BMS Systems and current fets!

[peterperkins]

Am i missing something? I keep seeing a multitude of BMS systems where the full battery current is controlled by fets?

Why do we do this?

Why dont we have a simple audible visual alarm for these scenarios, why do we feel the need to control/cut off the full battery current? It's expensive and unecessary IMHO.

A simple alarm and opto isolated output to the motor controller, throttle pot or charger seems much easier and cheaper.

Less losses, less heavy wiring. Cheaper pcb etc etc.

 

[neptronix]

It's for people that aren't as smart/cool/talented/big hit with the ladies as us

I agree, it is an expensive way to prevent overdischarge.
However it serves a good function during charge. a BMS takes a voltage in and balances the cells on it's own.

This is how lifepo4 has historically worked as no 10/14s balancing chargers were around until recently.

 

[boostjuice]

Unlike all the Chinese BMSes iv'e ever seen, the self contained versions of the Fechter/Goodrum BMSes only regulate the charging current with a FET during balancing. Discharge current has no such unnecessary resistive element inline (separate cabling). Cell voltages are monitored and LVC trips the 'e-brake' line rather than switching the discharge lead.....The smart way (As long as your motor controller has an 'e-brake' line).

 

[Jeremy Harris]

The simple answer for using active FET power switching is ease of use for people who just want plug-and-play battery packs.

Given the relatively low current that normal ebikes operate at, the BMS FET losses aren't great, plus using active power switching allows the BMS to protect the cells from excessively high rate discharge, so providing a measure of short circuit protection, as well as just a cell going too low.

All told, it's a good solution for ordinary consumers, maybe less so when it comes to those looking for a lot more power and who are happy to intervene themselves if given a suitable warning.

FWIW, I've built an ebike pack for a non-technically minded person and I used the LVC signal to just turn the controller off using the ebrake signal, rather than turn the pack off. This requires a degree of integration between the pack and the controller though, so doesn't necessarily make for a plug-and-play, stand alone, battery system.

Jeremy

 

[biohazardman]

What they said and most of the inexpensive lifepo4 battery packs from China have a very low safe discharge rate of 1 or 2C which is plenty for the average e-biker. Still, if the juice is allowed out at more than this trickle the packs are quickly rendered unusable. The BMS systems keep the packs running much longer than they would otherwise sometimes for years. Those of us using the A123s or Lipo are far less likely to have problems as the discharge rate is so much higher with these batts. We just need to stop when we reach the correct cut-off voltage so as not to damage things. I only use my BMS for balance charging every now and then and my controller cuts me off at a safe LVC.

 

[GGoodrum]

I think the Chinese BMS designs originally had to limit the current to keep our bike applications from killing cells in the low-C LiFePO4-based so-called "duct tape" packs. Why they continue to do this is a mystery.

Richard and I actually have a version that has an active cutoff feature, but it is somewhat "smart" in operation. For "normal" LVC trips, under load, it will still pulldown the throttle/brake signal. For slow drains, where a controller was left on, for instance, The cutoff will kick in, to prevent the cells from draining down dead. Many, myself included, have not had this sort of issue/problem, though. In my case, I always use a pair of big Andersons to connect the pack to the controller, and I've just gotten into the habit of disconnecting them afer a ride.

-- Gary

 

[texaspyro]

A proper BMS turns your battery pack into a magic black box with a charger input and a voltage output. It should protect the battery from what goes in the charge port and what comes out the output. Anything that is not kosher gets blocked to protect the battery. That includes trying to pull more current out of the black box than it can safely supply.

One great advantage of a BMS that can switch off the battery output is that it will protect your battery if you leave the controller on. MANY people have killed packs that way.