Directly replacing NiMh cells with Li-Ion cells - bad idea ?

[sam_sam_sam]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

Besides, if I needed to run this vacuum more than even 5 minutes at a time, I might well try to use an AC powered vacuum. Or even the 12VDC vacuum I have...

You have a good point

[eccerr0r]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

Yeah the battery was covered in paperboard (which was around the heatshrink), so I couldn't get a brand or model number without ripping it apart. I figure let sleeping dogs lie, it works, don't fsck with it or fsck with it minimally. I guess I have to stick with my conclusion even if it's unfounded, at least until it stops working... mainly had to make assumptions because I needed to charge it and didn't want to buy/integrate a fast charge system for it.

Besides, if I needed to run this vacuum more than even 5 minutes at a time, I might well try to use an AC powered vacuum. Or even the 12VDC vacuum I have...

[sam_sam_sam]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

No, it could not have been a cheap cell. The internal impedance of this cell necessarily is low because of the heavy load it needs to drive. Remember this thing needs to discharge at 8C. Surely if it's discharged at 8C, it can be charged at 1C at least?
Does that cheap battery you buy get hot at 2C load? Probably does if it gets hot at 2C charge.

If this is case then I am confused about why charge the battery at such a low rate

I curse what brand name is this battery that is in this unit

[eccerr0r]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

No, it could not have been a cheap cell. The internal impedance of this cell necessarily is low because of the heavy load it needs to drive. Remember this thing needs to discharge at 8C. Surely if it's discharged at 8C, it can be charged at 1C at least?
Does that cheap battery you buy get hot at 2C load? Probably does if it gets hot at 2C charge.

--
Oh BTW:

I charged it with a CC-CV charger when I first got it as a test, and it did not get warm at all!

[sam_sam_sam]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

Wow... :confused
The question was always: why did they build it this way, why didn't they put in a CC/CV 2-stage at least, if not a full 3 state charger so that the battery could be charged much faster. The current system, due to limit of 150mA, requires at least 10 hours to fully charge, though more like 8 hours perhaps - read on.
More design tradeoffs?

I have a feeling that the type of battery that used is the same type that they use outdoor solar power lights

Because the charging curve is a lot less than the normal ( meaning that you can not charge them with the same current maybe the same voltage ) Li-ion cells and if try to charge them this way they become very warm very fast

I bought some one time and change them on a Li-ion charger never again ( set at 2 amps )
( set at 200 milliamperes maybe )

The cost for 4 of this type of battery is about $10.00

For a good quality Li-ion cell is about $7.00

So $2.50 versus $ $7.00 this your trade off you were asking about

[eccerr0r]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

the answer to the question is in the datasheet,
because it's not safe to fast-charge lithium with just a set current - you need a profile curve.
a controller that lowers the current as the voltage aproaches the max so you dont go over it and damage/heat the cell.

Wow...
Backing up a bit, as far as I can tell the system that was sold as is for the vacuum is this:

Say we have the 15V wall wart, CV. Has 100 ohm resistor in series or is its ESR to limit it to 150mA. Straight wired to charge controller, that apparently cuts off charging once it gets to ~ 4.2V. The charge controller has only SOT23s on it, and has no inductors - so it could not possibly charge at high rate. The battery, 1.5Ah, is straight wired to the switch/motor. This is a slow two-state charger system, and won't go past 4.2V (actually I think less) because it cuts off charge there and doesn't try to charge if it's even close to 4.2V. So the battery should be protected against overcharge.

The question was always: why did they build it this way, why didn't they put in a CC/CV 2-stage at least, if not a full 3 state charger so that the battery could be charged much faster. The current system, due to limit of 150mA, requires at least 10 hours to fully charge, though more like 8 hours perhaps - read on.

Again my take is that this was done to extend battery life as well as being cheap. The sarcastic reason is detailed above due to limiting the lifetime number of discharge/charge cycles because the charge cycle takes 40x the discharge cycle. However the other aspect I just thought, since it does not do the CV absorption charge, it never fully charges - and hence only gets to 85% charge or so. Because of this, the battery is less stressed, and can sustain more charge/discharge cycles than if left fully charged. The manufacturer thought this was "okay" to take overnight to overcharge just because "it's always been done this way" since NiCd was typical like this.

More design tradeoffs?

[stj]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

Eh no, the question was not how to charge it faster than 0.1C, but rather why they limited the li-ion charge to 0.1C from the factory -- it would have been a more attractive product if it charged faster... but would wear out the battery in fewer months, perhaps increasing warranty replacements. They probably figured that it was no worse than the old NiCd "overnight" chargers so that's the way they designed the Li-ion, and hope that the battery will last longer than warranty.

the answer to the question is in the datasheet,
because it's not safe to fast-charge lithium with just a set current - you need a profile curve.
a controller that lowers the current as the voltage aproaches the max so you dont go over it and damage/heat the cell.

[budm]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

Still on the topic of batteries, my colleague got his hands on two rather beefy SLAs which saw hardly any use, essentially brand new, and he claims their full charge should be around 14v+. As they stand, after being on a charger today, they peaked at 13.9v which to him is not satisfactory...I call that BS, as 13.9v is more than enough voltage to call those cells perfectly fine and healthy.

https://www.powerstream.com/SLA.htm

[eccerr0r]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

if it's a single cell, use those smart usb charge modules i posted an ALI Link for.

