V regulator: +ve, -ve and Ground

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Hi Erosennin and Eric

Thank you for your replies.

In the past I have thrown together a single cell recharger based on the MAX1555:
https://www.electro-tech-online.com/custompdfs/2007/11/MAX1551-MAX1555.pdf


This was ultra simple. I was recharging a 1000mAhr battery.
Thus the rate of charging using the MAX1555 (see page 4 graph on top right): 280/1000=0.28C So I guess that this is ok?

Now I need to charge 2 batteries in series.
I have been trying to come up with 2 MAX1555 circuits working separately on 2 batteries, but the batteries are connected.
The charging will be: 280/170=1.65C so this is too fast?
However if I refer to that graph, I see that the MAX1555 will cut the current once a certain V is reached.
Basically I am very confused on two levels:
(a) Can 2 MAX1555 in "separate" circuits be used to charge 2 170mAhr batteries that are connected in series?
(b) Is the charging rate too fast?

Thank you for all your help, I hope that you can help me understand this problem.

Best Regards
a.

 

[erosennin]

a) I don't believe so
b) It's slightly high for my comfort...

The problem is, with the two batteries in series, their cumulative voltage is now 7.4V - the thing is, both batteries will not discharge at exactly the same rate although we would all like them to, and this gets worse as time goes on. This creates a problem when charging as one battery may be charged and the other still partly charged.

Is it vital that you charge the batteries in circuit? Otherwise with the max1555, 2 of your LiPos in parallel will work fine... It's just that I haven't experienced a battery charger that operates with the cells in series... (Might be due to my age???)

 

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Hi Erosennin

Yes it is important that I charge the two batteries which have to be in series (I need 7.4V regulated down to 5V) in circuit. (I tried using dc-dc converter and one battery to double my voltage, but that produced huge amounts of noise throughout my circuit).

I will charge the two batteries until they are fully charged. That way their difference will not matter (I think).

Thank you
a.

 

[Speakerguy]

With lithium polymers I would by a good charger, their failure modes include anything from puffed up cells to fire balls. A commercial charger is worth the expense and they are very flexible.

Also, I agree not to charge faster than 1C. Discharge rates can be up to 20C these days, but I have not seen charge rates listed at much more than 1C.