nimh charger

[donkey]

Heys guys stupid question is charging a nimh battery as simple as supplying a voltage?
or is there a circuit that someone has?
the power packs will be about 4.8volts and charging them will have to be unsupervised so a cut off wold be nice. I will be charging it from a 12 volt solar array with SLA battery. I was thinking of a power regulator but couldn't figure out the cut off part. if anyone has aschematic it would be greatly appreciated

 

[BobK]

NiMH batteries are the most difficult type to charge correctly. A good charger will detect charging is complete by either detecting a rise in temperature or a drop in voltage. An overchange will cause the battery to vent and damage it. I have made one that charges at a constant current of 1C and detects the voltage drop, which is about 8mV per cell, so 32mV in your case.

A simple charger can charge it at 0.05C, taking abot 24 hours to charge.

If you are always charging from a known state, you could use a constant current source and a timer to end the charge.

Here is a good site for information:

Battery University


Bob

 

[(*steve*)]

which is about 8mV per cell, so 32mV in your case.

I've always wondered (but never really bothered to research) how this 8mV drop manifests itself.

Does it occur relatively quickly? Is it maintained until well into the overcharge? And presumably if you have a set of batteries in series (and they have slightly different states of charge) you will see a series of 8mV drops?

 

[poor mystic]

I'd probably get a commercial unit from a hobby shop, but then again I find myself preferring guaranteed results over my own designs. is there a sheepish emoticon?

ps It sounds to me like you can't charge them in series at all, if this 8mV drop needs such careful monitoring.

 

[donkey]

what about NIcd? are they any easier to charge?
The only reason I am building my own is I am making a few battery powered lamps that will be charged via solar and I don't want to have to unplug and replug the items in and out

 

[KrisBlueNZ]

Temperature rise is the best way to detect end of charge. The voltage drop method doesn't work reliably below 0.5C charge rate. And you need to monitor the battery temperature anyway so you don't charge them outside the recommended range. It doesn't take much to overcharge them, and they must use a special type of magic smoke, because it smells awful!

I designed a NiMH pack charger but I can't post the schematic. In any case, most of the smarts is in the firmware. I based that on the behaviour of the Maxim DS2715, which is described well in the data sheet. There are dedicated charge controllers available from other manufacturers as well. Look for battery charge controllers on Digikey or wherever.

 

[poor mystic]

It sounds a tad problematic in the extreme temperature ranges to be encountered by the average Australian battery set, especially in direct sunlight.

What will you use the lamps for, Phil?

 

[KrisBlueNZ]

Right. NiMH cell temperature should never exceed 50 deg.C, maybe lower (see the cell's data sheet).

 

[poor mystic]

So what about 6V SLA's?

 

[KrisBlueNZ]

Temperature affects all batteries, because it affects chemical reactions. I don't know offhand what the requirements are for SLA; it will be described on the manufacturer's data sheet. I know their capacity drops significantly at lower temperatures.

 

[donkey]

mark I got kids that have no idea about turning of things like power hungry old plasma TV's. I am going to be giving thema battery powered lamp each and removing their lights... I am mean but I figure its a great way for them to learn to turn things off to save electricity. if they leave it on they have no light after a very short period of time lol
I was looking at SLA's but the problem is weight. getting it down to the charger every day will mean more moaningthan its worth lol.I might reconsider it though if my original plans don'twork.

 

[poor mystic]

Often, young people left in the dark turn to candles when the batteries run out. For this reason, I kept power on at one of our houses last year - I was fearful of fire.

 

[donkey]

lol well my kids are 17 and 19 and if I see either with a candle, lighter, matches or anything else flammable I have apowder fire extinguisher to use on them..... I mean the fire.... yes the fire

 

[poor mystic]

Better make a good lamp set then! Why not try and copy a commercial solar power installation, using 2nd hand parts?
I suggest this because I think it's often the cheapest option.

