# Have Discarded Battery Powered Vacuum Cleaner, Will Make NiMh Launch Battery

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#### brockrwood

##### Well-Known Member
TRF Supporter
Found a discarded “Shark” brand battery powered vacuum cleaner near my home.

No charger. Most attachments missing. So, what to do? Get the battery out, of course!

@FrugalGuy

The battery is a 10.8V, NiMh (9 cells) pack rated at 2,100 mAh. Nice.

Hooking up the voltmeter showed a charge of 11.8V. Hmm. Already charged to over the “nominal” voltage rating of the pack of 10.8V (9 cells times 1.2V per cell).

So what is the proper charge voltage for a 10.8V NiMh battery pack to get it to 100 percent charged?

A Google search turned up answers all over the place. Some people said charge it at 2.8V per cell.

What?!!

So I pulled out my trusty home-built battery charger. It is based on a circuit in the LM317 voltage regulator datasheet. See attached pic.

The 12 ohm resistor senses the current and turns on an NPN transistor to limit the current to the battery to 50 mA.

That’s a slow enough charge, right? I set the output voltage to 12.25V with the trim pot. I guess I could go higher, since the current limiting resistor and transistor should pull the output voltage down if the current exceeds .050 Amp. But I wanted to start out slow and cautious.

Once charged to whatever the 100 percent charged value is, this battery will become part of a simple homemade launch controller.

See the white wire? That is to sense the temperature of the battery. The original charger circuitry had some sort of temperature sensor that would stop the charge when the battery hit a certain temperature.

With a NiMh battery you can charge it at .05 times the amp hour rating, correct? This is a 2,100 mAh battery. So .05 is 105 mA. I am charging at 50 mA. That is only .025 times the rated capacity. That has to be gentle enough, no? Can I just leave it hooked up indefinitely?

Hmm. With output voltage set to 12.25V (no load) and current supposedly limited to .050 Amp, the output voltage at the banana plug sockets is 12.02V.

That means it’s charging?

Ok. I cranked the output voltage of the charger up to 12.69 volts (current limited to .050 amp). That should eventually get the battery up to 1.41 volts per cell. That ought to be a full charge.

1.2v per cell is the nominal voltage. Depending on the definition, a NiMH cell is dead when it drops to 1.0, 0.9 or 0.8v per cell. A NiMH cell fresh off the charger (after a peak charge) will be around 1.4v-ish.

As for how to tell if a battery pack is charged, what happens is the voltage will quickly drop.

So with your 10.8v NiMH pack, as it's charging, the voltage will gradually go up. Then, when the pack reaches a voltage of about 12.6 or so volts, it will suddenly drop a few tenths of a volt. That's when it's peaked and is fully charged.

However, note that the above info assumes you're charging at the correct voltage. Not sure how a charger decides what the correct voltage is, though. I presume it's the nominal voltage of the battery pack, but don't hold me to that. The above info also assume you have a battery pack with healthy cells that are comparable in terms of resistance, voltage and capacity.

As to the charging current, it depends on the cell. Most R/C hobby NiMH packs can be charged safely at 1C to 2C, no problem. Once you get to 3C, I think the packs may get fairly warm. It's fine to do this on occassion, but the pack won't last as long (in terms of overall life).

The supply voltage to the LM317 needs to be appx 3 volts higher then the battery voltage. I would recommend a 0.1C (210ma) charging scheme for 14 hours using this simpler current regulator circuit shown in the data sheet.

The supply voltage to the LM317 needs to be appx 3 volts higher then the battery voltage. I would recommend a 0.1C (210ma) charging scheme for 14 hours using this simpler current regulator circuit shown in the data sheet.
View attachment 617836
Like this?

Charging at 58.2 mA. That would be .0277C. A trickle. I don’t want to have to put a heatsink on the LM350 or use a higher wattage resistor (this is a 1 watt, 22 ohm resistor). Plus, if I accidentally leave it plugged in for a week, the battery can probably withstand 58.2 mA indefinitely, no?

The red wire coming off the black power socket is hooked up to the ring of the power plug from the wall adapter. It is connected to the negative rail of the breadboard. That is a bit confusing, I know. The reason that it is red, is because I stole this power socket from another breadboard project. In that project, the ring of the wall adapter power plug was positive.

Like this?
Must be. The current draw is correct for a 22 ohm resistor.

Must be. The current draw is correct for a 22 ohm resistor.
I read somewhere that NiCd cells are tougher and can stand overcharging when “full” more than NiMh cells can. Basically, when NiMh get to “full” the charging needs to stop. That is where “smart” chargers and chargers that sense the temperature of the battery pack are helpful.

