Battery Crisis

[blueroomelectronics]

It's probably fine. PA is pretty neat, bought my digital calipers there.

 

[bloody-orc]

It's probably a "stupid" charger then. you must make sure that the AA batteries you are going to charge are as empty as possible to avoid memory effect (that NiCad batteries have). also you must make/have a timer to make sure you wont overcharge those little bastards (or undercharge).

 

[Omar.M]

So every time I want to recharge the batteries, just hook each up to a motor until it is almost drained THEN recharge?

The timer shouldn't be much of a problem, I may need to calculate exact times though.

So other than this memory effect (which should not be a problem if I nearly drain the cells each time and do not under/over charge), is there any other problems with these NiCad batteries?

-Omar

 

[mneary]

Modern NiCads shouldn't have a memory effect prominent enough to be worth all that effort.

IMHO, the risk of damaging the battery by reverse charging one cell in a pack is greater.

 

[Omar.M]

Oh ok. Thank you VERY much .
I think I will go with the NiCads then.

Bill discussed this, but it seems that there is no LDO voltage regulators at Active Electronics (my only electronics supplier around here).
So for the LM7805 regulator I currently use, I do need an input voltage of above 7.5V.

Bill said he connected the PIC directly to the power supply (with a diode for the ripple regulation). I don't think this will work for me, 6V-- isn't the PIC made for just 5V?

Are there any solutions? I am thinking of giving them a call and seeing if they have any LDO voltage regulators in stock (that are not listed on the website).

Omar

 

[blueroomelectronics]

NiCAD & NiMH are 1.25V per cell. 4x1.25=5V

 

[Omar.M]

AYYEEE! Me being stupid again, I just overlooked that. I thought they were 1.5V like the alkaline rechargeable batteries.
Silly me, thank you sir!

 

[Nigel Goodwin]

NiCAD & NiMH are 1.25V per cell. 4x1.25=5V

Their nominal voltage is only 1.2V, so only 4.8V for four in series - as with any battery, voltage will change with degree of charge or discharge, but is nominally:

NiCh/NiMh = 1.2V
Lead Acid = 2V
Normal non-rechargeable = 1.5V

 

[bloody-orc]

for my experience the recharger batteries have 1.36V in them (NiMH at least). this is on the very edge of the PIC voltage range (5.44V) so a 0.7V drop on a normal 1n4148 diode makes it all good luckily you have some power hoggers on the circuit (motors) so the voltage will drop pretty fast back to the normal 1.2x V and you get your 4.8V per pack again. It would be better to use a PIC that takes anything from 3.3V to 5.5V so the pic will not go bananas when the voltage drops too low. might also invest in a brown out detector (if you don't have one built in the pic); the solution for the bananas problem.

anyway good luck


PS yes the memory problem seems to be a little problem, but better safe than sorry you know especially for those cheap ones. they probably have that memory effect in them... (why else are they so cheap then ). and yes running a small DC motor for some time will do the discharging. you could actualy also make a discharger for faster work, but that's not a must.

 

[mneary]

PS yes the memory problem seems to be a little problem, but better safe than sorry you know

Memory effect takes hundreds of cycles to even become noticeable. If you reverse a cell's polarity even once it is gone!

The cells are probably cheap because they come from a batch that don't meet spec in some way, such as meeting the advertised capacity, integrity of the envelope, etc.

 

[William At MyBlueRoom]

NiCAD are often 800mah where as NiMH are much higher current capacity 2600mah is not uncommon.

 

[Omar.M]

Hmm, well I don't know if you know this store, it is called Giant Tiger. Just some cheap stuff here, and most of the times it is not all that bad. The batteries that I saw had "For Solar Powered Lamps" thing on them.

Since I am not going to be a using a voltage regulator and expecting the stuff to run on the 4.8-5V batteries, do you think I will be spending more time recharging then actually testing the robot? Because if it drains too quickly, the batteries might go into the unusable output voltage range and so I'd have to discharge and recharge.

So right now, is this my best solution... running the things off four NiCd batteries?

 

[mneary]

I think that four cheap NiCds is probably your best solution. The NiCds will stay near 4.8V until they are almost depleted.

As for spending time recharging, I suggest two sets.

You mention discharging as if it were a part of recharging.... I maintain that discharging is a colossal waste of time and effort. A "memory effect" is really uncommon in normal use. I have a lot of NiCds in use, for a long time, and have never experienced it.

 

[Sceadwian]

In 'normal' use yes memory is rare. I think Nasa had a big problem with it because the charge cycle occured at exactly the same point in a batteries discharge state and it eventually developed a voltage dip at that point. NiMH's and Nicads alike will have a similiar problem with a generic charging circuit in things like cheap cordless phones. Because a lot of the time they're only used for a very brief period of time and then thrown on a charger again, these chargers are seldome fit to know the cell was only depleted only a fraction of a percent of it's charge and they tend to constantly trickle charge the batteries, which is just as bad.

 

[Souper man]

I'm running the Mongoose directly off the 4x NiMH batteries. I have a 1N5819 diode to avoid reverse polarity for the PIC. Works fine. No Need for 9V.
The only 9V rechargeables I could find were NiCAD @ 250mah.

O GOD!

Cadmium is seriously bad for environment, although its already pretty messed up.

Good example is lake erie in the 1970s lol!

 

[blueroomelectronics]

Well I don't like NiCAD, I use NiMH when I can find them.

 

[Hero999]

The best deal I've seen for NiCad batteries is from Maplin
A real NiCd D cell 5500mAh for just £2.98!
**broken link removed**

 

[Sceadwian]

Nicads aren't as energy dense or light as NiMh's but they're much more robust. Some RC fanatics still use them because they source current better. That's a very good price, especially for a D cell, I'd snap some of them up. Great for high current portable supplies.

 

[mneary]

If you can afford the size and weight of NiCd, then consider gel-cell (Sealed Lead Acid). 20% larger, 30% heavier, less than half the price.

D-sized 5500 mAH, £2.99 x 5 = £15, 4.8 oz x 5 = 1.5 lbs.
6V 4.5 AH Gel cell, £5.99, 0.9 kg.

 

[psecody]

Wow I'm supprised no one has mentioned Li-Poly batteries yet, why is that? Are they not as good for robots? (Well besides the fire hazards and stuff) Because I use these for micro airplanes and such and they seem like they would be ideal for small lighter weight robots.