Trickle charge for 4.5V LED fairy lights or 3xLED candles (parallel) - normally using 3xAAA

Thread Starter

advarp

Joined Jan 12, 2018
57
Hello there,
I am using sets of copper thin lead LED fairy light strings for my kitchen & bar benchtops, mirrors etc. They are usually operated from 3xAAA batteries (so 4.5V) but of course do not last very long on batteries so I have attached 5V mains adaptors (ordered 4.5V adaptors as well now but 5V is a lot more common due to Phone etc USB charging).
I'd like to keep the control boxes (which hold the batteries normally) for the LEDs due to the included timers (6hrs on, rest off, cycle). The prob is of crs that if the power fails, when restored, the LEDs never come on (one needs to press a button on the controller to turn them on perm or on timer) or they come on permanently. So I would like to have 3xAAA rechargeable batteries in the housing as backup for temporary power failures.. The problem is, I have no idea what circuit to use, preferably a very simple, few components one that fits inside the case as well (maybe). So I would like the rechargeables to be trickle charged permanently (or the charge to switch off when they are full but to restart of course when charge drops).

Any suggestion for a simple circuit I can build for this purpose (maybe a link etc)? So feed in 5V or 4.5V DC, power the LEDS but also trickle charge / keep charged the rechargeables for eventual power failure so the timing is not lost.

Right now I have just soldered the adapter terminals on top of the battery terminals for the fairy lights and for the LED candles, I have fitted DC sockets that switch off the battery side when the DC plug goes in. All good... until the power fails :)

Many Thanks!

PS Of course I can have mains timers switching the power adapters - I am using that for some 'dumb' LED candles (no own timer). The mains timers have battery backup themselves so they do not lose the timing info, but still the candles do not light up of course if mains is not present.

PPS some lights use 3XAA, probably no difference int the trickle charge circuit (?)
 

LesJones

Joined Jan 8, 2017
3,510
Insert a diode (1N4001 for example) betwee the battery positive and the connection the the electronics in the LED lights. (The end of the diode with the band to the electronics) Connect another diode between the positive of your 5 volt power supply and the band end of the first diode. (The two band ends connected together.) The LEDs will be slightly dimmer when working from the batteries due to the forward voltage drop of the diode. (0.6 - 0.7 volts.) The diode from the 5 volts will also give this volts drop so the LEDs will now ve fed with 4.3 - 4.4 volts.

Les.
 

Thread Starter

advarp

Joined Jan 12, 2018
57
Insert a diode (1N4001 for example) betwee the battery positive and the connection the the electronics in the LED lights. (The end of the diode with the band to the electronics) Connect another diode between the positive of your 5 volt power supply and the band end of the first diode. (The two band ends connected together.) The LEDs will be slightly dimmer when working from the batteries due to the forward voltage drop of the diode. (0.6 - 0.7 volts.) The diode from the 5 volts will also give this volts drop so the LEDs will now ve fed with 4.3 - 4.4 volts.

Les.
Thanks for that, this is to feed the circuit separately but I woudl like to trickle charge the backup batteries at the same time with some simple circuit (monitoring the charge current or something?)
 

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LesJones

Joined Jan 8, 2017
3,510
Ni-Cd and Ni-Mh cells have a lower voltage output than primary cells. They are about 1.2 vots comapred with 1.5 volts per cell. So you would only get 3.6 volts from 3 rechargable cells. You could trickle charge using just a resistor and diode provides the charging current was less than C/10. To avoid the voltage drop of diodes you could use a small relay that was held closed by the 5 volts supply (Selecting the 5 volts supply.) but you would also need quite a large capacitor to power the lights for the time taken by the relay to switch over to batteries.

Les.
 

dl324

Joined Mar 30, 2015
13,286
I have some of those LED candles and they work fine with NiMH batteries.