I read somewhere that NiCd cells are tougher and can stand overcharging when “full” more than NiMh cells can. Basically, when NiMh get to “full” the charging needs to stop. That is where “smart” chargers and chargers that sense the temperature of the battery pack are helpful.
That's true, in that NiCd cells are more forgiving to non-peak charging in terms of overall cell life.

But almost no one charges batteries with a dumb charger. However, in some applications, it's probably not cost effective or practical to use a smart charger for a given battery pack. Think solar powered garden lights which have to deal with humidity, temperature extremes and inconstistent charging. In those types of situations, a dumb, trickle charger is probably about as effective as a smart charger in terms of overall performance and cell longevity.

What kind of current can it put out? I've had not-very-good results using NiMH batteries in my PSII controller; they didn't really do any better than alkalines despite the info I had found that suggested they should do quite a bit better. I didn't pursue it too hard becuase, well, I never rarely *use* my PSII controller because I never launch on my own. Mainly I do igniter tests once in a while.

What kind of current can it put out? I've had not-very-good results using NiMH batteries in my PSII controller; they didn't really do any better than alkalines despite the info I had found that suggested they should do quite a bit better. I didn't pursue it too hard becuase, well, I never rarely *use* my PSII controller because I never launch on my own. Mainly I do igniter tests once in a while.
According to the label on this 10.8V, NiMh battery (all I have to go on), this is a 2,100 mAh (2.1 Ah) battery. That means that, supposedly, the battery can deliver 2.1 amps of current for an hour. That seems ambitious to me. Still, this battery came out of a rechargeable, battery powered vacuum cleaner, so it probably does have a pretty high current rating, at least for short periods.

A replacement battery for this one sold on Amazon says it can discharge at 10C for short periods. That would be an impressive 21 amps! Other things I read on Interweb say typical NiMh max discharge rates are more like 3C to 5C.

If it'll do 10C that's very good. Try it on some Estes igniters and see how fast it'll fire them.

1.2v per cell is the nominal voltage. Depending on the definition, a NiMH cell is dead when it drops to 1.0, 0.9 or 0.8v per cell. A NiMH cell fresh off the charger (after a peak charge) will be around 1.4v-ish.

As for how to tell if a battery pack is charged, what happens is the voltage will quickly drop.
If it's dropping quickly, it's too late and you're in overcharge territory. At that point it's also getting hot in a hurry.
So with your 10.8v NiMH pack, as it's charging, the voltage will gradually go up. Then, when the pack reaches a voltage of about 12.6 or so volts, it will suddenly drop a few tenths of a volt. That's when it's peaked and is fully charged.

However, note that the above info assumes you're charging at the correct voltage. Not sure how a charger decides what the correct voltage is, though. I presume it's the nominal voltage of the battery pack, but don't hold me to that. The above info also assume you have a battery pack with healthy cells that are comparable in terms of resistance, voltage and capacity.
Not quite. Peak charging requires charging at fairly high rates....back in the NiCd days of electric powered RC we routinely charged at 4C, so 8.4A into this pack. The input voltage is really irrelevant as long as it's well above the pack's voltage as the pack being charged will regulate what's actually happening.

NiMH cells, when they came around, were found to be somewhat fussier in that they didn't have as sharp a voltage drop at the peak. Fast chargers that appeared after NiMHs were widespread typically had a smaller peak threshold (a few mV per cell) before they shut off.

How high the voltage per cell actually got under these circumstances was related to the internal resistance of the cells. For NiCds again, 1.7V per cell at peak was not uncommon. NiMH would be similar.

A pack will never peak at trickle rates or lower (< 1/10 C).

The OP should be able to charge that pack at 210 mA all day and all night with no issues.
As to the charging current, it depends on the cell. Most R/C hobby NiMH packs can be charged safely at 1C to 2C, no problem. Once you get to 3C, I think the packs may get fairly warm. It's fine to do this on occassion, but the pack won't last as long (in terms of overall life).
Agree with this, especially with respect to NiMH. 3C is probably as far as you want to go. Also, the heating happens almost entirely as it is getting close to the peak. So...a simple way to charge this pack might be to put 1C into it (so 2.1A) then pull off the charge current once it gets, say, 10-15 degrees above ambient. That, of course, will mean babysitting it as it gets near full though.

At .210 amp that would be a 5.952 ohms resistor. I^2 * R = .262 watts power dissipated by the resistor. So I have to use a 1/2 watt resistor. Or maybe a 1 watt resistor, to be safe.

