Lithium versus AGM - the great debate

First of all, a quick bit of definition: 

agm = absorbed glass mat or absorbent glass mat = the most common type of sealed lead acid battery (or SLA for short). The basic idea is the use of a glass weave between the battery plates that absorbs the acid and, for a slightly higher manufacturing cost than a "wet" battery, delivers lower internal resistance (faster charging, able to deliver current faster when needed), deeper cyclic capability, less prone to sulphation, ability to handle deeper discharge without damage than a "wet" battery, lower release of acidic gasses, longer potential battery life. A technology developed in the 1980s and widely accepted as very effective in many applications.

Lithium = (in this case) usually means LiFePO4 - which is lithium iron phosphate or lithium ferrophosphate - commonly used for storage batteries, solar and the like (and not Lithium Polymer - used for radio controlled cars and planes etc, or lithium nickel manganese cobalt or lithium nickel cobalt aluminium - both used in (full-sized) electric cars).

So, lithium batteries are known for their higher price than AGM (maybe 200% higher) and their lower weight (maybe 65% lower). They are typically able to charge very quickly (depending upon cell design and battery construction) and they are often able to deliver up to about 95% of their capacity without a significant change in voltage. The voltage delivery of a lithium battery is a very gentle slope followed by a plummet, whereas an AGM is a curve that steepens as the battery is approaching exhaustion. Good lithium batteries are very stable, with excellent internal circuitry, and dodgy lithium batteries are very volatile and, if you are using a cheap one under your seat on a motorcycle, some fire retardant undies might be in order.

In terms of the expected battery life (cycle life or number of cycles) that you might expect, a huge amount of influence comes from quality of the battery charger, speed of discharge, speed of recharge, how the battery is maintained when not in use and, importantly, the depth to which it is discharged. The latter is a topic that deserves a blog in its own right, but the basic point is that a good AGM battery might do 1200 cycles if it is only drained by 25% (ie 75% remaining) each time, and this might drop to 500 cycles at 50% depth of discharge each time, or 200 cycles at 75% depth of discharge. A little golf cart battery (hand cart, not ride-on) might get your clubs round the course 300 times (recharging every time) if you play to a low handicap, and 150 times if you spend most of your time foraging in the rough and walking in zig-zags. By comparison, a good lithium battery might deliver 3000 cycles at a 50% depth of discharge before the battery gets to a point where it no longer does its job well enough to be worth keeping. 

Two caveats though! Firstly, it all depends on how often you'll work the battery and where your personal threshold for battery replacement is. A good AGM on a boat used once per week might do 500 cycles, about 10 years. In the same example, a lithium might be expected to do 3000 cycles - 60 years - but the question is whether you really need that (or whether you believe it, come to that).

There is a huge amount of research on battery performance which, spookily, seems to give differing results in some cases depending upon who was paying for the research. There are also claims about cycle lives of 5000 or more so, just so we are clear, that is either someone charging and discharging a sample of batteries every day, 7 days a week, for over 13 years, or it is someone drawing some conclusions based on shorter testing, or possibly the work of an overactive imagination. We can each form our own judgment on that. 

Let's look at some real world examples:

• starter battery, everyday car - lead acid batteries have been doing this for years and there is no real need to add several hundred dollars to the car price or shave off a few kg. Lead acid wins (but super-capacitors are coming, and that's another story)
• starter battery - racing car or racing yacht - weight saving is important, playboy lifestyle means deep pockets - lithium wins
• grid-tied property, storing solar energy to cover some of the shoulder period between dusk and cheap tariff late at night - no concerns about long-term storage or failure of the grid, just need to charge something and cycle it deeply for a few hours - lithium probably wins, especially as it is very low maintenance, but take a good look at the true payback period on this sort of installation
• Solar farm, storing power to feed back commercial quantities to the grid - lithium wins by virtue of being a regulatory requirement in many cases
• Off-grid house, self-sufficient with plenty of solar - weight isn't important, cost and payback is. Need to store enough power for 3 or 4 days of poor sunlight and happy to do a bit of light maintenance. Lead acid (2 volt cells) probably wins by a good margin, unless you are taking a very long view on the payback period
• Caravan - weight is usually pretty important, so lithium is often the choice for brand new vans but, mainly because old chargers and solar regulators won't look after lithium batteries, changing from lead acid to lithium isn't always easy, and a caravan designed for lead acid batteries has probably already had the weight compromises dealt with. Lithium wins, but it isn't clear-cut.
• Golf buggy (hand-buggy) - lithium is MUCH lighter, goes better, especially when the course is wet, and will last longer, but the cost is around 2.5 times that of lead acid. Lithium wins, especially if you need to carry the battery around between car and clubs, but I use an old lead acid battery that has done about 250 rounds, faultlessly, since I took it in as a warranty because the original owner couldn't get round 18 holes with it and I don't feel the need to change just yet, and my handicap isn't particularly low!
• Boat - house batteries - live aboard used every day - lithium wins by a country mile because of expected cycle life. You really don't want to be changing lead acid batteries every couple of years (and, yes, I know there is someone reading this who has a live-aboard with batteries that they purchased in the last century, but they aren't the norm)
• boat - house batteries - used at weekends - weight saving not critical, happy to accept a battery change every 5 years or so. AGM wins, saves money, just makes the poor installers do a bit more heavy lifting

So, horses for courses, and there are dozens of other scenarios to consider. As always, buy quality and you'll likely be happy, buy cheap and you are only kidding yourself, good chargers and solar regulators are important, and time spent sitting flat is the grim reaper for any battery, but particularly for lead acid. One of my favourite analogies is the pet rabbit. You wouldn't buy a rabbit, forget to feed it for 6 weeks and then take it back to the shop for a warranty claim. Batteries are a lot like pet rabbits. Keep them well fed and they will, generally, reward you with loyalty and long service.

One final thing. There is another lead acid battery technology out in the last couple of years that has some wild claims made about it. I won't name it here, because I don't want to be sued and I haven't got anything good to say, but if a fellow caravan or boat owner starts waxing lyrical about the new lead XXXXXXX batteries that they have just purchased, tell them to keep their receipt and make a diary-note to tell you, in 18 months, how pleased they are. You can guess how likely that it to happen and the moral is clear. If the top twenty battery manufacturers, with all their R&D power and decades of experience, haven't brought the product to market, there's probably a really good reason.