Rechargeable batteries dont work with PIR lights

By a process of trail and error I worked out why my LED lights activated by motion sensors stopped working.

They worked fine for a few months. Replaced the batteries with rechargeable ones and the lights kept coming on even without any motion. They are designed to stay on for 20 secs and shut down if no motion detected. But the lamps would come back on immediately after 20 secs and keep doing this draining the batteries.

Changed the rechargeable batteries to normal ones and they are working fine.

No mention of this anywhere on the internet. So you heard it first here.

Any reason why they should do that?

Just want to point out that rechargeable batteries are usually lower initial voltage than primary (non-rechargeable) batteries. That might be an issue.

I have a digital radio scanner that has a switch to select NiMh cells or alkaline so the battery indicator reads correctly.

Do you have a link to your particular light?

Yeah, nickel metal hydride rechargeables are nominally 1.2 volts, and have a kind of really flat discharge curve that looks like an integral symbol on its side. Which means that they start out at like 1.5, then very quickly discharge to 1.2, where they stay for something like 95% of the battery’s charge, and then at the very end, it drops like a rock from 1.2 to something much lower - 0.8 or something like that.

By comparison, alkaline batteries start out at 1.5 volts, and have a discharge curve that’s more like a line than a curve- the voltage is directly proportional to the degree that the battery is discharged. So if the battery is at 1.5v, it’s 100% full. If it’s at 0.8, it’s entirely discharged (0% full), and if it’s at 1.15v, it’s 50% full.

So devices that look at the voltage to determine remaining battery capacity get all screwed up with NiMH batteries, because they go from full to like 50% capacity really fast, keep that 50% for the entire time, and then go dead with no warning.

For what it’s worth, it depends on the light in question too. I’ve had several that work fine with Eneloop rechargeables. This light is great.

I think that on most scanners, the switch disables the internal recharging circuit. Which radio do you have?

Grecom PSR-800 that I run Eneloops. I recharge them with a Nitecore iD4 charger. The manual says the switch “enables the correct low battery detection level and allows recharging in the radio”

I never recharge in the radio though. I swap in a fresh set of Eneloops.