How to design a circuit that protect 7 lithium-ions in series

[Rich Grise]

Sorry for these multiple posts, OE got a hickup and kept that post in the
outbox and whenever I changed reading the NG it synched and sent a copy, but
never cleared the outbox. Seems to have to do with my newsserver. Now lets
see if everything is ok

OK so far from here - hitting "next" and watching headers change was
interesting to the extent that watching the timestamp of the message ID
increment (and noting the delta minutes) was fun, but I already knew you
wouldn't do anything so lame on purpose. ;-D

Cheers!
Rich

 

[Fred Bloggs]

There are many ICs that protect Li+ battery string(<=4 cells in series).

Now I have a project that I have to design a circuit that protect 7 Li+
in series. There is no IC which is designed for string larger than 4 cells.

The way I choose to design the circuit is to use 7 single cell protection ICs
in series, one chip protecting one chip. But the problem I met is the
circuit can not provide overcharge protection when the battery voltage go up
very fast.

Is the way I choose wrong? Anyone could give me some advice?

Thanks in advance.

Is that the only protection you need, overvoltage on charging?

 

[Qizhang Du]

Is that the only protection you need, overvoltage on charging?

No,I want overcharge and overdischarge protection. My circuit is ok for over-
discharge control,but has some problem with overcharge control. When the battery
voltage go up slowly, it can give the right protection,but when the voltage go
up very fast, it can not catch it.

 

[Mac]

This is not accurate. 4 cells in series give exactly the same current as
7 cells in series provide.

Perhaps you mean that, because the terminal voltage of a 4-cell pack is
4/7 the terminal voltage of a 7-cell pack, that the current _demand_ by
the load will be 7/4 times (plus inefficiency) what it was at the
higher voltage? This is true, for the same power delivered.

I'm pretty sure that is what the OP meant. He or she just chose the wrong
verb (should have been "require," not "give."). But I did have to read the
post twice before I came to that conclusion.

Eek! OK, I would be speaking out of turn if I tried to make any
suggestions as to designing a switcher that could substitute 4
cells for 7. Six to Ten _Amps_?? You're obviously not talking
about 2200mAh 'A' NiMHs. =:-!

Sorry.
Rich

[snip]

Yeah. That shut me up, too. ;-)

--Mac

 

[Fred Bloggs]

Thank you. But 4 cell pack will give twice the current than using 7 cell pack.
Also,the best converter's efficiency is 90%,the other problem is I have to
handle is the heat if I go in this way.

BTW,my 7 cell pack have to output 6-10A.

I don't see that any of the monolithic protection products handle
anything near 10A, so you will have to go custom anyway. The
architecture of these IC's is straightforward, the only tricky part is
handling sleep mode and other considerations to minimize battery current
drain overhead.

 

[Roger Hamlett]

I don't see that any of the monolithic protection products handle
anything near 10A, so you will have to go custom anyway. The
architecture of these IC's is straightforward, the only tricky part is
handling sleep mode and other considerations to minimize battery current
drain overhead.

Also though, you can do a _lot_ better than 90% efficiency. With
reasonable design, you can easily push past 95%, and on larger switchers,
figures over 98% become possible. The idea that 90% is a 'best' figure, at
these sorts of currents, needs to be definately put to rest...

Best Wishes