Rechargeable Life Span

George Sephton

Senior Member
Hi,
Ive got a MAX712 and Im about to buy a set of 4 AA NiMH batteries for it (http://www.amazon.co.uk/gp/product/B00008GSXV/ref=olp_product_details?ie=UTF8&me=&seller=) but Im getting confused with the times. I read the manual (http://datasheets.maxim-ic.com/en/ds/MAX712-MAX713.pdf) and saw that different pin connections relate to different settings. However table 1 talks of fast charging but Im not bothered about this. It says C/4 is slow and below that (Page 6) it has an eqaution with "charge time in hours", is that optional or maufacturer specific because I'd like the charge time to be about 36 hours or so not fast charge. Having said that does that mean the MAX712 is not right for me? If not I can still put up with the 4 hour charge cycle but in looking into this I looked at wikipedia (http://en.wikipedia.org/wiki/Trickle_charging) and that says
A trickle charger, on the other hand, will charge no matter whether the battery is fully charged or not. So it needs to be connected and disconnected periodically. If left in place too long it'll eventually boil the electrolyte out of the cells or damage the plates. Trickle chargers will work to keep the battery charged if used once a month or so for a day or 2, but the float chargers can be left connected indefinitely without potential harm to the battery.
which is now confusing.
My application is a product that will sit for most of it's life in a dock connected to a transformer, and that can then be taken out of the dock and moved around with so the battery needs to be charged but not charged all the time. Looking at that, does this mean my batteries would break and that I need to use a float charger? Or is there a way of waiting for the battery to be charged and turning off the charging unit and recharging the battery again in a week when it has discharged slightly if not been used or discharged fully if used? I also have a battery monitor if this comes in useful.
George S.
 

papaof2

Senior Member
In the intended configuration of the MAX712, the "trickle" charge would also be supplying power to the circuit the batteries are used with, so not all the "trickle" is being usd to charge the batteries.

Remember that you can set the charge current by adjusting Rsense - thus you can set the "fast" charge to any level and the "trickle" charge will be 1/16 of that value.

The AA cells you reerence are 2100mah, so the C charge rate would be 2100ma. Unless the batteries need to be fully charged in one hour (and IF they can take a one hour charge), you can set the "fast" charge at C/10 (210ma) and the "trickle" charge will be 210/16 = 13ma.

So, how much current does the device draw? Compute Rsense to deliver 16 times that value at fast charge and the trickle charge will be just enough to operate the device (give or take a little ;-)

John
 

George Sephton

Senior Member
@Dippy: it will be receiving a IR input signal every millsecond a second (or so) and sending one on a button press. This being done using a IRDA tranceiver I have.

@John: Like I said I really don't want it to be mega-fast I was just worried that my batteries might die. Don't know about current just yet. What is Rsense? Also how could I set the fast charge to be slow? Table 5 shows how fast it should be, if so does that mean the slowest is C/2?
 

manuka

Senior Member
George- we're rough hewn men of the world - please spill the beans on the entire circuitry . The snippets given are akin to talking about just the wheels on a car.
 

George Sephton

Senior Member
There isn't much to the circuitry really yet. Im about to mill my base station ive been working ages on, that uses the IRDA Tranceiver and somehow Im going to create another once my base station is setup to use a 20M a tranceiver, LCD screen, 2-wire HD____ (can't remember) a buzzer, and an LED. The tranceiver works with TX + RX so strings of data can be send instead of just numbers :D that's all there is to it. It's basically a remote control with a LCD screen on it. There's another LCD screen on the base. The base has an LCD Screen and 10 LEDs. It uses the DS18B20 for temp, in and out, LDR in and out, DS1307 for time, DS1807 for volume of media apps and a i2c port expander to control 10 leds, 3 buttons all used with a counter and transistors to control several audio inputs to one speaker output. Also includes other functions: alarm, time, metronome and an instrument tuner (soon)...Ive had most of the base functions working on a breadboard so Im going to make a PCB of it all. Then when that'd one ill make the remote but whilst I wait I decided to look at the battery charging facility. The base uses a 28X1.
 

goom

Senior Member
You may be over-complicating things. Why not just build a constant current source using a linear voltage regulator (i.e. a float charger). At 50mA for example, you could probably leave the battery pack on charge indefinitely without any damage. It certainly will not overheat at this level. If you want to get more sophisticated, you could turn on/off the current at preset voltages via a MOSFET or relay controlled by a PICAXE reading the voltage via ADC and a bit of simple programming. It would then be very easy to have an LED indicator showing whether it was fully charged or charging.
 

