On my electric gate project I need a 12 volt battery charger circuit that will monitor the 12 volt wet cell battery continuously and charge as needed. I have a circuit that uses a LM338 regulator but I'm not sure how well it regulates given that it uses a start switch. The transformer secondary is 20 volts and gets rectified to DC.
I can find a lot of circuits but they all seem to be light on charging regulation info. The one I'm looking at is on the LM338 data sheet and uses a LM301A op amp.

 

Hello,

What kind of battery are you trying to charge?
Each type needs its own type of charge control.
See this page for more info:
http://www.batteryuniversity.com/

This is a link from the EDUCYPEDIA on batteries:
http://www.educypedia.be/electronics/componentbattery.htm

Bertus

 

I suggest that you start with finding the manufacturer's datasheet for your particular battery. The exact charging procedure, voltages and currents can vary significantly from model to model, even in a given manufacturers' own product lines. Batteries are not all created equal.

On top of that, the charge/float voltages will vary over temperature, at a rate of -3mV per °C.

[eta]
See my post in the Tips & Tricks thread:
http://forum.allaboutcircuits.com/showpost.php?p=262143&postcount=38

If you don't know how to find the datasheet for your battery, post the manufacturer's name and the battery model.

 

Just to put something out here, attached is a fairly simple constant current constant float voltage charging circuit using an LM138 (higher spec version of an LM338). It does not include temperature compensation nor equalization charge capability. The input side assumes a voltage source that is already rectified and filtered DC.

 

The battery is a Yuasa motorcycle type, NP7-12, 7.0 Ah. Yuasa's web site only says keep it fully charged. The battery isn't exposed to temperatures below about freezing, a high of maybe 120 degrees for a couple hours on hottest summer days, inside the black plastic (vented) control box. My concern is that I don't build a charger that doesn't recognise when to stop charging.
Is the purpose of the transistor in these circuits to act as a monitor, the base input increasing or decreasing charge flow according to the level of current flow out of the battery, and thus "reading" the state of charge?

 

The battery is a Yuasa motorcycle type, NP7-12, 7.0 Ah. Yuasa's web site only says keep it fully charged.

Seems like you're using a starting-type battery where a deep-cycle battery may be called for. You may wind up with a shorter service life due to this.

You might consider a higher-capacity sealed lead-acid type battery. Your motorcycle battery will require periodic servicing. Sealed lead-acid batteries are basically maintenance-free.

The battery isn't exposed to temperatures below about freezing, a high of maybe 120 degrees for a couple hours on hottest summer days, inside the black plastic (vented) control box.

Black tends to absorb heat, making the inside hotter. High temps lead to shorter battery life. If you haven't done so yet, please read the post I linked to, above.

My concern is that I don't build a charger that doesn't recognise when to stop charging.

The circuit I posted maintains the battery at a "float" level that you pre-set. However, you really need a temperature compensated charger, since the temperature will vary so widely.

Is the purpose of the transistor in these circuits to act as a monitor, the base input increasing or decreasing charge flow according to the level of current flow out of the battery, and thus "reading" the state of charge?

In the circuit I posted, the transistor serves to limit the charging current until the float charge level has been reached.

You really haven't said much about your transformer/supply/whatever it is. Do you have any specifications on it? What's the 20V rating; is that at a certain current, and is it AC or DC? Do you have a manufacturer and part number for it?

 

OK, I sit corrected; the NP7-12 is a VRLA battery (Valve-Regulated Lead-Acid), which should be OK for your application - depending on how much current your motor for the gate will draw and for how long a period of time, and how often the gate will be opened/closed.

 

It has worked fine for some years so I guess the battery works well in this application. The electrical input is 120 AC transformed to 20 VAC at the house with a large wall wart, sent to the gate control box at 20 VAC, converted to 20 VDC at the control board. The transformer was (unfortunately) installed under the house way far back (leaving 30' of unused low voltage cable under there that could have been used to place the transformer in the outside laundry nook) so I don't have any specs on it. The rain is going to start flying soon and I want to get this thing operational so the two gals on the property can enter and exit without having to leave their cars in the dark. I think it's gonna be some time before I can successfully build a charging circuit so today I bought an inexpensive Schumacher 1.5 amp maintainer designed for these batteries. It charges, then floats, and is temperature compensating. It appears to use a 20 volt secondary transformer so after I ascertain that for sure I'll just remove it's transformer and use the 20 VAC arriving on the low voltage wire from the house. Then this winter I can mess with a charging circuit and take my time until I have something I can trust.