BMS FET frying and Amp limits on Duct Lifepo4

[SniperGaulois]

I'm getting a 35a 72v cont for my 20ah 48v batt with a standard BMS from anna from january... The forum is bit foggy about Amp Limiting of the duct batts. One guy fried his BMS straight away by going at 36a momentarily, and BMS have either 30 or 35a of limiter.

might i fry the bms?

will there just be a ceiling to the draw?

what should i be worried about?

what are you all getting from the duct BMS?

Thanks!

 

[kyakdiver]

I've been running my clite 36-72 40A controller with the ping pack now for just over 1000 miles now. No problems. I have a doc watt and it will hit 40A draw and doesn't cut out. I don't know if pings BMS is more robust or not though. I think you can buy one from ping for less than $50 from what I've heard on this board though.

My BMS does not cut out even up to 40A's for short bursts and will pull 35A's continueous on hills. It did cut at first because I had 4 ft of light wire coming from my hub... upgraded the wire and all has been golden since.

kyakdiver

 

[Rassy]

I use a 48V 20AH Anna pack. My controller is a 36V 35Amp WE being used at 48V with no modification. During acceleration or hill climbing the BMS will shut off and I have to stop, turn everything off, then restart everything to reset the BMS. No apparent damage is done.

My poor man's solution was to rotate my thumb throttle so that it can not go full on (the thumb button stops when it hits the shifting stuff). This has solved the problem, with very little performance loss, and I may just leave it at that.

Tomorrow my Watts Up should be delivered and I will be able to see what is happening and fine tune a little more.

 

[fechter]

If you try to draw too much current from the pack, the BMS is supposed to trip and cut the power to protect the pack. It is supposed to be 'idiot proof'.

If the BMS trips all the time when you don't want it to, that would be understandable at high current, but it should not fry.

You can adjust the current limit on the controller to keep it below the BMS trip point. Adding a large capacitor across the battery output can help prevent the BMS from tripping too quickly.

 

[dogman dan]

I also run the brushed wilderness hub, with a ping 36v-20ah. I get no cut outs at all. And If I understand it right, if it did that is to protect the battery cells, and a good bms does so without cooking itself off. The bms frying should be more related to it's quality, more than over amping it. Of course if you use the cut off enough you may wear it out. In my opinion those with really big amperage demands are poorly served by the duct tape battery unless they use really large sizes, or a brand name that is known to like paralell connecting.

 

[dogman dan]

Reading the first post again, I'm now confused. The bike will still be running 48 volts correct? But the new controller will be able to draw more amps from the battery. That should only result in a cut out if the amps are too high. You are not over volting the battery by using a controller that could take more volts. Many solutions exist to lessen the amperage you pull, including fuses or breakers. I really like the throttle mounting idea, its the only way I would use less throttle.

 

[recumbent]

I can set the max amperage on the "Cycle analyst" meter, maybe you can also with your meter.

 

[SniperGaulois]

thanks for that, just wrote a long reply and got a 404 error!

also if you put a 10k resistor onto the throtle sensor wire, cant remember which color, it will limit the max amps to about 32, and you can also pout a switch between 10k and 150k on the handlebars to switch between 32 and 10amps naximum, useful for idling with the full range of the throttle for maximum control.

i did some measurements already on a pot going to the controller,

350k the motor will make a tiny sound and not turn,
270k, the motor will do about 3kph on flat but not start from standstill
100k is about 50% power
50k is about 75% power
30k is the motor on full power

pretty cool

 

[recumbent]

I'd love to have a switch on my handlbars that would give me 50% power with full movement of the throttle, "zzoing" that's a totaly cool idea.
Useful in places like populated bike lanes or parks I frequent where there are many pedestrians around.

 

[SniperGaulois]

WEIRD RESULT- 27A limit on anna's 48V-20Ah BMS and 25A max continuous output on the battery...

i have borrowed a 35A controller, and the BMS cuts off around 27A, but also when i go up a hill, the level really struggles in between 25 and 26, 26.5, whereas you would expect to be able to max out the throttle to 35A straight past the BMS limit really easily. this must be because the battery can't actually output more than 25A for more than 2 seconds, hence 1.25C rating...i havent actually been able to see 27A on the screen once. when i'm on flat, the controller seems level off at max 22A... why?

isnt this very surprising? how come i have different from everybody else? the cycle analyst limit is 50A.

 

[fechter]

The maximum amps the motor wants to draw is a function of it's loading. Once the thing gets going above a certain speed, the motor will draw less current than the limiter rating. You want this for efficiency. Going downhill, the amps will drop to zero (or even go negative).

On a steep hill, the motor will try to draw more current than the limiter is set for, so the limiter kicks in.

It would be interesting to determine if the BMS is kicking out from high current or from the cell voltage sagging so much that the LVC is activated. The current shunts in the BMS are not really very precision devices. It might just be set too low. Got solder?

 

[SniperGaulois]

yes i have solder, are there any diagrams about the lvs?

i went up a steep hill with grass and the amps still leveled off around 25...

 

[fechter]

I guess you would want to know if it was the battery BMS or the controller that was limiting.

Usually when the battery BMS limits, it drops all power and you have to disconnect the load to reset it.

Your description sounds more like the controller is limiting.

You could try connecting the controller directly to the cells just long enough to get a current measurement (use a fuse).

If the controller is running too low, you'd solder the shunt in the controller.