How to look after 12V Lead Acid batteries?

[rupertlssmith]

I bought 6 12V lead acid batteries, 7.0Ah. The project I want to use them for has not materialized yet so they are just sitting around my workshop. I feel like I should have them on a float charge to keep them in good condition? If this is the case, where can I buy a charger that will keep all 6 on a float charge?

 

[chemelec]

For 6 batteries, it is Probably Better to Build One.
And it is also better to put an occasional load on them.
You could also Change the Charge Current if you want.

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Seeing as you just want to Maintain a Charge, Something like this should work OK.
It Splits the Charge, so if the 6 batteries are all Low, it will only charge Each One at a Lower Current.

 

[rupertlssmith]

For 6 batteries, it is Probably Better to Build One.

Would love to but no time at the moment for a project like that. Thats why I am asking if there is a charger I can buy where I just leave all 6 on it on a float. My physics teacher at school built a simple system like that and stored all his batteries on it, just a trickle charge but kept them stored at 12V or a little over.

 

[BradtheRad]

A certain amount of trickle charge is advised when storing motorcycle batteries. It's a good idea for yours as well. You do not want to overcharge them. Maybe 1/10 or 1/5 of an Amp. If it is too high then you start losing electrolyte.

The easy way is to hook up all your batteries in parallel, then send a regulated voltage to them. If none of them go bad, then all should stay healthy. Normal resting charged voltage is 12.8 (for ordinary lead-acid batteries). However this could be thrown off by temperature. Accuracy of your voltmeter makes a difference. That is why you should also track charging current. No more than a fraction of an Amp.

Sulfation is a common problem when a battery sits a long time. The cause is not always certain. It is a good idea to discharge each battery through a load (as suggested in post #2). See if it still has healthy capacity. Then charge it back up. Battery chemistry needs to be exercised, the same way a car engine ought to be run every so often.

 

[chemelec]

Normal Charge Rate for a SLA Battery is 1/10 Capacity. So for a Standby Charge, Use a 13.6 Volts regulated supply with a Maximum of 700 Ma.
Even if Only one battery was Charging, this would still be Safe for your Batteries.
Charging at LESS than 12.6 Volts will also result in Sulfation.

Direct Parallel Connection of those batteries is Not Recommended.
If One Battery was to go Bad, it could cause Full Current Draw from the Other Batteries, Resulting in Serious Damage.
Use DIODE Isolation, Like I show on my Schematic.

 

[Warpspeed]

Many people find that just connecting a battery to a float charger that maintains a constant never changing voltage for months or years, will still slowly degrade the battery which very slowly lose capacity.

A method I have had great success with is to charge the battery up to its full recommended charging voltage, perhaps 14.2v, and at a reasonable current.
Once it reaches that voltage switch off the charger completely.
Allow the battery to settle, and monitor the voltage.
When the voltage has fallen to something like 12.6 volts, switch the charger back on.

If the battery is in pretty poor condition it will struggle up to 14.2v then drop like a rock back to 12.6v, sometimes in only a couple of minutes.
Over time, the charging cycle becomes much shorter, and the settling time becomes much longer.
Eventually it will rocket up to 14.2v in minutes, and stay in the settling mode for a great many hours, sometimes even days.

This is great for the battery, it gets a bit of occasional exercise. And after a year or two left like that, it will probably be in even better condition than at the start.

 

[c_mitra]

The easy way is to hook up all your batteries in parallel, then send a regulated voltage to them..

Lead Acid batteries are quite a fussy beast- but then which rechargeable battery is not?

If you put them in parallel, surely they will see the same voltage but it is no guarantee that they will share the current equally.

I would have suggested that put them in series and feed a const current of 20-50 mA constantly.

But then you need to load them with a real load and discharge them to the recommended level, say 12V and then charge them again as usual.

They may not live long on a trickle charge, but every battery manufacturer has his own advice on how to maintain a battery. I think the fellow who made it knows best.