 

[donkey]

yeah I got an idea on the setup
but kris and I are discusiing a few things, like if I have the one panel (its 2 panels 200watt each) how can I charge the SLA batteries that I introduce daily?
so letssay I have 3 batteries from inside the house that need charging, all will have different levels of discharge. can I hook it up to the charge controller or will I need to hook something up so it doesn't try charging the extra batteries while detecting the big one is full? this might cause and over charge issue which I don't want

 

[KrisBlueNZ]

Perhaps you can find a charger that supports multiple batteries and will charge them in sequence.

If not, you need a way to detect end-of-charge and switch over to the next battery. Luckily with SLA, the charging algorithm is simple and there's no history involved, so a charger can be switched from one battery to another and will automatically do the right thing. Chargers for other types of cells need to be reset when you change to a new battery.

You could detect end of charge when the charge current drops below a certain value, although you'll need to adjust that value as the batteries age because their on-charge leakage current will increase as they age. Or you could watch for no increase in terminal voltage over a certain period of time. You could use relays to do the changeover.

It would be much easier to try and find a charger that can do it automatically.

 

[Mongrel Shark]

I have had a lot of sucsess rejuvenating old Ni-mh and Ni-cad with simple self blocking oscillators. Got some really old Ni-cad 9v's off a mate, after 20 years in a multimeter. Would only hold 2v in one, 5v in the other. A few days on my Zenner tester boost circuit (basically a joule thief, with un-equal winds, that makes 400v from AA with low current) Used 2 flat AA's to fix the 2 9v's one has limited amps now, only delivers 100ma short circuit, but the Amp hours are pretty good, not much less than my brand new 9v rechargeable (I go through about 8 DMM batterys a week, in 5 meters, testing stuff) I have only charged Ni-mh a few times (don't have many), but some friends assure me they respond really well. They wont get hot either. Same for lithium.

The circuit I am using was designed to put a couple of hundred volts into a .2µf(ish) cap, for testing Zeners. so I used really thin wire (.2mm). so I could get the 400 turns I wanted on a really small toroid. It has lousy amp delivery, sometimes it brings the voltage of a battery up, and doesn't fill up the amp hours, if that makes sense. So I recover the voltage with zener tester, then put the amps in with my bought charger. Works really well. I will be doing a new one soon, with battery reconditioning and ordinary charging in mind. They work on any voltage too. I don't think they can overcharge stuff, but leaving one on would waste power. Not that they draw much.

There are a bunch of people on youtube re-charging old stuffed drill batterys etc. They have to be really bad to not recover to some extent. A lot of people experience faster charging too. Although I have not been able to re-produce that. I need to try thicker wire on my inductor before I can confirm/deny.

Tons of pattens and research docs available on the net suggesting square wave pulses are the go for most recharging (the flammable kind of lithium cell may be an exception, I know 2 people have had explosions, although they where pushing. The newer kind of Lithium rechargeable seems to be a lot less explosive from the reports I am getting). Freq is important. No one seems to know why, but some batteries like certain freqs better. I think its partly plate spacing, and partly to do with max current absorbance and partly recovery/re-gen rates. I could be wrong though.

I'd be inclined to have pulsed charging, and a pulsed LED output on the lamps. So 2 custom joule thief's. Although it might defeat your lesson teaching. I can get good light for a couple of days from a full AA, and have them just not go flat for dim light. The back emf from the inductor seems to excite the chemical recovery in the batterys, at around .5v, and they just don't get any flatter. just hover up and down by a few milivolts, a few times a day. Put one of those tiny solar cells from garden lights on the lamp, and make them charge their own lights. Go for homemade lead-alum-carbon batterys, and it will go forever. Like Lidmotors forever light and forever motor.... With the cells you have, I could make a ridiculously efficient lamp. If you wanted to make them go flat quicker, you could go for a high voltage output oscillator, and run CFL's for a couple hundred ma from you battery pack.