The Bioenno Power LiFePO4 batteries seem to be very popular. They are lighter, per amp hour, then the sealed lead acid batteries. They also have a slightly higher voltage then a SLA battery. But, they are very expensive. LiPo batteries, as used in model aircraft, are also good choices but the amp hour capacity is very limited. They are very small in physical size, great for backpackers and perfect for QRP radios.
When it comes to cost per amp hour, I found this to be the best alternative:
The above battery is an LifePO4 (Lithium Phosphate Battery) that is rated at 20 amp hours. This battery has a built in BMS (Battery Maintenance System) to protect the battery from over charging, draining the battery to a voltage lower then 10 volts and temperature control. The actual weight of the battery is less than 4 pounds. That's at least half the weight of a SLA battery of the same capacity.
When I received my battery, the first thing I did was to fully charge it and then load test it. I put a 2 amp load on it and it came out to 19.76 amp hours. That's close enough for me. I like it so much I ordered a second battery. I now have a 40 amp hour total capacity for setting up portable and being able to run 40 watts out for an extended period of time.
These batteries are 60% cheaper then the Bioenno Power batteries. You can get them on Amazon. When I purchased mine, the cost was $81.99.
Maybe 20 amp hour is a tad too small for your needs? If so, they also make a nice size 36 amp hour battery. Weighing in at only 9.5 lbs, it sure is a lot ligher than a SLA battery. At the time I created this web page, the selling price on Amazon, was $129.99.
If a 20 amp hour battery is overkill for your needs, maybe a 10 amp hour battery would be a better choice. I purchased one of these to be used with my QRP only radios. The battery shown below is a 12.8v 10ah LiFePO4 . Again, Amazon sells them for $59.99.
A word about charging these batteries. First off you need a charger that is designed to charge LiFePO4 batteries. The voltage of these batteries is slightly higher then SLA batteries. You can use an SLA type charger on these batteries but they won't charge the LiFePO4 batteries to their full capacity. It's best to use a charger designed for these batteries.
The charging rate can be anywhere for 10% to 30% of the batteries rated capacity.
Example: To charge a 20ah LiFePO4 battery, the charging rate can be between 2 and 6 amps. 2 amps is 10% of the battery's capacity and 6 amps is 30% of the battery's rated capacity.
Pictured above right is a good little dual purpose charger. It has a mode change button so it can be use on SLA as well as LiFePO4 batteries. Again, it can be found on Amazon.
It's well worth the $22.99 Amazon charges.
I use mine to not only charge the above batteries, but I also put it in the SLA mode and trickle charge my motorcycle battery and garden tractor battery while in winter storage.
A word on life span:
Most SLA batteries last anywhere from 4 years up to 10 years. Depending on where they are used, you may not notice the actual capacity decreasing.
When it comes to LiFePO4 batteries, if not abused, they can easily last up to 20 years.
Bottom line, LiFePO4 batteries may cost two to three times the cost of SLA batteries, but in the long run you are saving money. You are really getting a big bang for the buck !
A word of caution:
The above batteries are not designed to be use for starting engines. DO NOT replace or try to jump start your garden tractor with any of these batteries. They do make packages using LiFePO4 batteries for motorcycles, cars and garden tractors, but the above are not them.