Lead-Acid vs Lithium-Ion Rechargeable Batteries

Question

I originally planned to use Lead-Acid rechargeable batteries in a medical device prototype, but it seems that nowadays most devices on the market are using Li-ion rechargeable batteries, which are definitely more expensive and generally have less capacity.

I also realize that many lithium-ion rechargeable batteries are labeled as smart batteries and have more communication interfaces like SMBus. My goal is to build an uninterruptible power supply with rechargeable batteries and a power management module, and wonder how much of a help those smart features would be.

Is it outdated now to use lead-acid battery in (medical) devices? I am new to the field and would love your input on this matter.

Answer 1

Lead-Acid is dependable, easy to use (i.e. easy to recharge, and easy to stay within its Safe Operating Area), very safe, and very heavy. Despite the rise of Lithium-chemistry batteries, it still has a place in various applications, including medical (especially for backup/UPS purposes), where weight isn't so much of an issue, or indeed where weight in, for example, the base of a medical product aids in its stability against tipping (think: portable rolling monitors, medicine dispensers, etc). It's also by far the most recyclable battery chemistry; worldwide 90+% of LA batteries are almost completely recycled.

Lithium-chemistry batteries (and there are several chemistry varieties with various pros & cons) have a much higher energy density (contrary to what you say in your question), which makes them appealing for some applications where weight or volume are premium concerns. However Lithium-rechargeable batteries have a MUCH tighter Safe Operating Area, and thus require more elaborate charge management electronics. Some Lithium chemistries are also readily prone to thermal runaway if operated outside their narrow Safe Operating Area, whereas other chemistry variants suffer this thermal runaway much less or not at all, and are thus preferred for safety-critical applications (like medical environments).

Because of Lithium's narrow Safe Operating Area and more elaborate charge management electronics, AND because active cell balancing amongst cells in a series string is necessary to keep all the cells in a series string in roughly the same state-of-charge, AND because of Lithium's flatter discharge curve, this more elaborate charge management circuitry (including MCUs with firmware) is ALSO needed to track state-of-charge, i.e. what % of battery capacity has been used/remains, which can be a valuable thing to know in many applications, to estimate run-time and advise when recharge is needed. Hence why you'll see some commercial Lithium battery packs with SMBus interfaces, which the UPS can use for its own purposes, and which can also be relayed to the host device being powered by the UPS.

Answer 2

The characteristic of these two technologies should be considered to make the best choice. As far as my experience goes, you can choose them by considering this:

Lead-Acid: (Specially sealed ones)

Pros:

• Easy to use
• Very stable
• Low maintenance
• Low cost
• Suitable for standby charge mode (like in UPS, constantly in charge and used once over a long period)

Cons:

• Low capacity to weight ratio
• Low peak discharge current
• Slow charge

Lithium-Ion: (Or similar technology like Lithium-pol)

Pros:

• High peak discharge current
• Fast charge
• High capacity to weight ratio
• Available in various compact packages
• Suitable to be used in cycle use mode (mostly discharging to 50% and recharge again will be better to extend life-time)

Cons:

• High maintenance
• Very unstable and may explode on overcharge/over-discharge (Needs the smart parts to prevent this)
• Low life cycles

So, if you're looking for a solution for cyclic use, Lithium-ion batteries will be a good solution as it has a lower weight. They provide more capacity and can be charged faster. However, you can expect about 500 life-cycle at the best condition from a lithium-ion battery. Also, it's very vulnerable to icing and cold will decrease the life-time.

But for standby use, the lithium-ion battery only will have a good life-time if you store them at 50% charge. So You will be losing much of the capacity if you need longer life-time. On the other hand, the lead-acid battery is very suitable for standby use as they have a charge mode for standby use. These batteries can be used fully charged and will have a longer life-time.

