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I have a rechargeable vacuum cleaner with NiMH batteries, 1800mAh and 14.4 volts, 12 cells. The charger is 18v 200mA. The batteries have now died.

Can I replace the batteries with 4 cell Li-ion in series?

That will be 14.4 volts, if considered 3.7v each. Of course, I know it will be roughly 4 volts when fully charged.

Li-ion batteries in my mind: 18650, 2500mAh.

Also, can I charge them with the same charger?

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  • \$\begingroup\$ TL;DR. No. Don't mess around with things like these if you are a hobbyist. It is very dangerous and you'll cause fires. \$\endgroup\$ – Lundin May 28 at 13:45
  • \$\begingroup\$ @winny The answer you cite is not a duplicate. And fwiw it is fundamentally wrong in it's assumption re LiIon cell voltage - taking the cell average voltage as it's maximum voltage. The latter MUST be used for charging and cell count considerations. In this case the OPs requirement CAN be satisfied - but he would not establish that from the cited answer. \$\endgroup\$ – Russell McMahon May 28 at 14:13
  • \$\begingroup\$ @Russel Right. There might be better examples. Variation of the same basic question is asked over and over again here. \$\endgroup\$ – winny May 28 at 14:36
  • \$\begingroup\$ @Winny True. Do you think any of them come as close to providing a specific answer that would meet his needs as mine below does? [Others may. I don't know]. \$\endgroup\$ – Russell McMahon May 28 at 14:39
  • \$\begingroup\$ what sort of cordless vacuum cleaner are we talking about here? And why would one need one? Would one not have access to a lead acid battery in some remote location? \$\endgroup\$ – Indraneel May 28 at 15:08
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Try to avoid asking "can I replace.." because sure, you can.

The real question is: Is it a good idea?

Unfortunately the answer is a solid NO.

Reasons why:

  • The NiMh cells are very likely high current cells, meaning they are designed to be able to deliver the high current that a vacuum cleaner motor needs. If you use "standard" Li-Ion cells these will not be suitable for such high currents so even if they do work they will wear out quickly. You can buy high current Li-Ion cells but these are more expensive than the "normal" variant.

  • You must add a charge controller circuit. The original NiMh cells are very easy to charge if you charge them slowly. Which is the case as your charger only delivers 200mA. When a NiMh cells is full, at 200 mA it will just get a bit warm and that's OK (not ideal, just OK).

  • Li-Ion cells cannot be charged with a continuous 200 mA, charging MUST stop when the cells are full. If charging does not stop you risk damaging the cells maybe they overheat and could start smoking or catching fire. As you will need some Li-Ion cells in series to make the voltage required, the charger needs some balancing circuit as well. That's complex and expensive.

You're better off replacing the dead cells with new, same model, NiMh cells or just buying a new vacuum cleaner.

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  • \$\begingroup\$ Good points. Interesting reading (a typical Li+ 4-cell, 2-Ampere-hour (Ah) battery pack stores an energy level of about 100kJ, while a hand grenade (150g of TNT) has 600kJ) maximintegrated.com/en/app-notes/index.mvp/id/4169 and a typical balanced charger at electronics.stackexchange.com/questions/397790/… \$\endgroup\$ – Peter Smith May 28 at 11:00
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    \$\begingroup\$ Charging can be easily solved by bringing out the original LiPo connector and by using an external charger. It will be much faster and safer. \$\endgroup\$ – FarO May 28 at 13:41
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    \$\begingroup\$ Data point only: For AA NimH up to about 1800 mAh, trickle charging at <= 0.1C is bearably acceptable. Or used to be. Above that mAh rating (and maybe now at or below that rating due to it being what's generally done nowadays) you cannot trickle charge long term at ANY level whatsoever, and may perhaps be allowed to trickle at 0.01C for a few hours. (Some manufacturers allow this - the large majority do not). Older small Ah NimH had chemicals and structures to provide recombination of evolved gases. Newer/larger ones have removed it in the quest for greater capacity. ... \$\endgroup\$ – Russell McMahon May 28 at 14:02
  • \$\begingroup\$ ... This is based on an exhaustive literature search some years ago when I was designing solar NimH chargers as part of lighting solutions. \$\endgroup\$ – Russell McMahon May 28 at 14:02
  • \$\begingroup\$ @RussellMcMahon For AA NimH up to about 1800 mAh, trickle charging at <= 0.1C is bearably acceptable In the end that is of course the manufacturer's choice. I do not expect a manufacturer of cheap devices to bother with this rule and just keep trickle charging at 0.1C like they've done since the days NiCd cells were used. But when you want to get the longest lifetime out of your NiMh cells then indeed it is a different story. Some cheap devices even use the battery protection as charging cut-off. No good for the cells but still that happens. \$\endgroup\$ – Bimpelrekkie May 28 at 14:16
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I'm about to attempt almost exactly what you describe.
Wish us both luck :-).

