Hello all,
I had a couple of questions about which components I should use for a homemade UPS that will output 5Vdc.
I recently picked up some discarded 6v lead acid batteries. They are rated for 7.25-7.45v @ 1.35a max for constant voltage charge.
The way I intend to set this up is as follows:
7.5v/800ma wall wart --> 220nf cap --> 6v lead acid battery --> 100nf cap --> DC/DC buck converter --> RPI, Arduino or any other 5v device.
My questions are:
Do I need to put diodes between the wall wart and the battery?
Will constantly applied voltage damage the battery?
Is this even a feasible approach?
TIA and I hope this is the right message board.
Edit: I should also note that the above charging voltage is the cycle voltage. A standby constant voltage is listed as 6.8~6.9v so I may need to either use a 12v wall wart and regulate it or if I'm lucky I can find one at the correct voltage.
Boardburner - Yes I am very aware that those magical devices, known in some very exclusive circles as "chargers", do indeed exist!
IIRC(not home ATM) the battery is 6v(actually 6.28v fully charged)/4.5ah. I have no link and at this very moment cannot provide a pic. However it is very similar to the link below.
srnet - All I can say about the wall wart is that printed on it's label is 7.5V/800ma. If need be I can always put a DC/DC buck converter between the power source and the battery and limit the current. As to where I live; I suggest you check your attic :o
On a more serious note I am simply trusting the ratings of these devices.
Also, at least from what I've read, sealed lead acid batteries will do just fine with constant voltage/constant current. Whether that is true or not I cannot say.
Perhaps I could leave it on a continual 6.8V float charge?
My other question still remains however; should I place diodes between the power source and the battery?
nothingClever:
srnet - All I can say about the wall wart is that printed on it's label is 7.5V/800ma. If need be I can always put a DC/DC buck converter between the power source and the battery and limit the current. As to where I live; I suggest you check your attic :o
Yes, the need for a current limiter exists. Add a current limiter! If you don't know for sure that there is a current limiter already in the circuit and you know that the charger is only good for a maximum of 800 mA, make sure that your circuit will never draw more than that limit.
On a more serious note I am simply trusting the ratings of these devices.
Burning down your house was already very serious. Is this more serious? I would say equally serious and the latter the root cause of the former. Do NOT trust the ratings of these devices.
Perhaps I could leave it on a continual 6.8V float charge?
6.8v is probably way too high for float charge. Charge the battery to 6 volts. Then, put an amp meter between the "charger" and the battery. The MOST amperage you want to see flowing while "float charging" is 0.225 amps (5% of the Ah rating max). If you see more current than that while float charging with 6.8v, then 6.8v is too much. We are getting back to the current-limiter/dedicated charger thing, aren't we? You want higher voltage which is current limited while the battery is being charged and you want lower voltage which is current limited to a lower value when the battery is almost full.
My other question still remains however; should I place diodes between the power source and the battery?
It depends on the charger's design. Since you are adding a current-limiter (right!?!) between the charger and the battery, it probably has the protection you want already built in.
What you need is a design which is current limiting, voltage limiting, and knows when either is the most important thing for you current battery's charge level. So, now, why not just buy a battery charger? Either buy a battery charger or design a battery charger. Your choice. Do NOT just plug it into a wall wart and say "DONE!".
ty_ger07 - after speaking with my cousin who is an electrician that initially got his education in the army working on helo avionics it was suggested that I not only have some form of current limiting but also employ a full wave rectifier (though he did mention that at lower power usage diodes would likely be sufficient).
My other idea was to use a 120VAC->5VDC/1A wall wart and run it through a DC/DC buck converter so I am able to step up the voltage to 6V and also limit the current. Does this seem more feasible?
Also if I am using a buck converter the diodes are built in. What has been a bit confusing is that I have read blog posts indicating that a LAcd battery can be charged by a simple bench PSU.
Also, because specs were requested, I've attached an image of both the battery and 7.5V wall wart below.
nothingClever:
helo avionics it was suggested that I not only have some form of current limiting but also employ a full wave rectifier
Full wave rectifiers are employed on transformer circuits normally.
Most PSU bricks these days do not use them, they are high frequency converters.
You need voltage/current regulation.
Power supply to buck converter to lead acid battery to buck converter to 5v device?
I should mention that he normally deals with high voltage AC circuits these days and that he was doing avionics work back around 02-04 in Baghdad. My(very uneducated) guess is that much of the military avionics tech in those days and likely currently is a bit old but also time tested and somewhat reliable.
The full wave rectifier was mentioned when I asked about diodes between the "charger" and battery. Apparently this has to do with something about diodes being half wave? As you can probably tell I don't have a freaking clue as to what I am discussing and there is a very probable chance that I misunderstood him as well.
A full wave rectifier or bridge rectifier employs 4 diodes , you can google it.
Used after a mains transformer normally or possibly with avionics a 400 Hz transformer of unknown voltage.
Things have moved on.
Modern wall warts are devices that employ a transformer but work at many 10 ' s of KHz so the transformer is very much smaller.
A slightly different but low weight and cost to transform voltage.
For charging a battery you need voltage/current regulation depending on the battery type.
EDIT
Most fixed buck or boost converters supply a fixed voltage for supplying various circuits.
For battery charging they are specifically designed for the battery in question and as previously mentioned are called battery chargers.
nothingClever:
srnet - All I can say about the wall wart is that printed on it's label is 7.5V/800ma. If need be I can always put a DC/DC buck converter between the power source and the battery and limit the current. As to where I live; I suggest you check your attic :o
On a more serious note I am simply trusting the ratings of these devices.
Big mistake, very big mistake.
The wall wart is probably capable of 800mA.
But its unlikely to be limited to 800mA.
So connect it across a flat lead acid battery and the battery will act like a short, the amount of current it can pull could easily damage the wall wart, and worse.
There MUST be a current limiter in there somewhere.
Boardburner2:
A full wave rectifier or bridge rectifier employs 4 diodes , you can google it.
Used after a mains transformer normally or possibly with avionics a 400 Hz transformer of unknown voltage.
Things have moved on.
Modern wall warts are devices that employ a transformer but work at many 10 ' s of KHz so the transformer is very much smaller.
A slightly different but low weight and cost to transform voltage.
For charging a battery you need voltage/current regulation depending on the battery type.
EDIT
Most fixed buck or boost converters supply a fixed voltage for supplying various circuits.
For battery charging they are specifically designed for the battery in question and as previously mentioned are called battery chargers.
This is the buck converter that I'll be using:
I never knew what the center trimpot was for but now that I've taken a look it appears that I can calibrate it to a full charge. I use the same one in a small bench power supply and never knew what the center trimpot did as it was never needed. I'll have to take a look at the lm2596 data sheet. It seems as if I may have had what I needed all along. I also have a small volt/current meter that I could Incorporate so that I can visually monitor the values.
