Something I've 'known' but never, until today, seen described concerns how much current you can draw from rechargeable batteries. Especially when what you're drawing is a high percentage of a a battery's amp hour rating.
For instance supposing you had an FT817 that draws something like 2A on transmit. You might think that a 2AH sealed lead acid battery is sufficient to keep you transmitting for 2 hours (I'm assuming a continuous duty cycle mode like FM).
But in practice you'll get less. Why? It's something called Peukert's Law.
I hadn't heard of it up to today (where I was researching a question I got asked).
To summarise you need more battery capacity than you might have calculated when you're drawing lots of current, eg when using a radio transmitter, particularly on a high duty cycle mode eg FM, SSTV or WSPR.
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