Pulse Charger for reviving tired Lead Acid batteries
If you own a motorcycle, a motor home, a caravan, a lawn mover, a day cruiser
or maybe a vintage car you must at some point had to write off a lead acid
battery. When a battery is improperly charged or allowed to self-discharge as
occurs during non-use, sulphate crystals build up on the battery's plates. The
sulphate preventing the battery from being fully charged and therefore it is
unable to deliver its full capacity. When trying to charge a battery in this
state it only gets hot and looses water, the gravity of the electrolyte is not
increasing to its normal “full charge” state. The only thing you do is killing
the battery completely. If a battery has a resting voltage of at least 1.8 Volts/cell
and no cells are shorted, desulphation of its plates can be done. This circuit
is an add-on and part for a modification of a normal charger and it takes care
of the sulphate problem.
CAUTION: Before you begin a project like this remember: mains voltage is
dangerous so if you are not 100% sure of what you’re doing consult a friend who
has the skills or, don’t do it at all !
The project: get hold of an old charger, big or small it’s your choice depending
on the size of batteries you normally handle (bigger is better). There are some
tricks to boost the performance if you need it. Start by ripping out everything
except the transformer and the rectifier. Some older chargers are equipped with
fin rectifiers, which have high voltage drop and must be replaced. Replace with
a rugged bridge rectifier that can cope with the amperes. All wiring on
secondary should be short and heavy wire. The rectifier should be bolted to the
chassis to keep cool. If the charger have a high/low switch it’s a bonus, if not
you can in some cases add a few turns of wire on the secondary winding. The
circuit; a 14-stage ripple counter and oscillator IC 4060 produce a pulse, which
is the heartbeat of the circuit. The pulse is feed to the 555 timer that deicide
the length of the active output. With the switch you can select long or short
pulse output. The output of the 555 timer triggers the zero-cross optoisolator
triac driver MOC 3041 via a transistor. This gives the charger transformer a
soft start via the triac and the snubber circuit. A small power supply is
necessary for the circuit and consists of T1 a transformer 15V 0.1A secondary, a
bridge rectifier, a regulator and two caps. Because this project include a
charger that is (X) the outcome can differ in performance from one case to
another. However this do not mean that your project doesn’t work, but the
efficiency can vary. Some notes the snubbercap is a high voltage AC type (X) and
the resistors on the mains side is at least 0.5W type. Use a triac that can take
400V+ and 10A+, I use BTA 25.600 but this is overkill in most cases. No PCB
sorry!
How it works:
Well the short version. The object is to get the cell voltage high enough for
the sulphate to dissolve without boiling or melting the battery. This is
achieved by applying higher voltage for shorter periods and let the battery rest
for a while. The pulses on short range is about 0.5s on / 3s off and the long
pulse range is 1.4s on / 2s off. These times can vary depending on component
tolerances. Start on long pulse and if you discover “boiling” (more than with
normal charging) in the electrolyte switch to short puls. Don’t leave the
process unattended, at least until you know how your specific version of this
project turns out. I built ver.1 of this circuit some 10 years ago and have
experimented with it but I’m sure someone can improve it further.
Good Luck! Ante
Ante135@hotmail.com
Download this project in doc format
Title: Pulse Charger for reviving tired Lead Acid batteries
electronic circuit
Source: www.electronics-lab.com
Published on: 2005-02-03
Reads: 1609
Print version:
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