Why can i not slow charge a lead acid battery?

[calvingloster]

ive been trying to figure out how i can build a simple charger for a 12v 7ah lead acid battery but i do not understand the circuits on the internet. I feel very stupid but i feel like there is this huge gap between people like me that barely know anything about electronics and people that know. The explanations of diagrams always leave me lost. I feel like people dont explain simply enough or cater for people that do not have an electronic background and just assume you know the bare basics.

Ive been trying to pass this gap now for about a year and im still struggling an it frustrates me to a point that i feel like just forgetting about electronics cause im not clever enough.......  =(

Anyways what i have read is that to charge a lead acid battery there are 3 basic stages. this is already to complex for me to build so what i want to do is charge the battery slowly from start till end. If i am correct a slow charge is at a 10th of the batteries ah capacity. So for a 7ah battery i will charge it at a constant current of 7/10 = 700mA?

Now i think i can use an LM317 to regulate the current to 700mA by using a 2Ohm resister to give me 625mA, ok so it will be an even slower charge then. What confuses me is this:

1. Can i do this, plug the battery in to charge at 625mA for 14hours? simple as that?
2. what voltage must that amps be at?

if you had to charge a battery in 3 stages like they say you must how can u know when one stage must be finished and the next stage must start? by measuring the battery voltage? but how do you do that when the battery is on charge? will you not measure the charging units voltage? or must you first unplug the battery from the charger and then measure the voltage then decide what stage it should be on?

[majenko]

1: yes.

2: Whatever the battery requires at that moment in time.

With a power source only one of either the voltage or current is "constant" - i.e., fixed at a specific value by the supply - the other value is free to wander around to whatever the connected device requires it to be.  Normally it's the voltage that is fixed, and the current is determined by the circuit attached to it.  With a constant current source the current is always the same, but the voltage is set by the circuit attached to it.

That said, there will be an upper limit that the supply can give for the "variable" value, and this has to be taken into consideration (i.e., you can't draw 40A from a 1A, 12V power supply or it will melt).  So for a constant current supply you have to know what the upper voltage is likely to be and cater for it.  With a 12V battery that's likely to be as high as 15V while charging, plus you have to factor in the 1.25V drop of the LM317T.  So you are likely to need at least 17V going in to your current regulator - better to be safe and have a few volts more.  A good choice would be an old laptop power supply as these are typically 19V.

However, there is one more gotcha in all this.  The LM317T is liable to get VERY hot.  You will need a Big Ass™ heat sink on it.  Personally I would go for the LM338T as it has a mich higher current handling capacity and shouldn't get quite as hot.  You'd still probably need a heat sink though, but not quite as big.

[majenko]

if you had to charge a battery in 3 stages like they say you must how can u know when one stage must be finished and the next stage must start? by measuring the battery voltage? but how do you do that when the battery is on charge? will you not measure the charging units voltage? or must you first unplug the battery from the charger and then measure the voltage then decide what stage it should be on?

The battery voltage varies all the time you are charging.  As I mentioned earlier the power supply's voltage is variable, so the voltage at any one point in time will be the charge voltage.  You can measure this at any time while charging quite happily.

[polymorph]

There is also the problem that the voltage while charging is higher than the rest voltage. And after you remove the charging current, voltage drops slowly. I'm designing a lead acid cell charger/conditioner/rescuer for Enersys  Cyclon 2V 2.5Ah cells, and one of the specifications is that the charge percentage can be found by looking at the voltage 4 hours after any charge or discharge has been terminated.

In any case... a float charge is typically done after the cell or battery is mostly or fully charged. In that case, the float charge is applied as a constant -voltage-, compensated for temperature.

The continuous application of a constant current without limiting the voltage to the float charge voltage, will eventually ruin the cell or battery.

YMMV, but here is the datasheet for the lead acid cells I'm using, it should open to Page 10 which covers constant voltage charging and float charge:

http://www.datasheetlib.com/datasheet/1170821/0810-0004_enersys.html?page=10#datasheet

[calvingloster]

1: yes.

2: Whatever the battery requires at that moment in time.

With a power source only one of either the voltage or current is "constant" - i.e., fixed at a specific value by the supply - the other value is free to wander around to whatever the connected device requires it to be.  Normally it's the voltage that is fixed, and the current is determined by the circuit attached to it.  With a constant current source the current is always the same, but the voltage is set by the circuit attached to it.

That said, there will be an upper limit that the supply can give for the "variable" value, and this has to be taken into consideration (i.e., you can't draw 40A from a 1A, 12V power supply or it will melt).  So for a constant current supply you have to know what the upper voltage is likely to be and cater for it.  With a 12V battery that's likely to be as high as 15V while charging, plus you have to factor in the 1.25V drop of the LM317T.  So you are likely to need at least 17V going in to your current regulator - better to be safe and have a few volts more.  A good choice would be an old laptop power supply as these are typically 19V.

