Hi folks,
I found this transformer from an old 120V 700VA UPS. Took me some time trying to figure out which was the input. Finally figured it out and powered it with my APC 500 inverter(actually I had initially plugged the secondary as primary accidentally but the UPS tripped, guess I was lucky) before trying it on mains. The Yellow, Brown and Red wires are the outputs. Red being the centre-tap. The voltages across output leads are as follows:
YELLOW-BROWN=13.7V AC
YELLOW-RED & BROWN-RED=6.8V AC
Can I use this to make a charger for 12V and 6V SLAs? Also is it possible to use just a D3SB60 rectifier at the output and connect it to a SLA battery?

 

The voltage is about 10 or 20% to low to fully charge them.

You need about 16 VDC as a max. to charge a 12 volt battery

 

It was made to USE battery voltage. 12.6 down to 12.0 VDC.

Charging requires more than that.

 

The voltage is about 10 or 20% to low to fully charge them.

You need about 16 VDC as a max. to charge a 12 volt battery

Hi,
But the would will still be around 14V to 17V after rectification right?

 

Hi,
But the would will still be around 14V to 17V after rectification right?

You should probably use some form of voltage regulation - breaking open SLA vent caps to replace gassed electrolyte would void the warranty.

Commercial "smart" chargers bulk charge then throttle back to 13.6V to reduce gassing - The voltage specifications may be different for SLA.

Constant current charging is common practice with SLA - but you still need some form of voltage management to avoid boiling it dry.

 

You should probably use some form of voltage regulation - breaking open SLA vent caps to replace gassed electrolyte would void the warranty.

Commercial "smart" chargers bulk charge then throttle back to 13.6V to reduce gassing - The voltage specifications may be different for SLA.

Constant current charging is common practice with SLA - but you still need some form of voltage management to avoid boiling it dry.

Yes. That is what I was worried about too. I was thinking about adding a buck converter after rectifying using a high current rectifier. The biggest Ah SLA I've is 6V 12Ah and 12V 7AH. So which buck converter is better in your opinion. The reason I'm asking this is because I'd previously tried using an LM2596 with a 18.5V 6A SMPS supply and both of them blew up while charging. This transformer I've shown in this thread is easily capable of 8-10A. I tried connecting it to a 12V 95W cigarette lighter and it worked. So, I don't want to blow up another LM2596. Just want to make sure I get the right one.
Thanks.

 

Yes. That is what I was worried about too. I was thinking about adding a buck converter after rectifying using a high current rectifier. The biggest Ah SLA I've is 6V 12Ah and 12V 7AH. So which buck converter is better in your opinion. The reason I'm asking this is because I'd previously tried using an LM2596 with a 18.5V 6A SMPS supply and both of them blew up while charging. This transformer I've shown in this thread is easily capable of 8-10A. I tried connecting it to a 12V 95W cigarette lighter and it worked. So, I don't want to blow up another LM2596. Just want to make sure I get the right one.
Thanks.

Sla batteries need Constant Voltage, so your 6v sla needs 6.9V, and the 12v needs 13.8V, so thats cutting it close to the limit direct from the transformer, ideally a 15V or use the laptop charger with a Lm2596 or Lm317.

 

Sla batteries need Constant Voltage, so your 6v sla needs 6.9V, and the 12v needs 13.8V, so thats cutting it close to the limit direct from the transformer, ideally a 15V or use the laptop charger with a Lm2596 or Lm317.

Hi Dave,
Yes I'm aware about that. The label on my batteries say the voltages you mentioned for standby use and slightly higher for cycle use. I've used a laptop charger previously with LM2596 for a smaller 6V 4Ah battery and it worked fine. But anything higher than that and the buck converter blows up.Even LM317 has somewhat similar current limit. As of now I've only 6V 12Ah and 12V 7Ah cells that has higher current draws when charging from 30% etc. Plus I was thinking of adding an ammeter and make this transformer with a rectifier + voltage regulator to make a battery charger. Like making a case for it.

 

Hi folks,
I found this transformer from an old 120V 700VA UPS. Took me some time trying to figure out which was the input. Finally figured it out and powered it with my APC 500 inverter(actually I had initially plugged the secondary as primary accidentally but the UPS tripped, guess I was lucky) before trying it on mains. The Yellow, Brown and Red wires are the outputs. Red being the centre-tap. The voltages across output leads are as follows:
YELLOW-BROWN=13.7V AC
YELLOW-RED & BROWN-RED=6.8V AC
Can I use this to make a charger for 12V and 6V SLAs? Also is it possible to use just a D3SB60 rectifier at the output and connect it to a SLA battery?

The answer is "yes" and "Yes", almost... If the output from Yellow-brown is 13.7VRMS - which it almost certainly is if you used a DMM to measure the voltage, then that's about 19.7 volts, peak.
If you half-wave rectify that 19.7 volts with a single diode you'll lose about 0.7 volts across the rectifier, which will leave you with about 19 volts pulsating DC, which is way more than enough to charge your 12 volt SLA.

By the same token, If you half-wave rectified the output from the center tap to the grounded end of the secondary you'd wind up with about 9 volts pulsating DC which would be more than enough to charge your 6 volt batteries.

The caveats for SLAs are that there's an initial charging current limit for the batteries which must be adhered to, and after that the batteries need to see a voltage limited source until they're fully charged.

 

Below is the LTspice simulation of a way to get both voltages from the CT transformer using a single bridge rectifier.
It uses the 4 diodes in the bridge for the full voltage, and 2 diodes (D3 and D4) as a full-wave rectifier for the half voltage.
It has the advantage of equally using both halves of the output winding for either output current.

C1 and C2 would not necessarily be needed for battery charging.



A further explanation of the circuit can be found here.

 

Yes. That is what I was worried about too. I was thinking about adding a buck converter after rectifying using a high current rectifier. The biggest Ah SLA I've is 6V 12Ah and 12V 7AH. So which buck converter is better in your opinion. The reason I'm asking this is because I'd previously tried using an LM2596 with a 18.5V 6A SMPS supply and both of them blew up while charging. This transformer I've shown in this thread is easily capable of 8-10A. I tried connecting it to a 12V 95W cigarette lighter and it worked. So, I don't want to blow up another LM2596. Just want to make sure I get the right one.
Thanks.

Yuasa published various pamphlets about battery care, the one that would be most use to you seems to have become scarce online - I'd recommend writing to them.

A simplified outlay of procedure is to charge from flat current limited to 1/3Ah rating and a maximum voltage capability of 29V. As it begins to recover charge current the CC source will be pulled down to somewhere around 10.8V you should switch to constant voltage - I think its 14.4V for flooded cell batteries and different for SLA.

Most "smart" chargers detect full charge and throttle back the constant voltage to 13.6V to minimise gassing losses.

 

Yuasa published various pamphlets about battery care, the one that would be most use to you seems to have become scarce online - I'd recommend writing to them.

A simplified outlay of procedure is to charge from flat current limited to 1/3Ah rating and a maximum voltage capability of 29V. As it begins to recover charge current the CC source will be pulled down to somewhere around 10.8V you should switch to constant voltage - I think its 14.4V for flooded cell batteries and different for SLA.

Most "smart" chargers detect full charge and throttle back the constant voltage to 13.6V to minimise gassing losses.

As the battery is discharged its fine.But once the voltage across the battery increses as it reached 80-90% charge, then the voltage needs to be limited.

 

Thanks for the suggestions and explanation everyone.