Eh no, the question was not how to charge it faster than 0.1C, but rather why they limited the li-ion charge to 0.1C from the factory -- it would have been a more attractive product if it charged faster... but would wear out the battery in fewer months, perhaps increasing warranty replacements. They probably figured that it was no worse than the old NiCd "overnight" chargers so that's the way they designed the Li-ion, and hope that the battery will last longer than warranty.

[Dannyx]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

Still on the topic of batteries, my colleague got his hands on two rather beefy SLAs which saw hardly any use, essentially brand new, and he claims their full charge should be around 14v+. As they stand, after being on a charger today, they peaked at 13.9v which to him is not satisfactory...I call that BS, as 13.9v is more than enough voltage to call those cells perfectly fine and healthy.

[budm]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

https://www.ebay.com/sch/i.html?_fro...P4056&_sacat=0

I bought some of these PCBA based on TP4056 but never have time to try them out yet.

[stj]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

if it's a single cell, use those smart usb charge modules i posted an ALI Link for.
you can set the charge current with one resistor upto 1Ah and they are $1.60 for 10!!!

attaching the DS.

[budm]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

I need to brush up on my battery basics - haven't cared about them too much TBH, so what is C ? Coulomb ? Rate of charge ?

https://batteryuniversity.com/learn/..._is_the_c_rate

[eccerr0r]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

This is only a 1-cell system so no balancing is required. AFAIK no real reason why one couldn't charge faster than 0.1C...

... other than if there is some penalty involved with doing fast charging and fast discharging... which may be the case, but not sure what the limitations are with the cell.

Perhaps the main reason is to slow down usage of the vacuum cleaner, so that the 400 charge-recharge cycles takes a year to wear out the lithium ion cell, instead of just weeks by finishing a charge-discharge cycle every 3 hours...

[Dannyx]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

I need to brush up on my battery basics - haven't cared about them too much TBH, so what is C ? Coulomb ? Rate of charge ?

[stj]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

how many lithiums are we talking here, 1 or higher?

[eccerr0r]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

Actually, the wall wart looks like a SMPS with a very high impedance as far as I know. At least the pictures that I've seen, the wall wart is tiny, much like a modern phone charger... That being said I've not had my hands on the real wall wart.

The main reason for saying that the 0.1C charging is not any worse than the original NiCd is that it won't take any longer for the Li-ion system to charge, 10 hours (typical of old dumb NiCd chargers) when we typically can charge li-ion in less than 3 hours! :o The smarts in the charging system is all in the "protection" board as far as I can tell, but it sure is not capable of handling high current. At least it does protect the Li-ion from overcharge, as li-ion will go "fuming" mad at 0.01C or even 0.001C overcharge.

[momaka]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

The question is why was my vacuum designed with such a slow charge?

If I had to guess... probably so that they could use a cheap wall-wart adapter for the charger instead of a more beefy SMPS adapter. Make sense from a cost stand-point at least: just tell the user to keep the vac always plugged in and charging so that it's ready when needed. Since most people don't user these kind of vacs too often (like you said, it's only good for the occasional small stuff), chances are they won't have a problem with leaving the vac to charge somewhere at its own slow pace.

I suppose this is not any worse than the original NiCd system that also charges probably at 0.1C.

The reason you sometimes see this slow "trickle charge" on Ni-Cd and Ni-MH batteries is because they are generally much harder to charge than Li-ion. By that, I mean the charge process is more complex. A good (proper) Ni-Cd / Ni-MH charger will not only limit the current going to the battery (charge rate), but also look at battery temperature *and* specific battery voltage curves of the battery. Depending on how fast you charge a Ni-CD / Ni-MH battery, the charge voltage will rise and then fall a bit when reaching near full charge. But how much this voltage "fall" will be, tends to vary with the charge rate and temperature. So a good Ni-CD / Ni-MH charger needs to have a not-so-simple micro-controller that can calculate when the battery is actually fully charged based on the parameters above - that is, if you want to optimally charge a Ni-Cd / Ni-MH battery and minimally affect its life.

But because Ni-CD and Ni-MH chemistry is a lot more forgiving to over-charging and under-charging than Li-ion (at least when it comes to safety), some manufacturers just choose to use a "dumb" charger that charges the cells at a specific charge rate or for a specific amount of time... or a combination of both. And if the cells are aging and not taking the charge too well from this "dumb" charger, they will over/under-charge... which will then decrease their life further as time goes on.

That said, trickle-charging on Ni-Cd and Ni-MH batteries can be quite safe (to the battery's optimal life) if the charge rate is very, very slow, as the battery will dissipate the extra charge as heat (which there wouldn't be too much of, really.) I don't remember exactly off top of my head any exact values, but I think something like 0.01C, you can keep the batteries on the charger indefinitely and they won't get damaged.

[sam_sam_sam]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

Yes I know and I seen them to

[stj]

Re: Directly replacing NiMh cells with Li-Ion cells - bad idea ?

This is the type of charge I use for a one cell 18650 battery they work very well But I check to make sure that it cut the voltage at level it suppose to

https://www.ebay.com/itm/283423505664

I am sorry but I just bought the last two that they had

But I will still leave here so you can see what type of battery charger I use

One note I do recap them before I use them

those are what they supply with the lights, but i dont like them so i just kept the cable and binned the psu.
have a zener diode decide if my lithium is charged - hell no!
btw, they cost a couple of $ - not 10!!