Battery Charger

[wreck-it-steve]

Hi,

I have some 12v batteries (the type used in alarm panels) that I need to charge. Sitting on my shelf I have a 50VA 12v AC Transformer and a bridge rectifier... If i put the two together would this safely charge the batteries?

If not what is the best way to do it?

Thanks

 

[ericmark]

A 12 volt RMS transformer will give around 17 volt peak. The battery needs to float at around 13.2 volt. So it would over charge.

The normal way to get 13.2 volt is to use a 7812 voltage regulator and put a red LED on the common pin which has a threshold voltage of 1.2 volts raising to output to approx 13.2 volts. Also the LED gives an indication that it is running.

This does not work with all 7812 regs but will work on most. So full wave rectified then capacitor then voltage reg with red LED on common. Different colours have different voltage so must be red.

 

[wreck-it-steve]

A 12 volt RMS transformer will give around 17 volt peak. The battery needs to float at around 13.2 volt. So it would over charge.

The normal way to get 13.2 volt is to use a 7812 voltage regulator and put a red LED on the common pin which has a threshold voltage of 1.2 volts raising to output to approx 13.2 volts. Also the LED gives an indication that it is running.

This does not work with all 7812 regs but will work on most. So full wave rectified then capacitor then voltage reg with red LED on common. Different colours have different voltage so must be red.

The transformer is a wire wound panel transformer (the type used in electrical control panels). Is there any easy "cheap" way to charge these type of batteries?

 

[bernardgreen]

Is there any easy "cheap" way to charge these type of batteries?

The suggestion from Eric is about the cheapest you can get a decent charger. The 7812 controls voltage and limits current to a ( reasonably ) safe level if a battery goes faulty during charging.

If you try and cut the price lower you might charge the batteries but at the cost of reducing their life and/or capacity significantly. You might even explode them due to severe overcharging.

 

[ericmark]

A battery charger like this costs £4. It is unlikely you can build one for that price. To build items for the fun of building yes but by time you buy a box to mount it all in, cable, rectifier, voltage reg, and LED to try and build for less than you can buy is not really going to work.

I found this out a long time ago. I lived on a caravan park full of contractors caravans most touring vans and we all wanted to charge batteries to run our lights and water pump. The cheap battery charger has no control it is designed that way as in the main they are designed to put on a car battery every 6 months to give it an equalling charge. Mainly from dynamo days.

This would result in one of two problems. Either the battery would go flat or run short of water due to being over charged. Either way batteries did not last long. Today you can buy regulated battery chargers without a problem but back then they were rare.

But if one tried to put a transistor in the supply the 0.6 volt drop was too much for a standard battery charger to then give full output. So I made a unit which would turn excess into light and heat. An Opamp compared the voltage output to a zener controlled standard and when the voltage raised it would turn on a transistor (2N3055) which powered a 36W lamp. This stopped the overcharging and also gave visual indication that the battery was charged. First one made with bits out of junk box. It worked well so I decided to make more. However this is where it all failed. It would cost around £10 and by time I added my time it just was not worth making.

The battery charger typically has a very poor quality transformer and it uses this to produce a natural steep cut off at the battery becomes charged so for a 8 amp car battery charger at 12v it will supply 8A but as the volts rise by time it reaches 14.5v the charge rate is around 0.5A.

So there is the unregulated. The float voltage regulated. The 3 stage step battery charger, and the pulse battery charger each getting more expensive. The last two are designed to quick charge. In the old days of open cells the hydrometer allowed us to monitor the battery. Today however with sealed batteries not so easy. So we have to decide good charger or short battery life.

Last Radio Rally I went to regulated battery chargers for small 12AH batteries were going for around £10.

 

[maltaron]

SLA batteries should be charged from a constant voltage supply, normally 13.8 volts.

 

[ericmark]

Although the guy has said they are small gel filled lead acid batteries he has not said what they are used for.

With a standby system then using a 13.2 volt regulated supply is fine. Where the battery is going to be used from time to time one may wish to increase that voltage to 13.8.

Where the battery is used for deep cycle then a completely different approach is used. One method is to have a voltage triggered delayed switch so when for example 16 volt is reached it give 15 minutes more and switches off. This is the basic way a forklift charger works and is nearly consent current.

But it's more down to what the battery is used for than the type of battery used. Clearly batteries used for deep cycle are built slightly different to those used for standby but from outside they look the same.

A kids toy could have a trickle charger with instructions to charge for 16 hours only. That's common but could not do that if used in an alarm panel. My mothers mobility scooter has a switched mode three stage charger as one does not want to wait 16 hours before using it again or have to remember to take it off charge.

Narrow boats often have a very costly unit which pulse charges it stops and quickly measures voltage drop on battery to sense the charge state so can charge a bank of 600AH batteries over an 8 hour cruse up the cut. Not unknown to have have two 120A alternators. The big problem with the narrow boat is the battery is still being used so the standard step charger can get foxed and overcharge the battery.

Which brings me back to, "It's all dependent on what the batteries are being used for".