R
Ross Herbert
Guest
On Wed, 13 Feb 2008 02:50:02 GMT, ehsjr <ehsjr@bellatlantic.net> wrote:
:Ross Herbert wrote:
:> On Tue, 12 Feb 2008 01:30:43 GMT, ehsjr <ehsjr@bellatlantic.net> wrote:
:>
:>
ave.H wrote:
:> :> How would I go about building a charger for a lead acid battery.
:> :> Jaycar Electronics sells a 5 amp hour 6 volt SLA battery for use in
:> :> flashlight lanterns, but as far as I can tell they don't sell the
:> :> charger.
:> :>
:> :> Battery
:> :> http://www.jaycar.com.au/productResults.asp?FORM=KEYWORD
:> :> (CAT. NO. SB2498)
:> :>
:> :> Thanks
:> :
:> :
:> :Very simple charger:
:> :
:> : -----
:> : +9 ------Vin|LM317|Vout---+
:> : ----- |
:> : Adj [2.5R]
:> : | | 1N400x
:> : +----------+---->|---+
:> : | +
:> : [Batt]
:> : |
:> : Gnd --------------------------------+
:> :
:> :You need a 9 or 12 volt wall wart capable of
:> :at least 500 mA, an LM317 chip, a heat sink for
:> :the chip, and 2 5 ohm, 1 watt (at least) resistors
:> :in parallel to make the 2.5 ohm resistance, and
:> :a 1N400x diode. Charge for 12-14 hours.
:> :
:> :Ed
:>
:>
:> While that might work, it is a constant current source more appropriate to
:> charging Ni-Cd or Ni-Mh cells, not a lead acid battery.
:>
:> LA battery is best charged with a constant voltage source or better still, a
:> regulated 2 or 3 stage charger.
:>
:> Read Jaycar's own reference sheet on SLA batteries.
:> http://www.jaycar.com.au/images_uploaded/slabatts.pdf
:
:
:Read Jaycar's own description for the specific battery.
:It states: "Charge current 500mA for 10- 14 hrs"
:
:There is no "might" about it (as in your statement
:"While that might work"). CC charging *does* work on SLAs.
:Note that this is not comparing CC charging to other
:methods. It is correcting the "might" to "does".
:
:If you want to talk about "best" chargers, don't snipe
:at my post which addressed a "Very simple charger"
:It's simple, it works, and it matches Jaycar's description.
:
:Ed
I said "might work" for the following reasons.
Provided that the OP does in fact stick to a "wall wart" transformer limited to
not very much more than 500mA capability, it will probably work ok.
Unfortunately, I don't know if the OP will not use a 12V "wall wart" or indeed
any other higher VA rated transformer which is capable of 1A or more, and this
is where the problem can arise.
Remember, your circuit is an add-on and CC circuits attempt to do do just that,
ie. source a constant current irrespective of load conditions.
The problem with such a simple circuit is that it depends to a large extent on
the specification of the DC input source.
If using a higher VA rated transformer (than a 500mA wall wart) the actual
output voltage of the BR and the output current rating of the transformer will
definitely have a say in how well it will work and whether the battery will be
damaged if left connected too long.
If we accept that the CC circuit you proposed is set at 500mA with such a
transformer, then it will pump this much current through the battery no matter
whether the battery is fully charged or not.
Consider the case where the transformer is capable of putting out 12V at say 2A
from the BR. The CC circuit will successfully limit the charging current to
500mA and the majority of the source voltage will be dropped across the BR and
CC circuits. When the battery is fully charged to say 6.8V or thereabouts, there
is still a surplus of 5.2V available from the BR which will continue to pump
500mA through the battery. It will destroy the battery if left connected.
You might remember those cheap and nasty Arlec chargers
http://cgi.ebay.com.au/ARLEC-BATTERY-CHARGER-6V-AND-12-VOLT-CAR-BATTERY-CHARGE_W0QQitemZ320217446445QQihZ011QQcategoryZ79253QQcmdZViewItem
They had nothing more than a transformer, a rectifier and a thermal current
limiting switch to control the charge rate. They depended upon the fact that
when the battery was fully charged the DC voltage from the rectifier and the
battery voltage were almost equal and thus very little additional current was
pumped into the battery. Unfortunately, depending on the local AC supply voltage
the transformer output voltage was usually several volts higher than the fully
charged battery voltage, so it continued to pump quite a hefty current into the
battery thus destroying it. Your CC circuit is an electronic equivalent of the
thermal current switch in those cheap chargers.
