# Simple Circuit question - charger for 12V battery

#### Energy forever

Joined Sep 11, 2021
46
Hello. If i build a curcuit that has a 12 volt 4 amp adapter (which internally is just a 120 to 12 volt step down transformer and a diode) plugged into mains and on the output of the 12 volt adapter i hooked up positive to positive and negative to negative on a 12 volt 1 amp-hour rechargeable battery, and then from the battery positive to a 12 volt light positive and from the battery negative to the light bulb negative. (It could be an incandescent 12 volt light, i just wanted to be clear on wiring). The question is, will the battery get damaged from having current constantly flowing from the power adapter to it then to the light bulb? The purpose of the circuit is in the event of a power outage, the light would remain on until the battery dies or power gets restored. I attached a real basic drawing of the circuit for clarity.

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#### Tonyr1084

Joined Sep 24, 2015
7,454
Welcome to AAC.

First thing you need to know about transformers and AC power is that when you change it into DC power the voltage will be higher. 12VAC when converted to DC is going to be (12 x 1.414) 16.968VDC. Rectifying that voltage through a diode will drop (or lose) 0.6V leaving 16.368VDC on the battery. That will certainly harm the battery. Overcharging it will dry it out and it will not be able to deliver any power.

Your approach is well explained, but not fully explained. This is why a circuit diagram is useful. Yours is more of a block diagram and it isn't done correctly as it is. But we can decipher what you are looking for and can help you.

Again, welcome to AAC.

#### Tonyr1084

Joined Sep 24, 2015
7,454
The question is, will the battery get damaged from having current constantly flowing from the power adapter to it then to the light bulb?
To answer this directly - yes, it will damage it as explained above.

Joined Jul 18, 2013
27,653
Normally emergency lighting, keeps the batteries trickle charged, then detects when a power failure occurs and switches over from charge to battery power.

#### Tonyr1084

Joined Sep 24, 2015
7,454
In this schematic (circuit diagram) mains power is transformed into 12VAC. The full wave rectifier (bridge rectifier) supplies the most current available because when using a single diode you chop the negative half the wave out of the circuit. The bridge rectifier captures that negative wave and converts it to a positive wave. Thus, in theory, you don't lose half the power from the transformer. However, you're always passing current through two diodes. Each diode will typically drop about 0.6 volts. So two of them means you drop 1.2 volts.

In my diagram I've included a coil. That's to capture and filter out any spikes in voltage while the capacitor is there to both act like a battery and to filter out any drops in voltage. You can ignore the coil (the bumpy thing) if you're only lighting up a light bulb. You also need to understand that mains voltage, while commonly referred to as 120VAC can vary greatly from neighborhood to neighborhood. Even home to home; and yes, even from day to day. Since you're lighting up a light bulb - those variances shouldn't be a problem. But if you want to apply this to a circuit then you need to consider regulation of some sort.

Joined Jan 15, 2015
7,220
Depends on what the battery is. Just as an example a 12 Volt SLA (Sealed Lead Acid) when fully charged:
"The typical charging voltage is between 2.15 volts per cell (12.9 volts for a 12V 6 cell battery) and 2.35 volts per cell (14.1 volts for a 12V 6 cell battery). These voltages are appropriate to apply to a fully charged battery without overcharging or damage".

Different battery chemistries require different charge voltages and when fully charged have different voltages. A 12 VDC wall wart or 12 VDC power supply is not a good idea for charging a battery. Other batteries depending on chemistry require more attention to detail.

Next, discharging a battery under a load can destroy the battery if the load is not removed when the battery voltage drops below a certain level, again depending on battery chemistry.

Normally in cases like you mention where a backup power battery is employed there is much more to it than simply placing a battery charger on one side and a load on the other side of the battery. A full detailed description of your project including battery type would go a long way in a solution.

Ron

#### Energy forever

Joined Sep 11, 2021
46
Welcome to AAC.

First thing you need to know about transformers and AC power is that when you change it into DC power the voltage will be higher. 12VAC when converted to DC is going to be (12 x 1.414) 16.968VDC. Rectifying that voltage through a diode will drop (or lose) 0.6V leaving 16.368VDC on the battery. That will certainly harm the battery. Overcharging it will dry it out and it will not be able to deliver any power.

Your approach is well explained, but not fully explained. This is why a circuit diagram is useful. Yours is more of a block diagram and it isn't done correctly as it is. But we can decipher what you are looking for and can help you.

Again, welcome to AAC.
What if i use a power supply that outputs a stable 12 volts, even under load?

#### Tonyr1084

Joined Sep 24, 2015
7,454
What if i use a power supply that outputs a stable 12 volts, even under load?
That won't harm the battery but that also won't charge the battery sufficiently.

