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NiMH charging circuit.

Hi, I have a small electronic device (portable DAB radio), that runs on

2xAA batteries, but for some unknown reason (cost??), the manufacturer decided not to provide an external power source socket.

The battery life is horrendous.. 6 hours max..

Anyway, figured it would be very handy to "upgrade" it. would like to put a couple of NiMH AA batteries in there, a small socket on the side, and make a in-situ charger from a PC USB supply.

Got a couple of questions, not very good at circuit design, and wondering if a guru here can help.

1/ Is is OK to charge these sort of things in-situ? I mean charging the batteries, wilst the player is in use, or will I need to devise a switching mechanism? (the small connector I have is a 3 way, so I can do this if I need to, but I want to keep it simple as can be).

2/ As I understand it, NiMH batteries use constant current, how can I achieve a simple circuit to charge this?

Any help, MUCH appreciated..

Reply to
mark.gillespie
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Reply to
Derek

two things

1) set up the charger so it chargesd at c/10 rate or less...if your cells are 2000mA hours, then set up the changer for 200mA and it will take about 14 hours to charge them....do not overcharge them or let them get hot, a lower current is better (safer re overcharging) but takes longer...

2) be sure that the voltage out of the charger will not FRY your unit if the charger is hooked up and the batteries are removed... you may need a zener diode of something to limit the no load voltage...

Mark

Reply to
Mark

You got it right, cost is the issue. To charge them properly and safely, you need a microcontroller. The micro needs to monitor the battery voltage (DAC) and activate an adjustable current source. The most important part of all, shut off the current source once the battery voltage reach the rated maximum. Otherwise, you are detonating an explosive device. There are known to be explosive, even when not charging.

Be careful about claims for circuit charging NiMH, they are probably designed for NiCad (less explosive).

Reply to
linnix

Is that why my Hazbolla Barbie came with NiMH?

Luhan

Reply to
Luhan

Absolutely, each 2.6Ahr (typical) cell is packed with 2.6 * 60 * 60 =

9360 Joules of power. Back to the OP's charging question. A fast charger can charge it with 2.6A in 1 hour, and a slow charger with 260mA in 10 hours. You are not going to be able to pull more than 100mA from the USB power source. It would be easier to pull power from the PC's 12V source, with a modified auto socket.

Reply to
linnix

Don't you think that we have 24 hour news services out here in the wild? CNN, for starters. Fox News and I'm sure there are others that I can't get on basic cable.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
Reply to
Chris Jones

You've received some answers. Here's one that works, is simple, won't explode your batteries etc.

-----

6V (+)---Vin|LM317|Vout---+ ----- | Adj [15R] | | +---------+--------> To battery (+)

Gnd ------------+------------------> To battery (-)

Use NiMh cells rated at least 1000 mAh. The charge rate is approximately C/12 for 1000 mAh cells. If you use 2000 mAh cells, use two 15 ohm resistors in parallel.

It does not use the USB port - instead it uses a 6V DC wall wart. With a 15 ohm resistor, it will provide 83 mA, and you do not need to switch it out when playing the radio. Unless your radio draws more than 83 mA, it will charge the batteries and play the radio at the same time. The larger the capacity of the cells, the longer it takes to charge them, unless you reduce the value of the resistor. Playing the radio while charging also lengthens the charge time.

Ed

Reply to
ehsjr

Estimate of 300mA (2Ahr / 6 hrs).

Not enough to run a 300mA radio. Too much for fullly charged batteries while radio is off.

Reply to
linnix

Right. It's not a battery eliminator, it's a battery charger, that can be used while the radio is playing. If the radio draws 300 mA, the suggested charger will provide only 83/300 or about 27% increase in run time - about 1 1/2 hours more - if the radio is constantly on for the full time.

What do you have in mind - both as to why you say it is too much for fully charged batteries, and as to a better circuit for the OP?

There is a better circuit that retains some of the practical benefits of the posted circuit, at the cost of more parts/bigger footprint/less simple. He said "I want to keep it simple as can be" and it doesn't get any simpler than 2 common parts. So I'd really like to see the engineering trade-offs you have in mind.

Ed

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Reply to
ehsjr

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