LED from battery - odd behaviour

I am getting ready from my solar install in my shed in the coming days. As a test I got together some old batteries (some UPS and an old golf trolley) and charged them up from the mains. The golf trolley bat is in the best condition and after charging I fully charged a large USB power bank (over 8 hours) from it and the voltage is still sitting at 12.8, 2-3 days later.

I have some low power (4.5W) LED strips that I hooked up on a switch from the battery to see how they'd look. There are 4 in total. When I switched them on the first one was bright, the second one not as much as the way to the last one which was noticeably dimmer. Maybe I wired them wrong, in series maybe? No, they are wired correctly.
So, I went right back and hooked up one LED on it's own with a switch between the battery and LED. If I switch on the switch and hook up my multimeter it shows the same ready as the volt monitor on the battery ~12.7. As soon as I touch the + on the LED, it illuminates but the voltage drops to 11.0. Obviously, when I add the next LED the voltage is down to ~ 9.0V and so on.

Does anyone know what this might be? I know they are wired correctly but I am surprised to see the voltage drop so much when only LED one is wired up (and all the others are disconnected).
BTW, the voltage monitor on the battery stays at 12.7 (ish) the whole time. The voltage only drops on the LED side (on the multi meter) when the LED is powered up.

I hope this makes sense! If it does, has anyone any ideas?

thanks

You either have a bad connection or the wire is too thin.

BiduleOhm said:

You either have a bad connection or the wire is too thin.

Click to expand...

Yes, thanks. The wire that came with the LEDs was hair thin - terrible stuff. I put some decent cable on there and we're getting the right voltage across the all the lights.