Maker Pro
Maker Pro

Is this circuit compatible for charging my gel cell?

Status
Not open for further replies.

Farukh Khan

Jun 12, 2015
160
Joined
Jun 12, 2015
Messages
160
As I have already told you that my power source is a 24V 20A power supply. But the schematic posted by me in post #1 have a power source voltage range of 15v - 18v . So, as per discussion I will need some sort of heatsink on the LM317T for the 24V source. But now the question of mine is using a 1 ohm resistor I will put the circuit in a safe charging mode for my battery in case of Amps. But what is the calculation for voltage here? What voltage might get out from the schematic in my post #1 for charging my battery? And will the voltage be safe for charging a 12V Gel Cell? What is the ideal charging voltage measurement for a 12V Gel Cell battery?

The next thing I want to know is if I go to Maxim Chip for charding my battery rather than the schematic of the post #1 then will I be at a safe and fast charging side? And is the maxim circuit is more efficient, fast and most importantly safe for charging my 12V 7.2AH Gel Cell Battery?

Please clear my confusions in these case.

Thanks in advance.....
 

AnalogKid

Jun 10, 2015
2,884
Joined
Jun 10, 2015
Messages
2,884
The LM317 can operate with up to 43 volts between its input and output pins, so the circuit does not need any changes because you have a 24 V source rather than an 18 V source. To be clear, you would need a heatsink for the 317 even with a 15 - 18 V source, just not as large.

The circuit uses he 317 to create a constant current, not a constant voltage, so the calculation is for the current output, not the voltage. The way a constant current source works is that the output voltage varies automatically to hold the output current constant even if the load resistance or impedance changes. So when the battery is completely discharged, the output voltage might be only 4 V but the current will be 650 mA. When the battery is fully charged the output voltage might be 14 V, but the output current still will be 650 mA. This can create a problem, which I will get to later.

To your next question, the calculation for setting the output current has to do with the BC547 transistor and the 1 ohm resistor. That can be almost any small signal NPN transistor such as 2N2222, 2N3904, 2N4401, etc. While all transistors vary, a good estimation of the base-emitter forward voltage is 0.65 V, or 650 mV. The charging current through the battery returns to the source through the base-emitter resistor, 1 ohm on the schematic. When the current is large enough that the voltage across it is 650 mV, the transistor starts to conduct and turn down the 317 output to maintain that 650 mV across the resistor. So the output current calculation is:
Iout = Vbe / R --> I out = 0.65 / 1 = 0.65 A or 650 mA.

Is this safe? Don't know. While the term "gel cell" tells us many things about your battery, it doesn't tell us everything. So we don't know the best safe maximum charging current for fast charging. But generally speaking, many manufacturers recommend that the max charging current be no more than 1/10 the battery's capacity. For a 7.2 A h battery this would be 0.72 A or 720 mA. This is why I think a 650 mA charging current is safe, but note that this is only a guess.

A problem with this type of charger is that it does not automatically reduce the charging current when the battery is fully charged. Leaving a battery connected to this circuit for a long time probably will shorten the life of the battery. This is why I said this circuit needs an alarm clock. The smart battery charging chips from Maxim and LTC have voltage monitors, current monitors, and power control circuits to prevent overcharging and overheating. So they are absolutely safer IF you know the charge details of your battery. The chips are not magical; they limit the charge current and voltage and time and power levels to the values you set with the external components. Also, the circuits are more complex, the chips are more expensive, and can be difficult to purchase in low quantities like one or two pieces depending on what country you are in. And, going back to post #3, you still will need a heatsink.

One way to stay with your original circuit and reduce the risk of damage to your battery is to add another LM317 as a voltage regulator before the one in the schematic. So two 317's in series: the first one limits its peak output voltage to about 16-17 V, and the second one limits the peak output current to 650 mA. The second on requires 2-3 V across it to function, so if you want a max voltage across a charges up battery to be 14 V, its source has to be 16-17 V, the output of the first one. This is actually a fairly common technique. If you search the web for schematics of power supplies using the 317, you'll see many that have two in series like this to give independent control of the max output voltage and the max output current.

ak
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
the simple answer is that you adjust the pot for 13.4V as specified.

