[just_lkn]

I should know the answer to this one... but I don't... ...I picked up 2100 MAH NiMH Batteries and plan on glueing together a few battery packs for my receivers and transmitter.

Should I charge these batteries individually, before I assemble the pack. Just to sure they are topped up before I do all this soldering. Or simply connect the require amount in series and charge as one lot.

[DanKitching]

You should definitely charge them all individually (back to back, with the least time possible between each) before building the pack.

Dan

[Gary Maker]

Actually I just assembled several NiMh battery packs this evening. I do not charge the cells individually first. When you purchase a new pack, it only comes with a residual charge, which means that the cells have never been charged before assembly! New packs must be charged from 20 to 24 hours the first time to "form" the pack, then thereafter, charge as you normally would!

I made up a jig to hold the cells square and use thin ca to hold them together permanently, then solder the tabs and the leads on. Next I shrink wrap the pack. THEN I charge the pack. I have been doing it this way for years and have literally sold dozens of packs! I have never had one pack returned defective....ever! It works for me!

Just a quick reminder if soldering cells without tabs. Use a hot, quick blast of heat from the soldering gun on the "Positive" end of the cell so as not to soak in too much heat, which could possibly melt the vent seal!

All the best with the project.

[DanKitching]

My bad, I saw 2100mAh and thought LiPo.

[Tomahawk]

That would take a long time if you had to charge each nimh cell individually at a 1/10 c rate and then solder them together.

Especially if you were building a 20 cell motor pack.


Chris

[Wayne MIller]

Hi Jst_Ilkn,

This only refers to NiCd and NiMh batteries NOT LiIon or LiPo batteries.

The ideal situation is to have all the batteries at the the same voltage after charging the first time. To get them there, we usually do a "forming" charge, this is a low ma charge for a longer period of time to allow all the batteries to come up to the proper voltage.

If they are not at, or near, the same voltage after the first charge, the low cell may never get fully charged. This is due to a "smart" charger averaging the voltage for all cells and it may determine the pack is charged, even though one cell may be a little low (the others a little high).

This is hard to explain, but I'll try. If one cell is low when you start to use it, it will still use the same number of ma out as the other cells. This means its voltage will always be slightly lower than the others when being used, then, when charging on a "smart" charger, all batteries will receive the same number of ma in untill the "smart" charger averages the voltage untill it figures the pack is fully charged - the low cell may never get up to the same voltage as the others due to the averaging of the voltages.

A simple way to form (or balance) a battery pack is to charge a completed pack at a low charge rate for a little longer than a normal charge time. The low charge rate should be low enough as to not damage any cells that reach a full charge before the others, that is usually 1/10 the ma rating of the battery max.

The reasoning is a low current charge (1/10 max of the batteries rating) can be left on the NiCd and NiMh batteries longer without any damage. If there is one low cell, the other cells will not be damaged during a low current charge, and it allows the low cell is still coming up to proper voltage.

Remember, a fully discharged battery will take 1.5 times its ma to fully charge. It should be noted that usually the batteries are never fully discharged, in fact, you should never fully discharge a NiCd or NiMh battery, they may never charge again if you do.

An example of charge times from fully discharge (remember it takes 1.5 times capacity in ma for a full charge): a 1000mah pack will take 10 hours at 150ma or 15 hours at 100ma for a fully discharged battery - you can work out the different rates and times for your charger.

Most packs will charge OK after they are soldered together with no problem. The easiest way to check is to not put heat shrink on until after the first charge (which should NOT be a quick charge) and measure each cell. If all cells come up to the same voltage, then everything is fine. if not, continue a slow charge (max 1/10 of their ma rating) untill each cell is fully charged - an overnight wall charger (nicknamed "wall wart") is better than a quick charger for this.

This is as simple as I can put it and I hope it isn't too confusing.

I hope this helps.

Fly4Fun,

Wayne Miller

[Vulcan1]

Gary,
Just to let you know what I do with my packs also. I form them in a square but I use a little Goop to hold them on the inside then shrink wrap after charging. CA glue also works but it is reccommended also for mounting carbon probes on a sinker EDM. This means it is very conductive so if somebody gets to much on they could have a short or an accelerated discharge.
Not to take anything away from you as that is how I did mine for quite awhile until I found out from other sources about the CA being conductive.
John

[Graham]

Hi Gary..

Is the "Cold Heat" gun you have "Like seen on TV" ok for that type of soldering?

Graham