R. I. Nelson wrote:

> I have about  8 different 12 volt led acid batteries that I wish to 
> charge.  some are deep cycle some are regular.  They are all lead acid 
> type. None of them are the sealed maint. free type.
> I also have a 50 amp 24 volt DC power supply that a friend gave me.  
> (He needed the space, no longer needed it and did not want to throw it 
> out.)
>
> The question does nay one know of  plans for a multiple battery 
> charger that I could make.  I would love to have something I could 
> just hook up the wires to the batteries and not have to worry if I 
> forget them for a day or 2.
>
I wrote because I dont see any replies to your mail .......
 (and because I just read this now)

The cheapest way time-wise and money-wise to get your batteries charged 
is to hook them up via a big resistor.
And buy a watch with alarm.
You can guess the charging time for a battery from the battery-terminal 
voltages.

General: If you dont have the possibility to switch off in time (auto or 
manual), do not go over 1/10 AH charge current. It is dangerous.
"1/10 AH current" for a 100AmpHour battery is 1/10 x 100 that is 10 Amp 
charging current.

Automation 1: Use a timer (and high-contact-current relay) to switch-off 
after/at an estimated time. One for each charger, or charge one battery 
at a time.

Automation 2: Measure the voltage at the terminals and switch off when 
the fully-charged-voltage has been reached. It works much better (and it 
is good for your batteries) if you combine this with 'Improvement 1' 
below (tapered-off charging current).
You can keep this simple or make it more sophisticated. (simple is 
probably better)

Improvement 1: Taper the current after bulk-charge. Lower the current 
once the terminal-voltage is over 12.8 Volt or so. (the lead-acid 
batteries I have charged all accept their bulk-charge around 12.6 - 12.7 
Volt so cut the bulk-charge-current above that voltage).

Switching off in time is not critical for non-sealed lead-acid batteries 
(it IS for sealed batteries). You can be several hours late switching 
off) if you limit the current below 1/10 AmpHour rating (max. 10 A 
charging-current for a 100 AH batt).
With a little experience it is also possible to estimate the charging 
time from the voltage at the terminals. And set a timer for 
auto-switch-off. You can check afterward if the battery is fully charged.

Practical hints:
Battery chargers often have few controls because charging is a 
non-critical proces. There are clear limits though.
Automotive battery-chargers (manual and automatic) limit their max 
charge-current by selecting a transformer with the right voltage. To be 
used without filtercapacitor.
This causes auto-taper-off of the charging current. As the 
battery-voltage increases the current reduces (as the charger is 
basicall constant-voltage).

As the design is fixed (as per the above) the process produces 
predictable voltages.
It is possible to switch off at the familiar voltage of 13.8 Volt. For 
one-time charging, followed by auto-charger-disconnect a higher voltage 
is used. Like 14.5 Volt.

If your 24VDC powersupply has a way to lower the voltage, you save energy.
Removing the filter-capacitor in the powersupply will help to lower the 
average voltage.

More details for more specific situations.

Herman in PHL (Philippines).

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