouldn't be too affected,
That depends on the actual contact resistance of the switch. If it is perfect and always switches closed correctly....if not...? Same thing for if it doesn't open correctly, and leaves a resistance that creates a voltage drop--will the BMS be able to tell that it is open, and shut off?

Iit probably still would, if the switch were on a simple switch input...the question is more complex on a balancer input with cell voltages present, and detection and balancing circuitry, etc.

I prefer to develop ideas trying to keep in mind all the things that can go wrong, even if they're unlikely, because the least likely things often have dramatically bad consequences. :(

The simplest way to do a function with the least possible complications and least possible failure modes is usually the best way to do it, especially if it is a safety-critical system. ;)
 
They're tiny bimetal clickers (which should make them as mechanically distinct as microswitches or tactile pushbuttons) with a guaranteed max contact resistance after 10k clicks. I'll put all of them (NC contacts) in series and connect them to the on/off input of the BMS. If any thermostat trips or anything's imperfect, the loop will break and the BMS will turn off.

What would be a good triggering temperature do you think? 60°C?
 
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