Home-made battery charger - batteries getting hot

[Jack.Straw]

I built a battery charger based on the MAX712 IC from Maxtor. Below are the specifications and my circuit design.

Maxtor MAX712 Data Sheet
Batteries: 6x AA NiMH 2100mAhr (Rayovac Hybrid)
Power Supply: 12v 2A DC Regulated

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The batteries are getting really hot. I'm not using any temperature sensors, just voltage slope and a 180 minute time-out. Charging around C/2 how warm should the batteries get? They are getting hot enough that you'll want to remove your hand after 10-15 seconds. Any ideas what i could have done wrong?

Thanks for your time,
-Scott

 

[MrAl]

Hi,


I did not check your circuit yet, but perhaps you should check and double check all your connections first.
Secondly, you could monitor the charge going into the batteries and calculate the actual charge getting to the batteries and see if it is MUCH greater than what they should actually be getting to fully charge. If it turns out to be much more, then something is wrong, but if not, it may be that they cells themselves are not very good. Older cells will act very strangely sometimes and i have curves that show that some of them do not exhibit minus delta V, so make sure you have some good cells, maybe brand new ones, to test with.

To monitor the charge you'll need to be able to measure the current to the cells.

LATER:
It looks like VLIM is supposed to be tied to VREF. That's not what your circuit shows so maybe try that first.
Also, your PG3 is not connected to BATT- like it should be for 180 minute time out.

 

[Jack.Straw]

MrAl, thanks for the reply. Both the connection from VLIMIT to VREF, and PGM3 to BATT-, are shown below the chip... it's a little hard to see in my diagram. I built several of these chargers in one sitting, and all of them are causing the batteries to get hot. So long as my circuit design is sound, i believe my connections are correct and good. I'm afraid I cannot test current right now as my tester only goes up to 200mA. It is possible that the cells are bad as you mentioned, as i have over-heated them several times. I do not have new ones to test with, but i will order some today. I believe these cells are fully charged right now, but that the charger is not recognizing it... and wants to charge for the full 180 minutes. It takes about 10-15 minutes for the batteries to get really hot.

Thanks for your time,
-Scott

 

[MrAl]

Hi Scott,

When you get the new cells you can try them but this time monitor the temperature of one of the cells and when it goes up by say 20 degrees C stop the charging even if you have to disconnect them. The cells should not get warm or hot until after they are fully charged, so if they start getting hot you know they are done. At least this way you can protect the new cells while you are testing.

Here is a graph of measured data, taken on two cells while measuring the temperature of one cell and ambient.
Note how the temperature rises more sharply once the voltage plateau is reached (end of charge).
This suggests a 10 deg C rise is enough to tell us that the cell should be disconnected.

 

[audioguru]

I charged old Ni-Cad and new Ni-MH AA cells side-by-side. The Ni-Cad cells became cooler than the ambient temperature and the Ni-MH cells became warm.
Both types got hot when they were fully charged.

 

[Jack.Straw]

So it sounds like the charger isn't sensing that the batteries are fully charged. I'll test with new batteries when i get them and make sure they don't overheat. I know the chip has inputs for thermal protection... but i have no experience with such things. What sort of parts would i need for that?

 

[Dragon Tamer]

Did you make sure that all of your calculations are correct. This sounds like the problem with the voltage-slope-termination, measure the voltage of V-Limit and V-Ref, and tell us what value you got.

 

[Jack.Straw]

Thanks guys, 2.25 volts across VLIMIT & VREF

 

[MrAl]

Hi again,


Usually for these circuits a thermistor is used to measure the temperature. It looks like that chip accepts a thermistor too so that would monitor the cells temperature and shut down if it got too hot. The drawback is that the thermistor has to be mounted so that it always touches the case of the cell, and probably only one cell can be monitored with a chip like that so we would have to assume that that cells is representative of all the cells in the string being charged.

 

[Dragon Tamer]

The MAX712 can accept thermistors, and since the V-Ref is 2.25V then it means your chip should be working properly. I would recommend increasing your charge rate to 264 minutes. It will lower your charging current and it will allow you to cut back on the number of resistors you are using for Rsense. (replace with 2-1 ohm resistors in parallel) This won't have any major affect on the batteries performance while it's powering the load. It won't affect the battery during backup unless the value of Rsense is higher than the internal resistance of the battery (4.3 ohms).

