Temperature compensation problems!!

Hey guys. So I'm designing a battery charger for one of our products, and one of the features I want to include is temperature-compensated charging voltage. Doing a bit of math I found some thermistors that I could integrate into the feedback path of my design in order to compensate for higher temperatures than the nominal 25 deg C.

I then realized I made an oversight. The internal temperature of our product under normal operating condition will be 45 deg C! This means that when the battery is at 25 deg C, the thermistor will be at 45 deg C. I could compensate for this by using higher value thermistors with higher B values, but then when the product is not at operating temperature it could be charging the battery at dangerously high voltages. No bueno.

The solution is that I need an NTC thermistor or temperature-compensating circuit that has its nominal value at 45 deg C, i.e it does not change resistance below 45 deg C.

Anybody have any recommendations, suggestions, comments?

If you want to compensate the charging voltage of your battery, the corresponding sensor should feel the battery's temperature, isn't it?

There's no reliable temperature relationship between your charger and the battery (accu).

You need to mount the thermistor remotely, directly in contact with the battery. It might be close enough if you mounted it on the terminal/clamp that connects to the battery terminal.

The internal temperature of my device is proportional to the outside temperature. Example: 25 deg C outside, 45 deg inside. 35 deg C outside, 55 deg inside.

The device is placed in a sealed box along with the battery. The compensation circuit is based on the ambient temperature inside the box.

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erikl said:

If you want to compensate the charging voltage of your battery, the corresponding sensor should feel the battery's temperature, isn't it?

There's no reliable temperature relationship between your charger and the battery (accu).

Click to expand...

The temperature inside the device and outside is proportional...it is not incredibly accurate but will give me a rough value. The thermistor or probe must be located within the device.

SWAGGANEER said:

The internal temperature of my device is proportional to the outside temperature. Example: 25 deg C outside, 45 deg inside. 35 deg C outside, 55 deg inside.

The device is placed in a sealed box along with the battery. The compensation circuit is based on the ambient temperature inside the box.

The temperature inside the device and outside is proportional...it is not incredibly accurate but will give me a rough value. The thermistor or probe must be located within the device.

Click to expand...

If the battery is inside the box then it's temperature should be close to the box's inside temperature. I don't see the problem.:?:

SWAGGANEER said:

The internal temperature of my device is proportional to the outside temperature. Example: 25 deg C outside, 45 deg inside. 35 deg C outside, 55 deg inside.

The device is placed in a sealed box along with the battery. The compensation circuit is based on the ambient temperature inside the box.

The temperature inside the device and outside is proportional...it is not incredibly accurate but will give me a rough value. The thermistor or probe must be located within the device.

Click to expand...

If you're sure of this fact, you can perform one of these possibilities:

1. Redesign your max. voltage control on the basis of the thermistor's 45°C resistance
or
2. Use a resistor corresponding to the thermistor's (45-25)°C difference resistance in series with the thermistor.

Why is your regulator so hot? Why is your sensor reading the hot spot rather than battery temperature.

Obviously both can be resolved with more efficient design. WHere is your design?

Another thought is to have a second thermistor which senses the charger temperature (could be a thermostat on the heat sink) and only when this temperature is reached does the voltage rise to that set by your existing thermistor. Use a thermostat to switch in or out a series resistor with your normal thermistor. Batteries for electric drill have a thermistor or diode attached to one of the battery cells in side the battery pack, so three terminals to the charger. Its de rigour for all fast chargers, i.e. <2 Hr chargers.
Frank