Transformer for recharging 6V Sealed LEad Acid Battery

[hairydogs]

I have a 6V rechargeable spot light using a 4AH SLA battery but forget which transformer should be used to recharge it. Probably it should be a 6V transformer but I am not sure.

Does it harm to recharge it with a 12 V transformer?

Another question: can I replace the 12V600ma of the LSI spotlight with the 13V 1.2 A surefire transformer? would it do any harm to the SLA battery?

Thanks.

 

[Evan]

There isn't enough information here. I have a Chinese-made lantern with 2 4 w flourescents that uses a 6 volt sealed lead acid and it uses a 12 volt 500mA wall wart to charge it. Inside the light is a bridge rectifier (so the polarity of the wall wart doesn't matter -- it could even be AC!) followed by a few diodes to drop voltage and finally some resistors to limit charging current, all of which together drops about half the 12 volts and leaves the rest for the battery.

On the other hand, a kiddie car runs on a 6 volt 10AH SEL, too, but it uses a transformer that is 6 volts 1.2A and plugs directly onto the battery instead of the car's wire harness.

You could open up the light and trace some wires, if the charging jack is connected directly to the battery + & -, then then a charger about 6 volts is probably right. If the charging jack goes to a circuit board, then it's harder to say without tracing out what is in the circuit.

Another point: Wall warts are rarely the voltage stamped on them. They are a voltage source with a lot of internal resistance. So a 12 volt charger open circuit may measure 17 volts. Hooked to a discharged battery, it will be loaded down to arround 13, and as the battery charges the voltage will rise to something in between.

How safe it is just to try a wall wart depends on the power. Lower current ones, like 500mA or less usually have no fuse and so much internal resistance that even a short won't hurt them. Higher current ones often have a fuse inside the sealed case. Of course the other half of the story in inside the device you plug the transformer into, and it may not like the voltage of a transformer other than the one it came with.

If you can remove the battery, it could be charged externally. Most SEL batteries say what they like right on the case, even thought they all want almost exactly the same thing. For example, a 6 volt 4AH I have here says:"cycle use 7.2v-7.5v, Standby use 6.75v-6.9V Initial current less than 1.35A". If you have a volt meter, you could verify the battery is getting about the right volts using this information, and check the current to the battery tapers to some low "trickle" value once the battery is charged. If, after a reasonable charge time the charge current is still high or the voltage across the battery exceeds the maximum "standby" voltage, there is some danger of overcharging the battery. This does not necessarily mean the charger is wrong or broke, many cheaper systems are a bit of a compromise and depend on you to stop the charge after some reasonable time.

I'd expect it would be OK to substitute the 13 volt 1.2A transformer for the 12 volt 600mA transformer, but you need to know what is inside the device being charged, 1 Amp diodes are used everywhere and 1.2 amp would be a slight overload. On the other hand I doubt the full 1.2A is actually delivered for most of the charge cycle. So you either need to take it apart and find out, or wait to hear from someone who knows those specific lights.

Sorry I can't give a more certain answer.

 

[Evan]

Maybe the simplest thing is go find your light in a store and see what the volts/amps are for the transformer that is packed in the box. (Also don't forget polarity, center minus into center plus won't go)

 

[hairydogs]

Thanks a lot!!!

I think I shall go down to the shop and see what the transformer is.

I also intend to see whether I could replace it with a PIAA superwhite automobile bulb.

thanks again.

 

[hairydogs]

went back to the shop and found the transformer is 9V 500ma while the one supplied with the spotlight is 12 V 300 ma.

It seems the charging voltage is not relevant to these sealed lead acid batteries.

Am I right?

 

[Evan]

Partly right. When charging a dead battery, it will pull the voltage of a typical transformer down, but unless something limits the voltage it will rise as the battery becomes charged and loads the transformer down less. For a 6 volt sealed lead acid battery, you want the final voltage when the charge cycle is done to be around 6.8 volts where the battery will float, neither self-discharging nor cooking away the electrolyte. So in a pinch you could charge with a variety of transformers, but you must manually limit the charge time, disconnecting the charger when the voltage gets up to 7 volts or so, or guesstamate when you think you've replaced what was used. Since Lead-Acid generally has very low self-discharge, manually limiting the charge may suit you just fine, you don't need to keep it always on the charger to be fully charged.

Generally Lead-Acid likes a constant voltage charge and the current tapers down. NiCad generally likes a constant current charge, time limited to about 40% more charge than was discharged. NMH and Lithium I know less about. A low rate trickle charge like 1/50th of capacity is slow but seems to be safe on everything.

 

[hairydogs]

How do I know when enough is enough. The intruction leaflet says 24 hr in the initial charge and 10 to 12 hours for the subsequent charge. The latter seems refer to fully discharged batteries. Do I need some sophisticated instruments? or put a volt meter between the charger and the batteries?

 

[Brock]

For sealed lead acid, you want to stay below 2.25v per cell. In a 6v battery that should be 3x2.25v 6.75v. So try not to go much above that when charging

A good rule of thumb for charging acid batteries is C/10 or C/20 meaning you charge them at their rated amp hours divided by 10 or 20. So again you have a 6v 4AH battery. You want it to be about .25 amps or 250mA for about 15 hours if you have a 6v charger.

You can charge faster, but the faster you charge the less life (cycles) a acid battery will have. The most I would put in that battery is 1 amp for 4 hours.

Hope this helps