# NiMH battery charging by MAX712 IC

I am using MAX712CPE(DIP) version for charging 1.2 volts × 8 batteries. Using the datasheet and available information on the Internet, we made the circuit schematic and are about to implement it. But still I had a small confusion in my mind with respect to V+ and Vin of the MAX712 IC. After reading the datasheet, I came to know that wall cube voltage (i.e. input voltage) needs to be at-least 1.5 V more than the series voltage of the batteries that we are charging, which in my case is 9.6 volts.

Now my question is, what is the difference between Vin and V+? For reference, I had taken this schematic information into consideration. I don't understand what V+ is used for. If V+ needs to be +5v, why we are considering wall cube voltage to be 1.5 V more than the net voltage of the batteries we are charging?

The 1.2 V number you're using is just a nominal cell voltage, picked because that's basically the midpoint of a NiMH cell's discharge curve:

The graph shows an 8-cell "9.6 V" NiMH battery, which happens to match what you're doing, but the principle applies to any NiMH battery. The vertical axis just scales according to the number of cells in the battery.

The charge curve looks a bit different from the discharge curve shown above, but the fast rise near the end of charging also happens.[*] The point is, your charger needs to be able to supply that peak voltage per cell, plus some additional overhead.

I've found 1.45 V per NiMH cell to be a good starting value when calculating this sort of thing. That means you are going to need at least (8 × 1.45) + 1.5 = 13.5 V to charge your battery to maximum.

This back-of-the-envelope sort of scribbling is a useful starting point, but I find that it's best to simply test it.

There's an easy way to do that, if you have an adjustable-voltage bench supply.

Build your charge circuit, attach the battery, and turn on the bench supply. Have the bench supply set to constant-voltage mode, with the voltage starting fairly high. In the case of this chip, I'd start near its max, 20 V.

You should be able to observe that the chip is charging as it should based on the current reading on the bench supply. It should be very nearly identical to the charge current you programmed the charge controller for. (There is usually a little bit of extra current on top of that, so don't worry if it isn't precisely as calculated.)

Once you're sure the circuit is charging the battery as it should, start slowly dialing down the voltage on the bench supply. The charge current will remain steady for a while, but at some point it will begin dropping as the voltage drops. Turn the voltage back up again past this magic point, and the current drawn from the power supply will stabilize again.

There's really nothing magic going on. You have simply found the minimum charging voltage needed by the circuit at that point in time.

Set the voltage a bit above this minimum point. Say, by 0.5 V.

Now, walk away from the bench for a while. If it's set for a 4-hour charge, go do something else for half an hour or so. When you come back, you will probably find that the current reading has dropped again. This is because the charger has increased the voltage across the battery, so the charging voltage needs to rise to compensate. Tweak the voltage up again until the current stabilizes again.

Repeat until the charge controller stops charging. Your final bench supply voltage will be set near the optimum point.

You should repeat this cycle a few times, covering the normal operating range of your charger. If the charge controller has to be generic, try different brands of NiMH cells, for example. If it will be used in unheated rooms or outdoors, you need to test over the normal operating temperature range, too.

Once you're satisfied that you've found the minimum charging voltage, you can design a custom power supply that provides exactly the right voltage, or buy/build one that happens to exceed it by as little as possible. The greater you exceed this optimum charging voltage, the more heat your charge controller will have to throw off, which shortens its life, and the life of the nearby NiMH cells.

[*] The peak cell voltage during charging is followed by a drop-off, which some charging controllers detect; for instance, the MAX713. This is called the -ΔV charge termination technique.

• This seems a complex way to find out information which is already known: the maximum voltage which NiCd or NiMH cells will reach during charge. I've seen numbers ranging from 1.6V-2.0V per cell and it does depend somewhat on charge rate, temperature etc. Trying to reinvent this information experimentally in the manner described is bad practice, considering it creates effectively only one data point. Better to read the data sheet for the actual battery cells being used in the design. Commented Feb 11, 2014 at 21:44
• Your chart shows a curve for discharge, not charge, which looks different. batteryuniversity.com/learn/article/… Commented Feb 11, 2014 at 21:44
• The MAX712 assumes it is not operating from a current-starved wall adapter. It terminates fast charge when it sees the battery voltage stop rising, at the top of the curve on the Battery University chart linked above. If you dial down the bench power supply voltage until it goes into voltage-limit mode, the charge current will decrease, battery voltage will decrease (as it does by itself when charge current decreases), and probably the chip will stop fast charging as its algorithm will be fooled. So your experiment might not work at all. Commented Feb 11, 2014 at 21:51
• @MattB.: Re: Empiricism vs received wisdom: By all means, read stuff. That's no argument for not trying things and learning by doing. Re: The curve. Yes, I know it's a discharge curve, because I drew it, and I labeled it. Yes, I know about the -∆V characteristic of a charging Ni chemistry cell. I put the curve in to disabuse the OP about "1.2V" only. Re: Experiment stoppages: Yes, you may have to restart the charge cycle a few times while zeroing in on the optimum charge voltage. It won't hurt the battery, or the charger. Commented Feb 12, 2014 at 5:37
• There's nothing wrong with learning by doing. What I was trying to dissuade is taking one data point experimentally, extrapolating it to be "optimal" and designing it in as though there's no scenario where it could fail to work properly. Running your same experiment at a different temperature, charge rate, on older cells, or so on, could give a fairly different result. The circuit won't vary so much over different operating conditions, but the battery will. It's worth finding that out for yourself, definitely. It's also something you can find out from people who already did the tests. Commented Feb 12, 2014 at 6:44

The chip takes the battery voltage as an input and regulates it to 5V with an internal shunt regulator. It uses this voltage internally as its own power rail, also exposing it externally on the V+ pin, where it acts as a voltage reference for the program pin(s) e.g. PGM3 which sets the trickle charge rate. Looking at the circuit diagram on page 1 of the data sheet, current will flow from the wall cube DC IN through R1 into the chip. Choose R1 resistance value such that the current through R1 is between 5-20mA, as indicated in item 6) on page 6 of the data sheet.

You do not need to "make" V+ as 5V, the chip will do it for you. The 1.5V minimum overhead between wall cube voltage and maximum battery voltage is to cover the voltage drop across transistor Q1 & diode D1 in the part of the circuit which regulates current flow into the battery during charging.

You mention 9.6V, but NiCd/NiMH will go higher than 1.2V during charging, so you need a wall cube voltage more like (1.8V x 8 cells) + 1.5V = 15.9V. The chip maximum input voltage is 20V so a wall cube output something like 16-18V should be ok.

http://datasheets.maximintegrated.com/en/ds/MAX712-MAX713.pdf