DDabs, I need to be up front with you. My knowledge is based on what I have read here, Wikipedia, white papers, etc. Not practical experience. But having opened Pandora's box, I feel obligated to tell you what everything I have read agrees on.
LA (lead-acid) batteries need to be charged in a rather complex way to get the maximum number of charge / discharge cycles before they give up the ghost. That number is measured in the hundreds. If you charged with solar cells during the day then discharged at night, you would be facing a maximum of 365 cycles in a year of cruising. Not good.
Taking a LA battery below 50% is damaging to the battery. They cannot be charged quickly because of their high internal resistance. This high IR also reduces their efficiency to giving back at most 80% of the energy put in. They are heavy and contain two very toxic substances: lead and sulphuric acid. They give off explosive hydrogen gas when being charged. A hydrogen flame is invisible making it particularly dangerous.
In contrast a LiFePO4 (not to be confused with several other lithium-based batteries such as a lithium-cobalt battery) is not subject to thermal run away, weights one third of a lead battery, has no toxic chemicals, does not out gas when being charged, works just fine completely submerged or in any orientation, has almost no internal resistance -- meaning that it can be charged many times faster than a LA battery and is almost 100% efficient. They are happy if never discharged below 20% or charged above 80% giving them a greater usable charge than an equivalent amp-hour LA battery. In this mode they are usually guaranteed to over 2,000 charge / discharge cycles. Some variations are good to 6,000 cycles if never discharged below 30%.
A strong point and weak point is that a LiFePO4 battery maintains nearly constant voltage until it is almost completely discharged. That means that ideally you would have some sort of meter that knows how many amps have left the battery and protects the battery before it is taken too low. The more common solution, since the voltage does drop somewhat, is a low voltage warning and cut off device. The good news is that these meters do exist and are recommended for LA batteries as well. In contrast, with a LA battery, the state of charge (SOC) can be determined simply by looking at the voltage across the terminals. The advantage of this nearly constant voltage output is that it is much easier on your electronics.
Another consequence of this steady voltage output is that your charging
electronic controller is simpler. But the voltage does raise with SOC. So it is possible to have a high voltage cut-off (HVC). Once again, these devices are available and some of those used for LA batteries can be configured for the LiFePO4 batteries.
As I understand it, a LiFePO4 is even more vulnerable than LA to being charged at too high a voltage or trying to cram too many amp-hours into it.
Another quirk of the LiFePO4 batteries is that their low IR means that there is no reason at all for 'deep cycle' vs. 'starting' batteries. They all can produce prodigious wattage without over heating
or being damaged. This also means that an accidental short of even a small LiFePO4 battery could be quite damaging.
voltage, depending on manufacturer, varies from 3.75 to 4.0 per cell, as I recall
. Once charged, they hold their charge for years. In fact a trickle charger
would damage them. Their voltage, when not under load or being charged, is nominally 3.2 volts. This means that a 12 volt battery would need 4 cells in series. That puts its output in the high 12 volts, much like a freshly charged, healthy six cell LA battery.
If you read some of the threads on this forum, you will find a discussion about balancing the cells. Don't be put off by this. LA batteries should also be balanced. If one cell is more fully charged than the others when the charging starts, it could be over charged before the other cells reach their full charge. To prevent that, part of the installation
procedure is to insure that all cells are at the same SOC before being put into service
. This is no different than LA batteries. It is just that you can buy one cell at a time with a LiFePO4 allowing you to balance the cells. You may not be able to with a multicell LA but you probable should with a bank of batteries.
In LiFePO4 batteries and their one cell at a time offering, it is easy to build circuitry that monitors each cell and shunts some of the electricity around a fully charged cell so the remaining cells get charged without over charging the already charged cell. This would eventually 'balance' or match the SOC of all the cells after a few cycles.
Well, there's a brain dump. I hope is makes sense and is correct.
Best of luck,