lead acid battery cutoff+current limiting schematics for busk amp

[fatecasino]

I am building a 15W guitar busk amp and now I am focused on the battery charging circuit.
After some study I realized some basic factors in lead acid battery healthy charging.
-charging voltage
-charging current
-charging cut off

My battery is 12V and luckily I have found a laptop power adapter which gives exactly 14.2V which is best for powering up my amp + charging my battery.
I am trying to find separate schematics for automatic cut off and current limitimg (I need 0.7A) but I can't find any because most of the are based on LM3xx voltage regulators which need +3 volts in the input 14.2+3=17.2v. Unfortunately 17.2V exceeds my amps upper voltage limit and I would like to stick with the adapter I already have.

So my main question is,
can you suggest schematics for auto cut off and current limiter WHICH  give in the ouput the same voltage as the input (14.2v)?

[anotherjim]

Nothing off the top of my head, but, are you tied to DIY for this? Camping/leisure stores will have suitable chargers in stock.

[R.G.]

What he said.

Charging a lead-acid battery is simple if you can afford the time to do it slowly and don't mind the imperfections leading to shorter battery life. Charging one fast and doing so in a way that preserves the longest battery life is the task done by specialized battery charger chips. I've programmed a battery charge controller for a 12V sealed lead acid battery for a friend. He was losing a battery a year in his remote, solar-charged setup. I dug through the literature on the internet and programmed a PIC to monitor it charging and keep it in the "long life" zone.

While just hooking it up to a 14V or so voltage source will recharge a lead acid battery, it may not be all that good for it, especially as it passes through full charge into overcharge. Overcharge eats away at batter life.

Another big part of lead acid life is to never completely discharge it. And "completely" means "below 11V". Each excursion below that terminal voltage takes some of the possible life away. Do you monitor discharge very carefully?

Getting these answer wrong will eat up more money in replacement batteries than you saved using a surplus power supply.

[PRR]

> exactly 14.2V which is best

"Best" how?

The internal chemistry is 12.6V (dropping when cold).

If you apply a higher voltage, current wants to be "infinite", which is bad for the battery.

You set some limit on current. If overnight charge is OK, 1/10 of the AH rate is generally safe for lead cells. 1/10AH "can" be applied for days with no further limiting. 1/100AH can generally be applied all winter. (There are different opinions on this; also many different battery chemistries so everybody may have a point.)

If you keep going, above about 14V the energy storage is filled and after that you are breaking the water to hydrogen and oxygen. The battery loses water, bad. The O & H gasses are VERY explosive. Explosion in a lead-acid battery is not just the BOOM, it is also the strong acid thrown all over.

The usual trick is to start from say 15V. Say your batt is 10AH. Safe charge is 1A. Assume battery may come home at 10V. (This is low, you should not make a habit of it.) You have 15V-10V= 5V excess. You want at-most 1A current. 5V/1A= 5 Ohm 5 Watt resistor. When the battery comes up to 12.0V the charge current falls to 3V/5r= 0.6A. When it comes to 14.0V the charge falls to 0.2A; 14.5V 0.1A.

The only reason to mess with voltage or current limiting is to get FAST charging. And maybe so you don't burn-up headlights. Your car has more or less voltage cut-back and some current limit. It is not unknown for a car to charge a weak 50AH at over 30A, but battery voltage should rise quick and after 10 minutes the voltage cut-out kicks-in and current falls. Note that this only puts a small 5AH charge into the 50AH battery. Car batts stay full only on extended cruises, and because we should NEVER run them down much.

If you busk 16 hours a day, you maybe need a fast charge. If you busk 24 hours a day, such as police walkie-talkies, you instead keep 2 or 3 batteries for each unit and swap them into a charger every shift. But for less intense busking, I should think overnight charge ought to put-back what your fingers take-out in the day.

Playing while charging is dubious practice. But if you put your amp directly across the battery, it should "never!" get much over 14.4V. If it does, battery un-happiness will be much worse than chip-trouble: chips don't spew a quart of strong acid.

