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battery backup circuit???

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chamilackjm

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i want a circuit when main power is broken down power provide by 12v battery to the target circuit, the circuit should also be included battery charger and battery level indicator, please help me .... thank you!!!!
 
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the easiest method i can think of is using a microcontroler. and run the current through that and when it senses decresed power let power come through the 2nd power source. i will add a diagram later.
 
You simply connect the 12v battery in this circuit to the project you want to back-up, via a diode:

**broken link removed**

BATTERY CHARGER - world's simplest automatic charger
This is the world's simplest automatic battery charger.
It consists of 6 components, when connected to a 12v DC plug pack. The plug pack must produce more than 15v on no-load (which most plug packs do.) An alternative 15v transformer and a centre-tapped transformer is also shown. A centre-tapped transformer is referred to as: 15v-CT-15v or 15-0-15
The relay and transistor are not critical as the 1k pot is adjusted so the relay drops-out at 13.7v.
The plug pack can be 300mA, 500mA or 1A and its current rating will depend on the size of the 12v battery you are charging.
For a 1.2AH gel cell, the charging current should be 100mA. However, this charger is designed to keep the battery topped-up and it will deliver current in such short bursts, that the charging current is not important.
This applies if you are keeping the battery connected while it is being used. In this case the charger will add to the output and deliver some current to the load while charging the battery. If you are charging a flat cell, the current should not be more than 100mA.
For a 7AH battery, the current can be 500mA. And for a larger battery, the current can be 1Amp.

SETTING UP
Connect the charger to a battery and place a digital meter across the battery. Adjust the 1k pot so the relay drops out as soon as the voltage rises to 13.7v.
Place a 100R 2watt resistor across the battery and watch the voltage drop.
The charger should turn on when the voltage drops to about 12.5v. This voltage is not important.
The 22u stops the relay "squealing" or "hunting" when a load is connected to the battery and the charger is charging. As the battery voltage rises, the charging current reduces and just before the relay drops out, it squeals as the voltage rises and falls due to the action of the relay. The 22u prevents this "chattering".
To increase the Hysteresis: In other words, decrease the voltage where the circuit cuts-in, add a 270R across the coil of the relay. This will increase the current required by the transistor to activate the relay and thus increase the gap between the two activation points. The pull-in point on the pot will be higher and you will have re-adjust the pot, but the drop-out point will be the same and thus the gap will be wider.
In our circuit, the cut-in voltage was 11.5v with a 270R across the relay.
Note: No diode is needed across the relay because the transistor is never fully turned off and no back EMF (spike) is produced by the relay.
 
the easiest method i can think of is using a microcontroler. and run the current through that and when it senses decresed power let power come through the 2nd power source. i will add a diagram later.

thank you for your reply, please add the circuit diagram as soon as possible.............
 
You simply connect the 12v battery in this circuit to the project you want to back-up, via a diode:

**broken link removed**

BATTERY CHARGER - world's simplest automatic charger
This is the world's simplest automatic battery charger.
It consists of 6 components, when connected to a 12v DC plug pack. The plug pack must produce more than 15v on no-load (which most plug packs do.) An alternative 15v transformer and a centre-tapped transformer is also shown. A centre-tapped transformer is referred to as: 15v-CT-15v or 15-0-15
The relay and transistor are not critical as the 1k pot is adjusted so the relay drops-out at 13.7v.
The plug pack can be 300mA, 500mA or 1A and its current rating will depend on the size of the 12v battery you are charging.
For a 1.2AH gel cell, the charging current should be 100mA. However, this charger is designed to keep the battery topped-up and it will deliver current in such short bursts, that the charging current is not important.
This applies if you are keeping the battery connected while it is being used. In this case the charger will add to the output and deliver some current to the load while charging the battery. If you are charging a flat cell, the current should not be more than 100mA.
For a 7AH battery, the current can be 500mA. And for a larger battery, the current can be 1Amp.

