# Max713 NiCad/NiMH smart charger.

I made a battery charger with some free samples of the maxim max713 chip.

Chip details.

Datasheet.

The 713 supports both NiMH and NiCad while the 712 is for NiMH. Both can fast charge up to 16 cells and support V/T, temperature and time out charge cut off. It can charge the battery while still powering the load. If you don`t need NiCad support get the 712, it`s better for charging NiMH safely because of the way it measures the cut off voltage.

The basic linear circuit is fairly easy to make up, you must use the formulas here to find the values for R1 and Rsense though. Do this with the biggest battery pack you want to use.

1. Choose how many cells to charge. Minimum Input Voltage = Number of cells x 1.9 + 1.5
2. Find out R1. R1 powers the chip. R1 in ohms = (Minimum Input Voltage - 5) / 0.005
3. Decide on a fast charging current. Ifast in mA = Battery capacity in mA / Charge time in hours
4. Find the Rsense resistor. Rsense in ohms = 0.25 / Ifast in A
5. Set PG0 and PG1 to the cell number according to datasheet Table 2.
6. Set PG2 and PG3 to set the cut off time according to datasheet Table 3. Cut off should be slightly higher than charge time.
7. PNP power dissipation. PDpnp =(Maximum Input Voltage - Minimum Battery Voltage) x Charge current in A Check this against the PNP datasheet. This is wasted heat and depending on your cell count range you will need a heatsink and/or fan.

For my charger I chose up to 6 cells (the picture shows jumpers up to 8 cells but it`s not wired up yet). Fast charge current and Rsense aren`t set in stone because they can change if you charge different capacity battery packs.

1. Minimum input voltage = 6 x 1.9 + 1.5 = 12.9v
2. R1 = (12.9 - 5) / 0.005 = 1600. I picked the next lowest resistor at 1.2k.
3. Ifast = 2500mAh / 2 hours = 1250mA.
4. Rsense = 0.25v / 1.25A = 0.2 ohms.
5. PG1 and PG0 both unconnected.
6. PG2 connected to BATT-, PG3 connected to REF pin. With a charge time of 2 hours, the timeout is the next highest at 132 minutes. There will be losses through heat so it`s fine. Also voltage slope cut off is enabled to turn off automagically when the voltage stops rising.
7. PDpnp = (13 - 4) x 1.25A = 11.25W. 2N6109 maximum PD is 40W but it gets lower as it gets hotter. For every degree C above 25 minus 0.32W from 40. If I think it could get up to 60 degrees.. 40W - (60-25) x 0.32 = 28.8W max power dissipation. Well over 11.25W.

Actual charge current above is about 900mA because the fan sucks up a bunch. The jumpers and resistors are really fiddly, I still have to get around to putting the temperature probes on it, changing the jumpers to dip switches or rotary switches, adding 8 cell battery support, and mounting it in a box. The current charge labels on the right are only accurate for a 1000mAh battery but it gives me an idea of what kind of charge rate to expect.

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Really cool project, can come in handy!

This is great! I've been wondering how to build a fast charger that can charge all the cells in one go.

If you were to use this charger in a docking station for a robot, how do you prevent too high voltage to go into the bot? E.g. a bot with 6 cells (7.2V) that uses a simple voltregulator, would have to take in 12.9V when charging. That would seriously heat up the regulator. Any ideas?

Don`t worry, the full input voltage doesn`t go into the battery.

When the charger has power but no battery is connected the output voltage is 2v per cell, but once a battery pack is connected it starts the charging voltage immediately. Charging voltage depends on what the battery voltage is like, but it starts low then gets up to a max of 1.65 per cell before cut off.

Hi,

Im a newbie with an insatiable lust for RC and electronics alike...

Ive had max712/713 ic's since I can remember and have been looking on building this charger (http://www.qsl.net/eb4eqa/batt_charger/batt_charger.htm) but was worried with the number of negative feed backs to it...

Would it be so much if I asked you for a copy of the schematics of your circuit?

Respects,

Joseph

Nice work!!!

**Off of the data sheet**

Why is "Batt" and "Load' appearing on the schematic? Which is which? Gimme what you got.

LOAD is just where your voltage regulator circuitry goes, thats if the battery will be charged while it`s still powering the robot. You could have the charger in a base station but you would need 3 contact plates for V+, GND, and Sense and if you wanted temperature monitoring you`d need even more contacts. Or you could chuck the whole lot on the bot and just have 2 contact plates for raw input voltage to the charger.

If you`re doing a standalone charger you can ignore C3 and LOAD and just hook up some screw terminals or some kind of plug jack where the battery is.

Where is the info about the temp-probe? Really, if I am not shoving tons of current to the batts, for some sorta 10 minute charge, do I even need to worry about this?

Oh, and this is an awesome post.... A good item, great walk through, nice neat math and formula... Awesome!!

The temp probe calculations aren`t very detailed in the datasheet but what they do have is on page 12 and schematics on 13.

You need NTC thermisters, all the same. Resistors and NTC should be between about 10k-500k resistance. Look at the whole schematic (I`m talking about 9a) as just 3 voltage dividers. Battery temp, hot, and cold.

Before charging T1 and T2 should be about equal at ambient temp and because REF outputs 2v, TEMP will be about 1v. REF divided by the T1 and T2. As T1 gets hotter the voltage goes up.

THI made by R3 and R4 divider says what voltage is too hot. You could calculate it by looking at a temp vs resistance curve for your NTC or just heat T1 to about 40°C and measure it`s resistance then pick 2 resistor values to give the same voltage.

TLO is to prevent charging while the batts too cold. I reckon you could just replace T3 with another resistor like the 2 resistors on THI. I don`t know what temp is too cold for charging. It don`t think it even gets cold enough where I am to worry about this.

If you`re just giving it a boost for 10 mins you wouldn`t need to put temp probes on it. Don`t forget it`s charging though :p

I got it now... The first time I downloaded the PDF of the datasheet, my computer choped some of it off.