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Forum Index : Electronics : Battery charging question

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Senior Member

Joined: 24/09/2017
Location: Australia
Posts: 129
Posted: 07:51am 19 May 2020
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Gidday guys,

Story is I have several 100Ah 12v and some 6v 400Ah FLA batteries that are going to rot away in my shed.
I've simply not got the time go get any meaningful projects underway. So in the meantime I want to use gear I already have to charge the batteries up a bit
now and then so they don't go to ruin.

I do have a 20A 12v adjustable power supply which I can muck around with, although I think it's a bit dodgy (Jaycar unit), but it is TL494 based which has allowed be to make the output voltage adjustable quite easily. I don't think there is any current limiting designed into the unit.

I have several 24v industrial SMPS units I'd like to hook up using arduino and simply PWM a FET or BJT or the like.

Despite my brief bits of time to look up circuits I'm not too sure if I should use an inductor (buck style) after say a FET to PWM charge a battery or if I can just PWM a FET or BJT without the whole buck carry on. I haven't got anything like the time make anything beautiful, I only get some free time now and then and never seem to make any progress.

Sorry if this question is silly or convoluted, just thought I'd raise my hand and ask out.

P.S. I even have the Mad-inverter 'kit' from Madness I bought about 2 years ago and have not even had the time to get underway, argh.

I do however have a reasonable collection of electronics and can order more from RS/aliexpress if I need to.



Joined: 02/02/2017
Location: Australia
Posts: 758
Posted: 08:17am 19 May 2020
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This is more like my kind of problem.

You have some 24V supplies already? excellent
maybe get one of these
dc-dc converters
and get the biggest power step down thing.
It will take the 24V and convert it to something like 13.7V (ideal float for 12 SLA battery). And do it at a decent current too. You adjust the output voltage via a trimpot, all you need is a DMM.

The 6V can also be looked after, just get another one and set it to the desired output voltage. It too will take the 24V you have as input.

These dc-dc converters are cheap at $10 and have inbuilt current limiting.
You can set the max. current they will deliver.

I would go that way. There is no need at all to charge your batteries with a high current, only 5 or so Amps is enough to get them charged and keep them there.
Of course others will state that "you need much more mate"
Nah, it's slow and easy is the way to go.
5 Amps for 24 hours is 120 Amp.hours
wronger than a phone book full of wrong phone numbers

Joined: 09/08/2007
Location: Australia
Posts: 3267
Posted: 09:43am 19 May 2020
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It takes very little to keep healthy batteries healthy.
You can float charge them, but I have had much more success with regularly cycling the batteries automatically.

Charge the battery fairly slowly up to some maximum voltage, say 14.2v. When it reaches that voltage terminate charging completely, and let the battery just sit and self discharge back down to perhaps 12.3v.  That could take from many hours to many days, depending on how good and how large the battery is.

When it finally reaches the lower threshold, switch the charger back on automatically. The process just repeats forever.  This cycling activity seems in some way to be much better at maintaining battery capacity over long periods rather than just holding the voltage at some constant value with a float charger.

What I use for charging is a 15v one amp dc plug pack in series with a 12 volt 18 watt car tail light bulb.  The filament varies over about a 10:1 resistance range, and it acts as a very robust and simple current limiter and short circuit protector for the charger.

The filament glows faintly dull red hot when charging so you can monitor charging without an amp meter. It will charge at typically several hundred milliamps, and short circuit current will be just over one amp with a very bright bulb !

Filament resistance is something like a bit less than one ohm cold and maybe ten ohms at full brightness.

What I use to control the switching is a programmable voltmeter. These are not cheap, there may be something out there much less expensive, but it is what I am using.


The specifications say 0.1v resolution but its four digits, and there is an 00.00 to 99.99v dc range which provides 10mV resolution, and alarm trip voltage settings.
Edited 2020-05-19 19:50 by Warpspeed
Cheers, Tony.

Senior Member

Joined: 24/09/2017
Location: Australia
Posts: 129
Posted: 10:36am 19 May 2020
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Hi Poida and Warpspeed.

Must say I'm a big fan, have been following the forum since the days of the beginning of the Ozinverter (hope Oztules is still OK these days).

Thanks for that link Poida, I'm so used to the little LM2596 based units I have kicking around that I can't trust past 3 amps continuous. And I guess I was trying to make things too complicated already considering my time constraints.

I also like the lightbulb idea Warp, I used the same thing repairing my Jaycar power supply last night to limit inrush 'just in case'.

I might use a blend of the ideas. Initially just charge them to float (then disconnect) for now with a cheap and cherry chinese pack.

And then in time make a voltage divider to Arduino to switch in/out charger(s) set at the bulk rate and allow cycling down through self-discharge.

I had a Trojan J305P-AC battery lose 1 cell I think due to neglect, and I hope not to lose any more too quickly, although they were free    

Keep up the great progress fellas, it's been some great reading  
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