charging a NiMh

[xtal_01]

I have read a few posts on this forum (and other forums) ... looked at data sheets .... found a good description on PowerStream ... but still have a few questions I am hoping someone can answer.

First, I should say I only dabble in electronics. I usually build industrial control panels .. anything from a simple starter to a PLC / touch panel HMI monitored and controlled via the web.

I was asked to duplicate a simple box that monitors air flow with a differential pressure switch.

The originals were made some 25 or 30 years ago and have started to fail.

The person who build and designed these (I worked with him) has long since retired and passed away ... so I can't just ask him what he did.

The circuit is simple ... transformer supplies a 12 volt regulator .... a relay is held on and alarms if it looses pressure or if it looses power.

The factory almost never looses power ... and if it does, there are three generators to bring it back up within about 10 min.

There is an adjustable voltage regulator charging a 9 volt NiMh battery.

The client wants to keep the design .... they say if it worked for 25 years, they don't want to change it.

Anyway....

So, if I understand the circuit correctly .... R4 (1K) limits the charging current.

R2 (1K) is listed as "optional" on the drawing but it is installed on all the boards. I am not sure if it is needed ... it actually causes a problem (from what I can tell) jumping around the diode D5 as it will "bleed" power back to the Yellow power LED (about 3 mA).

My best guess is that R2 is there to "force" a trickle charge onto the battery .. about 1 mA .... as the battery reaches the supply voltage, some power will still flow through the resistor as a result of the 1 volt drop across the diode.

So questions:

1) do I need R2 ?

2) What voltage should I charge the battery at .... rated voltage is 8.4 (7 cells at 1.2 volts ?) ... charge limit is 10.5 .... if I assume 7 cells @ 1.4 V each when charged then 9.8 volts ....discharge -off is 7 volts

3) where do I measure the voltage ... at the regulator output or at the battery (after the diode) ?

4) when setting the supply, do I have the battery in or out of the circuit?

Just as a side note .... I am still scratching my head over the D6 (it is across the contacts of the pressure switch) and there is a 1K resistor (R&) that does not show up on the drawing.

The "see note 2" is interesting as there is no note 2 ... there are lots of traces on the PCB that are not use ... even an unused 6 pin IC socket !

Any help would be greatly appreciated!

 

[D.A.(Tony)Stewart]

1. R2 is bad for reverse bias on U2 and raises battery voltage , not needed
2. Adjust to 8.4V at battery charged ,
3. slightly less than 9V at regulator due to battery leakage and VI drop on diode. 500 mV @ 110 uA, 400mV @ 14 uA
4. Leave battery in., but doesn't matter much if you wait for steady charge voltage
5. D6 is useless as the flyback coil voltage is negative absorbed by D6 and R7.

Why did it fail?

 

[xtal_01]

1. R2 is bad for reverse bias on U2 and raises battery voltage , not needed
2. Adjust to 8.4V at battery charged ,
3. slightly less than 9V at regulator due to battery leakage and VI drop on diode. 500 mV @ 110 uA, 400mV @ 14 uA
4. Leave battery in., but doesn't matter much if you wait for steady charge voltage
5. D6 is useless as the flyback coil voltage is negative absorbed by D6 and R7.

Why did it fail?

Thanks so much!

I was planning on leaving D6 and possible R7 out of the circuit (arguing with myself about R7). D7 should take care of any flyback coil voltage.

I honestly don't know what failed.

I tried to get one of the failed units but the maintenance guys replaced them with spares out of the store room and throw out the old ones. No one said anything till all the spares were used and they could not get new ones.

I am guessing after 25 years maybe the capacitors went. I am planning on using 10,000 hour ones in the new boxes.

 

[D.A.(Tony)Stewart]

It could be the battery or oxidized contacts too. A simpler solution would be more reliable. Just a lithium primary cell a buzzer and the pressure sensor with a test switch.

 

[xtal_01]

It could be the battery or oxidized contacts too. A simpler solution would be more reliable. Just a lithium primary cell a buzzer and the pressure sensor with a test switch.

I thought about a lithium cell but the charging looks difficult.

I am adding two test buttons ... one for the sensor and one for power failure (to test the battery).

Thanks

 

[betwixt]

Usual failure method over time is capacitors drying up. Before condemning the old units, I would change the battery, C1 and C2. They are the parts known to change characteristics over time.

I would guess the intention of R2 (across D5) is to trickle charge the battery, if its voltage is too low, additional current would flow through D5 and R4 to speed up the charge. What surprises me is there is no mechanism to speedily recharge a flat battery. It could take many hours or even days to recharge a good but discharged battery. Utilizing the N.O pin on the relay to inject the charging current would seem more sensible.

Brian.

 

[xtal_01]

Usual failure method over time is capacitors drying up. Before condemning the old units, I would change the battery, C1 and C2. They are the parts known to change characteristics over time.

I would guess the intention of R2 (across D5) is to trickle charge the battery, if its voltage is too low, additional current would flow through D5 and R4 to speed up the charge. What surprises me is there is no mechanism to speedily recharge a flat battery. It could take many hours or even days to recharge a good but discharged battery. Utilizing the N.O pin on the relay to inject the charging current would seem more sensible.

Brian.

I did notice that even in the units that are still working, the batteries have all been changed .... they are not original.

I wish I had an old failed unit .... as I said, maintenance got rid of them just thinking they would order new units. It was not till the store room used all the spare units and tried to get new ones that they realized they had a problem as they were custom made.

There is no hurry to charge the battery. If I had to guess, t here is only one power glitch per year ... maybe not even that many. These are on 24/7 over years and years.

I agree, it could be done a lot better but they are just trying to reproduce what they have and works for them.

Thanks ...