George Sephton

Senior Member
Yeah I was wondering after the wikipedia page if I should use a float charger. I'll look around for a float charger ic and see what I can come up with.
 

papaof2

Senior Member
Rsense is the resistor that sets the fast charge current (from the datasheet - if you've gotten to the equations portion of it). You don't have to use the full rating of the batteries as the C value in the equations - just whatever level you're comfortable with. C/10 is always a safe "fast" charge rate ;-)

John
 

papaof2

Senior Member
"How do I set it to be C/10? It only says you can do C/2."

First rule of using datasheet equations: learn when to lie to the chip ;-)

C is the rating of the batteries - 2100ma in your case. C/10 is therefore 210ma.

In the equation for determining fast charge current, use 210ma as the maximum charge current and set the chip leads for C. Remember that the chip does NOT know the rating of the battery you are using, so you can use ANY value (within the chip limits) as C.

John
 

BeanieBots

Moderator
I'm with Goom.
You've said you don't care about fast charging. You only need an "intelligent" charger when charging at rates higher than C/10.
A simple constant current of C/10 to C/20 would be your best bet. The batteries should last about 5 years before dendrites grow large enough to puncture the insulation layer at those rates.
If your supply voltage is reasonably stable, you could do it with nothing more than a resistor. Choose the value to give C/10 for a flat (1v/cell) battery.
If you want to put in a little more "intelligence" then simply add a turn off when you get to 1.4v/cell.

If you want to charge faster than C/10, then stick with the chip.
 

George Sephton

Senior Member
That's what im going to do then. Im going to use my method from my other post about battery monitoring to see when the battery is at very low charge and activate the charger. Just by connecting the positive of a battery to an adc pin and running readadc10, if the result is less than 10 I can give power to the charger, should it be in the dock, otherwise display an error. I will read the datasheet on how to set C/10 to be my rating as you said and will post back later.
 

BeanieBots

Moderator
Please be careful with your expectations.
Measuring voltage does NOT indicate state of charge.
It WILL give a vague indication and MIGHT be good enough for your purpose but don't expect accurate results.

You might like to consider an adaptation of this for a more accurate indication of state-of-charge.
http://www.picaxeforum.co.uk/showthread.php?t=7746
 
Last edited:

BeanieBots

Moderator
Simple resistor for charging and ReadADC for checking should do it then.
Don't forget that the PICAXE ADC reference is its own power supply.
Therefore, you cannot use ReadADC to check it's own power supply.
You will need another reference to do that. An LED works quite well if made into one leg of a divider.
 

jglenn

Senior Member
Putting 50mA in the battery forever may actually overcharge it. I have had good results with float chargers for lead acid and nicad batts that simply have a regulated output set for the full charge voltage of the battery, a 12V lead acid will be happy sitting at 14.4V forever, that's 2.4V per cell. Nicads run 1.2V, 1.4V per cell float should be ok. If you use a reg with current limit, then hooking a discharged batt will not hurt anything, just take 10 hours or so to charge. This technique works well with solar panels, to avoid overcharging. One project is a chopper/regulator built with an integrated power switching IC, a powertab, just add a couple parts. Adjustable output. :D
 

evanh

Senior Member
There is a distinction between what is usually meant by float charging and trickle changing. It comes from the two differing charge modes employed by Lead-Acid vs Ni-Cd. Once the battery is charged, with a Lead-acid the charger limits voltage, this is referred to as floating it. With a Ni-Cd there is no voltage limit, it just limits current to a trickle.

These two modes carry over to Li-Ion and Ni-MH respectively. So, you very much want to decide on the battery type you are using.

The Wikipedia article is vague on this point.
 

jglenn

Senior Member
My view of float is tight voltage regulation. Lithiums might be dangerous to charge casually, we used to have a lot of ignitions of model plane packs, people picking the wrong # of cells on the charger. Probably best to use a chip or intelligent charger for them. If they burn, can melt a hole thru tables, floors, all the way to the basement.
 

BeanieBots

Moderator
There does seem to be some confusion in this thread over "trickle" vs "float".
Also, I could never advise letting a 12v Pb battery float at 14.4V. That will cause irreversible oxidisation.
Li Ion & LiPo ONLY IF YOU KNOW WHAT YOU ARE DOING.

This is a topic which could go on forever and has done many times in the past. To save cluttering the forum with another set of repeats, for those who want to know more:-

http://www.batteryuniversity.com/

Just remember, the money is made by selling batteries and not chargers.
Work it out for yourself what that means when reading seller's specs.
 
Top