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... for the battery, it gets a bit of occasional exercise. And after a year or two left like that, it will probably be in even better condition than at the start.

I do not know what is the design life of the Lead Acid storage battery but I would guess that it is hardly more than 3-4 years. My car batteries have never lived more than 4 years. I do not know why the Lead Acid battery finally fails but I guess it is more chemistry than electrochemistry.

These days our power supply is improved and my inverter is rarely on battery. I shall try to see that the battery is exercised a bit more on less regularly.

 

[BradtheRad]

If you put them in parallel, surely they will see the same voltage but it is no guarantee that they will share the current equally.

I would have suggested that put them in series and feed a const current of 20-50 mA constantly.

I agree, your method is better than mine. My simple method is too easy. It assumes the batteries are all healthy. My method does not reveal a battery that's bad.

It is best to give them identical charging current as you recommend (in series). Furthermore it is much easier to spot a declining battery. One voltmeter (applied 6 times) can compare all six batteries and tell us if a battery is going bad.

 

[rupertlssmith]

Use DIODE Isolation, Like I show on my Schematic.

With a charger like this, is it possible to put >1 battery on a bank, with diode isolation, or would this allow only 1 batter per bank?

**broken link removed**

 

[BradtheRad]

The Noco unit looks ideal, from the description. It makes excellent claims for all the things it does but at a price of $225? I wonder if the price of six 7AH batteries compares with that.

You would need two Noco units if you want to let them manage 6 batteries unattended for a long time.

 

[rupertlssmith]

It makes excellent claims for all the things it does but at a price of $225?

Yeah, I know. Is there something simpler and cheaper that anyone knows of? chemelecs circuit looks good, but I just don't have the time to knock that together at the moment.

 

[BradtheRad]

Post #7 is reasonable. You'll need to make an 80V DC supply. Add a series diode (per post #5) to prevent reverse current. Put a resistor in the path, value chosen to allow 20 or 50 mA.

Your 80V supply does not have to be smooth. It can have a wide range, many volts above and below 80V. Perhaps 79V will do. The waveform can have crests and troughs. The chief thing it needs to do is push a few tens of mA average into a stack of six 12V batteries.

It may not be easy to make such a power source. 80V is an oddball figure. It's a bad idea to draw directly from your AC house voltage. It's safer to run it through a step-down transformer.

Someone may suggest a capacitive drop supply. It is feasible since you are drawing little more current than a few led's. However capacitive drop does not provide isolation from mains AC. It's a nervous thing to allow high voltage (even through a capacitor) in contact with lead-acid batteries. If a spark occurs within the battery (or close to a vent), then it can cause a battery explosion.

How to proceed next depends on what you believe you can construct, to do the task.

 

[BradtheRad]

An alternative is to make a 40V supply. Connect your batteries in 2 strings of 3 each.

Do you have a 30VAC transformer in your basement? The type that provides power for doorbells and furnaces? It should be no problem if you draw 100mA from it. Peak voltage reaches 42 V or so. You need to put in a diode and resistor. Since you'll have 2 battery banks you need 2 diodes and 2 resistors.

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Since you'll keep the battery strings separated electrically, it's a good idea if you make the supplies anti-parallel, in terms of polarity. This draws transformer current during both halves of the cycle, instead of just one half.

A doorbell transformer often can be found mounted on the joists in the basement near a furnace, central A/C, etc. It may be rated 1/4 or 1/2 A.

 

[c_mitra]

Your 80V supply does not have to be smooth. It can have a wide range, many volts above and below 80V. Perhaps 79V will do. The waveform can have crests and troughs. The chief thing it needs to do is push a few tens of mA average into a stack of six 12V batteries..

In good old days, simple (very simple) battery chargers were made with a transformer, a half wave rectifier and a carbon filament lamp. They are no more seen but I was curious about the carbon filament lamp (because that was the only place I have seen them). Batteries were charged in series and voltages were monitored regularly with a simple hand held meter.

Similar idea may be used for trickle charging.