Schematic Circuits

Low-loss Car Battery Charger

Low-loss car battery charger presented here can be used for a lead-acid battery with liquid acid, typical size 12V / 50-80Ah.

How it works

The basic charge is performed by a transformer and rectifier without any control through transistors or thyristors. You can accept any current from ca 4 to 6 A. The direct charge is very efficient, as only the inevitable losses of the transformer and rectifier are involved. To further get the losses down, I replaced the silicon bridge rectifier with Schottky diodes, and for the transformer I employed a cut-band core transformer. Also a ring core transformer would be good.

Schematic of the Car Battery Charger Circuit

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Adjusting the charge current

Using a 160W transformer with 15V winding gave automatically a charge starting with 6A (battery on 11V) then going down to 3A with rising battery voltage. If you need to make adjustments, it is easy to put some additional windings on the transformer. Typically 3-5 windings give one volt, which you can connect in phase or anti-phase with the existing secondary winding.

Switching over to buffer charge

During the direct charge phase, the relay is active. When a voltage level of 15.5V is reached, the comparator LM311 switches the relay off to buffering with 14V. If we would just give 14V to the battery it would take its final charge very slowly, creeping into the full-charged state, taking several days.
During charging, the battery is full with 14V but the voltage rise through the charge current (which is the internal resistance multiplied with the current) pulls up the voltage to a higher level, this is why to adjust to some 15.5 V (match with your specific battery and temperature). This additional voltage collapses in a few seconds when the charge is switched off.


The acid has to bubble regularly to mix it up, to avoid a harmful stratification. This is one more reason why not to use a creeping charge with 14V. Take out the plugs, else acid may be pressed out. Put a kitchen tissue over the openings, else the bubbles cause fine splashes of acid, with subsequent corrosion everywhere around. Measure the density of each cell and perform an individual charge if you find weak cells. Listen to the bubbling by using a thin plastic hose on your ear, all cells must bubble with equal intensity. Don’t put your ear over the bubbling battery, the fine splashes attack the skin, you get “batteryflea bites”. Add distilled water when needed.

Temperature influence

The voltages are given for 20C, but in fact there is notable temperature dependence. The diagram below (somewhere from the internet) shows this. If we are somewhere between 20 and 30C, everything is ok with 15.5V and buffering with 14V. If you happen to live in Oymyakon, add about 1 Volt. If in Timbuktu, go down by half a Volt.

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Maintenance – free car batteries

There are maintenance-free batteries, using liquid acid but having a recombination catalyst, they do not lose water and need no refill of distilled water. They are not sealed. They have to be operated upright. Charge as normal.

Sealed car batteries

They are rare for cars, mostly for motorcycles. The acid is in a kind of sponge or felt, they can be operated sidewards, the acid will not flow out. They are sensitive to water loss. The charge stop voltage must be lowered to some 14.5V and the buffer voltage to some 13.2 to 13.6V. Reconditioning is mostly based on filling up lost water. Although they are “sealed” they have pressure release valves.

NiCd, NiMH or Lithium-based batteries

This type of charger is not suitable.

Keywords : Low-loss, Car, Battery, Charger
Writer : delon  |
12 Jul 2019 Fri   
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