battery for long term trickle charge

[James S]

Hi Folks,

Been experimenting with LED nightlight circuits and the next logical step is to add a battery so that they can continue to run when the power goes out. The requirements of such a system are different from a flashlight since they batteries can be constantly trickle charged and will only be actually used very rarely.

making things that fit into a regular night light sized housing is not completely necessary. I am currently thinking of adding the circuit to the base of some lamps or even to mounting it behind a switch plate near the floor for lighting stairs and such. So the batteries don't need to be small.

I know that you can trickle charge a small gell cell batt for many years without any problems, but can I do the same for Nicads or NiMh batts? For the gell cell I need a slightly higher voltage than they are rated for at a very low amperage. Some things I read here seem to say that the current isn't important, but that bothers me a little bit.

So if I had a power supply that supplied 14v at 30ma or so, to light a small string of white LED's, could that just be wired directly to the battery to keep it charged up at the same time?

Some diodes to isolate it from the power lines of course😉

I'd like to avoid using transformers as I feel like that would add a lot more phantom loads to the house, like I dont have enough already! But the night light circuits that block the high voltage with a capacitor seem like an excellent way to provide a tiny amount of current for the trickle charge.

If I wanted to use Nicads or nimh batts in something similar how do I figure out good trickle charge rates and what kind of life from them can I expect?

If anyone has already build similar circuits I'd love to see the schematics.

Thanks,
James

 

[Albany Tom]

I don't know enough about nicad/nimh to comment on them, but for lead acid batteries, you want a contant voltage of 13.8 for a "float" charge. Use a digital meter, as 14.2 would be enough to cook away the electrolite on a gelled battery. The battery determines the current it needs to maintain the float, so you don't need a current limit. You do, however, want a nice regulated voltage. I would think the maximum current drawn from a charged battery would be about 1/100th of the AH rating of the battery, but that's a guess/estimate. I'm certain it's less than 1/20th of the AH reading. (In other words, for a 40 Amp-hour battery, maintaining a float would be less than 2 amps.)

For Nicad/Nimh, the trickle charge/float charge would be some percentage of the AH rating too, but a contant current rather than constant voltage. I just don't know the number, or what the damage is to the cell from overcharging...just heating? Maybe a C/50 rate? (Where C = the 1 hour discharge rate)