battery charger question

[RT]

does anyone have a circuit schematic for a battery charger ,for chargeing 2 6 volt/ 4ah batteries wired in paraellel at the same time,also would like the charger to show 2 states, off ,charging . also want the circuit to automatically shut off when the batteries are fully charged, red led on showing full charge but charger is automatically turned off..chargeing, yellow led on, hope that makes sense,i know u can buy them there called smart chargers but makeing one might be cheaper then the one i found for like 40 bucks.any ideas or help would be appreciated **** THANKS***

 

[k7elp60]

I think you are talking about Lead-Acid type batteries. Whether the batteries are flooded(liquid) or gel-cell they can be charged at 6.75 to 6.9V and they will never overcharge. This is called float charging. It is not a good idea to parallel charge the two batteries, unless they are the same date code and have been discharged in parallel. They can be charged seperately or together if they connected to the charger with a diode inseries, then the charger voltage has to be increased to offset the diode drop.
If you have a digital voltmeter, I can post a schematic of a charger that you can build that uses a wall wart for power. You will have to adjust the output voltage.
A lot of the smart chargers charge the battery at a slightly higher voltage, sense when the current drops to a certain level, and then either remove the charge voltage or step it back to the float level.

 

[RT]

yes any schematics would be great ,and i do have a multimeter ,,as far as adjusting the output ,im assuming with a voltage regulator and what out put would it need to be set at 7 volts or so.and the battires are identical bought as same time,i no it dont nessasary mean same date they were manufactured. all double check but im willing to bet there pretty close to identical ,....once again *****THANKS*****


And yes they are SLA battires......

 

[TheOne]

Here is the basic idea
https://www.aaroncake.net/circuits/charger1.htm

 

[RT]

would 7.5 volts /800 mA be sufficent to charge my 2 batteries in parallel 6volts/4ah(x2) or do i need to boost the amps some more there SLA






*****THANKS*****

 

[k7elp60]

7.5 volts is too much, maximum recommended is 2.45 volts per cell or 7.35 for a 6V battery. Then once the battery is charged(current drops to .01C the charger needs to be removed or set back to the float voltage of 6.75 to 6.9 volts.
Here is a schematic of a 6V charger. The 1N5400 diode is very important as it protects the LM317 if the AC power is lost. Adjust the pot for 7.6 volts at the junction of the 240 ohm resistor and the 1N5400 diode. That is without the charger connected to the battery. The LM317 should be on a heatsink. The charging current depends on two things the current capabilities of the input voltage source and the state of charge of the batteries.

 

[audioguru]

The LM317 controversy again.
It is spec'd with a minimum load current of 10mA. The 240 ohm resistor from its output to its adjust terminal and to the pot provides only a 5.2mA load. If the IC is low-spec, its output voltage will rise.

Many engineers make this mistake because the 240 ohm resistor is shown on National's datasheet right on the cover page. They don't read the fine print that the circuit uses a premium priced LM117 (minimum load current of 5mA).
Use a 120 ohm resistor and a 1K pot. Then you can measure its output voltage without any other load and it will be accurate.

 

[FRIED]

Put the diode across the in and out of the regulator (reverse biased)

 

[RT]

ok great guys thanks for the info and the circuit diagram going to start working on this ************ Thanks for the help**************

 

[k7elp60]

Put the diode across the in and out of the regulator (reverse biased)

The National data book that I have also says if the output capacitor is less than 25uf the diode is not needed.

It is spec'd with a minimum load current of 10mA. The 240 ohm resistor from its output to its adjust terminal and to the pot provides only a 5.2mA load. If the IC is low-spec, its output voltage will rise.

The same date book also lists a chart for minimum load current, the minimum load current depends in input-output differential voltage and temperature. For 40 volts the typical for the 317 is 3.5mA. For this input-output differential voltage and up to 150 degrees C the typical minimum load current is 1.5mA.

 

[audioguru]

For this input-output differential voltage about 4V and up to 150 degrees C the typical minimum load current is 1.5mA.

Hi K7,
I don't want to argue about a few ohms of load, but:
You are talking about the premium-priced LM117's (150 degrees) and you listed "typical" spec's. I would think that National selects the "best" ICs and sells them as LM117, the lower spec'd ones are labeled LM317.
They guarantee a minimum load current of 10mA or less for the LM317 (with a 40V differential) so if you want to gamble, use just 5.2mA and hope that the ICs you get are "typical".

I always use "minmum and maximum" spec's to avoid problems like the big shipment you recently received had ICs all with the same date code, and they were all "low spec". The next shipment might be "typical" if you can wait that long but what are you going to do with the low spec ones if your circuit is designed for typical ones?

 

[k7elp60]

Audioguru,
You may be right. A better solution would be a 560 ohm resistor in series with red, green, or yellow LED and this assembly in parallel with the 1.0 uF tantalum capacitor. Then there would be a power on light for the charger, and there would be a 10mA load on the regulator.

 

[audioguru]

Hi K7,
Good idea! Use a pilot light as the minimum load.