how to deal with li-ion protection circuitry

i'd like to use a li-ion battery pack in my device

its a motor that will be PWM controlled

the problem is the li-ion packs have a protection circuit that has very sensitive and very fast reacting current shutoff points, which is hard to deal with when starting up a motor that has a heavy load. there is bound to be a large current spike and this trips the battery pack circuit breaker which then wont reset until the load is completely removed.

how do i deal with this? its really annoying. i wish i could just power the motor directly from the cells without a protection circuit, but i know its needed for safety.

failure-to startup is not an option in my application so whatever solution i use needs to be very reliable

Reply to
acannell
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Who is your insurance company?

martin

Reply to
Martin Griffith

schreef in bericht news: snipped-for-privacy@c19g2000prf.googlegroups.com...

Charge up a big capacitor slowly from your Li-ion battery, and use the capacitor to source the start-up current.

A small NiCd or NiMH button cell might also serve in the same role - they have relatively low internal resistance, and correspondingly high maximum outptu currents.

--
Bill Soman, Nijmegen
Reply to
Bill Sloman

Not all Li+ batteries have this kind of protection, but it sounds like you've already commited to one type (packs?). The permitted current profiles are likely outlined in the part's spec.

The use of intermediate capacitive storage is going to take quite a bit of real estate, so it might be just as good an idea to change the battery type.

Have you tried limiting your pwm current limit? It's possible that the motor may actually start, though slowly, at a reduced but fairly high current that the battery will tolerate.

RL

Reply to
legg

This sounds reasonable. The problem is the protection circuitry reacts in 200us, so even a VERY short current pulse above 6A will trip the breaker. But maybe i can run a very short duty cycle somehow..

I cant use a pack that doesnt have protection circuitry...I really need that because these cells are dangerous and something has to make sure things dont get out of hand...EVER..ya know?

Reply to
acannell

This sounds reasonable. The problem is the protection circuitry reacts in 200us, so even a VERY short current pulse above 6A will trip the breaker. But maybe i can run a very short duty cycle somehow..

I cant use a pack that doesnt have protection circuitry...I really need that because these cells are dangerous and something has to make sure things dont get out of hand...EVER..ya know?

Sounds like you're trying to squeeze a quart out of a pint pot.....

Reply to
TT_Man

Sometimes the designers of these add-on widgets don't realise what the safety requirements actually are, only that it 'works' in the initial application. No skin off their nose.

Even so, if you can current limit at 5A8, the fast internal latch may ignore it. I'd say 200uSec was probably meant to deal only with failing external controllers that were normally expected to respond in a shorter time interval - not an unreasonable requirement, but not one looking for real internal battery pack problems, as such.

Another brand can easily have produced a more robust protection circuitry without ignoring real Li+ failure modes, which seldom stem from transient loading (in my experience and from the available literature).

RL

Reply to
legg

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