Despite my concerns I'm a big believer that a person "can't live in fear" as the expression goes and I do have 2 fire extinguishers on hand (one in house and one in vehicle). I also have long term plans for building a rideable electric go-kart with solar recharge capability and given the price points it looks like Lead-acid is a cheaper starting point on several aspects. The take away I'm really getting here is actively monitoring whatever chemistry in a well controlled manner and staying within spec is paramount for my safety. I've seen more than enough YouTube to know many people jump in full of assumptions and in a state of ignorance which is something I've never cared for.

After reading that reply i am confident that whatever you choose will be good because you obviously are looking into this adequately.

Just one little note on the fire extinguisher.
I'm reading that a class D dry chemical works on Li-ion but am also reading that you can flood them with water. The problem here is, it depends on the exact chemistry of the Li-ion because they are not all the same exactly. They use different metals for the cathode i believe so some can be flooded with water to put out and others the water will just make matters worse. I think many of the lower power cells are not as dangerous and they often have built in protection now too and they would say that when you buy them.
Doesnt sound very strange, but magnesium catches fire and so a Class D fire extinguisher is required and it may even have to specify for metal fires.
I'll probably upgrade my extinguisher too.

 

So, you are probably familiar with the fire triangle:

(there is also the fire tetrahedron which is more accurate but unneccesary for this discussion)​

Because the fire needs all three you can extinguish it by removing sufficient amounts of one or more of the sides of the triangle. Different extinguishers do different things to accomplish this.

Water attacks both the heat and oxygen sides. It removes the heat by evaporating and prevents free oxygen from getting to the burning material. Wood fires are well fought with water. Note, though, that a very hot wood fire can frustrate water extinguishers because there is simply not enough water volume to remove sufficient heat.

A dry chemical extinguisher primarily depends upon attacking the oxygen side. The powder covers the fuel and isolates it from the air. This smothers the fire since it can‘t get the oxygen it needs to continue the re action.

A CO2 extinguisher also primarily depends on attacking the oxygen side. It does this by displacing the oxygen with the CO2 gas It also has a lower temperature and may do some cooling as a secondary thing.

Class D extinguishers, intended for metal fires, also attack the oxygen side. The difference is the agent is specifically chosen to be non-reactive with the metals. Some use gases and others powders. Not all Class D extinguishers are the same and must be chosen for the target fire.

All this said, there is something very important to know about lithium ion battery fires: they are not lithium fires. Lithium batteries contain very little lithium. In fact, lithium battery fires are not considered metal fires, instead the are Class B and require an appropriate extinguisher, not Class D.

What makes lithium fires so problematic is how they happen and what is burning. When you consider the evolution of a lithi’m battery fire in light of the fire triangle, the trouble becomes clear.

A typical lithium battery fire happens when there is an internal short in a cell. It can be caused by physical damage, a manufacturing defect, or something else. The battery contains an enormous amount of energy per unit volume. The short leads to a lot of heat in a small spot, so we have the heat side. As the battery heats, the electrolyte boils pressurizing the cell. Very soon after the cell vents, often by rupturing.

The heat combined with the materials the cell are made from start a fire. Yes, it’s the battery’s internals that are burning, not lithium metal of which there is a very small amount in a battery. So we have our fuel. The battery parts.

While lithium metal is not there, what is in the battery are metal oxides. They are in the cathode including cobalt, nickel and manganese. This is very critical because these oxides contain a lot of… oxygen. And here is the last side, and our big problem. As you can see, most of the extinguishers try to isolate the burning fuel from the air which is very effective if that’s the source of oxygen.

But in a lithium battery fire, the oxygen is built in, and until the oxygen in the oxides in consumed, smothering is not possible. So, the only way to go is to attack the heat side of the triangle. And this can work but the fire is really hot! They burn at temperatures as high as 800ºC! It takes a lot of thermal mass to cool that down.

Water can work, but for an indoor fire of a small cell, the fire will burn itself out before you could use enough to stop it, and, the attempt will make a terrible mess. For larger fires, where you just have to put them out because of exposure, it will just take a very large amount of water to cool them to the point the fire will stop.

And, even after that, any damaged cells that didn’t participate can feel left out and decide to start their own party.

So… if you are concerned about using largish Li Ion cells or batteries (and the risk is actually not very high), there are two practical steps to take:

1. Ventilation
In the case if a battery fire there will be a lot of nasty gas produced, so you have to have some way to rapidly and thoroughly ventilate the area should that happen.

2. Containment
If the cell or battery is not very large, it will almost certainly burn itself out before you could out it out anyway, so it is important to contain the heat and flames until it does. A good metal enclosure, not in contact with flammable surfaces, since it will get very hot, will prevent spreading of the fire and damage to the surroundings. Note, however, that you do not want to make this container gastight, or even “well sealed”. See above about ventilation.

 

The great thing about lithium-ion cells is that you can harvest them for recycling bins. Although, admittedly, I've had to return about 90% of those cells because they were worn out when tested for capacity. But I did score one 56V pack that yielded 14 well-matched healthy cells. So you do need a way to charge and discharge cells and measure the capacity; the "Imax" B6 hobby chargers are good for that (they'll do every type of battery, so IMHO every electronics hobbyist or pro needs one or more). There are also multi-cell charger/analyzers; I got a LiitoKala Engineer Lii-500 that can charge and test up to 4 cells, but the B6 can discharge at higher currents.

Once you have some recycled cells, get yourself some 18650 holders, wire up a "BMS" (more properly called a PCM (protection circuit module)) which protects against over charging, over-discharging, and short circuits; if the PCM doesn't include balancing, add a module for that. (PCMs with balancing will have one physically large low-value resistor per bank, and usually will have two 6-legged chips per bank.)

Then get a thing to charge it; the simplest is a plug in "wall wart" or "power brick" (search for 12.6v 3s li-ion battery charger). If you have a source of 15V to 20V DC available (laptop brick, solar panel), there's modules based on MAX745 and other chips that will step that voltage down and charge the pack properly. There's cheaper modules that can step-up 5V USB C to 3S; charging current will be limited, but better than nothing as a back-up or for travel, or keeping the pack topped-up on the workbench or desk. (search for "2S 3S 4S 8.4V 12.6V 16.8V Step-Up Boost Lipo Li-Ion Dc 5V Type-C Charger")

SLA batteries are liable to die if they're not kept properly charged and can be killed by a single deep over-discharge. They also lack any protection from short-circuits, so it would be smart to insulate the terminals and add a fuse. I was tempted to suggest hooking it to a cheap solar charge controller to keep it topped up from a small solar panel (1 to 10 watts), but those PWM chargers can produce electrical interference. Maybe add a switch to disconnect the panel when you need quiet power from the battery?