What is the trick to replacing rechargeable batteries please?

No real tricks, just a few techniques that can make battery pack building/rebuilding easier.

Don't reuse old cells or the old welded connecting strips for new battery packs. If the old cells have leaked, the pack connector copper leads may also be damaged if the leakage has reached the ends of the stranded copper leads. The liquid that's leaked out of the cells can wick up into the stranded copper leads. Replace the pack leads with the same gage wire (or larger) if any damage is visible.

Choose new cells of the correct physical size and reasonable capacity, fresh, flat top cells with soldering tabs already attached. Solder the tabs to connect the cells together, not wires or other material directly to the cells. Some dealers will add tabs to cells for free.

Cells with tabs used to be made with the 2 tabs oriented 180 degrees apart (looking down at the top of the cell, one tab points left, the other tab points right). The cells I purchased recently from 3 different sources have the tabs both pointing in the same direction. Lay the cells out side-by-side in a line with alternating + & - tabs adjacent to each other, and all the tabs pointed upward. To get the tabs to meet and overlap, make a 45 degree bend close to the welds, which changes the direction of the tab to 90 degrees. By making the 45* bend close to the weld, the tabs won't stick out past the sides of the cells.

Clean the tabs and tin them with solder using flux.. as they overlap, tin the top of one tab, and the bottom of the other. With the cells neatly hot glue tacked together, apply an additional small amount of solder to the area where the tinned tabs overlap, and press the tabs together with a tool until the solder cools. I place a small square of fiberglass tape under the joined tabs as extra insurance so the + tabs can't short circuit to the cell case (even though there is an insulating washer under the plastic skin).

If the original battery pack utilized a thermal protection device, one should definiely be installed in the new pack. Many times a thermal switch is marked with the temperature rating. Temperature rise during charging can be detrimental to the useful life of the battery pack, and also to the capacity of the cells. Thermal protection switches with leads can be soldered to tabs of the cells. The polyswitch types of devices should be spot welded directly to the cells. Any other components (diodes, precision resistors) that were part of the original battery pack should be checked for normal values.

Connector leads can be soldered to the appropriate tabs, and shrink tubing over the soldered connection and the end of the lead will offer some support/strain relief for the leads.

There are dealers that also sell the shrink wrap to cover the cells, for making a neat and secure battery pack.

A few simple guidelines will prevent most problems. Don't solder directly to cell terminals. Don't allow cells to get hot while charging. Don't overcharge battery packs. One of the best ways to prevent overcharging is to use a manually reset timer to limit the amout of time for charging. Don't over-discharge battery packs. When the device/tool starts to run slower, stop using it and recharge the battery pack.

Many rechargeable consumer devices are supplied with very poor quality chargers (just a transformer and a diode was the cheapest method). If you can't get a reasonable service life from the battery packs, you might consider choosing another method of charging the battery, whether that means removing it to connect it to a better quality charger, or modifying the device with an external connector for safely connecting a better quality charger.

In some cases, it's just better to have more than one battery pack, that can be easily swapped by means of an external connector.

-- Cheers, WB .............

You

Perhaps. But the principle stands -- replace all the cells.

It's ampere-hour, not Ampere/hour. Big, big difference.

Cells with the same mAh rating from the same company nevertheless vary in capacity. I've had to return new cells for replacement because they were way out of spec.

You have it backwards. NiMH cells are usually pushed to a higher end-of-charge voltage than nicads. Charging NiMH cells on a nicad charger will, if anything, undercharge them.

Well, yes. I wasn't suggesting otherwise. The majority of cordless tools use sub-C sizes of different types and capacities - and of course number according to the voltage. If you can buy one on special offer it might be able to supply a set of cells to replace those in the faulty unit at a lower cost than buying individual tagged replacement cells. I've done just this a few times. For example, with the move to LI cells some are selling off their NiMH cells at discount. Same with NiCads.

Usually on the battery case or tool spec if not the individual cells.

Let me repeat this... The capacity printed on the cell is its nominal capacity. It is NOT the capacity of that particular cell. Individual cells can sometimes differ 20% or more from this value -- usually on the low side.

It's important to match the capacity of nicad and NiMH cells, because they have a very abrupt cutoff. This means that one cell can drop way low, while the other cells are at a voltage high enough to continue powering the device. If you let the device run long enough after this point, the weak cell will drop to zero volts, then reverse, possibly causing it to leak or explode.

"William Sommerwerck" wrote in news:hlocs4$nma$ snipped-for-privacy@news.eternal-september.org:

many NiCd chargers use a temp sensor to determine a full charge,and that will kill NiMH.

And you think makers of low end tools care about this?

Despite having used re-chargeable batteries from long before they became a domestic item, I've never known one to explode. But then I've always insisted on decent chargers.

Nicad chargers do not generally have temperature sensors, unless they're built into the battery pack. Furthermore, a NiMH cell can tolerate higher temperatures than a nicad so, again, a charger designed for nicads is likely to shut off too soon rather than too later.