SETTING UP
Connect the charger to a battery and place a digital meter across the battery. Adjust the 1k pot so the relay drops out as soon as the voltage rises to 13.7v.
Place a 100R 2watt resistor across the battery and watch the voltage drop.
The charger should turn on when the voltage drops to about 12.5v. This voltage is not important.
The 22u stops the relay "squealing" or "hunting" when a load is connected to the battery and the charger is charging. As the battery voltage rises, the charging current reduces and just before the relay drops out, it squeals as the voltage rises and falls due to the action of the relay. The 22u prevents this "chattering".
To increase the Hysteresis: In other words, decrease the voltage where the circuit cuts-in, add a 270R across the coil of the relay. This will increase the current required by the transistor to activate the relay and thus increase the gap between the two activation points. The pull-in point on the pot will be higher and you will have re-adjust the pot, but the drop-out point will be the same and thus the gap will be wider.
In our circuit, the cut-in voltage was 11.5v with a 270R across the relay.
Note: No diode is needed across the relay because the transistor is never fully turned off and no back EMF (spike) is produced by the relay.

oh it's a cool circuit can be make with least component, i am really appreciate about your reply, thank you very much,
 
The above circuit needs a way to limit the charging current into the battery (such as a series resistor). Otherwise the Plug Pack may overheat and fail since they don't necessarily have built-in current limiting.

The circuit trip point has no hysteresis so it will tend to rapidly switch between on and off if the battery is under a light load.
 
crutschow You are wrong on both counts.
The battery produces a "back-voltage" that allows very little charging current from a plug pack. You will be lucky to get 200mA from a 12v 500mA plug pack.
The circuit has hysteresis provided by the pull-in and release of the relay.
The best thing to do is build my circuits before you make any speculations.
I would not put a circuit on my site unless I had thoroughtly tested it.
 
I tried building the circuit, but I am confused on one aspect of its behavior. Whenever I unplug the battery with the DC from the outlet present, the circuit shuts off for a second or so. Do you have any ideas for avoiding this behavior? THanks

Sam
 
the easiest method i can think of is using a microcontroler.**broken link removed**
 
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crutschow You are wrong on both counts.
The battery produces a "back-voltage" that allows very little charging current from a plug pack. You will be lucky to get 200mA from a 12v 500mA plug pack. ...

No, Cruts is right. If you connect a discharged battery to this circuit, you are likely to have a fire in the plug-pack (wall-wart) because the only thing limiting the current is the output impedance of the plug-pack itself. Lead-acid battery chargers need to act as a constant-current source (actively limit the charging current) until the battery terminal voltage comes up. Some wall-warts are intrinsically current-limited by design (air gap in the magnetic path, core saturation); others are not, but might have a fuse (sometimes self resetting, but not always).
 
That's not really true. A discharged 12 V lead acid has a higher impedance than a charged unit due to the reduction in the specific grav. of the electrolyte. Therefore it self limits current.

That circuit depends on the battery Ah rating being small....ergo a relatively high impedance.
 
Hi colin55 thanks for the circuit, I made it but facing some issues.
The transistor is getting very heated up & the relay start chattering when battery near to full charge

Am I doing some mistakes?
 
The transistor is getting very heated up & the relay start chattering when battery near to full charge

The problem with ultra-simple circuits, is that one relies in "having luck" with the component tolerances for proper circuit operation.

For instance, your transistor overheating may mean that the transistor's beta is too low, and as the battery starts to charge it remain in linear mode for too long before switching fully.
Second, the chattering may mean that the relay's hysteresis is smaller and/or the battery's impedance is higher than the ones used by Colin55.
 
That's not really true. A discharged 12 V lead acid has a higher impedance than a charged unit due to the reduction in the specific grav. of the electrolyte. Therefore it self limits current.

That circuit depends on the battery Ah rating being small....ergo a relatively high impedance.

My statement is true; yours does not apply to 95% of the lead-acid battery charging process!

If you are discharging a lead-acid battery to where its impedance increases substantially, you are discharging it much too deeply! The charge acceptance rate (charging current) of a lead-acid battery that is discharged to ~ 5% to 80% (95% to 10% charge remaining) MUST be limited! If not, you will either overheat the transformer, blow up the rectifiers, or overheat the battery.

Besides, even if the battery is over-discharged to where it should not go, its charge-acceptance rate might be low initially, but as soon as the S.G. comes up, it will accept very high charging (damaging) charging currents. At least, the LM317 circuit Mosaic linked to has built-in current limiting, but it does not rise to some other requirements for a good circuit mentioned below:

The circuit posted by Colin has no current limiting other than what MAY be intrinsic in the wall-wart. It is an extremely poor lead-acid battery charger.
1. No current-limiting (likely to blow-up or overheat the wall-wart)
2. Very primitive voltage regulation (using Vbe of transistor, which has very soft knee)
3. Extremely temperature sensitive for ambient temperature changes (Vbe of transistor is very temperature dependent)
4. No hysteresis. It will constantly cycle on/off as it approaches the set point. (as the OP has found out)
5. No attempt to do anything but cut-off the charger. Six-cell Sealed lead-acid (and flooded ones) requires one voltage to reach full-charge (14.5 to 14.8V) , and a different float voltage (~13.0 to 13.6V).