However, there is one more gotcha in all this.  The LM317T is liable to get VERY hot.  You will need a Big Ass™ heat sink on it.  Personally I would go for the LM338T as it has a mich higher current handling capacity and shouldn't get quite as hot.  You'd still probably need a heat sink though, but not quite as big.

Thank you for your reply, i just have a few questions.

Firstly i do not understand how the voltage will change when charging. Say for example i have a flat battery and i put it on a constant current charger, will the voltage be highest or lowest and why? also if the battery is fully charged and i connect it the charger will the voltage be at highest or lowest? why does this voltage vary? does the battery act like a variable resister and voltage across it changes?

Secondly the heat that my LM317 will need to dissipate into the atmosphere is calculated how? for example if i use a 20v power supply and say for argument sakes the battery is charging at 13v. then the heat would be (20-13)*0.625A =  4.2W? and if i understand correctly the voltage at which the battery will charge will vary through the charge cycle is that correct?

[calvingloster]

There is also the problem that the voltage while charging is higher than the rest voltage. And after you remove the charging current, voltage drops slowly. I'm designing a lead acid cell charger/conditioner/rescuer for Enersys  Cyclon 2V 2.5Ah cells, and one of the specifications is that the charge percentage can be found by looking at the voltage 4 hours after any charge or discharge has been terminated.

In any case... a float charge is typically done after the cell or battery is mostly or fully charged. In that case, the float charge is applied as a constant -voltage-, compensated for temperature.

The continuous application of a constant current without limiting the voltage to the float charge voltage, will eventually ruin the cell or battery.

YMMV, but here is the datasheet for the lead acid cells I'm using, it should open to Page 10 which covers constant voltage charging and float charge:

http://www.datasheetlib.com/datasheet/1170821/0810-0004_enersys.html?page=10#datasheet

I dont understand the jargon you are using. what is rest voltage? and as you say if you remove the charging current the voltage drops? the voltage of what? the battery or the charger? if its the battery why would the battery voltage drop after you remove the charger if there is no load on the battery

[calvingloster]

if you had to charge a battery in 3 stages like they say you must how can u know when one stage must be finished and the next stage must start? by measuring the battery voltage? but how do you do that when the battery is on charge? will you not measure the charging units voltage? or must you first unplug the battery from the charger and then measure the voltage then decide what stage it should be on?

The battery voltage varies all the time you are charging.  As I mentioned earlier the power supply's voltage is variable, so the voltage at any one point in time will be the charge voltage.  You can measure this at any time while charging quite happily.

ok so for example if i have a flat battery, it measures 10volts and i put a 14v charger on this battery and try and measure the batteries voltage with the charger on i will not get 10volts. so how can you know what the batteries voltage is while it is on charge??

[Chagrin]

http://www.exide.com/Media/files/Downloads/TransAmer/Battery%20Care%20and%20Maintenance/Battery%20Charging%20&%20Storage%20Guidelines%20%205_9_13.pdf

If your battery is 12V, 7Ah I'm guessing that it's a sealed lead acid / gel cell battery. Per the doc above that should be charged at 14.1 - 14.4V, and stop charging at 13.5V. The doc also states the maximum charge rate, but for a consistent charge rate circuit the more practical rule is to charge it at a 10 hour rate, so for a 7Ah battery you would charge it at .7A.

Firstly i do not understand how the voltage will change when charging. Say for example i have a flat battery and i put it on a constant current charger, will the voltage be highest or lowest and why?

When you pulling a constant current out of a battery you expect to see the voltage of the battery drop gradually and measurably, don't you? When you're charging the battery with a constant current this is just occurring in reverse; the battery will again show a gradual and measurable increase.

[calvingloster]

http://www.exide.com/Media/files/Downloads/TransAmer/Battery%20Care%20and%20Maintenance/Battery%20Charging%20&%20Storage%20Guidelines%20%205_9_13.pdf

If your battery is 12V, 7Ah I'm guessing that it's a sealed lead acid / gel cell battery. Per the doc above that should be charged at 14.1 - 14.4V, and stop charging at 13.5V. The doc also states the maximum charge rate, but for a consistent charge rate circuit the more practical rule is to charge it at a 10 hour rate, so for a 7Ah battery you would charge it at .7A.

Firstly i do not understand how the voltage will change when charging. Say for example i have a flat battery and i put it on a constant current charger, will the voltage be highest or lowest and why?

When you pulling a constant current out of a battery you expect to see the voltage of the battery drop gradually and measurably, don't you? When you're charging the battery with a constant current this is just occurring in reverse; the battery will again show a gradual and measurable increase.

