Homemade SLA charger

[BatteryCharger]

I've decided I want to make nice SLA charger rather than using the wall wart I have been using. I want to do this because 1. I'm a cheap ******* /ubbthreads/images/graemlins/smile.gif and 2. I've done alot of electrical work but never really put together a circuit, so I want to try it. /ubbthreads/images/graemlins/smile.gif

I'm going to try and do this in 3 steps:
1. Constant current charger with LM317 and a switch to select different charging currents.
2. LED voltage meter using LM3914.
3. Full charge cut off using LED output from LM3914.

I'm still unsure of how to do number 3. Can somebody explain to me how I could cut off charging to the battery when the highest voltage LED in the meter comes on?

Any other thoughts on my charger? Good? Bad? Better way to do it?

Thanks /ubbthreads/images/graemlins/smile.gif

 

[Doug Owen]

[ QUOTE ]
BatteryCharger said:
I'm still unsure of how to do number 3. Can somebody explain to me how I could cut off charging to the battery when the highest voltage LED in the meter comes on?

Any other thoughts on my charger? Good? Bad? Better way to do it?


[/ QUOTE ]

Sure, several ways. Perhaps easiest is to add a resistor in series with the Adj. lead on the 317 (say a few thousand ohms). Attach the collector of a NPN to the Adj. lead (ground the emitter). The base is driven (through say one K) from the chosen LED output. A series diode in the charging path might be in order.

As far as a different approach, if you check the 317 specs you'll find a current limited, voltage regulated circuit using the 317, 3 resistors and a small signal NPN. Look for the 'Current limited 6 Volt charger' at the end, and scale as needed for 12 Volts. You could switch select the resistor for current sense, or just pick one based on the wall wart or battery capacity. You can also get a four level readout using the 339 a whole lot cheaper than the 3914. Four levels is typically enough. And you can get 339s four for a buck. You'll need a reference and a few resistors, of course.

Doug Owen

 

[BatteryCharger]

Thanks, Doug. I'm sure I'll have more questions once I get all my parts together. /ubbthreads/images/graemlins/smile.gif

Anybody else have any thoughts on my charger? Ways to improve it? If it works out I think I'm going to make a NiMh charger the same way...

 

[Doug Owen]

[ QUOTE ]
BatteryCharger said:
If it works out I think I'm going to make a NiMh charger the same way...

[/ QUOTE ]

I'd rethink that part. NiMH are a whole other kettle of fish, calling for an entirely different charge technique.

Doug Owen

 

[georges80]

I just made a few SLA chargers for my 6V 4.5Ahr gell cells like used in coleman rechargeable fluoro lanterns.

I used a BQ2031 (used to be benchmarq, now owned by TI) - very nice chip, has the hardware control to implement a buck converter. Also has inputs to set all sorts of parameters like bulk voltage, float voltage, current sense for when to switch. 3 LED output to show all phases of the charging process. Temperature sensor (thermistor) input for locking out charging when the ambient temperature is out of the safe region. I've used that chip for a few yrs and it works well. Finally ran some PCB's so I could make them easier. TI supplies free samples too... Visit www.ti.com and search on BQ2031 - datasheets & appnotes can be found there.

cheers,
george.

 

[BatteryCharger]

[ QUOTE ]
georges80 said:
I just made a few SLA chargers for my 6V 4.5Ahr gell cells like used in coleman rechargeable fluoro lanterns.

I used a BQ2031 (used to be benchmarq, now owned by TI) - very nice chip, has the hardware control to implement a buck converter. Also has inputs to set all sorts of parameters like bulk voltage, float voltage, current sense for when to switch. 3 LED output to show all phases of the charging process. Temperature sensor (thermistor) input for locking out charging when the ambient temperature is out of the safe region. I've used that chip for a few yrs and it works well. Finally ran some PCB's so I could make them easier. TI supplies free samples too... Visit www.ti.com and search on BQ2031 - datasheets & appnotes can be found there.

cheers,
george.

[/ QUOTE ]

Wow, that chip does look pretty good, but all the datasheets might as well been in chinese for me. /ubbthreads/images/graemlins/smile.gif

While we're talking about it, what is a good charging current for SLA's? What is the max I can use?

 

[georges80]

[ QUOTE ]
BatteryCharger said:

Wow, that chip does look pretty good, but all the datasheets might as well been in chinese for me. /ubbthreads/images/graemlins/smile.gif

While we're talking about it, what is a good charging current for SLA's? What is the max I can use?

[/ QUOTE ]

What SLA are we talking about. Model? Amp/Hrs? Voltage?

Most are charged at a fraction of their amp/hr rating - 1/3 or 1/4 for smaller ones. You need to limit the current to that level and also ensure you limit the voltage. Basically an SLA wants to be charged at constant voltage (with a current limit as above). Once the charge current drops to a very small percentage (some # of milliamps for example), you then switch to a float voltage which is less than the bulk voltage. You can apply the float voltage indefinitely, but not the bulk voltage.

george.

 

[BatteryCharger]

[ QUOTE ]
georges80 said:
[ QUOTE ]
BatteryCharger said:

Wow, that chip does look pretty good, but all the datasheets might as well been in chinese for me. /ubbthreads/images/graemlins/smile.gif

While we're talking about it, what is a good charging current for SLA's? What is the max I can use?

[/ QUOTE ]

What SLA are we talking about. Model? Amp/Hrs? Voltage?

Most are charged at a fraction of their amp/hr rating - 1/3 or 1/4 for smaller ones. You need to limit the current to that level and also ensure you limit the voltage. Basically an SLA wants to be charged at constant voltage (with a current limit as above). Once the charge current drops to a very small percentage (some # of milliamps for example), you then switch to a float voltage which is less than the bulk voltage. You can apply the float voltage indefinitely, but not the bulk voltage.

george.

[/ QUOTE ]

I will be using it to charge batteries from about 3 to 17 amp hours, and it will have a switch for different charging currents. So...C/3 will be ok?

 

[georges80]

[ QUOTE ]
BatteryCharger said:
I will be using it to charge batteries from about 3 to 17 amp hours, and it will have a switch for different charging currents. So...C/3 will be ok?

[/ QUOTE ]

Should be (C/4 or C/5 is safer), but I would check the specific datasheets of the SLA that you're using. Also, you need to ensure that the basic design is a constant voltage charger with a current limit. You can look at powersonic web site for datasheets for various SLA's. They are probably similar to the batteries you're looking at. They have reasonably comprehensive datasheets on their website and you can read more about their recommended charging requirements.

george.