 

[colin55]

Of course your cells are going to get very hot with a large charging current. Decrease the current if you don't want to limit the life of the cells. The normal charge-time is C/14 as this is a 10% charge and the rest is for losses.

 

[Jack.Straw]

Thanks for the replies guys. Contrary to my first post, i'm actually using the MAX713, not the MAX712. If i'm reading this chart correctly, the slowest rate of charge I can do with NiMH batteries using the MAX713 is C/2. Is that correct?

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If i get some of the MAX712 chips, would I do C/4 with a timeout of 264 minutes?

Thanks again for all your time & advice,
-Scott

 

[colin55]

Do you particularly want a FAST CHARGE?
Because that is what you are talking about.
Fast charge needs a lot of care.

 

[MrAl]

Hi,


Just to note, a 1 amp charge rate for a 2000mAh cell should not be a problem.

 

[Jack.Straw]

Yes, a fast charge is important to me for this project. I'm making these little keg amplifiers/media players and a charge time more than 4 hours would be pretty inconvenient. I would prefer 2 hours. I would be ok with putting thermistors on to protect the batteries. Was the comment above saying that the batteries only get hot after they are fully charged correct?

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[colin55]

They get hot during the charging process and the charger should detect when they are fully charged and turn off.
The only way to see what happens is to charge them at 1 amp and feel them after each 30 minutes and see if the charger turns off after about 2.5 hours - for nearly flat cells.
It's the "turning off" that's most important. That's the intelligent part of the charger. The cells get a very short life if the charger keeps charging them after they are fully charged.

The circuit needs two 1N4001 diodes in parallel for 1 amp. These diodes are really only 0.75 amp
At 1 amp they drop 1.1v = 1.1 watt and you need very good heatsinking.

 

[Jack.Straw]

The circuit needs two 1N4001 diodes in parallel for 1 amp. These diodes are really only 0.75 amp
At 1 amp they drop 1.1v = 1.1 watt and you need very good heatsinking.

Colin, thank you! I've updated my layout with the 2nd diode... is this correct? Would it be better for me to use a different diode? The heat sink i'm using at the moment is the largest one i could find that would fit on the PNP (TO-220). The heat sink i am using looks like THIS. Also, i bent the fins alternating forward & backward in an attempt to increase airflow between them.

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Thanks for your help! -Scott

 

[colin55]

Just feel the heatsink. If you can't hold your fingers on it after 10 seconds, it is too hot.

 

[Jack.Straw]

Ok, so i built another from scratch and got brand new batteries. I read online that parallel diodes don't always effectively distribute current between them, so i used a 3A diode. The batteries are still getting too hot when the batteries reach full charge. I don't think the voltage slope is being detected. I'm going to go ahead and try to use temperature detection.

I would like to use an absolute temperature shut-off at 40C, with low temperature disabled. I've got my schematic modified to do this. I need to replace T2 in the schematic below with a resistor, but i don't how to choose the correct resistor value to set the temp to 40C. Can anyone explain how I would calculate that? The MAX712 documentation uses a 10k ohm thermistor for T1 at 25C.

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Thanks again for your time,
-Scott

 

[Jack.Straw]

It's taken me all day, but i think i'm starting to get it. The Rayovac Hybrid NiMH see up to 45C during charge according to their datasheet. If I use a thermister with a 10k rating at 25c, B3887:

R(T) = Ro EXP {B * (1/T - 1/To)}

Where "Ro" is the thermistors nominal resistance, "To" is nominal temp, and "T" is absolute temp. So..

25C=298K (To)
45C=318K (T)
Ro=10k (thermistor nominal resistance)
B = 3887

So..
4.4k = 10 EXP {3887 * (1/318 - 1/298)}

So that means when the thermistor gets to 45C it should be at 4.4k resistance. The looking at the voltage divider at 45C:

Vout = Ro / (Ro + R(T)) * 2
1.39v = 10 / (10 + 4.4)) * 2

That means I want THI to be 1.39V so that the charger shuts off when TEMP exceeds 1.39V. The closest resistor pairs to set THI to 1.39 is 27k & 12k (1.384v).

So, my schematic looks like this so far:

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Did i get all the right? My brain hurts. Thanks for all your time & help with this project!
-Scott