Charging A Lead Acid Battery

"Dave.H"

** N19309

...... Phil

They have a truly irritating number of ads (site-loads-like-sightless-burrowing-rodent-posteriors), but EDN has an extensive archive of "design ideas" that can be useful to browse through.

Uses a dedicated IC, and is a 4 cell rather than 3 cell charger:

Here's one from a bicycle enthusiast - not terribly sophisticated, but should work:

Here's a more complex one from a Ham Radio enthusiast:

--
Cats, coffee, chocolate...vices to live by

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

:Dave.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 :>

:> (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.

: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 :

:(CAT. NO. SB2498) : :Thanks

There are several suitable Jaycar 6V SLA chargers. MB3516, MB3525 for example.

I noticed that, right after I sent the original post. I think the reason I missed it is because I was only looking in the "Battery Charger" section. I will use the 6V 500 mA (MB3516) one.

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

** Jaycar are a mob of dumb parts grocers - they are experts on NOTHING !! ** No - it will not work as a good charger should.

It will destroy the battery.

** It destroys them by overcharging - you ASS. ** YOU are a good match for the creeps at Jaycar.

In the KNOW NOTHING stakes.

...... Phil

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Why is your simple charger so complicated? Why not use a 12 volt DC wall transformer and 13 ohm resistor (5 watt)? You get 540mA when the battery is low at 5 volts, and about 400mA as the battery voltage rises to 7 volts.

-Bill

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If you forget to turn off a charger like that, it will seriously overcharge a 6V battery! On the other hand Ed's charger will not do that.

Mark

It won't? It should continue to charge the battery up to the wall wart voltage, which is still too high if you leave it plugged in too long. I would, and do, just use a CV source that is set to the float voltage of the battery. Pick a regulator that has over temp and current limiting and let it float.

Jim

What about all the voltage drops between the wall wart and battery:

1 to 1.5V drop-out voltage of regulator 1.2-1.3V between regulator "out" and "adj" pins (across 2.5R resistor) 0.6-0.7V diode drop across 1N400x

Mark

If you use a constant current source, make sure it can only rise to the trickle voltage.

Here is a charger that should work for you: (view courier)

12V ----o---------------o------. | | | .-. | | | |1k | | | | | | '-' 2N2222 | | | |c | o------o-----b| | | | |e | | | | |c | | '---b| 2N3055 (may need heat sink) | | |e | | | | | | | | 2N2222 | | c| | z |b-----. --- Your battery A e| | - 8.1V Zener | | | | | | | | | ___ | | GND------o------o--|___|-o------' 1R 5W (created by AACircuit v1.28.6 beta 04/19/05

This will supply a constant current of 700mA or so until the battery gets up to about 6.8V, at which time it'll keep the voltage about constant to trickle charge the battery.

You can affect the current by choosing a different sense resistor. The 1R resistor is convienient to obtain, but may not be what you want. Typically, you charge one of these SLA batteries with 500mA, so a 1.2R resistor might be better. Also, the zener diode is only required if your power supply isn't regulated. If it is regulated, you can use a pot and adjust the voltage to about 8V at the wiper instead.

Hang around while you are charging the battery for the first time. If it is getting hot, use a smaller current. You don't want those SLA batteries to start outgassing.

Regards, Bob Monsen

The circuit you describe would require *regulated* 12 volts, making it _more_ complicated than the "very simple" circuit. The very simple circuit is designed to match the charging requirements in Jaycar's description. Can't do that with the

12 volt DC wall wart and 13 ohm resistor.

Ed

"ehsjr"

** Jaycar are a mob of dumb parts grocers -

they are experts on NOTHING !!

** It destroys them by overcharging - you ASS. ** YOU are a good match for the creeps at Jaycar.

In the KNOW NOTHING stakes.

...... Phil

Can't do that with the

On Wed, 13 Feb 2008 02:50:02 GMT, ehsjr wrote:

:Ross Herbert wrote: :> On Tue, 12 Feb 2008 01:30:43 GMT, ehsjr wrote: :> :> :Dave.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 :> :>

:> :> (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. :> : : :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

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.

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voltage

Right! The "very simple charger" does not include circuitry for protection against overcharge.

Ed

Build it and try. As the current drops I'll bet you don't get the drops you think, especially across the resistor used for the current sense. You are also assuming that the 9V wall wart is regulated. A cheap unregulated wall wart that is a "nominal" 9V under X% of load will usually be quite a bit higher than you expect as the load drops. A CV float charger could be used with any wall wart that is the V drop of the regulator or higher.

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No, it doesn't require a regulated input.

If the transformer is rated at 500mA and 12 volts, the starting current will be 500mA. As the battery voltage rises, the load decreases, causing the transformer voltage to rise and the current to remain fairly constant, or more constant than using a regulated input. Therefore, an unregulated input is better than the alternative you suggest.

-Bill