picking switch-mode battery chargers

I'm planning to buy chargers for two types of sealed lead acid
batteries: one for a 12V, 4Ah battery, another one for a 12V, 1.5Ah
battery. I'm planning to buy from Mouser Electronics. From their
catalog shown in http://www.mouser.com/catalog/632/1852.pdf , I'm not
sure about the criteria in which I should be choosing between each
charger. Does anyone know how I should be picking the chargers?

 

>, sci.electronics.design,
says...

Dead link.

One SLAC charger should be able to charge either type. Lead Acid
batteries should be charged with a temperature compensated constant
voltage source. It should sense the terminal voltage and switch
to a float charge at something like 95% charge. SLACs should last
ten years if treated properly. Don't skimp on the charger.

 

Manufacturers of small SLA's recommend limiting the charging current
to 30% of the battery's capacity. So for the 1.5 Ah battery, a .5 amp
charger would do. For the 4 Ah battery, get a 1 or 1.5 amp charger.
You don't have to adhere to that 30% figure rigidly; it won't kill the
battery to go a little over, and going under just means it will take a
little longer to charge.

 

Thanks for the tips. According to the catelogue choices, I personally
think there's no point of me of buying a 0.5 A and 0.8 A charger
separately. I'm planning to use a .5 A charger for both, although it
will take the 4Ah battery about 8 hours to charge.

 

No, lead acid battery charging is about 66% efficient, it will
take about 12 hours.

The charger range you gave a link to is a boost+float type acording
to the brief description so it should get the battery up to close to
a full charge fairly quickly.
On float charge the current does not stay at the maximum capacity
of the charger then suddenly drop to zero when the battery is full,
the current drops slowly over a few hours.

I had one of those huge torches ("flashlight" to merkins) that
have a 100watt bulb and a small lead acid battery inside.
The unregulated wall wart charger produces a rarther
high voltage with little load. The torch was left on charge
for weeks. No wonder the battery failed after a few months.

Bob

 

Just got the battery charger. I've managed to charge my 12V 4Ah
battery in half an hour with the 500 mA charger, which I expected to
take longer. I got a suggestion that it may be charged beforehand and
to get the runtime, I would have to put a load on it. I removed this
battery from an alarm system that is saying I have low battery. Does
it mean the battery would have to be replaced as well?

 

Your gel cell was either almost fully charged, or it
is sulfated and needs to be replaced, assuming your
new charger is working properly. If you are interested
in digging in to learn a little more about your battery
read on. If not, the most likely problem is the gel
cell - just replace it.

Here's a circuit you can use to discharge a lead acid
battery to a specific voltage with a known load, to get
a good runtime test.

+----------------------------------o o---+
| | | |
| S1 |>| |
| __ |
+--o o---+--------+------------+---|<---+
| | | | |
| [1K] [1K] | |
|+ | | /e o
B | +---[1K]---| 2N5401 To Load & clock relay
A P | a \c o
T 1K O<----[TL431] | |
T T | +----+ |
| | | | | |
| | | [D1] [Rly] |
| | | | | |
+---------+--------+------------+----+---+

Use a heavy duty automotive relay with an 88 ohm
coil. The circuit, with no load connected, will
draw about 140 mA from a 12 volt battery, starting
when S1 is pressed and continuing until the battery
discharges to the voltage set by the 1K pot, at which
point it will turn off and draw absolutely no current
from the battery.

A second relay can be connected to the load terminals,
and that relay's contacts can be used to turn a clock
on and off. That way, you can automatically time how
long it takes to discharge the battery. You can (and
should) add a load resistance at the load terminals
to place the load you want on the battery.

Set the 1K pot to turn the circuit off when the
voltage drops to 11.5 volts. That protects the gel
cell from being discharged too far. Add a load
resistor and second relay such that the total draw
is around 700 mA. Connect the contacts of the second
relay to turn a clock on and off. Set the clock to
12:00 and press S1. Go about your business.
When you come back to the circuit, the clock will
display the elapsed time, and will stop running when
the circuit shuts off. You can add an LED and
resistor to the circuit to indicate when it is running
if you like.

Ed