Answer 3

There is much on this site already on this subject.
Also see Battery University and their LA and LiIon comparisons

Summary:

Lead Acid (LA) can be a good choice

• where initial cost is important and
• where mass or size for a given Wh capacity is less important.
• Lead acid has better very low temperature performance than most LiIon batteries.
• In applications usually in standby with high current standby occasional use (eg UPS, automotive starting)

Lead Acid batteries will almost always be a very poor choice.

• In modern equipment that is intended to be portable and/or have good energy density.

The large majority of medical equipment, laboratory equipment, scientific instruments and portable or mobile equipment and equipment using batteries for major energy storage nowadays used LiIon.

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If long lifetime is important lead acid needs attention to charge rates, proper boost cycles, limiting depth of discharge and more. Lead Acid can be a good choice for standby or cycled stationary use where size and weight are not overly important or mobile use (such as in vehicle starter, lighting and accessory use (but not usually for 'traction').

Lead acid batteries have have lower initial cost per nominal capacity but to obtain good cycle lifetimes they can be discharged to only a fraction of their full capacity.

Lead Acid have a role in emergency and standby applications where low cost is important and cycle life is less important. A UPS (uninterruptible Power Supply) may be rated at 10 kW output but be almost always in standby mode waiting the next power outage. Battery capacity will then usually be designed to provided rated power for a specified period using a large portion of the battery capacity. High DOD (depth of discharge) significantly impacts battery cycle life if repeated frequently but in most cases a UPS may operate in standby for months or years at a time. In areas where there are frequent power outages a much larger than usual capacity battery, or even a LiIon battery may be appropriate.

LA have lower energy density by mass or volume compared to LiIon, larger whole of life cost per energy stored, more complex charging requirements and lower allowable depth of discharge for good lifetimes.
If you want even longer cycle lifetimes than LiIon at still superior performance in almost every way them LiFePO4 may be a good choice.

Lead Acid often offer superior low temperature performance (well below 0 degrees Celsius), but this is not usually a factor in instrument applications.

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I also realize that many lithium-ion rechargeable batteries are labeled as smart batteries and have more communication interfaces like SMBus. My goal is to build an uninterruptible power supply with rechargeable batteries and a power management module, and wonder how much of a help those smart features would be.

Whether smart features are useful to you depends on how much effort you want to assign to the core task and how much to battery management. You have mentioned both medical equipment and a UPS. If the primary focus is implementing a UPS and volume justifies it then performing all the functions yourself using bare or protected cells with perhaps a pre-built BMS (battery management system) and any other functionality designed by you is liable to be both economic and allow you to achieve specific requirements. Batteries with "smart" facilities are often significantly more costly per Wh than bare battery based systems. This may be acceptable where an existing market price point can easily be met, but less so where the equipment must sell in a highly competitive market.

If the UPS mentioned is a sub-system for medical equipment where good power functionality is essential but is not the main role, then using what is available allows you to focus on your core task. For most applications charging and battery management requires current, voltage and temperature measurement, charge current control , battery isolation and perhaps discharge control if not part of the main task. Even with those capabilities you can implement coulomb counting for charge management, trickle up charging for undervoltage battery packs, pulse charging (if desired) and more.

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ocrdu asked: (S)LA has "more complex charging requirements" than Li-ion?

Yes, perhaps surprisingly. Despite the various issues that can occur LiIon charging is usually an extremely simple and well defined process. Lead Acid charging is not.
LiIon usually uses

• CCCV (Constant Current and then Constant Voltage) charging.
• Then terminate on preset current tail.
• You can add trickle up precharging if below some preset Vmin.

Much is available on CCCV charging on SE.EE eg here and here
and much on LiIon charging here

Lead Acid batteries have different boost and float charge rates.
And various charge rates by subtype.
And electrolyte mixing and diffusion if you want to get fancy.
And differing Vfloat for deep discharge and float modes.
Also cycle life with lead acid varies greatly with depth of discharge.
And lead acid must be maintained above a certain minimum voltage as much as possible to prevent damaging sulphation.

LiIon is about as simple as they come. There is a rule set that can usually be followed to get expectable results.