I recently purchased three Lead Acid SLA's .One Chinese 6V and two Yuasa SLA's a 6V and a 12V. I have two automotive chargers with me, a Bosch automatic charger with 6V/12V(0-8A) and a Lucas Heavy Duty Automatic Charger with 6V/12V/24V (0-12A).I have a tiny SMPS 12V 2A circuit combined with an LM2596 buck converter (output:6.82V, 628mA) also with me that i used earlier for a 6V SLA where i used to keep monitoring the current until it drops to ~50-70mA. But the charging takes a really long time, I wanted it to charge faster.So is it safe to charge any of the above batteries with the automotive chargers? I just want to be sure since i don't want to ruin those good Yuasa batteries.

 

No. Car batteries can take very large charge (and discharge) currents while SLA cannot. A car battery charger will easily damage a small SLA battery. Your buck converter will not fully charge an SLA as its output voltage is a little low (but it will do harm).

 

Best would be to buy a low-current charger designed for those batteries such as one of these.

 

For short term bulk charging to get them most of the way up to full charge they will work fine but I wouldn't use them for long term top off charging unless you are there to supervise things.

ALthough, if you are doing active voltage and current monitoring while charging and know where your charge cut off points should be it can be done without issue.

These guys have pretty easy to follow explanation of what you would be looking for and given their numbers if you know what your charger puts out it may be possible to make your own charging adaptor circuit of sorts to make your automotive battery charger work as a basic SLA charger.

http://www.powerstream.com/SLA.htm

 

No. Car batteries can take very large charge (and discharge) currents while SLA cannot. A car battery charger will easily damage a small SLA battery. Your buck converter will not fully charge an SLA as its output voltage is a little low (but it will do harm).

Thanks for the reply. Yes you are right I was worried too that if I connect the car battery charger that the SLA might heat up and explode or something. The buck converters output can be varied, I set the output to 6.85V just to be safe and not damage the SLA by overcharging in case I forget to remove the charger. I usually remove it from the charger when the terminal voltage reaches 6.7V and the current is ~4-3mA.
I tried to discharge one of the 6V batteries yesterday @ 2.6A and got the voltage to 6.11V. Today put the battery on the charger I made from buck converter for 3 Hours 40 mins. Here are the results below:
The first two pictures are the terminal voltage and the current while charging and the last two pictures are the buck open voltage and short circuit current.

 

Thanks everyone for the feedback. Guess I'll keep away from Automotive charger since there is risk of explosion in case I forget to remove it or something. From the link given above by @crutschow the cheapest SLA charger costs about $10 in US while the same model here on amazon.in costs about $45.The better ones are $100+. At what voltage and current is it safe to stop charging if I where to do it manually?

 

At what voltage and current is it safe to stop charging if I where to do it manually?

It often says this on the battery itself.

 

It often says this on the battery itself.

The battery label says
Cycle use:7.2-7.5V
Standby:6.75-6.9V
Max charging current:1.35A
So my charging current of 0.68A is safe I guess. But the charging current decreases as the battery reaches it full charge from what I have heard. So at what current should I stop charging? Like at 100mA or like at 4mA ? Currently I stop when the current draw is at 4-3.5mA.

Thanks.

 

Stop when the current stops decreasing.
The cycle use voltage is the charging voltage to use when you are charging, disconnecting the charger, discharging and then recharging.
Standby voltage is for when you are leaving the battery permanently connected to the charger so it is always ready to use.

So if I understand what you are trying to do then you should be using a charging voltage between 7.2V and 7.5V

 

Stop when the current stops decreasing.
The cycle use voltage is the charging voltage to use when you are charging, disconnecting the charger, discharging and then recharging.
Standby voltage is for when you are leaving the battery permanently connected to the charger so it is always ready to use.