R.G.> Getting these answer wrong will eat up more money in replacement batteries than you saved using a surplus power supply.

+++

[antonis]

can you suggest schematics for auto cut off and current limiter WHICH  give in the ouput the same voltage as the input (14.2v)?

If you use ANY passive element(s) you can't obtain this effect..!!
(nice and clear..) 

If you have availiable a slightly higher voltage you can use a series power transistor with another transistor controling it's base voltage via a current "sense" resistor between it's Base & Emitter..

If you want an "auto" cut-off charger you'll have to deal with 3 pin voltage regulator with Op-Amp current limiting and switching to floating charge state..
(actually a double voltage comparator with a BJT as switch..)

[fatecasino]

> exactly 14.2V which is best

"Best" how?

The internal chemistry is 12.6V (dropping when cold).

If you apply a higher voltage, current wants to be "infinite", which is bad for the battery.

You set some limit on current. If overnight charge is OK, 1/10 of the AH rate is generally safe for lead cells. 1/10AH "can" be applied for days with no further limiting. 1/100AH can generally be applied all winter. (There are different opinions on this; also many different battery chemistries so everybody may have a point.)

If you keep going, above about 14V the energy storage is filled and after that you are breaking the water to hydrogen and oxygen. The battery loses water, bad. The O & H gasses are VERY explosive. Explosion in a lead-acid battery is not just the BOOM, it is also the strong acid thrown all over.

The usual trick is to start from say 15V. Say your batt is 10AH. Safe charge is 1A. Assume battery may come home at 10V. (This is low, you should not make a habit of it.) You have 15V-10V= 5V excess. You want at-most 1A current. 5V/1A= 5 Ohm 5 Watt resistor. When the battery comes up to 12.0V the charge current falls to 3V/5r= 0.6A. When it comes to 14.0V the charge falls to 0.2A; 14.5V 0.1A.

The only reason to mess with voltage or current limiting is to get FAST charging. And maybe so you don't burn-up headlights. Your car has more or less voltage cut-back and some current limit. It is not unknown for a car to charge a weak 50AH at over 30A, but battery voltage should rise quick and after 10 minutes the voltage cut-out kicks-in and current falls. Note that this only puts a small 5AH charge into the 50AH battery. Car batts stay full only on extended cruises, and because we should NEVER run them down much.

If you busk 16 hours a day, you maybe need a fast charge. If you busk 24 hours a day, such as police walkie-talkies, you instead keep 2 or 3 batteries for each unit and swap them into a charger every shift. But for less intense busking, I should think overnight charge ought to put-back what your fingers take-out in the day.

Playing while charging is dubious practice. But if you put your amp directly across the battery, it should "never!" get much over 14.4V. If it does, battery un-happiness will be much worse than chip-trouble: chips don't spew a quart of strong acid.

R.G.> Getting these answer wrong will eat up more money in replacement batteries than you saved using a surplus power supply.

+++

If I am right, this type of usage of the battery is the cycle usage. The correct setting of the charge voltage is critical and ranges from 2.30 to 2.45V per cell.
2.30 x 6 = 13.8V
2.45 x 6 = 14.7V
My adapter is 14.2volts which I think is a good voltage for this battery. I am not speaking here about a sophisticated charging circuit.

If you busk 16 hours a day, you maybe need a fast charge. If you busk 24 hours a day, such as police walkie-talkies, you instead keep 2 or 3 batteries for each unit and swap them into a charger every shift. But for less intense busking, I should think overnight charge ought to put-back what your fingers take-out in the day.

Playing while charging is dubious practice. But if you put your amp directly across the battery, it should "never!" get much over 14.4V. If it does, battery un-happiness will be much worse than chip-trouble: chips don't spew a quart of strong acid.

R.G.> Getting these answer wrong will eat up more money in replacement batteries than you saved using a surplus power supply.
+++

I only want to be sure with 2 things which are almost included in your post:
In the scenario that I leave the amp charging for days:
1. The battery won't explode
2. The battery will have an extended life, not the maximum possible, but an average one.