What kind of battery are you thinking of using? All you've said is 12 volts and 1 amp hour. The type of battery is very important.

A few years ago I built a car radio into a box and powered it from a car battery. To keep the battery charged I took a 12V supply and modified it to output 13.8 volts. This is the proper voltage to maintain a car battery, it's called "Float Charge". If you charge a car battery to 12 volts it will have no power. It needs to be charged to a higher voltage. Cars can charge as high as 14.5 volts, but that's for a short period of time, and the charge voltage will drop down to around 13.6 volts. But charging a battery to 15.8 volts would boil out all the electrolyte, and damage the battery. But different batteries have different chemistries. That's why it's important to tell us what type of battery you're using.

Another thing about lead acid batteries - you don't want to discharge them below 11.8 volts. Doing so causes the plates to sulfate and that can clog the pores of the battery and prevent it from delivering its full power. So lighting a light from a battery during a power failure needs more control than just running the battery down. And again, chemistry is critical in giving you the best possible answer. And using Lithium based batteries - it's far more critical how you charge them and how low you allow the voltage to drop. Not only can you hurt the battery you can also end up with a fire. Charging batteries is no joke. It has to be done right.

Joined Jan 15, 2015
7,220
What if i use a power supply that outputs a stable 12 volts, even under load?
Nothing will happen, the battery will not charge. You do not charge a 12 Volt battery with 12 Volts. Again, battery type? Chemistry? When a 12 Volt battery drops below about 11.6 Volts it starts to destroy itself assuming SLA. There is a good reason battery backup units are not all that simple.

Ron

#### Energy forever

Joined Sep 11, 2021
46
Depends on what the battery is. Just as an example a 12 Volt SLA (Sealed Lead Acid) when fully charged:
"The typical charging voltage is between 2.15 volts per cell (12.9 volts for a 12V 6 cell battery) and 2.35 volts per cell (14.1 volts for a 12V 6 cell battery). These voltages are appropriate to apply to a fully charged battery without overcharging or damage".

Different battery chemistries require different charge voltages and when fully charged have different voltages. A 12 VDC wall wart or 12 VDC power supply is not a good idea for charging a battery. Other batteries depending on chemistry require more attention to detail.

Next, discharging a battery under a load can destroy the battery if the load is not removed when the battery voltage drops below a certain level, again depending on battery chemistry.

Normally in cases like you mention where a backup power battery is employed there is much more to it than simply placing a battery charger on one side and a load on the other side of the battery. A full detailed description of your project including battery type would go a long way in a solution.

Ron
So, if i am understanding you correctly, if i use a power supply that puts out a voltage, up to, but not over, the voltage of the fully charged battery, the battery will be fine, and the circuit i described will work as intended? So, for example, a battery when fullly charged is 14 volts, and i use a power supply that outputs a stable 13.5 volts, in the circuit i drew, the light will be on and the battery will stay charged up to the 13.5 volts, and in the event of a power outtage, the light will remain lit until the battery dies?

#### Tonyr1084

Joined Sep 24, 2015
7,454
So, for example, a battery when fullly charged is 14 volts, and i use a power supply that outputs a stable 13.5 volts, in the circuit i drew, the light will be on and the battery will stay charged up to the 13.5 volts, and in the event of a power outtage, the light will remain lit until the battery dies?
Essentially yes. But again, battery type is critical in considering how you charge and discharge the battery. Don't forge ahead with this project until you have a clear understanding of how everything has to come together. Otherwise you risk ruining a battery, or worse, have a fire.

Joined Jan 15, 2015
7,220
You posted right after I did. Yes, batteries have a low voltage limit just like a high voltage limit for charge. You get below the limits I mention above and the battery gets destroyed. Battery backup is not all that simple.

Ron

#### Energy forever

Joined Sep 11, 2021
46
That won't harm the battery but that also won't charge the battery sufficiently.

What kind of battery are you thinking of using? All you've said is 12 volts and 1 amp hour. The type of battery is very important.

A few years ago I built a car radio into a box and powered it from a car battery. To keep the battery charged I took a 12V supply and modified it to output 13.8 volts. This is the proper voltage to maintain a car battery, it's called "Float Charge". If you charge a car battery to 12 volts it will have no power. It needs to be charged to a higher voltage. Cars can charge as high as 14.5 volts, but that's for a short period of time, and the charge voltage will drop down to around 13.6 volts. But charging a battery to 15.8 volts would boil out all the electrolyte, and damage the battery. But different batteries have different chemistries. That's why it's important to tell us what type of battery you're using.

