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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Hi birdofprey. Sorry for not replying, I have very flaky internet service right now.

There really is no *wrong* Rsense because it depends on how fast you want to charge your batteries which is totally up to you. An Rsense of 0.3 ohms for a 2400mAh battery makes it a charge current of 833mAh. With that charge curent they should be fully charged in about 2.8 hours.

Thanks for the info.

What would a Rsense of 0.5 ohms do to the charge time?

also, Im using 4 AA batteries, each 1 is  2500 mAh, (I know this is a novice question) -so with 4 does it stay at 2500 or does it jump up to 10,000mAh?

An Rsense of 0.5 substituted into the calculation in step 4 gives an Ifast of 0.5A. Work backwards into step 3 to find charging time 2.5A/0.5A = 5 hours.

If your batteries are connected in series the voltages add but the capacity doesn't change. In this case your battery pack is 4.8v 2500mAh and your calculations are right as long as you set a cell count of 4 with PG0 and PG1. This is most likely what you have.

If your batteries are connected in parallel the capacity adds up but the voltage doesn't, and in THIS case you have a 1.2v 10000mAh battery and it would take 20 hours at 0.5A.

I was wondering how much current goes through the PGM0,PGM1,PGM2,PGM3 and would a transistor used as a switch be able to handle it?

I believe a possible 30V max can go thought those pins and using a transistor that can handle 40V would work but could they handle the current?

Just got my samples a few days ago!

I will study the circuit a bit more before I try anything with them but the more I look at it the more confusing it becomes... Will really have to study the PDF to understand as much as I can.

If I can get this to work with different battery packs it will be incredibly useful!

I finished the PCB layout and sent it off. I am going through Sparkfun's service so it will take up to 6 weeks to get them back... I'll know then. I checked and rechecked my work, I think i got it right. As soon as it is working, you will know!

Awesome! Can`t wait to get my grubby little paws on one :)

HI Ezekiel, I assume you're back in aust? I hope everything went as planned.

Are you regulating your input voltage? - well, obviously you are, but I see no regulators on your board, are you using a wall cube to rectify and transform? Does the wall cube heat up much? What is the amps (@ voltage) output of the wall cube?

Have you hooked up the temp probes yet? how do they affect the performance/result of the charge? -- apart from avoiding melting batteries...?

11 watts!??? you could fry eggs on that!! have you tried the switching mode to decrease the heat load?

Something I'm confused about is the voltages- the rechargable battery standard voltage appears to be 1.2 V (if not standard, then "many"). Even taking into account wobbles, etc, in the supply voltage, calculating the required input voltage by Nx1.9+1.5 seems excessive.  1.9 is a LOT larger than 1.2! I'm hoping to charge 8 cells, without melting a hole into the core of the earth, so I was hoping I could get by with a 20V supply - charge over 2 hrs and try to figure out how to cope with 12 Watts of heat -  (rather than figuring out what to do with 15 watts...) do you have any pearls of wisdom to offer on the viability of this? Perhaps I should check out switching mode....

Oh, one more thing - you use 4 v as your minimum battery voltage - presumably a minimum voltage of 1V/cell - why did you choose 1 V?

apologies for the looong post.

CTC, if you're reading this - did you complete your charger circuit? any luck?

Hey emuller, still in Japan. Am staying with my wifes parents probably until about early December and then moving back to Australia. Not sure when we will be back in Japan so we are getting some good family bonding time in before we leave.

The input voltage is regulated by my benchtop PSU which is the wooden box in the pics. I bought this kit, plus the biggest, oldest and heaviest junk plug pack that Hard Off had. I pulled the transformer out of the plug pack and built it into the wooden case. Later on I made a 4 digit LED segment display to show the output voltage.

In linear mode the amps out of your PSU should be almost the same as going into your batteries (which can be calculated). I guess a switched mode charger would be able to regulate the power better and as such lower the maximum voltage and current input needed. I haven`t made a switched mode one though and my understanding of it could be limited.

I haven`t hooked up the temp monitoring yet. I packed my charger and PSU in a box along with my CNC frame parts and shipped it back to Australia. It could be 3 months before I see any of it again :( Probes should be easy to add. The problem is finding a thermistor with a good enough datasheet to do the calculations, or just measure the resistance by hand.

NiMH and most other batteries need a decent voltage above their nominal to get charging. 1.9 is pretty high, but maybe some high performance packs require such a high voltage. I never saw my battery voltage go over 1.65v per cell so perhaps you could try 1.7v as a minimum and see how things go. The most power needs to be dissipated when the voltage drop is very high. So if you want your charger versatile and able to charge multiple sized packs on the same input voltage you will have to be careful of the PD. On the other hand if you only want to charge an 8 cell pack and use 1.7 as the max cell voltage the PDpnp could be as low as 8*1.7+1.5 = 15.1 min input. (16v-8)*1A = 8W PDpnp. 8W can easily be taken care of with a heatsink and small fan.

Why I used 1v per cell when calculating the PD is a good question! I must have had a reason but it escapes me right now, heh.

wow - lots of info, thanks for all of that. Ah - I thought you were leaving for aust in early nov. packing all that stuff away must have been irritating..

My intention is to permanently encapsulate the batteries in the vacboks, and create a single dedicated charging circuit. Space is limited in that thing so cooling may be a problem - HOWEVER - there ARE 4 cpu fans stuck on top, so perhaps I can find a cunning way to get them to cool the transistor by putting it on the outside of the special brain-box I'm making for it all