Lead Acid (LA): Cheap and robust, but low energy density and heavy, can easily supply very high currents, limited cycle life
Lithium (LI), including 18650s and all other formats: Higher cycle life if used properly, better energy density and weight advantages, somewhat unstable if mistreated, not so good at providing really high currents
LA is suitable for all applications where the operating area is a bit rough, where price is important, but weight and size isn't, and/or really high currents are needed and/or where a "controlled" charge and discharge cannot be provided or a simple charging process is needed
LI is suitable where size and/or weight matters, as well as cycle life, but the current needed is limited (although that scales with size of course, but then again with size and cost as well) and the cells can be operated in a more controlled environment in terms of temperature and controlling the charge/discharge
There isn't one better than the other, that totally depends on what you want to achieve. If your only concernis longest cycle life, then surely go for LI and run them between 3.0 and 4.0V. They will last virtually forever. Depends on your use of course, but you get the idea.
Korishan said:
I'm not sure how the LA's are rated.
Well, that varies a bit depending on manufacturer, manufacturers honesty/transparency, intended use and so on. The same is true for LI though. The topic probably has enough content to write a small article about it, but for now I will try to summarize it.
Generally speaking, it is very similar to how LI cells are rated. A manufacturer of a LI cell will give you a datasheet with discharge curves for certain circumstances and the manufacturer of a LA battery will do the same. However, the discharge rating is given in a different notation. And sometimes there is no datasheet available, just as it is the case with non name branded LI cells.
Let's just have a look at an example. I have a 12V 120Ah LA battery here from a brand called Ective Batteries. This is not a manufacturer, just a brand name that some store made up to sell cheaper, but still very cost effective LA batteries to customers under their own label. The battery is actually made by some kind of OEM. It is a bit of a guess, but I think this is what they are doing. And as a result there is very little info available in this case, there is no datasheet and you only get these three ratings on the battery:
120 Ah (C100)
105 Ah (C20)
90 Ah (C5)
"Cx" means full discharge over x hours. That of course translates to a certain load in amps that is applied to the battery. This is basically the same compared to a LI manufacturer giving you ratings for a "xC" discharge.
Now, there are certain "standards" but no one is forced to use them. Full discharge is usually 1.75V per cell, but anything between 1.60 to 2.00V can be found, depending on the manufacturer. C20 is another standard for rating and comparing LA batteries. If nothing is stated, then it should be a C20 rating. In this case my battery would only be a 105 Ah battery though and therefore they rate it as 120 Ah (C100). Usually you don't even state the C100 rating, you use C3 to C20. A bit shady, but at least they tell you about the C100 rating. You just have to know what it means. And if your application involves a rather slow discharge then this isn't even such a bad thing for you.
Another example I came across recently, because I thought about buying some used ones, is Exide, which of course is a well-known and trusted, high-end manufacturer of LA batteries, with their Sprinter series of batteries. These are made for all kinds of UPS and security-related applications and are probably two or three times as expensive per Ah as the batteries from the first example. The datasheet for these things is probably as extensive as you can imagine. Apart from the usual C3 to C20 they also give you C2 and C1. And also ratings for 3, 5, 10, 15, 20, 30 and 45 minutes (!). And all that for end of discharge voltages from 1.60 to 1.95V in 0.05V increments. And in two flavours, constant current and constant power.
Korishan said:
Noting that, lions are usually more accurate in their mAh ratings than the LA's. Lions also have a more stable voltage drop. They'll drop from 4.2V to 3.8V under load, and then very slowly drop to 3.2 or 2.8V over the course of the discharge. Whereas the LA's have a tendency to constantly drop voltage during discharge.
I wouldn't say that LI is more accurate in their capacity than LA. That totally depends on the manufacturer and isn't really an inherent property of the chemistry.
Same with the voltage drop, that depends. There is an initial and instant drop due to load, and that isn't always to 3.8V for LI, and a continuos drop over time as the cell discharges. It is basically the same for LI and LA. The size of the initial drop scales very nicely with the load. I have the datasheet for the LG HG2 at hand since I'm doing something with them at the moment. The start of discharge voltage for them is between ~4.15 and ~3.3V, depending on the load. A LA battery will do something similar.