Another thing about lead acid batteries - you don't want to discharge them below 11.8 volts. Doing so causes the plates to sulfate and that can clog the pores of the battery and prevent it from delivering its full power. So lighting a light from a battery during a power failure needs more control than just running the battery down. And again, chemistry is critical in giving you the best possible answer. And using Lithium based batteries - it's far more critical how you charge them and how low you allow the voltage to drop. Not only can you hurt the battery you can also end up with a fire. Charging batteries is no joke. It has to be done right.
Idk what battery yet, im just getting an understanding of this so i do know what i need. What i did learn so far is if i use a power supply up to but not over the voltage of a fully charged battery, it wont damage it. So, if i have a battery that is 14 volts when fully charged, i can use a power supply that outputs a stable 14 volts, even under load, which will keep the battery fully charged, and wont damage it, right? But i am confused about why a deep cycle battery would get damaged from being discharged. I know deeply discharging a deep cycle battery reduces its number of cycles, but i never read anything saying to not discharge till it dies. I thought the battery will stop lighting the light before it fully discharged because of what the voltage and amperage output of the battery would be after awhile.

#### Ian0

Joined Aug 7, 2020
8,406
I know deeply discharging a deep cycle battery reduces its number of cycles,
If you discharge it below 10.8V and leave it there for any length of time, it will reduce the number of cycles to ONE.
So, if i have a battery that is 14 volts when fully charged, i can use a power supply that outputs a stable 14 volts, even under load, which will keep the battery fully charged, and wont damage it, right?
If it's a lead-acid battery that voltage is 13.65V. A battery designed for float charge can last 12 years in that state.
You would have to make sure it is a current-limited supply, as it would draw a large current when the supply returns and starts to charge the discharged battery.
I thought the battery will stop lighting the light before it fully discharged because of what the voltage and amperage output of the battery would be after awhile.
If it's a filament lamp, then current will continue to flow until the battery voltage reaches zero.
With LEDs, firstly, you'll need a much smaller battery because they are so much more efficient, and a carefully chosen number of LEDs in series would stop drawing current at a certain voltage and might save the battery, but using a comparator to switch off the load is the way it is done professionally.

#### Energy forever

Joined Sep 11, 2021
46
If you discharge it below 10.8V and leave it there for any length of time, it will reduce the number of cycles to ONE.

If it's a lead-acid battery that voltage is 13.65V. A battery designed for float charge can last 12 years in that state.
You would have to make sure it is a current-limited supply, as it would draw a large current when the supply returns and starts to charge the discharged battery.

If it's a filament lamp, then current will continue to flow until the battery voltage reaches zero.
With LEDs, firstly, you'll need a much smaller battery because they are so much more efficient, and a carefully chosen number of LEDs in series would stop drawing current at a certain voltage and might save the battery, but using a comparator to switch off the load is the way it is done professionally.
Ok, lets say we use a lead acid battery, and a power supply with a stable 13.5 volts, and use led's in series so they auto turn off when the voltage goes below 13 volts. That should work, right?

Joined Jul 18, 2013
27,653
If this unit is for safety in advising the general public, it is subject to strict NFPA regulation, in N.A. at least.
Incidentally, there are many emergency lighting DIY circuits out there on the WEB.

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#### MrChips

Joined Oct 2, 2009
29,246
Build a SLAB charger.

Joined Jan 15, 2015
7,220
A simple Google of emergency exit light circuits will bring up plenty of examples. Again a description of exactly what your goal is will help. Things like how many LEDs? I can appreciate you are just in a rough planning stage but the more details the better. I also realize you want your load always powered not just in event of a mains failure. As you can see by suggestions it is not as simple as one may think. You may also want to just consider an over the counter solution. Note the frequently bought together in the link. You are looking at an under \$22 USD solution.

Ron

#### Ian0

Joined Aug 7, 2020
8,406
Ok, lets say we use a lead acid battery, and a power supply with a stable 13.5 volts, and use led's in series so they auto turn off when the voltage goes below 13 volts. That should work, right?
13V is too high. The battery should be allowed to discharge until the voltage is about 11.5V. It will reach 13V almost immediately when the charger is disconnected.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
There is a lot of stuff on batteries at www.batteryuniversity.com.

Lead acid is a chemistry. it does not necessarily mean the construction. Emergency Lighting has used SLA or Sealed Lead Acid which is rated for deep discharge. Capacity is in amp-hours. You can get a better idea of how long batteries can last using the curves. The rated capacity allows some relative comparison.

A car battery uses CCA or Cold Cranking Amps. This is really a measure of ESR or Effective Series Resistance. The plates have to be close together to get the low series resistance. It also makes the battery easily damaged. With the plates further apart, you get less instantaneous current, but the whiskers that get created don't get long enough to bridge the plates

car batteries get damaged when the cell voltage gets below a certain value.