Charger with protection.

At work, the equipment was written off and I got a power unit without guts, with an inscription on the front panel "statron TYP 2230". In the presence of the body and installed inside the power transformer. According to the inscription on the output terminals (13.8V / 10A), the transformer was quite powerful and the output voltage of the power winding at idle was about 16 volts. I decided to leave it for the time being.
Cold came and the relative asked me to collect for his fleet of cars a charger for car batteries, reliable and to normally charge the battery and did not fail for accidental shorts of output terminals and incorrect connection of batteries. The pair of impulse chargers "Orion PW415" purchased by him earlier did not work for a month in street operating conditions (one and a week), and burned well inside from dust and water drops that got there and subsequently occurred a breakdown, which had not yet been recovered. I remembered about the power supply left to the best of times, or rather everything that was left of it, it was ideally suited for the realization of the task, and there was plenty of space in the case.

После просмотра различных вариантов схем, в качестве основы была выбрана схема промышленного зарядного устройства "Барс-8А". Нового здесь ничего не открою, эта схема есть на просторах "инета", просто она удовлетворяла всем запросам заказчика.
Трансформатор, установленный в имеющемся блоке питания, имел одну силовую обмотку на 16 вольт ХХ и в принципе вполне подходил для зарядного, исходная схема которого имела трансформатор с вторичной обмоткой со средней точкой. Исходя из имеющегося трансформатора, исходная схема была немного переделана, а конкретнее были добавлены два диода VD7 и VD8 для обеспечения мостовой схемы выпрямителя от одной обмотки.

In principle, it is perfectly possible to use any suitable power transformers with one winding and with a winding with an average point, with voltage at XX within 16-22 volts - only the rectifying part of the charger will change. 
For example, if you use a transformer with a secondary winding with an intermediate point as a power transformer, the rectifier diodes can be removed, and the power part diagram will look like this.

Generally, the power of this circuit is limited by the power transformer and rectifying diodes and thyristors used in it. 
The circuit itself does not work if the battery is not connected to the output, or is connected in the wrong polarity. Just at the output in this case, nothing will happen, at least short the output terminals. 
An ammeter (milliammeter with a shunt) was inserted into the existing power supply, a regulator of the charging current (voltage) was installed, one more LED (green) for indicating the device's inclusion in the network. The existing LED (red) was used to indicate the correct polarity of the battery connection.

The circuit was assembled on the board by mounted mounting, diodes and thyristors were installed on a common radiator through insulating gaskets. As diodes and thyristors, we chose thyristors KU202 and 10 ampere diodes. Of course, it's better to put them more powerful, but based on the available trance with a voltage of about 20 V, let's hope that the maximum charging current of the device does not exceed 10-12 amperes, which is enough for them. For better cooling, a fan was installed in front of the radiator from the computer power supply unit.

The shunt for the ammeter was made of knitting iron wire, 2 mm in diameter. Its length is selected by an experimental method, according to the maximum deviation of the milliammeter needle at a current of 10 amperes. Low-power transistors here can apply any, appropriate structure, as KT815G - any medium (large) power. 
The device can be assembled on a printed circuit board, 50x65 mm in size. See below for the board variant.

I want to note that this charger is assembled from serviceable parts and without errors in the installation - it does not need to be adjusted and starts working immediately. The device is reliable in operation and will not accidentally disable it. For even more reliable operation, as power elements, it is better to use thyristors (diodes) for a current of at least 25 amperes. 
I hope this device does not disappoint you. 
Good luck in the assembly!