Will this work?

[pacificblue]

http://www.batteryuniversity.com/

 

[keantoken]

keantoken,

Here's some feedback.

1. The article you posted pretty well tells you what you need to do to charge SLAs. The battery will never reach full charge with only 13.8 volts applied, and you still need to limit the current; especially if the battery voltage is very low when the charger is turned on.

There is a current limit. Q1, Q2 and Q3 are actually a CCS, capable of being used as a follower. Q2 and Q3 turn on as the current get to the max and limit Q1's Vbe.

2. I'm not an expert on batteries, but for batteries in the 5 - 10 A Hr range, NiCad may be the next best choice. I believe NiCads require 2 mode chargers for best performance (life).

Thank you. I will look into this.

3. If you don't want to use an automatic dual mode charger, you might consider a manual one, where you manually switch in and out a high current charge circuit that will be in parallel with the 13.8 volt supply.

That sounds like a good idea. How about one that turns on when I close the switch?

4. You need to realize that batteries can put out very large currents under overload conditions and that could cause safety problems. Fuse the battery for safety.

I plan on doing this, just keep forgetting to include it in schematics.

5. Almost all JFETs (not MOSFETs) are depletion mode devices. This means that at Vgs equal zero the device current will be at the maximum (Idss). As you reverse bias the gate (wrt the source) the drain current will decrease (like a vacuum tube). (Thus the FET regulator you posted won't do what you want.)

Okay. I think using a MOSFET here would work, but that is not an option because I would need a larger value resistor, and this would require higher voltage rails.

6. All semiconductor devices can have wide parameter variations from device to device. (Check the data sheet on BJT current gain, or a JFET on Idss, or a MOSFET on Vt.) Any "open loop" circuit where it is desired to operate transistors in their active region will give disappointing performance. Therefore, feedback is almost always used. Note that a voltage follower circuit uses negative feedback. A common emitter circuit with a (bypassed) emitter resistor has negative feedback.

7. Precision analog circuits require feedback. Period

Rick

http://www.batteryuniversity.com/

I've already been referencing this site a lot, but thanks.

If I seem vague in this post, don't be fooled. I understand everything that has been said. Thank you for your comments.

- keantoken

 

[AndrewT]

Hi, a two stage/mode charger is fairly easy to assemble.

Use a 317 to current charge @ ~10% of the AH rating, i.e. 700mA for a 7AH battery.
Use a voltage regulator to limit the voltage and when you hit this limiting voltage (~13.8V to 14.0V) the CCS steps down to ~1% of AH rating.

Leave the battery charging at 1% and monitor the voltage. If it continues to rise or fall then adjust the trickle charge current slightly (+-20% ?)

The voltage regulator will run full open when the voltage is low allowing the 10% CCS to current charge.

 

[keantoken]

Thank you.

- keantoken

 

[TechGuy]

I don't like the idea of switching, because it's more complex, more expensive, and requires PCBs, which I can't make yet.

The simple switchers are extremely easy to use. The only difference between using a Linear Regulator and a Switcher is an external diode and the inductor. You can easy breadboard, wirewrap using simple switchers TO-263 devices. You just need to solder solid wire (22 Gauge) to the pins. It takes just a couple of minutes to solder wire to the pins. Cut a set of 1 inch (or less) 22 Gauge Solid Copper with the insulation removed. add solder to one of the ends. Bond the wire to the pins of the TO-263. If your going to push a lot of current thru the switcher you can solder on copper foil for the Heat Sink Tab. You can also purchase Simple switchers using the TO-220-5 or TO-220-7 packages that will use standard TO-220 heat sinks.


The Simple switcher will operate as a float charge. As the battery becomes fully charge the voltage will rise. the Simple switcher PWM will reduce the output current to zero as the duty cycle will drop to well below 1%.

The only disadvantage of using a multimode charger is the longer charging time. You could modify a simple switcher to operate in a dual mode charger by making use a current sensor that turns on\off a transistor that programmatically adjusts the output voltage (using the transitor to switch on a parallel resistor to increase the output voltage) . However, if you use a dual voltage charger you may need to add a secondary voltage regulator since I suspect your load is expecting the voltage to be at or near 27.6V. If you used a higher voltage (32V in fast charging mode), it may exceed the voltage limit of your load.

 

[ChrisA]

So I thought of this.

The batteries are kept charged constantly, and the trafo is protected by the current limiters. Large transients will be pulled from the batteries, with a limited amount of help from the input transformer.

You are making to way to complex. The common method is to make use od the lead acid battery chemistry to regulat the power supply. Build a SMALL low current 13.5V supply. That's way easy if you happen to have a 12V power cube (wall wort) handy.

When the battery is fully charge it's voltage will equal the power ciube's and no current will flow. When the battery is low current will flow and the battery will charge but the small transformer is current limited so the battery will not see to much current. A 500ma block would work.

To do this right you need a battery that can supply two days of typical usage without going below 50% charge and a power supply that can re-charge this in 24 hours.

Once you get past this very simple method them you are best (by far) using a micro-controller to control the charge curent based on characteristic of lead acid chemistry. These are common. People tried for years to make a good high powered charger and none really worked until micro controllers (like the PIC or AVR) became cheap jellybean parts. Some of these little $2 computers have ADC built-in so you can make an easy voltage or current sensor

Another option is to buy a larger battery. 200AH batts are not as expensive as a good charge

 

[pacificblue]

Re: Re: Will this work?

You are making to way to complex. The common method is to make use od the lead acid battery chemistry to regulat the power supply. Build a SMALL low current 13.5V supply. That's way easy if you happen to have a 12V power cube (wall wort) handy.

When the battery is fully charge it's voltage will equal the power ciube's and no current will flow. When the battery is low current will flow and the battery will charge but the small transformer is current limited so the battery will not see to much current. A 500ma block would work.

Don't forget to add a diode so that a new, fully charged battery doesn't destroy the wall-wart. But the diode would of course have a voltage drop, so the battery might not be charged completely from the wall-wart, so you need a slightly higher transformer voltage. Or maybe you just buy something like this (English description is on page 8), where everything is provided.

 

[WSJ]

The two-Step Constant Voltage Charging method uses two constant voltage devices. In the initial charge phase the high voltage setting is used. When charging is nearly complete and the charge voltage has risen to a specified value (with the charge current decreased), the charger switches the voltage to the lower setting. This method allows rapid charging in cycle or float service without the possibility of overcharging, even after extended charging periods.

Download Power-Sonic Technical Manual.
See Figure 19: Dual stage current limited battery charger.
http://www.power-sonic.com/index.php?doc_id=116

 

[AndrewT]

I find the terms used by Power Sonic to be confusing.

They refer to constant voltage charging and to two step constant voltage charging.

In both cases you can see from the plots of voltage/current vs time that the initial charging is constant current and not constant voltage.

 

[WSJ]

The LM317 output current is internally limited; an external pass transistor is used to increase the charger output current limit. When a discharged battery is connected, the charger starts charging the battery in the constant current mode (current limited output) due to the low battery voltage, as the battery voltage increase the charger comes out of the constant current mode into the constantan voltage mode.

 

[AndrewT]

exactly, it's constant current overridden by constant voltage to prevent overcharging.

 

[keantoken]

exactly, it's constant current overridden by constant voltage to prevent overcharging.

Yes, I understand perfectly.

- keantoken