What Bimpelrekkie says is "true enough" but there is a reasonable chance of getting a "good enough" result if you are willing to "play a little" and also to tolerate the possible worst case consequences.

The worst case outcome is a possible "vent with flame" incident - a rapid flaming discharge that may well destroy the vacuum cleaner and possibly do other damage. This is a possible consequence of most LiIon powered appliances, but in this case we are trying a little higher.

The cells used MUST be able to handle the load current, as noted elsewhere.
Some LiIon cells are rated at 10A or more discharge current. Others aren't. You need to know what your load is and what the cells are rated at.
If tthey are AA NimH cells they are probably not operated at more than 5-10A. If sub C's - maybe more than 10A.

I am using LiIon cells from faulty Dyson vacuum cleaner battery packs to repower an old Electrolux dustbuster - so in my case discharge current is OKish.

A LiIon cell MUST NOT be floated at its full charge voltage of 4.2V. However, it can be floated at say 4.0V. I plan to clamp the pack voltage at 4V/cell or less. In your example, 14.4V/4V = 3.6 cells. Using 4 cells you could charge to 16V (if the charger allowed it) and probably not harm the vacuum cleaner. This has a "flat" voltage of 4 x 3V = 12V.
Or, you could use 3 cells and clamp to 12V. This would provide 9V when cells were fully exhausted at 3V cell - stopping at a higher voltage prolongs cell life.
How well the vacuum cleaner works on 9V is TBD.

Using 4 cells and clamping at 14.4V gives 14.4/4 = 3.6V / cell which would result in an extremely low cell capacity. Increasing this to say 3.8V/cell = 15.2V gives you about 60% of rated capacity when the voltage reaches 15.2V, and about 65% after being allowed to float at this voltage for 2+ hours.

These tables (from www.batteryuniversity.com) show the capacity that you can expect from various endpoint voltages. Note that the figures in the two tables are close but not identical in some cases.

enter image description here

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Balancing is "a good idea" but may not be needed. Adding a 4V clamp per cell would allow eah cell to fully charge.

If yoi do not mind reduced capacity you could use something as simple as a zener diode per cell - selected to NEVER allow > 4V/cell at 200 mA (or at actual abs max charge current).

Zener per cell dissipation is 4V x 200 mA = 800 mW so a say 2W zener with sensible cooling should suffice.
While pack clamp dissipation is 12v or 16V x 200 mA = 2.4W / 3.2W respectively, so a suitable zener or collection of zeners should be "easy enough" to provide.

If you want a precision-bodge-job you could provide an eg TL431 + pass transistor clamp per cell to allow precision voltage control. Which is what I intend to try.

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    \$\begingroup\$ This doesn't answer the question. If you are editing a post, either refrain from posting it in the first place or keep it deleted until it contains something substantial. \$\endgroup\$ – Lundin May 28 at 13:40
  • \$\begingroup\$ @Lundin Are you an admin? If you had had as many answers closed just before you posted a perfectly good answer so that all your effort was wasted, as I have, then you too may take such steps. Long long long experience on this site with 2500+ answers has shown me that the likelihood of a question being suddenly kneecapped is not predictable using any normal sensible metrics. Alas. || Hopefully you approve of my answer. \$\endgroup\$ – Russell McMahon May 28 at 13:56
  • \$\begingroup\$ Yes, every user of this site is an admin, at some extent. It is custom to leave a comment when you flag/delete vote posts. \$\endgroup\$ – Lundin May 28 at 14:01
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    \$\begingroup\$ As for questions getting closed in your face, it sounds as if you have a habit of answering bad questions. So maybe instead of posting some placeholder answer, consider if the question should be answered in the first place, or if duplicates exist? This one has a potential dupe with 3 close votes, which you can see too. \$\endgroup\$ – Lundin May 28 at 14:02
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    \$\begingroup\$ @Lundin anyone with any experience knows that it's a regular occurrence for questions to get deleted as being "unanswerable" while someone is in the midst of writing a perfectly good answer, for no other reason than that it's much easier to find five ignorant people to flag than it is to find one knowledgeable person to write the answer. \$\endgroup\$ – hobbs May 28 at 22:03

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