replacing Ni-Cd cells on kitchen appliance

[spacedog]

hi, My wife has a small dust-buster type handheld vacuum cleaner not unlike this:



...which was dying. it uses 4 x 1.2V 1700mah ni-cd cells

so I ordered some replacement cells, the closest ones I could find are 1800mah, but what the hell, a little extra juice right?

I wired them up and installed them, inside looks like this:



but mine has a 4th cell positioned near the motor as well as the other 3 in the handle.

it works great. but it's never fully charged using the original base charger. I had it in there for 2 days and the charge light won't turn off.

is the extra 100mah messing with the cut off circuit on the charger? or do new ni-cds simply need more time to charge?

[T3sl4co1l]

Charge just means "it's charging"... you're deluding yourself if you think cheap Chinese crap is going to have a charge controller in it!  Bet you dollars to donuts.

I have a $20 Harbor Freight "18V" cordless drill... all NiCd, and the charger is just a DC wallwart, 30V O/C, "400mA" (I haven't measured if it actually delivers that while charging), and the charging base is a power LED and a 'charge' LED driven by a transistor that merely detects when the battery is connected.  No charge controller, no temp limit, nothing, just 400mA all the time.

Leaving it in the charger is the 100% worst thing you can do, because the cells overcharge, overheat and eventually dry out or fail shorted.

On the plus side, my drill's battery pack is easy to work on (it's a molded ABS case held together with screws, no potting or anything), and I was able to "zap" the one shorted cell, and recharge the others which were low.

The healthiest thing you can do is use the device regularly; charging to less than 100% full and discharging until the weakest cell drops out, and repeating until capacity is back up where it should be.  Which sucks, because you have no way to tell when it's fully charged..

Tim

[IanB]

the cut off circuit on the charger?

Right, as T3sl4co1l says there is unlikely to be any "cut off circuit" in the charger for such a device. If you leave it on charge the charger will just pump current through the cells until they are dead.

This good thing is that you can get an idea when the battery is fully charged, though. Since the cells are in the handle you can just feel the handle and wait until it starts getting warm. Once the handle warms up the cells are rejecting the excess charge as heat and you can unplug the charger.

To get maximum life out of something powered with 4 NiCd cells in series, make sure to recharge it as soon as there is the slightest sign of the motor slowing down. If you run it until the motor dies you will subject one or more of the cells to reverse bias and that is the surest way to damage a NiCd cell.

[spacedog]

I've not taken the base apart, but the with old batteries the light turned off when it was charged, worries me that it's not going off with these new cells, the cells don't get hot while charging, but they do get a little warm while discharging (the motor runs them down in 5 mins, can't compare to the old batteries as it had not much life with them) it's Phillips brand cleaner, they usually make decent stuff and are not considered cheap Chinese crap right? Anyway, I'm not used to using these kind of cells and thought I'd ask you guys.

I'll measure the draw tomorrow., see if it drops off or if I'm just killing off the new cells..

[IanB]

It's maybe harder to do now, but the best thing is to test new cells before installing them. You can check that you didn't get any duds (not uncommon with aftermarket cells), and you can check that they are reasonably well balanced. You can also balance and condition them before installing them. For instance, if you need four replacement cells you would buy six and pick out the four best matched ones.

How are the cells installed? Do they have welded on tabs? It is really important, if soldering, to solder to the tabs and not to the cells themselves. Applying excess heat to the end caps of the cells can easily damage the internals (there is soft plastic beneath that melts easily).

To check the cells on arrival, measure the voltage before doing anything else. Reject any cells that measure below 1.0 V on arrival as faulty. A good cell should measure from 1.0 to 1.2 V or more when delivered from the factory.

Next, condition each cell individually by charging it constant current at a 0.1C rate for 16 hours. Let it rest for an hour and then measure the voltage. If you have a suitable power supply you can put all four cells in series while doing this. After that charging process four good cells should have matching voltages to within about +/- 0.01 V.

Next, discharge each of the four cells individually at about 0.2 C (e.g. use a 10 W resistor of 3 to 5 ohms). Measure the discharge time down to 1.0 V for each cell. Ideally this time will be consistent for each cell to +/- 10%.

You may think this seems like a lot of fussing about, and maybe it is. But it is the only way to obtain a true understanding of what is going on with these cells.

[Alex Eisenhut]

Charge just means "it's charging"... you're deluding yourself if you think cheap Chinese crap is going to have a charge controller in it!  Bet you dollars to donuts.

I have a $20 Harbor Freight "18V" cordless drill... all NiCd, and the charger is just a DC wallwart, 30V O/C, "400mA" (I haven't measured if it actually delivers that while charging), and the charging base is a power LED and a 'charge' LED driven by a transistor that merely detects when the battery is connected.  No charge controller, no temp limit, nothing, just 400mA all the time.

Leaving it in the charger is the 100% worst thing you can do, because the cells overcharge, overheat and eventually dry out or fail shorted.

On the plus side, my drill's battery pack is easy to work on (it's a molded ABS case held together with screws, no potting or anything), and I was able to "zap" the one shorted cell, and recharge the others which were low.

The healthiest thing you can do is use the device regularly; charging to less than 100% full and discharging until the weakest cell drops out, and repeating until capacity is back up where it should be.  Which sucks, because you have no way to tell when it's fully charged..

Tim

I use my Imax B6 universal charger to charge my drill's battery, since the supplied charger is indeed just a wallwart and a resistor.

It's a nice business model; the charger reduces the battery useful life, they sell new batteries for like 5$ less than a new drill...

[flolic]

it works great. but it's never fully charged using the original base charger. I had it in there for 2 days and the charge light won't turn off.

I have exactly the same vacuum cleaner as pictured, with 3 cells. I replaced dead NiCd cells with NiMh ones and it work great since. Charging light also never goes off on mine, but I am quite sure it was always like that...

[G7PSK]

I have come across some cheap rechargeable units using NiCd 's that simply passed the charging current through the led, as the charging current dropped the led would dim and then finally go out unless you looked at it in complete darkness then you could see a dim glow, is that what you have going on here if so an increase in battery capacity might mean that the charge voltage never makes equilibrium with the battery voltage. 

[spacedog]

ok, this was bugging me again, so I opened it again as well as the base station, the base station is completely empty, the DC wall-wart goes straight to the 2 spring loaded contacts. the circuitry in the device itself is exactly as describes above, a resistor/LED/switch. I guess I was mistaken assuming that there was some intelligent charge circuit in play there. Thanks for the input as always, you guys rock!