Float charging multiple car batteries

[Jamie]

Hi,

Im designing a float charger to keep a heap of car batteries charged in a
car museum.

The batteries are connected to the cars, so they have minimal load (cars are
pre 70's so no ecu load ect)

I so far have made the variable voltage psu +6 and +12 volts, these rails
will be distributed around the building with 2.5mm cable.

The bit im currently deciding on is how to regulate the charge
I have two ways in mind

1) Use a diode and 18 ohm resistor (for 12v rail) on each fly lead from the
wall to the battery, this will result in around 50ma float current, and
voltage pre-set at the power supply

2) Use a diode, and LM317 and 16 ohm resistor between out and adj pin,
forming a constant current supply, voltage would be around 14-16 volts on
the input pin and the reg will provide the voltage drop to keep a constant
50ma current to the battery. - im worried this may overcharge due to the
higher voltage required.

Any suggestions are welcome.

Thanks
Jamie

 

[Ian Stirling]

Hi,

Im designing a float charger to keep a heap of car batteries charged in a
car museum.

The batteries are connected to the cars, so they have minimal load (cars are
pre 70's so no ecu load ect)

I so far have made the variable voltage psu +6 and +12 volts, these rails
will be distributed around the building with 2.5mm cable.

The bit im currently deciding on is how to regulate the charge
I have two ways in mind

1) Use a diode and 18 ohm resistor (for 12v rail) on each fly lead from the
wall to the battery, this will result in around 50ma float current, and
voltage pre-set at the power supply

This is more or less OK.
The power supply should have a voltage of around 13.6V, so after the diode
around 13V (not 13.8, the batteries will last a bit longer at not 100%
float voltage).

Say a 13.6V power supply, a 1A diode and a 12W bulb in series (the bulb
goes on if there is a short) for each battery.

 

[Jamie]

Thanks for the reply ian,

I would probably just use a 10 watt wire wound resistor and a diode for each
lead, this is a bit more reliable compared to a globe, because if I have
globes on the flyleads, I probably would be continually changing them if
the leads get dragged across the floor etc.

From my calculations I was going to use around 13.1 volts at the terminals
(after the diode etc), this will drop anyway once the terminals are
connected to the batteries due to the resistor.
50ma should be sufficient float current for 24x7 connection ?
Would I be right in having a load resistor across the terminals (say 100
ohms or so to provide some load to the diode etc) to set the voltage or
should I just connect the charger to a couple of batteries and measure and
adjust the voltage on the charger side of the resistors?

I was considering the constant current regulators, but after testing one, Im
not sure if it is a good idea due to lack of proper voltage regulation.

 

[Adrian Jansen]

According to some specs on long-life batteries, you should occasionally
charge the batteries to full float voltage, 14.5 volts or so, and let them
gas freely for a couple of hours. The idea is to stir up the electrolyte,
and prevent stratification into layers of different density in the cells.
Every 3 months or so is enough. Apart from this, you can hold them at
13.5-13.8 volts indefinitely.

Consult the battery manufacturers websites. Faure have a lot of data on
various charge modes.

--
Regards,

Adrian Jansen
J & K MicroSystems
Microcomputer solutions for industrial control

 

[Ken Smith]

Hi,

Im designing a float charger to keep a heap of car batteries charged in a
car museum. [....]
1) Use a diode and 18 ohm resistor (for 12v rail) on each fly lead from the
wall to the battery, this will result in around 50ma float current, and
voltage pre-set at the power supply

You can use a PTC "circuit breaker" to limit the current if the leads get
shorted. If you use a diode to isolate the batteries from the supply,
they won't be discharged if the power turns off.