When are NiMh cells at full charge?

[Simen]

I've just built a small, but fast solar charger for AAA/AA size NiMh cells; using what i had around.

Each round solarcell are 1V, 800-900mA, and the battery holder contains no electronics; not even a blocking diode, and the two batteryslots are in paralell.

(The Volt-meter are scaled 0-1.6V; 0.1V step, and the Amp-meter from 0-800mA; 50mA step.)

Those two battery-cells in the picture are 1.2V - 1800mA, and when i insert those when depleted, they draw around 600-700mA for 5-6hrs, then it slowly drops towards 300-400mA with a end-charging voltage closer to 1.5V.

Now i wonder: Is there a certain way to tell when the batteries are fully charged, just by watching the voltage/current with NiMh batteries? (Without any more electronics)

[Bruce S]

Simen;

  A short answer is: Not really an easy way to tell. NiMh along with NiCds will hold the voltage until just about completely drained.

NiMh can use the same style of charging that NiCds use, but have a much much smaller negative voltage dip.

Without a blocking diode, once the sun goes down, the batteries will drain back through the cells, kinda like putting them in circuit with a windmill without a blocking diode, turn the 'mill into a motor :-)

Find a schokty <-sp? diode and put one inline, the voltage drop (loss) is lower then normal diodes.

What you're seeing with the full out charge of the batts is correct, they are better off with constant current charging and will take ALL you got until they start to fill up.

Normally a C10 is more than safe enough to charge these.

The way you currently have these setup, you're feeding ~1Vdc along with say 800mA, this will be "okay" , however as the summer gets on, they could burn (overcharge).

I would be better to hook them up in series, they'll hold the voltage down enough, and 800mA is far more than enough to charge them.

Those are nice cells you have too.

Keep Smiling;

Bruce S

[Simen]

Thanks for the comment.

I'm aware of the drainback, but i have no Schottky diode right now, and with a normal diode, i wouldn't reach top charging voltage with only 2 solar cells. (Actually 4pcs. 0.5V 'halfmoon' cells in series; total 2.0 Open Voltage)

So for the time being, i'm just watchful. I'll never leave the batteries in the charger overnight, and i time the charging manually for now.

I've read a bit about NiMh cells, and what i understand is, that NiMh cells can actually handle C1 charging (even more), but time and temperature needs to be watched closely. At end of charge, NiMh cells will have a sudden temperature rise, which will be harmful for the cells over time. Also, NiMh cells should manage a charge voltage of 1.8V

Without any chargecontrol (Sensing temperature rise, and/or Delta Voltage), i'll  just perform a quick mA/mAh calculation, and keep an eye on the watch and the mA-meter.

I was just wondering if anyone had some experience with a end-of-charge voltage (not resting) for a single NiMh cell. My experience are a top charged 'resting' voltage at 1.370V

[ghurd]

My charge rate is lower. C/5 or C/10.

I just take them up to about 1.42V for a while and then change them out.

They hit about 1.30 or 1.35V and then voltage takes a long time to creep higher.

Seems to take forever to go from 1.40 to 1.44V.

They may rarely get a 100% charge but are none the worse for wear.  Still using the same AAs I used camping in 2005 (couple pics in my Diary).

I know I never over heated them!  

G-

[Bruce S]

It seems you have it under control, and understand them well.

Like G- said, real resting voltage displayed on our "tester" shows 1.42 at full test , I've seen the unit push them up to 1.47 during deep or Prime tests, but not any have I seen them pushed to 1.8V, at that point they'll be venting, especially the NiMh ones as they generate more heat as they charge then NiCds normally do.

The neat thing is that with your sweep meters, you can actually see when the longer period of max voltage is coming up.

When they are depleated and you insert them into the charger, as you noticed they take the full 800mA when they reach 1/2 full then the chemistry comes into play and forces them to not be able to charge as quickly, each time they get 1/2 fuller they will normally cause the incoming current to drop by 1/2, unless of course your setup allows it to be overcome.

This is where heating and venting comes into play.

They can safely take a full 1C but with your arrangement a timer should be fine, also since this is an open air charging you can get away with higher charging versus in-device charging. You should get more than 500 full cycles charging with this setup.

Any broken lawn light around? they're great for finding those pesky diodes

Keep Smiling;

Bruce S

[Bruce S]

G-

If I remember those pics, you are certainly correct, heat was NOT going to be a problem

When's the next  trip planned?

Bruce S

[Simen]

Thank you all for replies.

Now, the 'resting' voltage i have, are measured a couple of hours after they're charged. I've just charged a couple of 800mA AAA batteries now, and they're also resting at 1.370V

I take the batteries out of the charger when they're close to a charge voltage of 1.50V; it usually corresponds to 'cell mAh/Charge mA=hours charged'

Regarding heat; i've tried to let the batteries sit in the charger 'overtime', but i can't see to get any increase in heat. I guess the wind are to blame.

Alas; no solar gardenlight here...

Those two solar cells i'm using comes from two 'sunvents'; solar driven ventilators for boats/rv's etc... One of them, i shovel'd off the roof of my caravan one winter; it was 'hidden' beneath 2 feet of snow. (Forgot it was there... ) The other one i sacrified off my boat, replacing it with a passive ventilator.

I think i stick with my routine; i feel better now, now i know i'm in the ballpark.

[stop4stuff]

That's a nice looking little charger you've got there Simen.

The 2100 & 2300 mAh NiMh AA cells I use top out at 1.45v per cell when fully charged. They're usually charged up with a NiMH mains charger with the input rated at 12v 500mAh from the step down wall wart.

[Simen]

Thanks; if not estetically nice, it works nice...

Yes, if i measure the cells fresh out of the charger, they're around 1.45V; letting them rest for a couple of hours, they're down to around 1.37V.

I read somewhere that the self-discharge is 5-10% during the first day for NiMh; then 0.5-1%/day...

[ghurd]

July? August?  South to KY for a week on a house boat.  :/

At least the sun will be out more often.

G-

[Bruce S]

Simen;

  I've read that too, but have not seen this to be true in real life. The resting voltage reads about right.

I have a few of the higher density NiMh 1500,2000 & 2500s. I've let them sit after charging for week on end and had no noticable loss in use.

I actually prefer the NiCds for cameras as they have the ability to recharge the flash quicker than NiMh. I do realize that the Cd in them is horrible for humans, land, etc. However, I also believe that with proper recycling these both can be very viable altternatives to Lead based units.

Li-ion does seem to be the to go, for a ton of reasons, but way waste what we already have out of the ground.

Down off the soap-box now.

Keep smiling;

Bruce S

[Simen]

Maybe your NiMh batteries are of the newer 'Low Self Discharge' kind?

http://en.wikipedia.org/wiki/Low_self-discharge_NiMH_battery

Mine are not. Now, approx. 24hrs after full charge, they've dropped from 1.370V (measured 1hr. after charge) to 1.350V. That's only a 20mV drop. For my use, i do not worry about that.

My electric lawn-mower are powered with a 36V, 2.6Ah LiIon battery. The battery weighs around 1kg (2 pounds?), and it keeps the engine at constant speed/power all the way until it's discharged. Just long enough for me to finish my 200sqm lawn. Charge time are 1hr for 100% charge; 1/2hr for 80% charge.

(One have to use the supplied charger with LiIon batteries, so it goes without saying that i connect the charger to my solar-bank via an inverter... )

So yes, i do like the LiIon technology, except for the price...

I do agree with you regarding stuff already dug out; let's use, and re-use it over and over and...