If you want to trickle charge NiMH, I'd use C/40 because they don't tolerate overcharge as well as NiCd. For NiCd, you could go with C/10.

upload_2018-1-16_6-12-4.png
5V is from the adapter. D1 protects against reverse polarity. Wattage for R1 depends on current.

I chose the nominal cell voltage of 1.2V for the resistor calculation.

EDIT: D1 can be 1N4001. My schematic editor only has 1N4004 and I didn't change the value...

Caveats.
  1. Using batteries in series can cause the weaker cell to be damaged by stronger cells if the weak one is allowed to be reverse charged.
  2. A weak battery will spend more time in overcharge than the stronger cells. This will accelerate it's demise.
  3. There is no protection against high current if one or more of the cells are damaged or deeply discharged.
  4. There is no protection from overcharge with the adapter connected.
  5. It would be best to use batteries of similar "strength".
  6. Don't mix batteries of different capacities.
  7. Don't mix batteries of different chemistries.
  8. Don't attempt to charge primary batteries.
  9. There is no protection from reverse charging if batteries are inserted backwards.
 
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Thread Starter

advarp

Joined Jan 12, 2018
57
Thank you, so R1 value is...0.7 V/I ? Which would be...?
The rechargeables are 1.2V 900mAH, charging 14h@75mA or 4h@300mA.
I was thinking of a more sophisticated schematic where the charge stops altogether ig they are charged so they do not get damaged but if C/40 is Ok indefinitely... (C/40 is in this case 22.5mA, so the resistor should be..33.7Ohms?? and no idea how many Watt ?)

EDIT: I am reading that trickle charge is actually bad for Ni-MH (which these batteries are)...
http://www.candlepowerforums.com/vb/showthread.php?66261-NiMh-trickle-charging

And others recommend only 3mA for AAA and 8mA for AA, so 300Ohms and x Watt for AAA? I'm a bit lost here
 
Last edited:

LesJones

Joined Jan 8, 2017
3,510
To build a charger with proper end of charge detection I suggest you Google something like "Ni-Mh charger chips" End of charge detection is quite difficult for Ni-Cd and Ni-Mh cells. The negative delta method is normally used. You could also design your own charger circuit using a microcontroller with a built in ADC.

Les.
 

Thread Starter

advarp

Joined Jan 12, 2018
57
Thanks for that, well I am also assessing whether it is worth doing this in view of the need for a proper charger or 1) forget about it or 2) just have the batteries via a diode and just take them out charge them every now and then in a separate Ni-MH charger. Since I only need the backup for the odd event, maybe 2) is a solution or just reset the timer after power fail , i.e. 1). Or, just bypass their own timer and use a mains timer which has a backup and put up with no candle light when power is out (they are quite handy then actually!).
 

LesJones

Joined Jan 8, 2017
3,510
I think your decision will be based on how reliable your mains supply is in your part of the world. If it was me I would just use primary cells a backup as it is very unusual to have a loss of power as long as a second and this would only happen a few times a year.

Les.
 

dl324

Joined Mar 30, 2015
13,286
Thank you, so R1 value is...0.7 V/I ? Which would be...?
The rechargeables are 1.2V 900mAH, charging 14h@75mA or 4h@300mA.
You can't safely charge a NiMH battery at a C/3 rate without additional circuitry to detect NDV, temperature, and have a fail safe timer.
I was thinking of a more sophisticated schematic where the charge stops altogether ig they are charged so they do not get damaged but if C/40 is Ok indefinitely... (C/40 is in this case 22.5mA, so the resistor should be..33.7Ohms?? and no idea how many Watt ?)
Trickle charging for NiMH is not recommended for longevity, performance, or safety. In your case, adding the preferred charging circuitry would require you to build one, in a small enough package, for each of the candles you want to power in this way.

Trickle charging at C/40 is a compromise for your convenience. It lets you keep the batteries topped up, but does nothing to charge the batteries properly.

To charge a 900mA mAh capacity battery, C/40 would be 22.5mA. R = .7V/22.5mA = 31.1 ohms. The nearest 5% value is 33 ohms (you round up to the next value so the current will be less; going the other way would give greater than C/40).

