In a previous question, I asked about using capacitors as batteries in automotive applications.
- Are there any other alternatives to lead acid batteries?
- What are the pro's/con's to these other battery types?
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The leading alternative at the moment appears to be lithium style batteries. There do not appear to be any manufacturers who are putting them in by default, but you can get hold of them... for a price.
Diyelectriccar gives this diagram comparing power, cycles, availability, etc. for various different batteries:
So you see that the Lithium battery has a much better energy output compared to its weight, a way higher peak power, and way more cycles at Depth of Discharge. The disadvantage is cost and availability. They cost around 10 times more, and you can't just go down the the store and buy one... You'll have to make a special order.
In the aircraft world nickel cadmium batteries (NiCd) are sometimes used. But they are only a small improvement of lead acid in typical specs and offer nothing in terms of value when compared to lithium batteries (although they are a little less expensive).
Right now everything seems to stack up in favor of lithium except that they don't play nicely with others and have no tolerance at all for fools…
As @MichaelKjörling points out via the link in the comment below, LiPo batteries, while inherently dangerous, are in practice the safest batteries available – because they are so capable of death and destruction great care needs to be taken in designing around them and especially in the charge/discharge control. So we shouldn't fear LiPo, any more than we fear gasoline or compressed air, but you have to treat it with respect and if you're thinking of retrofitting a battery or experimenting some of the responsibility shifts to you.
I will add fuel cells to the mix. Portable fuel cells could replace batteries in many situations and I believe eventually will.
About five years back I had seen a portable unit being used for remote mountain top communications in a demonstration for military use. I have been watching these ever since waiting for cheap and easy commercial availability.
They keep offering the commercial "home" versions as being available for some locations in use with solar for storage and supply. But the negative side is they are just not easy to lay ones hand on at this time, unless you buy a kit.
Advantages are offer in a compact system that has charge feature to replenish the storage unit by means of just about any DC voltage source. With many I have seen if storage "charge" runs out they can be reloaded via several methods (depending on how they are constructed) either through a gas fill (hydrogen, propane, butane) and some even water.
A year back I had found a possible source for a unit that would fit in place of my RV batteries, it had a solar charger interface for the hydrogen bottle it used. I was leaning toward ideas of providing them to all the snowbirds that park here in the desert but wanted to make sure of backup fills if needed. And found the lack of filling stations a problem. If and when filling stations become more prevalent for bottle filling they should sell like crazy.
There are non battery+starter motor alternatives to starting an engine too... With multiple ways to store energy used for starting. They have starters based on large springs, compressed air, even gunpowder based engine starters. I remember there is even a direct injection engine that can just start by just spraying gas and igniting it in a cylinder on the power stroke.