High ampere current limiter to charge high AH battery?!

[Willen]

Hi,
I taught that we cannot connect power source to battery without current limiter. Even we cannot connect 10A 14V transformers' rectified DC to the 100aH lead acid battery directly without limiter (10A current is good current level for 100aH lead acid battery however).

Main matter is I got just LM317 based 1 or 1.5 ampere current limiter by searching. Feeling amazing that how power inverters charges 100aH or larger battery! What type of limiter they use! Faild to find circuit also! Wishing a simple charger system; thanks!

 

[audioguru]

An LM338 is the same as an LM317 but its max output is 5A. Maybe two 5A current regulator circuits can be connected in parallel.

 

[MikeMl]

Really high current chargers (40A+, like golf carts and fork lifts) do it by designing the transformer to be intrinsically current limited...

 

[Nigel Goodwin]

Either self-limiting transformer (MUCH more common), or a simple low value high wattage resistor.

If you don't use the correct transformer, then the resistor is the easiest option - you certainly don't need any electronic limiting.

 

[audioguru]

In Asia I doubt that a "correct" transformer is available. Also the mains voltage varies all over the place. I think a voltage regulator with current limiting is important to have in a charger there.

 

[Willen]

I thought there are some switching supply to limit high current.
@ AG, I just heared LM358 is an OpAmp.
@ Nigel, What is mean by 'correct transformer'?

 

[audioguru]

@ AG, I just heard LM358 is an OpAmp.?

It is an old low power dual opamp. Its opamps are the same as in the LM324 quad opamp.
It might be the first low power dual opamp ever made.
Because they are low power then they have crossover distortion and have poor high frequency response and slew rate above 2kHz.
Here is a sinewave showing the crossover distortion (that sounds awful):

 

[Nigel Goodwin]

@ Nigel, What is mean by 'correct transformer'?

A transformer designed for a charger - which is self-limiting.

 

[tcmtech]

I have built many battery chargers over the years and I have never concerned myself with current limiting other than having a properly sized circuit breaker on the output.

As far a s current limiting for a LA type battery it's rarely a real issue being that if the output voltage of the transformer is right the battery will level itself off on charging amps well before it ever does any damage to itself.

The way I have observed it is that whatever a battery is rated for on discharge amps it will also take on charging amps which means that if you have a 100 AH rated battery with a discharge rating of 600 amps and you discharge it at the full 600 amps you may only get a few minutes of run time at best. As that relates to charging at 600 amps the process and times will be proportional but in reverse meaning that the battery will level off on its own from the extremely high charge rate to a lower rate in a very short period of time.

Now as far as having the correct voltage of transformer its natural resistance of the primary and secondary will create just enough of a voltage drop that having a 10 amp rated charging system on a deeply discharged 100 AH rated battery will not be able to pull more than what the transformer can handle for a long enought time period to likely do any harm to either.

 

[Willen]

Hi AG,
You said LM317 is similar to LM358 but LM358 can charge to 5A. And now you are saying LM358 is similar like LM324. Here LM358 and LM324 both are low power OpAmp then how you said it can charge upto 5A? Being confused!

Hi tcmtech,
Did you mean that- if I have 15V and 10A rectified DC (from approx. 12.5V AC 10A output transformer) then I don't need current limiter to charge 100AH battery? Because output voltage and current from transformer is already suitable to charge 100AH lead acid battery.

 

[audioguru]

Hi AG,
You said LM317 is similar to LM358 but LM358 can charge to 5A. And now you are saying LM358 is similar like LM324. Here LM358 and LM324 both are low power OpAmp then how you said it can charge upto 5A? Being confused!

Sorry, I typed the wrong number. The LM338 is the 5A adjustable voltage regulator.

 

[tcmtech]

Did you mean that- if I have 15V and 10A rectified DC (from approx. 12.5V AC 10A output transformer) then I don't need current limiter to charge 100AH battery? Because output voltage and current from transformer is already suitable to charge 100AH lead acid battery.

More than likely yes. 14 - 15 volts peak is where I shoot for when building battery chargers out of old transformers.

 

[MrAl]

Hi,

A really really long time ago i used a light bulb in series with a high current diode to charge the car battery which went too low to start the car. I think it was a 100 watt light bulb but it was a long time ago and it could have been higher.

The 100 watt light bulb at 120v draws a bit less than an amp, but with the 14v drop of the battery it will allow maybe around an amp of current to flow. It takes a while, but it charges enough to start the car (i think it was a 390 cu in V8 engine).

It works, but you have to be careful because the bulb resistance is maybe only 8 to 10 ohms when it is first connected. That means the battery will get a large surge current at first and then it will taper off quickly. I was in the basement when i plugged it in just in case
So you do have to be careful when using light bulbs to limit the current.

Since you need more current you'd need more light bulbs in parallel or else a larger size light bulb. That could be a problem because then the surge would be higher. For 10 bulbs that would mean 10 times the surge which could mean 100 amps. A car battery could probably take that, but again you'd have to be very careful.

The other point is that without a transformer or other true power conversion the current the battery draws is the same current the line sees, which means the efficiency will be very bad. As others have pointed out, if you use a transformer that has a higher voltage and some power resistors, the line voltage fluctuations will affect the current. But this depends highly on what the secondary voltage is. The closer it is to the actual required voltage (like 14 or so volts) the more it will affect the current. So in some cases it will matter and some it wont.

Linear voltage regulators come at a price too, they need very good heat sinking or else they go into thermal cutout which means little or no current. Nice size heatsinks will work but small ones probably not. If it is a one time charge that does not have to be repeated very hour or something, then you might get away with immersing a smaller heat sink in a container of distilled water. That will keep it cool pretty long because the specific heat capacity of water is like 8 times that of copper (and aluminum). If it has all night to cool off, you can charge again in the morning.

 

[MikeMl]

Dont overlook another simple solution. Assume you have a basic transformer/rectifier (with or without a capacitor filter) which will deliver a maximum of ~40A into a mostly discharged (<12Vdc) LA battery. It should have an open-circuit voltage high enough so that it will eventually charge the LA battery to ~14.5Vdc.

Add to this a voltage detector (TL431?) and either a relay or SSR. Connect the relay or SSR contacts in the primary of the transformer. Make the voltage detector so that it has hysteresis with a cut-out voltage of 14.5Vdc and a cut-in voltage of ~12.5V (both independently adjustable?). The voltage detector circuit will drive the relay coil or the SSR control input.

Now the charger will charge until the battery reaches full charge, and will automatically turn on again as the battery is depleted. If you set the cut-in and cut-out voltages as described above, the relay/SSR will not cycle too frequently...