Power is P = IV = I^2R = V^2/R = (0.7V)*(0.7V)/33 ohms = 0.15W, so you could use a 1/4W resistor. An ultra conservative design would derate to 50%, but I wouldn't have an issue with 40% in an application like yours; 1/2W would be overkill.
EDIT: I am reading that trickle charge is actually bad for Ni-MH (which these batteries are)...
http://www.candlepowerforums.com/vb/showthread.php?66261-NiMh-trickle-charging
It's only bad in that it isn't a "proper" charging method. At C/40, you're in little danger of having a battery leak or explode while keeping it topped off.

If you really care about battery longevity and safety, you shouldn't attempt to charge the batteries and remove to charge in a proper charger.
And others recommend only 3mA for AAA and 8mA for AA, so 300Ohms and x Watt for AAA? I'm a bit lost here
A "safe" trickle charging rate depends on the mAh capacity of the batteries. I'd ignore the numbers given because the source is clearly suspect (or you quoted something that was correct with insufficient context).
 

dl324

Joined Mar 30, 2015
13,286
You could try a LiPo charge controller and an 18650 cell.
That would require the OP to retrofit all of the LED candles to accommodate 18650 batteries and include a charger. My impression is that the OP is looking for a simpler solution.
 

Thread Starter

advarp

Joined Jan 12, 2018
57
Yes well that could fit in larger candles, but really some are set on top of cupboards etc where a separate box is not a big problem. When using an adaptor, I would run cables between them and to the adaptor.

The problem is really that strings of LEDS do not run long on batteries (especially AAA) and LED candles also need recharging and then restarting at the right time so that the timer starts to cycle - and using an adaptor is fine as long as the mains does not else the candles / lights timers start cycling from the time power comes back...

Maybe using normal cells for backup is indeed the simplest solution, I was so taken with rechargeables etc that I overlooked that fact! Power fail may indeed be just a few times a year here in AU but it can take from 10 mins to even 2 days when it does happen (rarely over a few hours).

However, I have a fairly important question:

Now, if using the very first proposal with the batteries and adapter connected via diodes, will the batteries be drained? They woudl provide 4.5V (normal cells) as opposed to 5V coming from the adaptor, both via the 1N400x diodes. What does actually happen there? The batteries will nto be 'charged' due to the diode, but will they contribute to the power supply in any way considering their voltage is lower? I woudl think not, but can you confirm? A simple 'automatic switch' woudl be good, so the diodes provide that?
 

dendad

Joined Feb 20, 2016
3,924
That would require the OP to retrofit all of the LED candles to accommodate 18650 batteries and include a charger. My impression is that the OP is looking for a simpler solution.
That may be so, but I took it to be all the LEDs were connected to a 5V plug pack anyway so just use one cell to power them all. Put the cell with the charge controller in a box at the plug pack end and run the lot on it. Then don't put any batteries in each LED case.
 

Thread Starter

advarp

Joined Jan 12, 2018
57
I would need an answer to this if anyone woudl oblige please:

However, I have a fairly important question:

Now, if using the very first proposal with the batteries and adapter connected via diodes, will the batteries be drained? They woudl provide 4.5V (normal cells) as opposed to 5V coming from the adaptor, both via the 1N400x diodes. What does actually happen there? The batteries will nto be 'charged' due to the diode, but will they contribute to the power supply in any way considering their voltage is lower? I woudl think not, but can you confirm? A simple 'automatic switch' woudl be good, so the diodes provide that?
 

LesJones

Joined Jan 8, 2017
3,510
The batteries will contribute no power to the lights as long as the "5 volts supply" is higher then the "4.5 volts" from the batteries. I have measured new alkaline cells that gave 1.62 volts which would mean that your "4.5 volt battery" was supplying 4.86 volts. You would need to make that your "5 volt" supply was greater than 4.86 volts.
Les.
 
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