What voltage would you set the cutoff voltage of this simplistic charger to? If you say 13.5V, then the battery is only ~60% charged the first time that voltage is reached. If you say 14.5V, then this charger will turn back on after the battery drops a few mV, effectively holding the battery at too high a voltage for too long.!

Such a simplistic charger cannot do the job, and depending on how it is set up, will either damage the battery by chronically undercharging it, or damage it by chronically overcharging it. Put another way, there is no single voltage setting of the trimpot that will not ultimately damage the battery it is connected to.
 
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My statement is true; yours does not apply to 95% of the lead-acid battery charging process!

Most of the time or 95% of the time does not mean TRUE. Your statement would be categorized as false if a 96% Confidence were required.
 
Most of the time or 95% of the time does not mean TRUE. Your statement would be categorized as false if a 96% Confidence were required.

That is a BS response. Refute the point that a battery charger must be current limited.

Just because a battery may not draw much current when it is first connected to the charger due to being overdischarged, it will begin to draw current as the Specific Gravity of the acid increases. It will go through a region in the charging process where it cannot be charged with an ideal voltage source; the charging current must be purposefully limited.
 
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With small wall-warts there is no need to current limit. The wall-wart might have a problem if the battery voltage is really low, but it takes very little charge, and therefore time, to bring a battery up to 11 V or so. Cheap chargers like this are just a transformer and rectifier. The light is for information only. Chargers like that will usually deliver far less than their rated current when the battery voltage is anywhere near normal. Also they will tend to eventually overcharge the battery.

I agree about the other shortcomings of the circuit. It relies on far to many variables. The only hysteresis comes from the relay, which will mean that the relay isn't turning on or off as fast as it should, which could result in increased contact wear from slow connecting or disconnecting. That is the reason some car manufacturers won't use diodes across relay coils, because they slow the de-energising of the relay. A resistor of around 10 times the relay coil resistance is used instead. That keeps the voltage down to around 120 V, which is no problem for suitable transistors, while stopping the coil current 10 times faster than a diode.
 
That is a BS response. Refute the point that a battery charger must be current limited.

Duck and weave it seems. I point out the flaw in your statement and you switch the subject. Further you start to argue 2 years after my original statement is made.
Let me enlighten you.

Wall wart chargers are limited by voltage drop due to current draw. Cheap battery chargers rely on transformer voltage drop effect to limit the current without explicit electronics to do so. It's a matter of economics for those who don't care to spend money on a smart charger. Nothing will blow up, although trying to charge a large capacity battery with a wall wart might make it run hot for long periods (max load) and eventually die.
 
Let me enlighten you.

Wall wart chargers are limited by voltage drop due to current draw. Cheap battery chargers rely on transformer voltage drop effect to limit the current without explicit electronics to do so. It's a matter of economics for those who don't care to spend money on a smart charger. Nothing will blow up, although trying to charge a large capacity battery with a wall wart might make it run hot for long periods (max load) and eventually die.

Not enlightening to me. Earlier in this thread I said: "If you connect a discharged battery to this circuit, you are likely to have a fire in the plug-pack (wall-wart) because the only thing limiting the current is the output impedance of the plug-pack itself. Lead-acid battery chargers need to act as a constant-current source (actively limit the charging current) until the battery terminal voltage comes up. Some wall-warts are intrinsically current-limited by design (air gap in the magnetic path, core saturation); others are not, but might have a fuse (sometimes self resetting, but not always)."

I have a couple of wall-warts here that I tried to use for simple battery chargers without electronic current limiting. One had a non-replaceable fuse-able link buried inside the molded plastic housing, so once the fuse blew, the only way of repairing it was to take a hacksaw to it to open the case. The other had a bi-metal thermostat in the windings, so it kept heating up, clicking off, cooling down, clicking on... Both of these responded to current overload caused by the battery being a very low impedance during most of its recharge cycle...

Current thread on these forums dealing with this issue...
 
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