![]() | ![]() | ![]() |
| |||||||
| Electronic Projects Design/Ideas/Reviews Are you building an electronic project or want to? Maybe you need some assistance? Come and submit your electronic questions here and let our experienced members find a solution. |
![]() |
| | Tools |
| | #1 |
|
Hello. I am wanting to build a battery charger for a project i'm working on. First, the details: Charger IC: MAX712 Data Sheet Battery Pack: 6 AA Rayovac Hybrid NiMH 1.2v, 2100mAh Power Supply: Regulated Laptop Power Supply - 19vDC 3.42A Intended Charge Rate: 1C using voltage slope detection There are a number of calculations on the MAX712 datasheet that i'm not 100% sure about. Can anyone confirm these values for me? ![]() R1: R1=(minimum wall-cube voltage - 5v) / 5mA Is 19v my "minimum" wall-cube voltage since it's a regulated power supply? Assuming the answer is yes: (19v - 5v)/5mA = 2.8k ohm resistor? Rsense (current sensing resistor): Ifast=(capacity of battery in mAh)/(charge time in hours) Ifast=2100mAh/1 hour = 2100 Rsense=.25v/Ifast Rsense= .25v/2100 = .000119 = .119 ohm resistor? PNP (Q1) PDpnp = (maximum wall-cube voltage under load - minimum battery voltage) x (charge current in amps) The 2N6109 on the datasheet shows a max of 40, but i'm worried about heat. Will a normal TO-220 heatsink like the one below be enough?? ![]() PGM Settings: PGM0/PGM1 - Open/Open (6 Cells) PGM2/PGM3 - Ref/Ref (90 min timeout w/ voltage slope enabled) Other values (defaults): Vlimit = Ref TLO = Batt- C1 = 1uF C2 = 0.01uF C3 = 10uF R2 = 150 ohm R3 = 68k ohm R4 = 22k ohm D1 = 1N4001 Thank you for any advice you can offer! -Scott | |
| |
| | #2 |
|
I would have said that 19v input is a little on the high side for charging 6 AA batteries...... That said reducing the input voltage is going to be good for the power dissipation in the 2N6109....which is not operating in a linear mode anyway... Assuming a fully charged Nimh cell is around 1.2v,then 6 of these will equate to 7.2v.. Having an extra 4volts headroom for the circuit to function correctly I'd say 12v would be sufficient.. In which case R1 would be 1.4k (12-5)/0.005 Rsense stays the same although a preferred value would 0.12 ohms
__________________ Eccentric millionaire financed by 'er indoors Last edited by Chippie; 16th November 2009 at 08:12 PM. | |
| |
| | #3 |
|
Chippie, thanks for the response. I was goign with that 19v supply because I was having problems finding an inexpensive 12v, 2100ma+ power supply. I don't really need the 1C charge rate though. I found this 12vDC 1A switching power supply on ebay. If i used that one, would I need to do a C/3 charge rate? Last edited by Jack.Straw; 16th November 2009 at 10:15 PM. | |
| |
| | #4 |
|
Sounds better to me. Leave R1 at 1.4KΩ and increase Rsense to 410mΩ for a charge current of C/2. (600mA). That way you'll have enough headroom for both, supply voltage and current. Boncuk
__________________ Proper Planning Prevents Piss Poor Performance | |
| |
| | #5 |
|
Boncuk, thanks for the response. I'm confused by the 600mA part of your response. I was under the impression that C/2 would require 1050mA? (2100mA/2)
| |
| |
| | #6 |
|
The 600mA thing has thrown me for a loop. I was thinking C/3 would be my max rate with 2100mAh batteries and a 1A power supply. Am I not understanding something?
| |
| |
| | #7 |
|
Charging at 600ma just means the cells wil take longer to charge....3hrs..... Remember, the cells are 2100mah capacity, charging at 2100ma means they will be charged in an hour(thereabouts) The 1 amp psu is ok because the load current = the charge current ....600ma in this case
__________________ Eccentric millionaire financed by 'er indoors | |
| |
| | #8 | |
| Quote:
| ||
| |
| | #9 | |
| Quote:
In Energizer's Ni-MH battery applications manual they show that their AA cell's voltage peaks at 1.55V when it is charged at 1C. Then the total for 6 cells is 9.3V.
__________________ Uncle $crooge | ||
| |
| | #10 | |
| Quote:
But I then didnt specify any particular brand of cell..... Nothing wrong with erring on the side of caution......
__________________ Eccentric millionaire financed by 'er indoors | ||
| |
| | #11 |
| The brand doesn't matter. Ni-MH is Ni-MH. They are all the same. Energizer AA Ni-MH cells are made in Japan by maybe Sanyo. Their 9V Ni-MH battery is made in Germany by whoever.
__________________ Uncle $crooge | |
| |
| | #12 | |
| Quote:
![]() Anyway 1050mA will be too much for your small power supply. Go for 80 to 90% load of the rated power.
__________________ Proper Planning Prevents Piss Poor Performance | ||
| |
| | #13 |
|
Ok, thanks guys. I ordered a 12v 2A regulated power supply and am going with a C/2 charge rate. I designed and built this layout last night: ![]() I hope i got it right. The last thing is to add LED indicators. The datasheet gives this example: ![]() Does that mean i should put one LED between R1 and V+ without any other resistors? (i'm used to putting a 1k resistor before my LEDs, but think R1 handles it here?) Then a 2nd LED between V+ and FASTCHARGE with a 470 ohm resistor preceding it? Why does it say 470 ohm minimum? Should i just use the 1k resistors i have around? Thanks again for all your help! -Scott | |
| |
|
| Tags |
| batter, charging, max712 |
| Thread Tools | |
| Display Modes | |
| |
Similar | ||||
| Title | Starter | Forum | Replies | Latest |
| MAX712 Charger | Connor | Electronic Projects Design/Ideas/Reviews | 8 | 22nd July 2009 01:42 PM |
| laptop batter light blinks when charging | Marks256 | Chit-Chat | 9 | 8th July 2008 08:19 PM |
| MAX713/MAX712 in series | kein0r | General Electronics Chat | 13 | 30th March 2007 10:08 AM |
| Help with pbc layout and schematic matching...MAX712... | eyevancsu | Electronic Projects Design/Ideas/Reviews | 3 | 29th September 2003 09:15 PM |
| NiMh charger using MAX712/MAX713 | tibor | Electronic Projects Design/Ideas/Reviews | 3 | 26th September 2003 11:36 PM |