:Ross Herbert wrote:
:> On Tue, 12 Feb 2008 01:30:43 GMT, ehsjr <ehsjr@bellatlantic.net> wrote:
:>
:>
ave.H wrote::> :> How would I go about building a charger for a lead acid battery.
:> :> Jaycar Electronics sells a 5 amp hour 6 volt SLA battery for use in
:> :> flashlight lanterns, but as far as I can tell they don't sell the
:> :> charger.
:> :>
:> :> Battery
:> :> http://www.jaycar.com.au/productResults.asp?FORM=KEYWORD
:> :> (CAT. NO. SB2498)
:> :>
:> :> Thanks
:> :
:> :
:> :Very simple charger:
:> :
:> : -----
:> : +9 ------Vin|LM317|Vout---+
:> : ----- |
:> : Adj [2.5R]
:> : | | 1N400x
:> : +----------+---->|---+
:> : | +
:> : [Batt]
:> : |
:> : Gnd --------------------------------+
:> :
:> :You need a 9 or 12 volt wall wart capable of
:> :at least 500 mA, an LM317 chip, a heat sink for
:> :the chip, and 2 5 ohm, 1 watt (at least) resistors
:> :in parallel to make the 2.5 ohm resistance, and
:> :a 1N400x diode. Charge for 12-14 hours.
:> :
:> :Ed
:>
:>
:> While that might work, it is a constant current source more appropriate to
:> charging Ni-Cd or Ni-Mh cells, not a lead acid battery.
:>
:> LA battery is best charged with a constant voltage source or better still, a
:> regulated 2 or 3 stage charger.
:>
:> Read Jaycar's own reference sheet on SLA batteries.
:> http://www.jaycar.com.au/images_uploaded/slabatts.pdf
:
:
:Read Jaycar's own description for the specific battery.
:It states: "Charge current 500mA for 10- 14 hrs"
:
:There is no "might" about it (as in your statement
:"While that might work"). CC charging *does* work on SLAs.
:Note that this is not comparing CC charging to other
:methods. It is correcting the "might" to "does".
:
:If you want to talk about "best" chargers, don't snipe
:at my post which addressed a "Very simple charger"
:It's simple, it works, and it matches Jaycar's description.
:
:Ed
I said "might work" for the following reasons.
Provided that the OP does in fact stick to a "wall wart" transformer limited to
not very much more than 500mA capability, it will probably work ok.
Unfortunately, I don't know if the OP will not use a 12V "wall wart" or indeed
any other higher VA rated transformer which is capable of 1A or more, and this
is where the problem can arise.
Remember, your circuit is an add-on and CC circuits attempt to do do just that,
ie. source a constant current irrespective of load conditions.
The problem with such a simple circuit is that it depends to a large extent on
the specification of the DC input source.
If using a higher VA rated transformer (than a 500mA wall wart) the actual
output voltage of the BR and the output current rating of the transformer will
definitely have a say in how well it will work and whether the battery will be
damaged if left connected too long.
If we accept that the CC circuit you proposed is set at 500mA with such a
transformer, then it will pump this much current through the battery no matter
whether the battery is fully charged or not.
Consider the case where the transformer is capable of putting out 12V at say 2A
from the BR. The CC circuit will successfully limit the charging current to
500mA and the majority of the source voltage will be dropped across the BR and
CC circuits. When the battery is fully charged to say 6.8V or thereabouts, there
is still a surplus of 5.2V available from the BR which will continue to pump
500mA through the battery. It will destroy the battery if left connected.
You might remember those cheap and nasty Arlec chargers
http://cgi.ebay.com.au/ARLEC-BATTERY-CHARGER-6V-AND-12-VOLT-CAR-BATTERY-CHARGE_W0QQitemZ320217446445QQihZ011QQcategoryZ79253QQcmdZViewItem
They had nothing more than a transformer, a rectifier and a thermal current
limiting switch to control the charge rate. They depended upon the fact that
when the battery was fully charged the DC voltage from the rectifier and the
battery voltage were almost equal and thus very little additional current was
pumped into the battery. Unfortunately, depending on the local AC supply voltage
the transformer output voltage was usually several volts higher than the fully
charged battery voltage, so it continued to pump quite a hefty current into the
battery thus destroying it. Your CC circuit is an electronic equivalent of the
thermal current switch in those cheap chargers.