Should I search of current limiting and voltage auto cutoff or it's OK to connect directly the adapter to the battery?

If you use ANY passive element(s) you can't obtain this effect..!!
(nice and clear..) 

If you have availiable a slightly higher voltage you can use a series power transistor with another transistor controling it's base voltage via a current "sense" resistor between it's Base & Emitter..

If you want an "auto" cut-off charger you'll have to deal with 3 pin voltage regulator with Op-Amp current limiting and switching to floating charge state..
(actually a double voltage comparator with a BJT as switch..)

I "think" 14.2v is OK for a 12volt battery, please correct me if I am totally wrong. Can you please show any of these simple passive circuits?

[anotherjim]

Your PSU could make a fine trickle charger, but that's not the duty. As you say, it's cycle. Trickle charge is pretty easy, but cycle use is different.
With multiple cells in a pack, charging from flat needs to be done intelligently. A good charger can spot a higher resistance cell and try to bring charge up gently in the hope the lazy cell will catch up - it will ask itself if the charging current is consistent with the measured voltage across the pack. If it is, it will allow charge to increase. Similarly a low resistance cell will prevent voltage from every getting high enough to justify pushing in more current. Not only the cells go (or start out) bad - cheap construction can mean spot welded straps between cells which can easily break or have high resistance after physical shocks to the pack. The cycle charger must be fairly intelligent to keep things safe for you and your battery.

But if you want simple, a rough charge protection might be a 12volt/5watt auto type filament lamp in series with the supply. If the battery is flat, the lamp will be full bright and delivering 0.45amps. As battery voltage builds up, charge current falls and that lamp gets dimmer. The filament resistance falls as it cools, giving a little compensation for the drop in charging  current as the difference in battery and applied voltage reduces. You have to be the intelligence in the charger. You have to watch if the lamp doesn't get dimmer or battery voltage doesn't rise or the battery doesn't get too hot.

[fatecasino]

Your PSU could make a fine trickle charger, but that's not the duty. As you say, it's cycle. Trickle charge is pretty easy, but cycle use is different.
With multiple cells in a pack, charging from flat needs to be done intelligently. A good charger can spot a higher resistance cell and try to bring charge up gently in the hope the lazy cell will catch up - it will ask itself if the charging current is consistent with the measured voltage across the pack. If it is, it will allow charge to increase. Similarly a low resistance cell will prevent voltage from every getting high enough to justify pushing in more current. Not only the cells go (or start out) bad - cheap construction can mean spot welded straps between cells which can easily break or have high resistance after physical shocks to the pack. The cycle charger must be fairly intelligent to keep things safe for you and your battery.

I understand that this a more sophisticated charger than I want, but just for curiosity and future work, can you suggest a schematic of such a charger?

But if you want simple, a rough charge protection might be a 12volt/5watt auto type filament lamp in series with the supply. If the battery is flat, the lamp will be full bright and delivering 0.45amps. As battery voltage builds up, charge current falls and that lamp gets dimmer. The filament resistance falls as it cools, giving a little compensation for the drop in charging  current as the difference in battery and applied voltage reduces. You have to be the intelligence in the charger. You have to watch if the lamp doesn't get dimmer or battery voltage doesn't rise or the battery doesn't get too hot.

Sorry but my English is not that good and google didn't help...what is an auto type filament lamp?

[antonis]

If I am right, this type of usage of the battery is the cycle usage. The correct setting of the charge voltage is critical and ranges from 2.30 to 2.45V per cell.
2.30 x 6 = 13.8V
2.45 x 6 = 14.7V
My adapter is 14.2volts which I think is a good voltage for this battery.

Not quite...

Briefly, a lead-acid battery needs a 3 state charge procedure (at least..)
Bulk charge (constant current, voltage rising)
Absorption charge (constant voltage, current falling)
Float charge (constant voltage, maintenace current)
optional:
Equalization charge (constant current, rising voltage - constant current, constant voltage)

The highest Voltage value needed is in the Equalization phase (which is 14.5 - 14.6 V) followed by the Voltage value in Absorption phase (which is 14.1 - 14.4 V).
(value is followed, not phase..)

