NiMH battery charger for 12V pack

[chillysalsa]

Hey guys,

I have come across a bunch of 12V NiMH packs @ 550 mAh, and would like to use them as a power source. Was thinking 2 in parallel, for 1100 mAh should be perfect for my use...

I need a schematic for a charger... I know I can just use a 12VDC adapter that puts out <500 mA and leave that on for an hour or two... but I would really like to make a nice dV/dT or other 'fancy' auto-shut-off charging mechanisms.

Where should I start?

Thanks!

 

[Kenny12]

i too would like to know, but with a 9V battery

 

[tangent]

Quote:

Was thinking 2 in parallel

Discharging them in parallel isn't a great idea because if one battery goes bad, the other will likely cause havoc with it. Some of the things that can happen are that one battery shorts and causes the other to discharge very quickly through it, and one reversing the voltage of the other and damaging cells.

Charging them in parallel has similar problems.

If you want more mAh, get bigger cells, don't put multiple small cells in parallel.

Quote:

I need a schematic for a charger..

http://tangentsoft.net/audio/ppa/bb/

Quote:

i too would like to know, but with a 9V battery

9V battery is a different ballgame, because they can't be fast-charged. (Or rather, "shouldn't".) Since all the fancy charge controllers I'm familar with assume fast-charging, there are problems getting them to work in a slow-charging scheme. The biggest problem is that the overcharge timers on these chips top out at around 5 hours, but you need a good 12 hours or so for a slow-charge scheme.

At the moment, the best I can suggest is to either go with a trickle charger and remember to turn it off within a day, or design your own charge controller. I've been thinking of doing the latter for this very reason, using a PIC. It's one of those 'round tuit' things....

A good trickle charger is easy to make: configure an LM317 as a current source and force about 1.5V * N across the battery with it, where N is the number of cells in your battery. (6-8 for 9V types.)

 

[guzzler]

for the "9V" NiMH battery, a trickle current of between 10mA and 15mA is probably best. As tangent said the LM317 set to CCS mode is the best way to do this. A 100R resistor is going to give you 12.5mA output, perfect. It can be a 1/4W metal film, the power dissipation is only 16mW.

g

 

[hadron]

I got a MAHA MH-C777 charger from http://www.thomasdistributing.com/, was thinking about building my own, but for under $50, why bother. It uses -dV for the primary cutoff, plus a heat sensor as a secondary cutoff. I mostly use it with a 10 cell AA NiMH pack (for my DacKit), no problems in a several months I've had it.

Edit: I'm using the MAHA 1800 mAhr AA tabbed cells, the 650 mA fast charge rate might push a 550 mAHr pack to the limit, though most NiMH can handle ~1C charging rates. And even if this is a problem with your pack, the dT sensor will kick in and drop it down to the 40 mA trickle rate.

 

[tangent]

Quote:

why bother

The best reason I can think of to make your own battery charger is so you can charge your battery without removing it from the amp. If the idea is to make an external charger, then yes hadron, you're probably better off buying something commercial.

Quote:

most NiMH can handle ~1C charging rates

Excepting the small-cell 9V types, I think all NiMHs you're going to come across should be able to tolerate 1C rates.

 

[hadron]

Quote:

Originally Posted by tangent The best reason I can think of to make your own battery charger is so you can charge your battery without removing it from the amp.

Actually, I did exactly this, the MAHA comes with a handy alligator lead that I modified into a panel connector. Wired a double throw power switch so the charger is connected to the battery in the off position.