The FR-4x uses rechargeable AA batteries and those have a limited lifespan. Even using the BEST batteries on the BEST chargers, used in PERFECT circumstances, you will see battery life degrade within1-2 years and by the 3rd year, you will really need to replace them, no matter what brand.
For me, I separate rechargeable batteries in to 2categories:
- Batteries where I want maximum play time but regular recharge intervals. This is when you play regularly and recharge often.
- Batteries where I store for long periods of time (5 or more days) and do not want to charge all that often, but I don't play all that long.
In ALL cases, you want a quality charger. Speed kills. The slower you charge the battery, the less it heats up, the "fuller" the charge and the longer the battery lasts. Quick chargers can kill your batteries in a matter of a few months.
In the first case you want batteries that have the biggest "gas tanks" so last longer while you play. These are batteries like the 2650 PowerEX. I've used them for many years in high intensity photography sessions (it was common for me to shoot as many as 6000 pictures over a 2 day event). Normal batteries would last me about 750-1000 shots, but PowerEx batteries would last me in the 2500-2750 range or more when used in lower flash power settings. While using one set, I would be slow charging another at all times, but rarely needed that 3rd set even on the 2-day weddings.
Their one downfall was that if you charged them up and placed them in a closet for a month, when they came out for use, they'd be at about a 40%-50% charge, so they lose energy fairly quick over time.
For the times I had smaller events, like an engagement session, where 250-500 pictures were needed, I could use batteries like Eneloop. Eneloop batteries would get me to the 500-550 picture range before going flat, but I could recharge those several days in advance. Generally speaking they hold less "gas in the tank" but when inactive, don't lose it as much. Recharge a set of Eneloops, place them in the closet and a month later, they're at about 90% full.
Knowing the above will help you choose which battery to use based on your style and needs. In terms of chargers, I use a PowerEx MH-C800 smart charger and even in 2020, they are considered one of the top 3 best chargers on the market. For about $40, you cannot go wrong, and the 2 that I have are working perfectly now over 10 years.
No matter what battery you use, I strongly suggest you remove them if you are storing the accordion for more than a week. All batteries... let me repeat this... ALL BATTERIES have the potential to burst and leak. When they do, they leak battery acid on to the internals, and worse, its not just the acid, but the acid fumes that can cause eventual issues that can cost you hundreds or even thousands of dollars of damage to your 4X.
Hope that helps.