Hi,
I am looking for some help in the design of this battery charger, it should go to trickle charge when the battery bank is full. It will be used to charge two 150AH 12V truck batteries in series. These are "maintanance free" lead calcium batteries. This system is fitted on a standby generator.

 

According to Electropedia Lead Calcium batteries are:-

Lead acid batteries with electrodes modified by the addition of Calcium providing the following advantages:

• More resistant to corrosion, overcharging, gassing, water usage, and self-discharge, all of which shorten battery life.
• Larger electrolyte reserve area above the plates.
• Higher Cold Cranking Amp ratings.
• Little or No maintenance.

As these are a type of lead acid battery, charging with a constant voltage is the way to go. The charge current will automatically regulate. The normal charging voltage is 13.8V to 14.2V for a "12V" battery. Two 12V batteries in series double this voltage.

A simple charger could be made with a transformer, bridge rectifier, capacitor and series regulator. All sufficiently big enough to handle the current. In case the terminals are shorted, it needs to have current limiting.

If you have to buy the components to make it - it will probably be cheaper to buy a ready-made switchmode power supply. 12V or 24V types are normally adjustable to give the voltage you need.

 

How about some design on a series regulator?

 

Why not buy a ready made one. Modern commercial chargers use a micro-controller and switch-mode to implement very sophisticated chargers that are hard to duplicate at home...

 

One of the requirements of this system is that it must be repairable at ground level. Switchmode supplies are not. Please do not forget we are in Africa. It must also not use vey complicated IC's (Maxim) but rather easily obtainable parts like 723's and op amp's. Voltage regulators are not a problem.

 

Ok, but read the description of the charger in the link I sent. Since it will be unattended for long periods, your homebrew charger must be capable of switching from a preset "charge voltage" to a "float voltage". It also has to be "current-limited" to protect itself and the battery if it is connected to a discharged battery or one with a dead cell. That's a lot of stuff to do with just analog bits.

 

Well, instead of criticising, how about some ideas? Or is that not in your make up.

 

I see no criticism being given towards you, only helpful and correct advice has been given thus far. You need to be aware of those things in order to build a successful charger.

Have a look at this Projects Collection thread by Beenthere:
http://forum.allaboutcircuits.com/showthread.php?t=6099

Many parts can be substituted. It's an all-analog design.

I suggest that it would be better to build two 12v (actually 14v) chargers rather than one 24v (28v) charger.
If you're using a 24v charger and one battery develops a shorted cell, the other (good) battery will be over-charged, damaging it. Further, it will be easier to build a 12v charger than a 24v charger. Additionally, two 12v chargers will have more utility than a single 24v charger, as the 12v chargers could be used to recharge individual 12v batteries.

 

If you have to buy the components to make it - it will probably be cheaper to buy a ready-made switchmode power supply. 12V or 24V types are normally adjustable to give the voltage you need.

This is absolutely correct. You can buy an offline 250W switch supply that is cheaper than you could build.

 

How about some design on a series regulator?

A 250W series regulator design is not feasible. You would need a heatsink the size of a coke machine.

 

Well, instead of criticising, how about some ideas? Or is that not in your make up.

Why don't you buy a manufactured one and buy some spares? I spent 30 years designing power supplies, and a well designed one is very reliable. You could keep a spare if you need it to be field repairable. Send the blown unit back to depot or manufacturer.

I think you are under the myth belief that a linear would be automatically more reliable (and easier to fix) than a switcher. That is not true.

 

Thanks for all the input. I like the idea of 2x 12V (13,8V) chargers. I will try to work out how to connect this.

 

Thanks for all the input. I like the idea of 2x 12V (13,8V) chargers. I will try to work out how to connect this.

It's very easy. Just connect a single charger across a single battery, negative to negative, positive to positive. Do the same thing with the other charger/battery pair.

I do this all the time with some retired military vehicles a local museum has; they all have 24v/28v electrical systems; they have a pair of 12v batteries in series.

This also lets you use easily available 12v automotive chargers instead of trying to build a custom version.

 

Thank you I will do

 

Take a look at the PB-137 12V Regulator IC which is designed for battery charging. Using this $0.52 IC as a base, building your own charger is feasible.

Cheers, DPW

 

Take a look at the PB-137 12V Regulator IC which is designed for battery charging. Using this $0.52 IC as a base, building your own charger is feasible.

The PB137 is a fixed 13.7v regulator designed to be used as a battery charger.
It is limited internally to 1A output.
If left powered and connected to a battery indefinitely, it will overcharge the battery.
It is cheap, but very inefficient.
It does not meet our OP's requirements.

 

If left powered and connected to a battery indefinitely, it will overcharge the battery.

Sorry, but I have to disagree here. 13.8V is normally used to indefinitely float charge Lead Acid batteries - including the sealed types. Higher voltages can be use for faster charging but should not be left connected for long periods.

The charge current will control itself automatically using the correct constant voltage.

 

Sorry, but I have to disagree here. 13.8V is normally used to indefinitely float charge Lead Acid batteries - including the sealed types. Higher voltages can be use for faster charging but should not be left connected for long periods.

The charge current will control itself automatically using the correct constant voltage.

Float voltage really needs to be adjusted according to battery chemistry, construction, and the battery core temperature. Float charging at 13.7v when the battery internal temperature is 50°C will be much too high, and at 0°C will be much too low.

 

Sorry, but I have to disagree here. 13.8V is normally used to indefinitely float charge Lead Acid batteries - including the sealed types. Higher voltages can be use for faster charging but should not be left connected for long periods.

The charge current will control itself automatically using the correct constant voltage.

Float chargers have to be temp compensated and 13.8V is a little high for them for wet cell lead acid. I keep mine set at about 13.2V (25C) with about -20mV/C compensation. The batteries on my bike last 8 years or more with continual charge for topoff.

 

Hello:
I don't know if this is an option, but when I was a avionics tech in Vietnam, we used to charge 24volt batteries with a 28 volt 'raw' supply in series with a 24 volt landing light.

When the light went out, the battery was charged.

Marc