A more complex answer is that you find the datasheet for the battery and file it's recommendations. However gel cells are pretty similar to each other, so the general recommendation on the circuit is probably fine.

for more information on the voltage vs temp, read here
http://www.qsl.net/wb3gck/gel-cell.htm

and for more information about charging, read here
http://www.qsl.net/wb3gck/gel-cell.htm
 

Farukh Khan

Jun 12, 2015
160
Joined
Jun 12, 2015
Messages
160
I will be able to manage the maxim cheap easily in my country. So not worrying at all about that. But I really don't understand what you tried to say with "values you set with the external components". Please elaborate what you tried to say using that line.

If I follow : http://www.rason.org/Projects/gelcell/gelcell.htm

Will I be at the safe side for charging my battery?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
The circuit uses he 317 to create a constant current, not a constant voltage, so the calculation is for the current output, not the voltage.

Actually, in this circuit, the 317 sets the voltage, the transistor pulls the voltage lower if the current through the sense resistor causes a voltage drop of 0.65 child or more.

the 317 can't be pulled lower than 1.25V so a dead short will give you a higher than expected, approx twice the normal limit.
 

Farukh Khan

Jun 12, 2015
160
Joined
Jun 12, 2015
Messages
160
The circuit on my post #1 seems a little risky to me and need some configurations to made. Can you guys please suggest me a schematic like this one : http://www.rason.org/Projects/gelcell/gelcell.htm

So that I can charge my gel cells as per it's requirement and safety measures per indicated by steve in : http://www.qsl.net/wb3gck/gel-cell.htm

I am really willing to charge my 12V 7.2AH Gel Cell with Maxim chips rather than colin's schematic to be at the safe and fast side.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Will I be at the safe side for charging my battery?

If you set the voltage and current to reasonable values, the answer is yes.

Note that it would not be wise NOT to connect it to the battery before power is applied (i.e. power it up, THEN connect to battery)
 

Farukh Khan

Jun 12, 2015
160
Joined
Jun 12, 2015
Messages
160
But what will be the case when my power supply will gone due to load shedding? Still the battery will remain connected with the maxim charger circuit.
 

Gryd3

Jun 25, 2014
4,098
Joined
Jun 25, 2014
Messages
4,098
But what will be the case when my power supply will gone due to load shedding? Still the battery will remain connected with the maxim charger circuit.
Put a diode in-line with the charger and increase the charge voltage to compensate for the voltage drop across the diode, or use a normally open relay that closes when power is provided. This can sit on the output of the charger and will disconnect the battery completely from the charger when there is no power.
 

Farukh Khan

Jun 12, 2015
160
Joined
Jun 12, 2015
Messages
160
The maxim chip is quite intelligent for charging a battery as per described. It automatically turn off charging the battery when the battery is full. So can you please describe what is the risky thing here to keep the battery attached to the charging circuit? I mean where will actually the risk occur? and what risk actually?
 

Gryd3

Jun 25, 2014
4,098
Joined
Jun 25, 2014
Messages
4,098
The maxim chip is quite intelligent for charging a battery as per described. It automatically turn off charging the battery when the battery is full. So can you please describe what is the risky thing here to keep the battery attached to the charging circuit? I mean where will actually the risk occur? and what risk actually?
Talk about not leaving the battery connected was based on the circuit from your original post that did not use the maxim chip.
 

Farukh Khan

Jun 12, 2015
160
Joined
Jun 12, 2015
Messages
160
Ow got it...misunderstood about the maxim circuit. So, the maxim circuit is the most secure and fast charging circuit which can be used to charge gel cells? So shall I consider the maxim circuit as this posts conclusion? Does the maxim circuit suggested by minder to charge my gel cell have any risk? Does it need some extra configuration? Does the maxim charger schematic need some changes in order to improve it to further safety?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Out of the two circuits presented, the maxim one is probably better.

You would have to look at the datasheet to see whether it varies voltage with temperature (only a major issue if the battery will be exposed to a wide range of temperatures).

Have you read the datasheet at all?
 

Farukh Khan

Jun 12, 2015
160
Joined
Jun 12, 2015
Messages
160
I have read the datasheet. There are some little variations with temperate in some cases. But the battery and charger both will be kept on a normal temperature environment and if possible I will try to maintain the temperature inside the enclosure. So I get that maxim circuit is the one which is better. So the last thing I wanna ask you guys to look at the whole schematic for charging a gel cell given by minder. And see if any modification will be needed to made the circuit more reliable, fast and safe.


Thanks in advance....
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Either should be fine.
 
Status
Not open for further replies.
Top