Home     Forum     Projects     Experiments     Circuits     Theory     BLOG     PIC Tutorials     Time for Science

### Author Topic: Battery charger circuit!  (Read 5196 times)

0 Members and 1 Guest are viewing this topic.

#### alb1985

• Newbie
• Posts: 15
##### Battery charger circuit!
« on: July 15, 2013, 20:12:56 PM »
Hello,

I would like to design a charger that can charge two duracell, AAA 1.2v 1000mAh batteries in parallel. I would like 2 led indicators. One led can be a charging indicator while the other can be to indicate battery done charging(fully charged). In the design i would like for the charger to stop charging when battery is fully charged.
Can anyone help?

Thanks

#### cheerio

• Sr. Member
• Posts: 306
##### Re: Battery charger circuit!
« Reply #1 on: July 15, 2013, 21:03:00 PM »
what battery type is it? liion, nimh?

#### kam

• Hero Member
• Posts: 1849
##### Re: Battery charger circuit!
« Reply #2 on: July 15, 2013, 22:04:45 PM »
Generally, having batteries in parallel is not a good idea. Hooking them up into a charger is a bad idea. If one battery fails the charger will continue pushing current for 2 batteries.

#### alb1985

• Newbie
• Posts: 15
##### Re: Battery charger circuit!
« Reply #3 on: July 15, 2013, 22:08:53 PM »
The batteries are NimH.  In series would be ok than.  Any suggestions of how to do it?

#### cheerio

• Sr. Member
• Posts: 306
##### Re: Battery charger circuit!
« Reply #4 on: July 16, 2013, 00:45:29 AM »
it uses 1 led instead of two. but that should not be a problem. it can handle up to 4 cells in series afaik

#### Sir N

• Newbie
• Posts: 26
##### Re: Battery charger circuit!
« Reply #5 on: July 16, 2013, 02:07:37 AM »
Hi,

I would like to design a charger that [...]
Funny, but to me, it sounds more like you want somebody else to design it for you

I would like 2 led indicators. One led can be a charging indicator while the other can be to indicate battery done charging(fully charged). In the design i would like for the charger to stop charging when battery is fully charged.
Can anyone help?
Personally, I prefer a ยต-Controller for such, but if you prefer a different solution, the Maxim  MAX712 is a very good charge controller for NiMH (1..16 cells), where you can control sll parameters of the charge and its termination. It can even be configured for switch mode control in high current chargers.

A "charge in progress" LED can be driven from its fast charge status output and while you can use another LED driven inverted from the same output to tell when it's done, it won't be needed if you have a "power on LED", as you'll then know if it's fast charging, done or the charger simply lost power.

If you don't need fast charging, a "tapered charger", which switch off completely when full charge is reached, can be used - a voltage regulator, 2 small signal transistors, a relay plus a little "glue" components - tell me if you want a schematic for this.

#### alb1985

• Newbie
• Posts: 15
##### Re: Battery charger circuit!
« Reply #6 on: July 16, 2013, 02:34:18 AM »
You are absolutely right.  It does sound like I want someone to design it for me.  But no I dont I just need somewhere to start.
A schematic would be great.

Thanks

#### kam

• Hero Member
• Posts: 1849
##### Re: Battery charger circuit!
« Reply #7 on: July 16, 2013, 08:19:51 AM »
When you find the controller that you want to use, you then search for application notes or typical usage drawings. For example:
http://www.maximintegrated.com/datasheet/index.mvp/id/1666

You can trust application notes from the manufacturer. They usually work

#### alb1985

• Newbie
• Posts: 15
##### Re: Battery charger circuit!
« Reply #8 on: July 17, 2013, 19:48:32 PM »
Is that chip(max712) obsolete.  Maxim does not recommend it for new design.  what does that mean?
They recommend DS2711 to ds2715

#### kam

• Hero Member
• Posts: 1849
##### Re: Battery charger circuit!
« Reply #9 on: July 18, 2013, 07:54:01 AM »
"Not recommended: means that there is a better product and this will probably go obsolete.

#### Sir N

• Newbie
• Posts: 26
##### Re: Battery charger circuit!
« Reply #10 on: July 23, 2013, 03:01:34 AM »
Is that chip(max712) obsolete.  Maxim does not recommend it for new design.  what does that mean?
They recommend DS2711 to ds2715
They don't recommend it for new design... In an industrial setting. This only means that they don't wanna produce it for more than a number of years, but since it has been close to an industry standard for a number of years, they have to keep producing it for quite some time (I'd guess 4..5 years absolute minimum and hobby vendors will probably store enough of it to last for more than a decade).

For one-off hobby stuff, it means absolutely nothing - as long as you don't fry the chip (or get lightning damage - a couple of years ago, our TV set and HiFi took a hit under a thunderstorm and was beyond repair), it should last you a lifetime - buy an extra if you want to be on the safe side (and always use a socket of course).

You can find lots of circuits around this chip and its NiCd counterpart (MAX713). The only real difference between MAX712 and MAX713 is their dV/dT termination - NiMH needs a dV of 0 mV (i.e. no change of terminal voltage between two samples), while the charge of NiCd shouldn't be terminated until the dV/dT is slightly negative - apart from that, you can look at MAX713 schematics and just replace the chip.
The chipps they suggest as a replacement won't have an equally large following, so my recommendation is to stick with the MAX712, if you want the benefit of lots of designs floating around the web.

The datasheet gives you schematics and ample info to go beyond the basic circuit if you want more.
Googling for "MAX712 battery charger" gives you more circuits than you'd care to go through, if you need further.

BTW. What, in your opinion, constitutes the design (since you want to do it yourself but wants a schematic)?
(I'd say that whoever made that schematic from scratch is the designer )

If you really want to design the circuit yourself, I'd go with more basic components, like op-amps and comparators, as they'll still be around decades from now (they'll only improve, but keep standard pin-outs).

Whenever I design something for the web, that's my prime directive whenever possible, as special chips may be hard to get for a hobbyist living at the edge of the known world, while the basic stuff is more available everywhere.

For a NiMH charger, all the info you really need is how the charge parameters should be, like:
- Max current at undervoltage
- Max normal charge current
- Max trickle charge current *
- Max terminal voltage
- Max charge time *
- Max cell temperature *
- dV/dT terminating voltage *

*  If implemented

With that info, some op-amps and compartors, plus the usual glue (resistors, caps, diodes etc.) and a power transistor or two (whether FET or BjT), it's just like solving a puzzle, with the added fun of pressing buttons on your calculator