# Why would manufacturers use Ni-MH batteries in a robot vacuum cleaner?

In the late 1990s/early 2000s laptop and cell phone manufacturers started phasing out Ni-MH batteries in favor of Li-Ion. The main reason for this seems to have been that Li-Ion cells do not suffer from memory effect¹, which is the major drawback of Ni-MH cells: if they are frequently charged without having been completely discharged, their capacity drops rapidly.

During the transition period, Li-Ion batteries were mostly reserved for high-end devices, or the costlier option where a device was available in either configuration.

The main obstacle to universal adoption of Li-Ion seems to have been the cell voltage of 3.2–3.6V—in contrast to the 1.2V of Ni-MH cells, which is in the neighborhood of alkaline batteries and led to wide availability of Ni-MH cells in the standard alkaline form factors, whereas no such ”household sizes” ever evolved for Li-Ion.

Today, when I look at the specs of robot vacuum cleaners, I see that most come with Ni-MH batteries. Ariete offers one single model with Li-Ion batteries, which seems to be their low-end model. Also, they have proprietary battery packs—so neither cost nor the standardized form factor seem to have influnced the design decision.

So why are Ni-MH batteries so common in robot vacuum cleaners?

¹ Edit, since there’s some confusion about the term: For the purpose of this question, “memory effect” shall refer to loss of capacity (reversible or not) caused by frequently recharging a battery without fully discharging it first. (If there’s a more accurate, somewhat widely accepted term for this characteristic in Ni-MH cells specifically, a hint will be appreciated.)

• Wrongo, Batman. NiMh does NOT have a memory effect. – JRE Feb 4 '18 at 15:48
• It does. It's only less severe for NiMh than for NiCd – gommer Feb 4 '18 at 15:49
• NiMh has its own way of dying. When it goes, it is gone. A NiCad suffering from memory effect can be reconditioned. A NiMh that has lost capacity has lost it for good. – JRE Feb 4 '18 at 15:53
• All true. And what's you're point exactly? We were talking about memory effect of NiMh and not about possibly reconditioning NiCd – gommer Feb 4 '18 at 15:56
• You brought up NiCad. NiMh loses capacity permanently, which isn't the memory effect. NiCad has a reversible lose of capacity that is known as the memory effect. – JRE Feb 4 '18 at 16:06

## 2 Answers

You forget about other factors. capacity/volume/weight ratio's and price.

• Since it's a robot, discharge is controlled and not usually influenced by the user.
• NiMh for a given capacity is cheaper
• Higher weight is actually an advantage for this type of product.
• Less electronics for protection and possible liability issues for the manufacturer or distributor.

All these or a combination of several of these probably tip the scale.

• Li-Ion : a mobile fire starter running round the house while you are at work ... – Solar Mike Feb 4 '18 at 16:00
• @SolarMike fire safety might indeed be a point. The one Ariete model with Li-Ion needs to be started manually; the models with timers have Ni-MH packs. – user149408 Feb 5 '18 at 11:23
• Since it's a robot, discharge is controlled and not usually influenced by the user—as far as I can tell, the robot starts its cleaning cycle and returns to the base at the end of it. The model I’ve seen doesn’t seem to enforce a complete discharge before recharging. – user149408 Feb 5 '18 at 11:25
• Are you sure? Returning to the base station does not mean it actually charges. And on top of that: best technical implementation does not necessarily mean manufacturers actually do implement it this way. – gommer Feb 5 '18 at 11:27

Fire risk seems to be the main worry, at least that IRobot folks claimed it was.
I suggested LIFEPO4 which isn't a fire hazard. LIFEPO4 also last longer and have better high power characteristics.
I understand why not lithium ion, but not why they are avoiding LIFEPO4.