Thermal runaway and battery charger questions

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Has anyone ever experienced thermal runaway in a UPS battery system? We had one a couple of months ago in our C&D batteries. It is a 120 VDC system. After months of trying to find the problem it turns out it was a loose terminal in one of our control boards. It is a SCI charger. I also have a question about basic ohms law regarding the charger voltage. Figure you have a voltage of 100 and a 10 resistence, that equals 10 amps. Now you turn the voltage up to 200 volts and the resistence stays the same, now you have 20 amps. With a charger when you turn the voltage up and current goes down, the load (resistence) never changes. What am I not seeing?
 

broadgage

Senior Member
Location
London, England
If one considers a battery connected to a battery charger to be a load, then the battery is a non ohmic load, and the current absorbed by the battery will vary in a non linear fashion with changes in voltage.

For example a 12 volt nominal battery would normally be charged at about 13.8 volts. Once the battery is fully charged then the current will be very small, perhaps 0.1 amp for a medium size battery.
If the charging voltage is improperly raised to say 16 volts, then a very great current will flow until either a fuse blows, or the battery, or the battery charger is destroyed.

If however one is altering the line voltage supply INTO a battery charger, then with most designs, a higher line voltage will result in a lower line current.
That is because most modern battery chargers use a switch mode power supply, which draws the required wattage at any line voltage, with the design limits of course.
For example if the battery charger requires 120 watts, then that will be about 1 amp at 120 volts, but only about 0.5 amp at 240 volts.
(this all presumes that the charger is designed for multi voltage input, if it designed for a fixed line voltage then it wont work properly or at all at the wrong voltage)

The correct charging voltage varies slightly with temperature, and should be less at higher temperatures.
The simple basic chargers, such as one might use at home for a vehicle battery, simply assume an average room temperature.
This is liable to result in overcharging in hot conditions, and in a less than full charge in cold conditions.
Larger more sophisticated battery chargers, as used for large UPS systems, measure the room temperature, or better the battery temperature, and adjust the voltage accordingly.

An excessive charging voltage or excessive temperature results in the battery still absorbing current when fully charged, this additional energy input turns into heat which raises the battery temperature.
This increased temperature results in more current, which raises the temperature even more.
If allowed to continue, disaster will assuredly follow.

The original cause of the problem could be
1) wrong voltage battery fitted, such as 108 volt instead of 120 volt
2) battery temperature sensor in the wrong place, perhaps detecting cold air from an AC unit rather than a warm battery
3)one or more cells in a battery failing, so that a nominal 120 volt battery is in fact, perhaps a 114 volt battery.
4)defective charger, as appears to be the cases refered to above.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
I think what you are missing is there a several charging algorithms used to charge batteries.

For standby emergency service like a UPS or telecom battery plant the FLOAT algorithm is used. Float is a constant voltage method with limited current. Depending on the battery type of FLA, SLA, or VRLA, and cell temperature the cell voltage is regulated to 2,2 to about 2.3 volts per cell. So if you have an 88 cell plant using VRLA batteries the charger constant voltage will be 202.4 volts.

So when the battery is fully charge the charge current is very small usually called a trickle charge of around 1/100 or the amp hour rating of the battery. This trickle charge over comes the self discharge rate of the battery keeping it at 100% capacity. The excess current capacity of the rectifiers is then used to power the terminal equipment

Most modern Float service rectifiers have a temperature sensor that is installed on a pilot cell somewhere near the middle of the mass. As temperature goes up, the float voltage is reduced to prevent thermal runaway.

Ok as to the charge current in float service or any charging algorithm is determined by the difference in voltage between the battery, charge voltage, internal resistance of the battery, and the current limit of the rectifier. So to make things simple let?s use a 12 volt VRLA as an example. The battery state of charge is 12.1 volts, rectifier voltage is 13.8. So 13.8 ? 12.1 is 1.7 volts difference and the battery internal resistance is .034 ohms. So the charge current will be 1.7 volts / .034 ohms = 50 amps assuming the rectifier can supply 50 amps @ 13.8 volts. As the battery reaches full charge, its voltage is going up to meet the supply of 13.8 so the charge current will taper off to eventually reach a trickle charge current to overcome self discharge.
 

Besoeker

Senior Member
Location
UK
I think what you are missing is there a several charging algorithms used to charge batteries.

For standby emergency service like a UPS or telecom battery plant the FLOAT algorithm is used. Float is a constant voltage method with limited current. Depending on the battery type of FLA, SLA, or VRLA, and cell temperature the cell voltage is regulated to 2,2 to about 2.3 volts per cell. So if you have an 88 cell plant using VRLA batteries the charger constant voltage will be 202.4 volts.

So when the battery is fully charge the charge current is very small usually called a trickle charge of around 1/100 or the amp hour rating of the battery. This trickle charge over comes the self discharge rate of the battery keeping it at 100% capacity. The excess current capacity of the rectifiers is then used to power the terminal equipment

Most modern Float service rectifiers have a temperature sensor that is installed on a pilot cell somewhere near the middle of the mass. As temperature goes up, the float voltage is reduced to prevent thermal runaway.

Ok as to the charge current in float service or any charging algorithm is determined by the difference in voltage between the battery, charge voltage, internal resistance of the battery, and the current limit of the rectifier. So to make things simple let’s use a 12 volt VRLA as an example. The battery state of charge is 12.1 volts, rectifier voltage is 13.8. So 13.8 – 12.1 is 1.7 volts difference and the battery internal resistance is .034 ohms. So the charge current will be 1.7 volts / .034 ohms = 50 amps assuming the rectifier can supply 50 amps @ 13.8 volts. As the battery reaches full charge, its voltage is going up to meet the supply of 13.8 so the charge current will taper off to eventually reach a trickle charge current to overcome self discharge.
Good post.
I was in the process of typing a response but you said everything I was going to. Then some.
 

rattus

Senior Member
Please Sir, more:

Please Sir, more:

Well said Derek,

Now tell us how an electromechanical regulator works. You know, like they used on autos in the dark ages.
 

rattus

Senior Member
Anyone?

Anyone?

Actually, the early alternator regulators were very similar to the generator regulators. They used two relays instead of three.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Actually, the early alternator regulators were very similar to the generator regulators. They used two relays instead of three.
That's because the diodes in the alternator eliminated the need to disconnect the output from the battery at idle of when off.
 
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