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# designing a NiMH battery charger

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#### wheetnee

##### New Member
hey guyss
i have a project due in 2-3 weeks and me + group have to build a battery charger of 4 AAA Ni MH Batteries. It has to be computer monitered/controlled as well which makes the task even harder. We are using N704 ND AAA Ni MH batteries (you can find them on digikey.ca) The capacity of it is 900 mAH at the rated voltage of 1.2 V

(we hv a power supply which supplies 12 V DC)

right now me and the group are in the process of building a battery charger, but theres a problem, I don't know how/where to start for it. Our group is planning to use trickle/quick charging (programmed by matlab but we still dont know how to use it yet),, how would i connect the batteries in a way so they don't explode or overheat...do i connect them by parallel or series? do i have to add resistors between them to regulate the current? another thing is to consider what happens if there is 1 or 2 or 3 or 4 batteries charging??

this was our datasheet: https://www.electro-tech-online.com/custompdfs/2009/10/nh12-850.pdf

also, our Teacher Assistant said as the battery is charging, the resistance of it changes, so its hard to regulate the amount of current flowing through the battery. Efficiency is one of the areas we are marked on, So I'm not sure how to make it work ,I guess we need to build some sort of current regulator?

for the charging 1/2/3/4 batteries, my group was considering if we took one battery out or switched one off, how would the circuit be affected? would the current flowing in the batteries have to change?

should I add resistors between each battery if i were to put it in series? ?

oh and our group plans to charge the batteries in about 6 hours? (3 hours seems to hard, we calculated we needed 1.2 Amps flowing in the batteries and thats way too much from our supply)

if i were to draw a schematic on just the batteries alone,,, would it look like this?

**broken link removed**

i just want an idea on how to connect the batteries in the battery charger before connecting to the rest of the circuit. series or in parallel? with resistors or capacitors in between?i also have to implement DAQ in it too, where does that go? are they any ideas bout the battery connection? i'm really new to building stuff like this so im kinda confused!

i know its kinda hard but i really do need help, any suggestions / schematic diagrams anyone can offer will be greatly appreciated!!

wheetnee said:
right now me and the group are in the process of building a battery charger, but theres a problem, I don't know how/where to start for it. Our group is planning to use trickle/quick charging (programmed by matlab but we still dont know how to use it yet)

Trying to build something without first having a firm understanding of how it's going to work it almost always a big mistake. It looks like you'll need to do a fair bit of research; it's good idea to resources which give detailed information about how modern chargers monitor the batteries while they're charging, and with this knowledge in mind produce your own interpretation of the process.

series or in parallel? with resistors or capacitors in between?

This is the sort of thing you're going to need to understand for such a project, not just be told by someone on forums. I don't know much about the specific topic so I can't help with the particular problem, but with the amount of information you're going to need to complete this project it's likely that you'll need to do much more research/investigation before answers to smaller questions are going to be of much help.

Good luck with your project, hope you get it figured out.

The Energizer website has a Ni-MH Battery Applications Manual that has all the information about charging the batteries properly.
You could also go to the website of a semiconductor manufacturer like Maxim-IC and copy what they do in their battery charger ICs.

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