So if I understand what you are trying to do then you should be using a charging voltage between 7.2V and 7.5V

Thanks. So I'll try using one of the batteries at 7.2V like you mentioned for a week or so and see how it holds up. Also should I reduce the voltage from 7.2V or keep it at the same voltage after the current becomes stable?

 

Thanks. So I'll try using one of the batteries at 7.2V like you mentioned for a week or so and see how it holds up. Also should I reduce the voltage from 7.2V or keep it at the same voltage after the current becomes stable?

No, you shouldn't leave it at that voltage for long term (even a week).
That's what the standby voltage rating is for.

 

The battery label says
Cycle use:7.2-7.5V
Standby:6.75-6.9V
Max charging current:1.35A
So my charging current of 0.68A is safe I guess. But the charging current decreases as the battery reaches it full charge from what I have heard. So at what current should I stop charging? Like at 100mA or like at 4mA ? Currently I stop when the current draw is at 4-3.5mA.

Thanks.

Given that battery spec that falls right into normal 12 volt automotive charger ranges of 14.4 - 15 volts for bulk charge and 13.5 - 13.8 volt float.

If it was me I would run a battery on a normal charge cycle with the automotive charger and see where it's actually ars running numbers line themselves out. You may very well find that for your battery the charges you have will do just fine as long as you don't leave them on for days at a time.

The only thing I would initially watch for is not going over the 1.35 amp charge rating for more than a few minutes after you first connect. If the battery drops down to something close to or below that after the first 3 0 5 minutes you would likely be good to go.

 

No, you shouldn't leave it at that voltage for long term (even a week).
That's what the standby voltage rating is for.

Sorry, I meant charging it at 7.2V till the current drops and becomes constant and then use the battery(discharge and recharge) like this for a week and see how its holding up.

 

Given that battery spec that falls right into normal 12 volt automotive charger ranges of 14.4 - 15 volts for bulk charge and 13.5 - 13.8 volt float.

The SLA is a 6V battery and wants 7.2V to 7.5V. How does that 'fall right into' the 13.5V to 15V that an automotive 12V battery wants?

 

Given that battery spec that falls right into normal 12 volt automotive charger ranges of 14.4 - 15 volts for bulk charge and 13.5 - 13.8 volt float.

If it was me I would run a battery on a normal charge cycle with the automotive charger and see where it's actually ars running numbers line themselves out. You may very well find that for your battery the charges you have will do just fine as long as you don't leave them on for days at a time.

The only thing I would initially watch for is not going over the 1.35 amp charge rating for more than a few minutes after you first connect. If the battery drops down to something close to or below that after the first 3 0 5 minutes you would likely be good to go.

Okay I'll try it on the automotive charger at 6V and see how much current it draws and see if its dropping. Initially I was hesitant because most 6V 4.5Ah SLA chargers do not have large current capacities and are safe. I once accidentally shorted an automotive charger and the connecting clips where glowing red hot, which gave me the impression that if I connect it to such a small battery that I might boil the acid in it and it may explode.

 

The SLA is a 6V battery and wants 7.2V to 7.5V. How does that 'fall right into' the 13.5V to 15V that an automotive 12V battery wants?

I have a 12V 7Ah Yuasa SLA too with me that I recently purchased and haven't charged yet as I wasn't sure if I could connect on those chargers.

 

No both the chargers I have mentioned above one from Bosch (has 6V&12V) and the other from Lucas (has 6V,12V &24V) selectors on it.

Ah, well, that's better, but do keep an eye on that maximum charge current.

 

Ah, well, that's better, but do keep an eye on that maximum charge current.

 

Thanks guys. I'll try charging both the 6V &12 V batteries tomorrow and post the results.

 

The SLA is a 6V battery and wants 7.2V to 7.5V. How does that 'fall right into' the 13.5V to 15V that an automotive 12V battery wants?

The 6V battery label says
Cycle use:7.2-7.5V
Standby:6.75-6.9V
I would think the 13.5v to 15V for the 12V battery corresponds to both conditions.