I worked for several months at Microsoft Hardware. One of my projects was documenting Microsoft's rapid charger for its cordless mice. Though nicad and NiMH cells are more alike than different, the latter are usually pushed to higher charging cut-off voltages and temperatures. A nicad charger is not likely to damage NiMH cells.

"Wild_Bill" wrote in news:XWMfn.332226$ snipped-for-privacy@en-nntp-04.dc.easynews.com:

My wife & I each have coldless drills, which we use intermitently for household repairs & projects. The only way to make sure we have a charged battery handy for emergencies is to leave one in the chargers. Even though these are reasonably high quality chargers (Ryobi & DeWalt), I'm sure we are shortening the life of the batteries.

It occured to me that one option might be to put the chargers on a timer, so that they are on for half an hour a day. That should top them off, without cooking them. The catch is that I'm not sure what sort of load the charger presents when it is off. I could just end up discharging the battery through (at a minimum) diode leakage. As long as the leakage is low, that might actually be OK, ot it could produce a nasty memory effect in the NiCd's.

Any thoughts or comments?

Thanks!

Doug White

What does that have to do with the issue at hand?

Cells from the same production run are likely to have similar capacities. But this isn't always true.

If you are making up your battery pack, you should break in the cells and measure their capacities, assembling the batteries from cells within a few percent of each other.

I've never had the problem, either. But the charger has nothing to do with it. I was talking about discharge (see above).

The safest way to handle a multi-cell battery pack is to run it until it just begins to show a drop in performance, then charge it. This reduces the chance of any cell being driven into reversal.

The more cells in the battery, the greater the chance of reversal.

You probably are. I would not charge the batteries until they began to run down, then recharge no more than four hours (or whatever the manual recommends as the maxium charging time).

I own an inexpensive 12V Ryobi drill, which gets similar intermittent use. The charger /does not/ shut off automatically. Indeed, there is nothing to indicate that the charge is complete! I am very careful about when and how long I charge it.

The battery does not quickly self-discharge. I've never had a time it wasn't ready. So, after each use, I'll recharge it for 15 minutes or so.

A 12V battery pack has eight nicad cells. It's important not to repeatedly run down the battery, as you're increasing the chance of reversing the lowest-capacity cell in the pack.

A half hour a week -- or even a month -- would probably be enough.

That would be several hundred K.

"William Sommerwerck" wrote in news:hlosrd$bog$ snipped-for-privacy@news.eternal-september.org:

my 9.6V Makita stick has EIGHT cells,wouldn't a 12V pack have TEN cells?

I've often run re-chargeables flat - although not intentionally. There'd have to be a *vast* imbalance before one blew up. And indeed in practice the cells don't all fail together, so even the very best makes will exhibit just the effect you're talking about when old.

Yes.

I've not actually experienced it. So put it in the same sort of pigeon hole as memory effect. ;-)

"Jim Yanik"

Temp detection is the MAIN way of detecting the end point when charging NiMH cells.

The PROBLEM comes with NiCd chargers that use VOLTAGE peak detection - cos NiMH cells never drop voltage like NiCds do.

.... Phil

"William Sommerwanker"

Chargers that are looking for the battery voltage to peak then come DOWN will NEVER stop if NiMH cells are being used instead of NiCd.

..... Phil

Sorry, Phil, you've got it backwards.

NiMH cells can show a meaningful voltage drop at the end of charge. In general, the harder the cell is charged, the greater the drop.

If you can place a thermistor on or near the NiMH cell, detecting the temperature rise is usually the easiest and cheapest way to terminate charge. This is particularly true if you're not hitting the cell hard and don't have a big enough minus delta V to reliably detect.

Chargers that use voltage detection usually also use temperature detection, as a belt-and-suspenders safety feature. One exception is the MAHA C9000 which uses only delta V (as far as I can tell), because it's hard to read the cell's temperature through the plastic base of the charger.

Why don't you think before yelling?

NiMH cells can be rather brutally charged without damaging them, at a rate that most nicads would not tolerate. (MAHA recommends at least 0.5C for its NiMH cells, but I grew up with nicads, so I tend to set the charger between

usually

Again, you have it backwards. Perhaps, you brainless ignoramus, you might like to discuss this with the engineers I worked with.

You're an object lesson in the dangers of discussing something before being certain you're sure you know what you're talking about, in "open mouth, insert foot".

Why do you think I should be afraid of you, you pitiful excuse for a human being?

"You're nothing but a pack of cards." -- Alice

The thing to take away from this thread is that EVERYBODY knows exactly what to do, they just don't agree...violently!! Be VERY CAREFUL acting on ANY information from this thread.

You MIGHT get more reliable info from sci.chem.electrochem.battery

There's a reason that the label says, "don't open this battery pack." This thread is why...