These voltages must be aplicable on battery's terminals (charger output) which means that you must have greater voltages availiable on charger's input (depending on specific charger circuit).

For the simplest charging circuit you'll need a 0.6 - 0.7 V drop (BJT Vbe) plus the voltage drop on Zener's current limiting resistor (if you use one on transistor's Base) resulting in an inadequate source voltage of 14.2 V..

[duck_arse]

Sorry but my English is not that good  and google didn't help...what is an auto type filament lamp?

car indicator lamps and headlamps used to be incandescent bulb types, with filaments that got hot, and emitted light. hence - auto as in automotive, as in cars and trucks, and lamp as in globe or bulb.

[R.G.]

If I am right, this type of usage of the battery is the cycle usage. The correct setting of the charge voltage is critical and ranges from 2.30 to 2.45V per cell.
2.30 x 6 = 13.8V
2.45 x 6 = 14.7V
My adapter is 14.2volts which I think is a good voltage for this battery. I am not speaking here about a sophisticated charging circuit.

Yes, we understand that you are not talking about a sophisticated charging circuit.

We are telling you that you need a more sophisticated charging circuit to prevent you from killing lots of batteries, which can be quite expensive. The 14.2V charger will charge your batteries to a certain extent. However, depending on the use you make of the batteries and the type of battery, the battery life may be short.

There are at least four types of lead acid batteries. One of these you will not find; it's the large tank-type flooded-plate battery that was at one time used in telephone main offices. Of the other three, the most common is the automotive flooded-plate type. The next most common is the sealed lead acid (SLA) type, which are small, and used in lots of backup application. The other is the deep-cycle lead-acid battery, sometimes called a "marine battery".

Lead-acid batteries do not survive many cycles of being discharged to low voltages, then recharged. They have the longest life if they are kept nearly fully-charged and used for big pulses of power at intervals. Ideally, the pulses of power should not drain them very much. The chemistry inside is such that big drains, down to low voltages, will chemically poison the plates and electrolyte fluid. After a number of cycles of being deeply discharged, the poisons build up until the battery will start self-discharging, and it dies rapidly after that.

The deep-cycle battery is designed internally to withstand many cycles of being discharged to low voltages, then recharged. It is designed differently both mechancially and chemically inside to withstand this. These are typically more expensive.

You do not say what kind of battery you intend to use. Most buskers I've seen use sealed lead-acid, probably because these are smaller and lighter. It's quite difficult to get to a location for busking with a 50-lb/25kg lump of lead and acid; although once in New Orleans I did see a busker with a cart and a car battery in it. 

You also did not say anything about your plans for busking, in terms of how much battery drain you expect; how much current your amplifier will use, and for how long you will play before recharges. This translates to - how deeply will you discharge your battery? If you plan to discharge it below about 10.9 to 11.0V, the battery will wear out after few cycles and will no longer charge, and then you are done with busking until you get a new battery. The discharge cycle has a HUGE effect on battery life.

A sophisticated charging circuit can help, but not completely make up for deep discharges. A simple constant voltage charge, as you describe, will not help at all.

2. The battery will have an extended life, not the maximum possible, but an average one.

We're telling you that we think that you are exposed to the battery having a much shorter than average life because of the conditions of use. We think that deep discharges will be common. I think that unless you use a voltmeter on the battery and stop busking when the voltage drops to 11V, the battery will wear out quickly. A sophisticated charger will help, but no charger will keep deep discharges from damaging lead acid batteries.

I "think" 14.2v is OK for a 12volt battery, please correct me if I am totally wrong. Can you please show any of these simple passive circuits?

14.2V is OK for a "12V" battery, and there are ways to make this work, usually with a resistor in series or a variable resistor, such as an incandescent light bulb designed for automotive use, in series.

But that's not the bigger problem. The big problem is that batteries are expensive.