FLKiaEV wrote:No issues whatsoever. I tested them individually about a month ago and they were still showing excellent voltage (13.something IIRC). I will say that if you are planning on doing this with a parallel setup, do take the time to make sure that the batteries are voltage-matched before connecting them to each other (within a few hundredths of a volt if possible). For example, if you test one at 12.9 volts and the other at 13.3V, I would charge up the first one for a few minutes until it is closer to 13.3V so there are no imbalance issues right from the start (which can cause one to get a little hotter than necessary). Be sure to secure them to each very well and to the battery holder. You might have to get creative here...
And honestly, one 20AH battery will almost certainly be more than enough unless you're in the habit of running your headlights for a long time with the car turned off, or blasting the radio for long periods of time. I lucked out as to how I got the two that I have and was already expecting to use just one, which should be fine for virtually everyone.
Score! ReLiOn has rolled out a 50 Ah LT (low temperature) battery. It's about twice the price of the 20 Ah LT battery, so more desirable (in a few ways) than a 2x20 Ah set-up. Obviously your 2x20 set-up was better for you as one of them was free!
Interesting that the stock battery is in the 45 Ah range, despite your understanding that even 20 Ah would be satisfactory for most people. I guess they've built in some redundancy that way. The 50 LT is the sweet spot anyway, about double the price of the 20 Ah, but slightly less than half the price of the 100 LT.
How long is the stock 12v likely to last? It's commonly said that 3-5 years is typical for 12v batteries in cars, but the #1 stressor is being used as a starter battery in an ICE car. Perhaps it'll last even longer in an e-car?