kair.us/ projects/ maintenance_charger/
Maintenance charger
On this page you find all the information needed to build a
maintenance charger for lead-acid batteries. It has protection
against short circuit and reverse connection. The charger has
automatic temperature compensation, unlike most of the popular
CTEK chargers.
Background
In 1998 summer issue of Elektor Electronics, a maintenance
charger for lead-acid batteries was published. I built 4 pcs
of these chargers, and some years later, 3 pcs more. I have used
them on our tractors, combine harvester and summer car batteries
during winter. Before that, each year at least one tractor needed a
new battery. Now there can be several years that none of them needs
to be renewed. I just connect the charger in autumn, and the battery
and machines are in cold shed for the winter. When the machines are
needed for sowing in spring, I disconnect the charger and they are
ready for use. They don't need any attention during winter. After
over 20 years of service, all the chargers are still working. One of
them needed to be repaired however, since it got damaged when it was
connected with incorrect polarity.
Recently, I needed one more charger. The easiest solution nowadays
would be to buy a commercial charger, e.g. CTEK. Back
in 1998, commercial maintenance chargers were not available.
However, most of the CTEK models still don't have automatic
temperature compensation. And, can I be sure that it would work for
20+ years? Well, maybe the real reason of not buying a commercial
charger was that I wanted another project :)
Updated charger design
At first I was thinking of re-drawing the PCB to get Gerber files.
My old charger boards were hand drawing using Decon Dalo 33 pen, and
then etched at home. The original Elektor design is rather big work
to build, because the enclosure doesn't have PCB mounting posts.
Also, it doesn't have fool-proof protection. I decided to make
partial re-design of the circuit to make it easier to build and also
fix some problems of the original design. Changes compared to
original:
- Designed to fit into readily
available enclosure
- PCB transformer on same board. Eliminates one possible bad
connection (DC jack)
- Relay replaced with transistor to eliminate contact wear
problems (haven't had any during the 20+ years though!)
- Current limit protects against short circuit
- Reverse battery connection protection with buzzer warning
- If mains cable gets disconnected during use, charger doesn't
drain the battery (except the 0.1 mA battery OK LED current)
The PCB is designed with Cadsoft Eagle. Below is Eagle 3D rendition
of the board.

Downloads for the design files, Gerber files and documentation are
below. You can order bare boards from PCBWay using this link. Here
is TME
shared project which contains all the components for the
project.
maintenance_charger_v11.zip
Eagle .sch and .brd
maintenance_charger_v11.pdf
Circuit diagram, assembly drawing and BOM
maintenance_charger_v11_gerber.zip
Gerber files
Circuit operation
One method of maintenance charging is float charging. This means
that the charger produces constant voltage, e.g. 13.8 V, which is
continuously applied to the battery. This means that the battery
is constantly full. At least in theory, this could cause some
unwanted boiling in battery.
This charger stops charging when the battery voltage reaches 13.8
V. The charging starts when voltage drops to 12.6 V. This ensures
that battery is always full enough to prevent from freezing and
sulphation. These voltage switchpoints are temperature
compensated. In 0ºC, the switchpoints are and 14.3 and 13.1 V.
Build and adjustment
Assembly of the unit is straightforward. The heatsink for
transistor doesn't need to be isolated. Leave LED legs long
enough, so the LEDS can be bent to side seam of enclosure.
Before use, the voltage threshold needs to be adjusted. Measure
voltage from TP1, and adjust with RT1. Set the voltage to 2.1 V
when charge LED does not light. If ambient temperature is other
than +20ºC, compensate this reading -4 mV/ºC. For example, if the
adjustment is done at 0ºC, adjust TP1 voltage to 2.18 V. The
adjustment is easier if you connect the charger to a full battery,
or if you connect big enough capacitor to output terminals.
Otherwise, the charge LED is off only very briefly. The adjustment
procedure is described on schematic diagram and on bottom of the
board as well. Another tip is to keep the charger powered for some
time, with enclosure closed, before adjustment. This takes the
self heating of charger into account.
Usage
Using the charger is simple. Connect the charger to battery.
Ensure that the green 'BATTERY OK' LED lights. Then connect mains
cable. Ensure that yellow 'POWER' LED lights. All set! The red
'CHARGE' LED indicates that the battery is being charged.
If you are visiting the location where battery/vehicle is stored,
it is worth checking that both yellow and green LEDs are
constantly lit. It indicates that all is OK. If the yellow LED is
off, the charger is not getting power. If the green LED is
flashing, it indicates that connection to battery is not good.
kair.us/ projects/ maintenance_charger/
page created 29.1.2022
last updated 20.2.2022 webmaster@kair.us