Why don't nimh's need balancing?

[pinkpanda3310]

This is probably a strange question but I've not had much to do with nimh's. When I got into the hobby a few years ago, the very first run I blew the esc and didn't realize until I drove home and unplugged the batteries. So due to being a complete noob I killed two birds with one stone- esc and batteries (nimhs).

Since then I've gone brushless and lipo (barr the tx batteries). I have a few rechargable AA's that get used in various things including a toy for one of my daughters which takes 3 AA's. So I often have an odd number of cells to charge. I picked up a battery tray that takes 10 cells and was going to modify it so the cells were wired in parrallel so it didn't matter how many cells I had to charge. Plus I don't have to worry about how much charge is left in each cell.

All this caused me to think of the above question and what do you think about the parrallel tray idea?

[What's_nitro?]

So long as they are even to start with, I've never heard of Nixx cells going out of balance.

+1 to the parallel tray idea.

[pinkpanda3310]

That's the thing there not all even to start with 'cause they're coming from different devices. I thought the parralleling would eleviate any problems caused by this. Thanks for your input Nitro.

[_paralyzed_]

No no no. NO.

Serious racers do balance their nimh's. More expensive nimh's come in "matched sets" meaning they all have similar internal resistance and overall capacity.

Charging in parallel will guarantee the same voltage for all cells, but it is impossible then to vary how much of that voltage goes into the battery.

Say one AA battery is 500mah, another is 1500mah. Charged for 1000mah will cook and blow the first battery, and leave the second battery weak at only 2/3 the charge.

NO PARALLELALLEL TRAY

[_paralyzed_]

Especially if they have different amounts of capacity left. That would just compound the problem. (each battery requiring a different amount of the set voltage)

[pinkpanda3310]

Sorry I should have been more clear. They are all the same make with the same mah rating. They just have varying voltages at the start of the charge.

From what you say I understand parrallel charging nimhs is different to parrallel charging lipo. I didn't know that.

[What's_nitro?]

You could make a passive balancer like this (see attachment) It's meant for AA or other small-ish sized batteries, hence the low current charge. It will, however, keep them in perfect balance every time. As is would require monitoring so that the cells were not overcharged, but a circuit can be added to shut of the charge automatically. I was going for simplicity with this one.

[pinkpanda3310]

That's great Nitro! So I could use that with a delta peak charger and... happy days

[What's_nitro?]

Those resistors will probably get hot... So if you have a fan to aim at it that would help. You could switch them out for 5W resistors, but they're much bigger.

Here are the links to the resistors if you need them.

0.75 Ohm
3.0 Ohm

[pinkpanda3310]

Space is no biggy so if the 5w resistors run cooler I will try that. Thanks heaps.

[What's_nitro?]

No problem. I used this design way back when NiCd was cool and I had a Nikko truck.

[slimthelineman]

novak smart tray is the best way. used them for years when i raced heavy on ni-mh. allows you to choose cel voltage cutoff in .1v increments, will dead short if you need it to and does each cell indipendantly. only bummer is you gotta use side by side packs to use the tray. can find em around for like 25 bucks on fle bay. wanna make your nickel packs last a long time its essential they be balanced BEFORE charging, otherwise sooner or later you will hear the dreaded pop! and your done.

[What's_nitro?]

Slim, you're right. But he's talking about smaller AA size cells for Tx use.

Back in the mid 80's when we raced with the "hot" 1500 and 1600 SCE, SCR NiCds we used to balance them. The hot ticket was the "Cell Mate" charger. Every month/end of season we'd dissassemble our packs. It would charge the cell and give you all the readings. Hi/lo/avg/disc rate, etc. Keep notes and build your packs according. The "notes" were like the stickers you see on the NiCd cells these day. I still have a couple of theses packs and surprisingly one day I charged/cycled a couple and they did pretty good in my 'Pede (when it was brushed w/a Reedy) We used to time/gear for 6 minute races then too IIRC. Hell, when we raced "stock" class they stilll had fixed/crimped endbells. The only mods you could do was play with the brushes and springs.

[himalaya]

NiMH packs are self-balancing.

Being insensitive to overcharge, all NiMH cells in a pack can get fully charged just by slightly overcharge EVERY cell. i m sure you noticed all NiMH get warm at the last charging period, that's a sign the cell is full and is converting charging electric energy to heat instead of chemical energy form. This will ensure all cells are full/balanced.

Lipo doesn't have the capability to convert excessive electric charge into heat, they are strictly forbidden from getting overcharged. so a balancer is needed.

Being different from Lipo, NiMHs are not voltage terminated to detect the charge end point. All lipo from all manufacturer have a voltage threshold of 4.20V, but not Nixx batteries, their fully-charged voltage vary from cell to cell, and changes even kept untouched. so most chargers don't terminate Nixx charge by voltage, they do that by delta-V and/or temperaure. Parallel charge Nixx batteries can be practical but not acurate.