I am trying to design a 24 volt central battery emergency light system for a college project and finding it harder than I thought. Can anybody help me with information and diagrams for the charger system and a low volt cut out, or reccommend some good sites to visit. Thanks.
In message , dated Mon, 14 Aug 2006, Jim Crow writes
This isn't the best way to do a college project, is it? If you ask about a specific problem, you will get help here, not spoonfed the answers but help to work them out for yourself, which gets you credit. No credit for copying!
What sort of battery is it? If it's deep-cycle lead-acid, the manufacturers' web sites and literature give you a lot of information about care and feeding. You can often ask their Technical Support.
-- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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National Semiconductor
In message , dated Mon, 14 Aug 2006, Jim Crow writes
Good, but you query gave me that impression.
I asked you to tell us what sort of battery you are using. If you do THAT, we can direct you to sites with information, and even add to it. Without knowing the sort of battery, we can't help.
-- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
I have looked at some battery manufacturer sites and have think I should use 2 x 12 volt 7Ah Yuasa batteries which are deep cycle AGM VRLA type.
How do I know what current to start charging these at, is it 1/10th of the output ?
Hi,
I have used the UC3906 for charging and the MAX8212 for low voltage cut off in the past for a lead acid battery charging application. Checking the datasheets on these products would possibly give you some good clues on how things can be done..................... Assuming that you are using lead acid (I haven't checked the voltage rating on these parts).
Regards
AJ
In message , dated Mon, 14 Aug 2006, Jim Crow writes
I can't find a 7 Ah deep cycle VRLA battery on the Yuasa site. What is the Yuasa part number? 7 Ah seems to be a bit low on capacity. How many lights are you running, for how long and what current do they draw?
You won't get 7 A for 1 hour, or even 3.5 A for 30 minutes; that's a
20-hour rating (350 mA for 20 hours, which is just about four hand-torches).There is some flexibility. If we can identify the exact product you need, the data on charging is available from Yuasa web site.
-- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
The Yuasa battery I saw was a NP7-12 which it says is for strand by operation, it is to feed about 20 x 24 volt 8 watt fluorescent slave units for a minumum of 1 hour. I have had a look at the datasheet for this battery but do not fully understand all details it specifies.
Thnaks for that, I have got that downloaded now and have a basic diagram to start my project with.
Many thanks AJ. The UC3906 appears ok but the MAX8212 has a max input voltage of 16.5 volts and I am trying to charge a 24 volt lead acid system.
In message , dated Mon, 14 Aug 2006, Jim Crow writes
Each slave unit takes 0.33 A, so 20 take 6.7 A. The NP7-12 will only give 4.2 Ah at the 1 h rate. You need a bigger battery.
Go to:
and download the NP series data. That will tell you a great deal of what you need to know, including what charging currents and voltage limits you should use.
-- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
I haven't looked at the 8212 data sheet, but conceptually, you would put a
2:1 divider between the battery and the device.Tam
I thought the MAX8212 may be too low but I thought some of the circuit examples might be handy and perhaps you could pick another part or make you own. The following link shows and example of one used as a low battery disconnect...
The UC3906 might not be the best choice for your application but explains how a SLA is best charged. The info from that and the battery datasheet which John Woodgate provided a link to in a previous msg should give you plenty of info to get started.
Regards
AJ
If you don't find a specific part to do the job of low voltage cutout , maybe roll your own in the way of a lm3915 bar/dot driver. Basically a voltage comparator with 10 outputs and by setting the divider up to cover the voltage range of the battery - and by using an output to drive a transistor to switch off a latching relay to cutout the battery. Then you could have the latching relay energize when the mains come back up. Jtt
For emergency applications, a float charge on SLA would do the trick. What you want in the design of the charger is idiot-proofing. For instance, the battery eventually will get replaced. Is the new battery in backwards? Is the new battery too low in voltage to be deemed worthy of charging? Short circuit protection? Current limiting? A good design is one that won't cause a return or design out.
Most SLA designs disconnect the battery at around 10V to prevent it's destruction. In emergency lighting, they may not do this, since , well, it's an emergency. ;-)
In message , dated Mon, 14 Aug 2006, snipped-for-privacy@sushi.com writes
They should, but also have enough battery capacity so that even in the worst case, the voltage never drops to 1 V per cell.
The worst case is pretty easy to determine; it often isn't. The worst case is the maximum time it would take to evacuate the building. If it's empty, emergency lighting is not required. Some idiots specify that the system capacity must be enough for 12 or 24 hours!
-- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
It's worse than that. The fluorescent tubes are 8W rating, so the inverter draw will be somewhat higher.
I left out reverse power protection. This is where the battery is fully charged, but the charger itself isn't plugged in. It could be as simple as a diode, though in a high power charger you might do something more efficient. For a float charge, the diode is fine, but the voltage sense has to be after the diode to compensate for the drop. You do need to insure the sense circuit has protection. A reverse battery condition with the charger off is also needed.
Some designs take care of reverse power. This would be where some moron plugs in a battery that is higher voltage than the charger.
A robust charger takes lots of planning. Just ask Dell or Apple. ;-)
Note that I know (I think) all the pitfalls of a charger, and when I needed one for home use, I just bought one off the shelf. ;-) Full uP control and most of the protection I mentioned. Some manufacturers (boards and chip level) put in protection modes not on the datasheet. There are reasons not to mention the extra protection modes, but that would be a paragraph or two more.
Ah: emergency lights are SUPPOSED to be rated on consumption, not the lamp rating. Of course, they may not be. If so, 8.5 W is probably about right; the inverter is quite efficient.
-- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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