[simple question] Why schmitt trigger for battery charger, but not simple comparator?

Well, lead acid batteries may like to be trickle charged, but they can be re-charged much faster using a three stage charging regime.

--Mac

Hello Boki,

That's not a lot of information about your charger circuit ;-)

Anyway, Schmitt triggers in that application are sometimes used so that the circuit has a clean on/off status and doesn't go into a noisy linear mode when its threshold is reached. It depends on the battery. For example, lead-acid batteries as you find them in golf carts are usually charged in linear fashion, no Schmitt.

Regards, Joerg

Boki, The short answer is "Yes". ~ Schmitt triggers use positive feedback to provide a snap action that prevents them from staying in the linear region. It is the positive feedback that changes the threshold voltage as a function of the output state. Regards, Jon

Hi All, When we use schmitt trigger as charger circuit, does the reason is we need a different V_threshold to decide start/stop charge? : )

Best regards, Boki.

Right. They do like to be kept on a trickle, or "float" charge. It's not normal to shut off a lead-acid charger after a charge. It's much better keep it on a constant-voltage float.

--
  Keith

I spent years in 'the black hole of NiCad analysis' at a company that shall remain nameless.

Trickle charging a NiCad (at a rate of C/20 or less) had no measurable effect on device life, using an accelerated life test. I also did other tests. I devised a scheme (very accurate too) to determine the remaining charge state of a NiCad (which all the manufacturers say is impossible, of course - they want to sell batteries) and trickle charged NiCads for months and measured them. There was no substantive difference in charge accumulation between the control sample that had only been slow charged and the group that had been slow charged and then trickle charged.

I currently have NiMH stuff (legacy) but I have not subjected them to the same testing, so whether NiMH can be constantly trickle charged

*with no real effect on battery life* is not something I can make a complete statement on; the equipment that uses them, however, *does* trickle charge continouously and it has never had a battery failure in installed equipment (in 4 years). Whether it affects total charge accumulation is not something I have tested.

According to the manufacturers I use, Li+/LiPoly should NOT be trickle charged, under any circumstances. (I use those in newer equipment).

Cheers

PeteS

I'm not sure I would call myself an expert - after all, I'm a youngster around here.... (....)

On Li+, I use TI's BQSwitcher series, which works ok as long as you take precautions. I use the BQ24103 as an autonomous charger, and it seems to work well - it's in production units now.

My one real issue with it is it's complete lack of any real ESD (or anything else) protection on the pins, so I had to deal with that. This is in a shipping automotive design.

As to NiMH etc, I agree - a trickle charger will last the life of the unit without expensive schemes that are unnecessary in the vast majority of caes.

Cheers

PeteS

There are all sorts of things that are done that shouldn't be.

NiCds (or NiMH) should never be used in a "float" application. It is asking for a short life. If you take a peek at UPSs or emergency lighting you'll find lead-acid batteries. Better appliances that are designed to be "float" charged use SLACs.

I still see a lot of NiCds (more than NiMH). Perhaps they have environmental restrictions on NiCds in CA(?), which isn't a bad thing, IMO, since there are alternatives.

--
  Keith

Hello Keith,

I am no battery expert but AFAIK NiCd are often trickle charged. Cordless phones do that and in ours the batteries last around two years that way, sometimes more. NiMH are said not to like trickle which presents a problem in the real world where standby function without expensive charge control is required. Thing is, if you go to the store looking for rechargeable AA cells all they have nowadays is NiMH.

Regards, Joerg

Hello Keith,

True. But cordless phone batteries last a long time in that abuse situation. Ok, one just died a few minutes ago but that is fine considering it has lasted over two years with several partial charge-discharge cycles every day and continuous trickle charge. I bet this phone was picked up and placed back into the charger around 2000 times before the battery failed.

Regards, Joerg

Hello Pete,

Ah, finally a word from a real expert. Thanks.

That's what I also read in several quite credible scientific papers.

I do the same. I mean, what other option really is there besides a very complicated and thus expensive uC based charging scheme?

Regards, Joerg

Got it : )

Best regards, Boki.

"Jon" ???????: snipped-for-privacy@g43g2000cwa.googlegroups.com...