The current through the LM350 is .210 amp. The LM350 is dropping the voltage down from 18 volts (the input) down to 12.35 volts to charge the battery. Well I guess the resistor is dropping 1.25 volts of that. So the LM350 is dropping 18V - 12.35 which 5.65 volts. But subtract the 1.25 volts dropped across the resistor equals 4.4 volts being dropped by LM350. Times .210 amp equals .924 watt dissipated by the LM350.

So I need a small heatsink on the LM350 and a 1 watt “current sense” resistor. I don’t have a 5.952 ohms resistor but I can get close. Maybe a couple of resistors in series.

So be it.

6 ohms is close enough.

The 50mA constant current charger charged up the battery. Let it run all day (about 12 hours). Final voltage 13.04 volts. That is 1.45 volts per cell. Might be overcharged.

Charging an old 12 volt NiCd now. It was down to 7.69 volts! Not sure it is any good but we shall see. Pumping 58.2 mA into it.

The 50mA constant current charger charged up the battery. Let it run all day (about 12 hours). Final voltage 13.04 volts. That is 1.45 volts per cell. Might be overcharged.

View attachment 619177
How long did you charge it at 50ma? When you said" let it run all day" was there a load on the battery?
I don't think it's overcharged.

How long did you charge it at 50ma? When you said" let it run all day" was there a load on the battery?
I don't think it's overcharged.
I let the charger charge the battery all day. At a constant 58.2 mA. It was about 12 hours of charging.

The 12 volt NiCd, which is at least 12 to 13 years old, has charged up to 14.03 volts. That is 1.403 volts per cell.

My charger drops about 4 volts total, with an 18 volt input, so we might be at the max this charger can charge this old fossil to.

The chew mark is from one of the pet rats we had over a decade ago. Rats are sweet but they will chew on anything.

The constant current charger, with the 22 ohm resistor setting the output current to 56.8 mA was charging the 12v, 1,600 mAh NiCd battery pack too slowly.

I changed the resistor to 10 ohms. Now the constant current is 125 mA. That is a .078C charging rate. Still quite low but twice as much current as before.

I used a higher wattage resistor although a 1/4 watt rated resistor would have worked (resistor is only dissipatibg .156 watt). I put a heatsink on the LM350. Again, I don’t think it necessary. No harm in over-engineering a little. Better than under-engineering. Plus, maybe I will crank up the current a bit more in the future. Now the LM350 is heatsinked and ready for it.

I swapped out the 18VDC wall adapter for an old Dell laptop power adapter from the junk bin.

This old Dell power adapter puts out 19.5V DC at 3.34 amp. That should be plenty of overhead voltage for the LM350’s dropout voltage (max 3 volts) and the 1.25V drop across the current set resistor. And 3.34 amps is more than enough for this slow-charge, low-current charger. This charger circuit using the Dell power adapter as its input should still put out 15.25V of charge voltage, even under a worst case scenario.

I added a 1,000 uF cap (35V) and a 1 uF cap (ceramic) to the .1 uF cap to filter the input to the LM350. This is just because I don’t know how well-filtered the output of the Dell power adapter is. These three caps should filter any low, medium, and high frequency ripple on the input to the LM350. Again, probably not necessary.

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125ma for 24 hours should be enough to charge it IMHO.

125ma for 24 hours should be enough to charge it IMHO.
All charged up. Even at .125 amp the LM350 and the current limit resistor barely get warm.

125ma for 24 hours should be enough to charge it IMHO.
It’s weird. The Dell laptop adapter has three wires on the output side. A small, insulated wire carrying 12 volts. The bigger wire, wrapped in a sheath around the smaller wire carrying 19.5 volts. And the a bigger wire wrapped around that one that is ground.

So I am using the 19.5 volts wire and ground. I wonder what the 12 volts wire is for?

Something needing 12 volts for that specific model laptop I presume.
Does the connector have 3 pins also?

Something needing 12 volts for that specific model laptop I presume.
Does the connector have 3 pins also?
Oh, so it does! Tip, ring, and SLEEVE.

Something needing 12 volts for that specific model laptop I presume.
Does the connector have 3 pins also?
Ah! A search of the Interweb reveals that these old Dell laptop power adapters (for old Inspiron laptops I think) have a center “control pin” that tells the laptop what sort of power adapter is hooked up to it. That is what the center wire is for. Me no need. I will just tape it off.

https://electronics.stackexchange.c...-wires-on-a-bipolar-dc-plug-what-are-they-for