Voltage to maintain rechargeable cells.

[ericmark]

I have found a lantern, the batteries had burst, so the battery case useless, however it is a nice start to make an emergency lamp, it is 6 volt, used C cells so enough room, so 4 rechargeable cells and a relay will fit in the space. The idea is the relay holds the lamp off, so as soon as supply lost, it switches on. A simple battery eliminator will provide power, but how to stop the battery over charging is the problem.

After we left the NiCad era, I have not been involved with battery charging. It can be very slow, as long as the supply will hold the relay open, that's enough. 1 mA extra would maintain the batteries and in fullness of time recharge them, thinking 4 AA cells, so what voltage/current am I looking for?

 

[Wayners]

Maybe make a power bank like this.

 

[SimonH2]

A lot depends on the battery technology you want to use.
Lead Acid is relatively simple - look at the manufacturer's data sheet and it'll show the different charging regimes - including long term float from a constant voltage (CV) supply.
Anything else, and especially lithium, is ... not trivial. All the modern battery technologies need a dedicated battery controller, you can't just stick a CV supply on them and leave it.

 

[ericmark]

I have been looking at lead acid, 70 x 47 x 107 at 4 Ah is the smallest I can find. My Lidi smart charger would charge it, it's a little too tall, 60 mm would be better, 80 x 80 is space with battery pack removed, but when hunting I found a battery designed for the lamp, at £14 not worth messing around. But reviews are rather poor, found an advert here says 125 lumens high and 80 lumens low, with a 5 or 10 hour life, so assuming 100 lumens per watt, it would seem around 1.4 Ah so thinking about it.

The hurdle is charging the 6 volt lead acid, the Lidi charger would charge it, and I have 3 so not a problem, but one has to push a button to start the charger, basic thing is one needs to select 6 volts, so it will not auto start. So at £13.75 think better going for ready-made

like I already have. OK 3–6 hours only, but no messing around, just plug in.

 

[SimonH2]

There used to be lead acid 6V lantern batteries, and lead acid C and D cells. At one time I considered making a unit with 3xC or D cells (forget dimensions now) and the charge controller into something that would fit within the space of a lantern battery. These days easier to replace the incandescent bulb with LED and use a boost converter off one or two NiMH cells or something like that - if you have a lantern that means enough to you that you want to keep it.