a switchless battery charger. Is this circuit OK or is the author unreal?

[mik3ca]

You can charge NiMH at 4C if you know what you're doing.

This is why I come here for ideas. But I did think of one. This circuit that I just thought of. Thermistor is attached to the battery to detect heat and when the battery is hot enough then transistor turns off which makes input to IC2 low so battery won't charge. When IC1 is on then IC2 goes on and provides constant current via R1.

I may have R2 and the thermistor switched up and maybe my input voltage is too low but am I sortof in the right direction? (I still think its a bit ridiculous to strap a thermistor to a battery while charging but if it has to be done then so be it.)

 

[dknguyen]

This is why I come here for ideas. But I did think of one. This circuit that I just thought of. Thermistor is attached to the battery to detect heat and when the battery is hot enough then transistor turns off which makes input to IC2 low so battery won't charge. When IC1 is on then IC2 goes on and provides constant current via R1.

I may have R2 and the thermistor switched up and maybe my input voltage is too low but am I sortof in the right direction? (I still think its a bit ridiculous to strap a thermistor to a battery while charging but if it has to be done then so be it.)

View attachment 112733

If you want to charge 4C, use the MAX713. The raw circuit is beyond you for the time being and there is no room for error and different methods of detecting end of charge are used that don't work for lower charge rates. These are circuits that have timers, sampling, memory of initial conditions, and calculate slopes. Use it with a big transistor and a big heatsink and fan.

 

[ChrisP58]

I agree with dknguten. If you want fast charge, then you need a reliable, precise charge termination detection method. And, properly configured, the purpose built battery charge ICs will give you that.

The battery charge IC's use thermistors to look for a change in the rate of temperature rise, not an absolute level of temperature. In other words, at what point is the current into the battery no longer being converted to chemical energy, and is being converted to heat instead. That is the point when the battery is full. If you wait for the battery to reach a given temperature, like your circuit in post #21, you've probably already damaged the battery.

 

[MikeMl]

Look at any older power tool that uses fast-charging into NiCds. It has three terminals on the battery. Two terminals pass the charging current from charger to battery, while the third terminal is for the charger to sense the thermistor embedded inside the battery.

 

[mik3ca]

ok I found theres a max712 and a max713 IC for charging batteries and I think one is for detecting negative slope and the other for detecting positive slope. On futurelec.com, the prices of the two chips are $1 apart. Which chip is better to use?

 

[dknguyen]

Zero slope is for NiMH. Negative slope is for NiCd.

https://www.maximintegrated.com/en/app-notes/index.mvp/id/4496

 

[Nigel Goodwin]

You might like to look at the LTC4011 from Linear technology, this is a switch-mode charger IC and can be set for either NiCd or NiMh - it's come to my attention because I replaced a faulty one in a unit the other week. Having repaired a number of units that use the IC it seems to work pretty well, and has the required temperature sensor input for monitoring the batteries.

 

[alec_t]

Having repaired a number of units that use the IC it seems to work pretty well

So what was the problem which necessitated the repair? Hopefully nothing to do with the IC itself?

 

[Nigel Goodwin]

Only in one case, there's a 3V regulator inside the chip, and it was outputting 9V, it's used to feed the thermistor for the temperature sensing.

 

[AnalogKid]

I would not worry about efficiency. This is for a home-brew, one-off battery charger, not a cell phone. I think the complexity and cost of getting a switching regulator to work correctly far outweighs the little bit of heat saved. The LM317 is a power device; it can take the heat. When you have real numbers for the transformer-bridge-filter capacitor section, working out the heatsink required is simple math.

Schematics #1 and #3 are constant current circuits, which is the right place to start. Schematic #16 adds a form of fully-charged detection to back off the available energy and not overcharge (overstress) the battery chemistry, but is not constant current for the main charging cycle.

Note that Linear Tech, Maxim, and TI (formerly Unitrode) make chips designed specifically for "proper" charging of battery chemistries, if you want to go that route. If not ...

#21 is better, but not quite there. For semi-intelligent charging on the cheap, I like the two LM317's in series. The first is a standard constant-voltage regulator to set the maximum possible voltage across the battery when it is fully charged. The second is a constant current regulator to set the maximum possible current to the battery during the heavy part of the charging cycle. In your schematic, IC1 needs the two standard voltage-setting resistors, with the thermistor circuit added in parallel with the lower one. Depending on the thermistor, you might be able to simply place it in parallel with the shunt resistor, but that way might not have enough protection effect. That is, when the battery gets hot it might not turn down the output voltage enough.

ak

 

[gary350]

I don't see any reason to make a lead acid car battery charger complicated. 12 V 10a transformer with bridge rectifier & filter capacitor = 17vdc Car lead acid battery is self regulating the lower the battery is the more amps it pulls and the closer to full charge it gets the less amps it pulls. You can not over charge a lead acid car battery it will not take a charge if it is not low.

 

[AnalogKid]

You can not over charge a battery it will not take a charge if it is not low.

? ? ?

 

[dknguyen]

I don't see any reason to make a battery charger complicated. 12 V 10a transformer with bridge rectifier & filter capacitor = 17vdc Car lead acid battery is self regulating the lower the battery is the more amps it pulls and the closer to full charge it gets the less amps it pulls. You can not over charge a battery it will not take a charge if it is not low.

Well this is just blatantly false:

1. You CAN overcharge batteries. Just looking at lithium-ion and lithium-polymer. You can charge them above the voltage they should be at. They then get hot, puff up, and then explode into flames.

2. You CAN overcharge lead acid batteries. The difference is that lead-acid batteries are known for being unusually rugged amongst battery chemistries. It's just is more difficult to damage them, but by no means impossible. You just have to overcharge them hard enough or long enough before degradation and damage will occur. They don't fail as visibly, as quickly, or as catastrophically as something like a lithium chemistry battery.

3. The ruggedness of lead-acid batteries are the exception, not the rule. If it was, charging lithium batteries would be a lot simpler, a lot safer, and a lot cheaper.

4. The OP was not asking about lead-acid batteries. Even if he was, this advice implies bad things. Just because a battery is pulling a certain amount of charge current does not mean that it should be pulling it.