This document says you must charge the battery at constant current of half the ah rating which is not correct. it sais that if a battery is 100ah you must charge at 50A. Thats riduculous.

[jremington]

This document says you must charge the battery at constant current of half the ah rating which is not correct. it sais that if a battery is 100ah you must charge at 50A.

If you are referring to this link: http://www.exide.com/Media/files/Downloads/TransAmer/Battery%20Care%20and%20Maintenance/Battery%20Charging%20&%20Storage%20Guidelines%20%205_9_13.pdf , I don't see any such statement therein.

[Chagrin]

This document says you must charge the battery at constant current of half the ah rating which is not correct. it sais that if a battery is 100ah you must charge at 50A.

If you are referring to this link: http://www.exide.com/Media/files/Downloads/TransAmer/Battery%20Care%20and%20Maintenance/Battery%20Charging%20&%20Storage%20Guidelines%20%205_9_13.pdf , I don't see any such statement therein.

It's stated in the "Bulk charging" column, but that's stating the maximum charge current. I'm assuming they're stating that it's unsafe (explosion, etc.) to charge at a higher rate. "It is typically only used in applications that need rapid recovery for deeply discharged batteries." So if you're stranded in your car with a dead alternator then anticipate no less than 2 hrs to recharge the battery.

[calvingloster]

Ok yes they do say absolute maximum. Ok next questions.

Is a float charge only required if the battery is left unused for a long time?

Say i want to charge my battery in 2 manual steps, 1.....bulk charge(constant current) and 2.....topping charge(constant voltage). how exactly would i go about doing this. please correct me if i am wrong but can i not supply a constant current and constant voltage right from the beginning? this probably might just slow down the charging process but then i will be able to do the bulk charge and topping charge without any changes in my electric circuit?

[calvingloster]

Bump

[cjdelphi]

if you had to charge a battery in 3 stages like they say you must how can u know when one stage must be finished and the next stage must start? by measuring the battery voltage? but how do you do that when the battery is on charge? will you not measure the charging units voltage? or must you first unplug the battery from the charger and then measure the voltage then decide what stage it should be on?

The battery voltage varies all the time you are charging.  As I mentioned earlier the power supply's voltage is variable, so the voltage at any one point in time will be the charge voltage.  You can measure this at any time while charging quite happily.

ok so for example if i have a flat battery, it measures 10volts and i put a 14v charger on this battery and try and measure the batteries voltage with the charger on i will not get 10volts. so how can you know what the batteries voltage is while it is on charge??

2 ways.

1. Supply 14.2v, as the battery nears 14.2v  the current significantly drops and just floats, but this is constant voltage or cv, 14.2v.

2. Supply say 19v dc, regulate it for cv, then the current draw will be in the amps region, now we limit the current to say 500ma via circuitry or a simple resistor.  This then leaves you with a problem as you need to stop the charge, check the battery voltage, the enable charging again.... setting it to 14.2v and walking away is as easy as it gets.

[calvingloster]

if you had to charge a battery in 3 stages like they say you must how can u know when one stage must be finished and the next stage must start? by measuring the battery voltage? but how do you do that when the battery is on charge? will you not measure the charging units voltage? or must you first unplug the battery from the charger and then measure the voltage then decide what stage it should be on?

The battery voltage varies all the time you are charging.  As I mentioned earlier the power supply's voltage is variable, so the voltage at any one point in time will be the charge voltage.  You can measure this at any time while charging quite happily.

ok so for example if i have a flat battery, it measures 10volts and i put a 14v charger on this battery and try and measure the batteries voltage with the charger on i will not get 10volts. so how can you know what the batteries voltage is while it is on charge??

2 ways.

1. Supply 14.2v, as the battery nears 14.2v  the current significantly drops and just floats, but this is constant voltage or cv, 14.2v.

2. Supply say 19v dc, regulate it for cv, then the current draw will be in the amps region, now we limit the current to say 500ma via circuitry or a simple resistor.  This then leaves you with a problem as you need to stop the charge, check the battery voltage, the enable charging again.... setting it to 14.2v and walking away is as easy as it gets.

Why can I not just limit the voltage to 14.2volts and the current to a 20th of the battery capacity right from the beginning? Then I don't have to change a thing? For the bulk charge it will charge at 300mA at 14.2volts and the topping charge will charge at 14.2volts at 300mA? Why can this not be done?

For the bulk charge you supply max volts and max amps, but why can you not do a slow bulk charge? Supply a voltage and a current that is required for the topping charge. This is obviously much lower and might cause the bulk charge to take longer but it's more convenient cause you don't have to change anything? Is this not possible?