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Max712/713 Help!

I have been toying around with different power sources, and in my travels have made a few battery packs recently... The problem is, I havn't bought a charger. Awhile ago i got a few samples of Maxim IC MAX712/713 NiMih/NiCad battery charger ic's. I downloaded the datasheet for it here, but I have a few questions.

 First, i'm having trouble understanding in the schematic what exactly the "Battery" is for. I'm assuming that the "Load" +/- is where i would actually be placing the battery to be charged, but why the other "Battery" when the whole system will be powered by a wall unit?

Also, it talks about the RSense resistor, "Since V+ shunt regulates to +5V, R1 must be small enough to allow at least 5mA of current into the V+ pin."... Now does this mean that the charger would be trying to trickle charge @ 5mah? Wouldn't it be more efficient to charge for a few hours at 150-300mah?

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ezekiel181's picture

I posted some more information on my blog morris, I hope it helps.

ezekiel181's picture

Wall of text ahead! I got some samples of this chip a couple of years back and built a charger with it. I got the max713 which can charge both NiMH and NiCad before I knew much about them. The 713 uses a voltage drop to detect full charge, thats fine for NiCads, when they become fully charged the voltage drops slightly and the charger cuts off. Unfortunately, for NiMH the voltage drop at the end of charge is VERY small and hard to detect, ie. it`s easily missed by the charger which can result in overcharging. I should have got the 712 because I never use NiCads and the 712 ends charge when the voltage doesn`t increase any more, which is better for NiMH batteries.

MATH TIME KIDDIES!

First, decide the maximum number of batteries you want to charge. I chose 6. Formula below tells me my minimum input voltage is 12.9v.

  • Minimum input voltage = Number of cells x 1.9 + 1.5v

R1 must be chosen to allow 5mA into the V+ pin. V+ is to power the chip, not power to charge the batteries. The formula is in the datasheet.

  • R1 ohms = (minimum wall cube voltage - 5v) / 5mA.

So for 13v input R1 = (13-5)/0.005 = 1600ohm. I made it 1.2k. Close but no less than 5mA into that V+ pin.

Decide on charging rate, or how fast the battery charges.

  • Fast Charge Current mA = battery capacity in mA / charge time in hours.
This makes a lot more sense after you think about it for a while. A 1000mAh battery charged in 1hr means the fast charge current is... 1000mA! Duh! If you want it to charge slower, maybe in 1.5hrs then the charge current is less.. 666mA.

 

Rsense is the current sensing resistor, which lets the chip regulate the fast charge current to what you decided on in the previous step.

  • Rsense ohms = 0.25v / Fast Charge Current in AMPS

If you want to charge a 1000mA battery in 1hr then the Fast Charge Current is 1000mA and Rsense is 0.25/1.. 0.25ohm. Thats a very small resistor! You can easily make it by putting 4x 1ohm resistors in parallel. My charger has a bank of 1 ohm resistors which I can add or remove to to change the charging current.

Keep in mind, charging higher capacity batteries will increase the current draw if you don`t change Rsense, a 1500mAh battery with 0.25 ohm sense resistor will pull 1500mA charge current. Don`t try charging 9000mA D cells in 1 hour with this circuit :) might get some fireworks.

I did a TON of research on this chip, plus breadboarding it a few times before putting it onto some proto board. It still isn`t perfect, I want to add temperature sensing, support for up to 8 cells and if possible make it switched mode to lower the wasted power. It DOES work however.

Maybe I should post it in a blog or something?

morris4019's picture
Also i don't really see where it talks about R1, am i just suppose to put something in?