I was requested to post it as a new question here: Lead Acid Battery charging from laptop psu so here goes the copy paste with an added video link I just uploaded:

I am new to lead acid charging and have been using a 19 V 5 A vaio laptop charger to directly charge the batteries, 2x used 12 V 75 Ah in parallel (=12 V 150 Ah) for a few months which in turn power some low watt LED lights and a 12 V table fan when main grid load-shedding occurs in this part of the world. I've noticed that the batteries can initially draw around 8amps from the 19 V 5 A PSU without overheating the PSU. The batteries no longer boil as they used to when charged four in series with the huge 48 V controller which was required to overnight recharge an electric autorickshaw (their first hand use) and now remain cool whilst charging, I haven't needed to top up the water in over six months either despite the driver having had to top them up 2-3 times a week previously.

I am wondering if it's OK or whether I need to reduce the voltage output from the PSU to ensure longevity of the batteries since the charger is usually left on until I switch the connector between the two sets of batteries which I charge and use alternately as the lights from one set begin to dim after 2-3 days. Any suggestions and links to diagrams if voltage reduction is helpful would be appreciated.

BTW, I also charge another set of four in parallel with another 19 V Toshiba laptop charger (trickle charges a few points of a volt every day), I've noticed that smaller laptop chargers (I've got a small pile of them sitting around) tend to immediately begin to heat up upon plugging in (possibly due to lack of intelligence or inability to regulate amp draw?) So there appears to be some sort of intelligent co-ordination between the branded high amp chargers and the batteries - just added that in case it helps in drawing a clearer picture of how things are.

Here's a video with the pulsating voltage multimeter readings:

Charging lead acid batteries with laptop psu

I found this circuit for reducing the voltage, is it efficient enough to charge my larger batteries through? Or maybe if I added a few more LM317's in parallel in order to increase the load bearing capacity (datasheet shows max 1.5 A each) - and would the transistor cope despite this? Any other workarounds?


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    \$\begingroup\$ A "laptop charger" isn't actually a charger at all, the term is a misnomer. They are constant voltage power supplies, with the lithium ion charging circuitry being actually part of the laptop itself, not the power supply. While using a laptop power supply for directly charging lead acid batteries might work, it isn't good for the batteries or the power supply. \$\endgroup\$
    – jms
    Commented Oct 1, 2016 at 12:15
  • \$\begingroup\$ Thanks for the reply :) I know it's also called a psu but from the way the battery charges and the current slowly dwindles, it can be called a charger for all practical purposes, especially for the sake of Google searches. OK, now I've posted a 14v circuit in the op and would like to know your comments and recommendations. \$\endgroup\$
    – abz
    Commented Oct 1, 2016 at 15:05
  • 1
    \$\begingroup\$ Is there some reason you can't use a charger designed for lead-acid batteries? They are cheap - but don't go for one that feels too cheap. \$\endgroup\$ Commented Oct 1, 2016 at 15:36
  • \$\begingroup\$ I no longer have a PayPal account or active debit card, and as you can see in the video,I did buy a solar charger here but it appears to be a cosmetic brick which appears designed to display a few charging led's and reassure solar panel customers while in actuality it just routs the power straight through - unless it's so rubbish that it might have shorted when reverse polarity is sometimes accidentally applied. \$\endgroup\$
    – abz
    Commented Oct 1, 2016 at 17:34
  • \$\begingroup\$ @Andrew Morton On a cheerful note though, I just checked the voltage on the twin set which is charged and it is displaying 12.36v after a few hours on a light load - which is similar to how it was around six months ago when I attached the laptop psu with desulfater, bearing in mind that the psu provides a similar voltage and amperage to a 120w solar panel. \$\endgroup\$
    – abz
    Commented Oct 1, 2016 at 17:41

2 Answers 2


1 - Look, the first thing you need to do is get a cheap DMM. As it stands you have no idea what is actually going on in your batteries, and frankly I'm surprised at how durable lead-acids can be when subjected to the abuse that you are apparently putting them through. Once you have a meter, you can look up "lead-acid charging algorithm" online and learn how to do this right.

2 - Your 48-volt charger was an abortion. Never, ever, use a charger that boils the electrolyte. I suspect that the only reason the PSUs don't cause boiling is that they simply can't produce enough power, while the monster can. And does. Get rid of it, or learn enough electronics to figure out the charging circuit and modify it. You are causing long-term damage to the batteries if you use it.

3 - There is no "intelligent coordination" going on. All of your PSUs are in current limit, and the smaller ones simply don't handle it as well as the larger. The exact hows and whys depend on the circuit design and components/assembly techniques, but it's not a matter of coordination, intelligent or otherwise.

  • \$\begingroup\$ Thanks for the swift replies, although the batteries do seem to be performing better since they were decommissioned from the shop supplied fast charging 48v that came with the autorickshaw in series, the used to run down real fast until they took a few charges in this trial method which no-one appears to have had the guts to try :) I've found a couple of schematics online and wonder if they would limit the current draw: uploading links in a few mins \$\endgroup\$
    – abz
    Commented Oct 1, 2016 at 12:54
  • \$\begingroup\$ Here, though it looks like that transistor would severely limit current draw even if I put it through three of them in parallel the layout of my posts could be better but the type box hits send when I hit enter on mobile : i248.photobucket.com/albums/gg195/abz2000123/10-06-2011/… i248.photobucket.com/albums/gg195/abz2000123/10-06-2011/… renewablekinabalu.blogspot.com/2013/07/… \$\endgroup\$
    – abz
    Commented Oct 1, 2016 at 13:13
  • \$\begingroup\$ @abz - If you know that your PSUs can provide 8 amps, that implies that you do have access to a meter of some kind. Stop guessing at what's going on and start paying attention to the details. Charging lead acids is hardly a black art - folks have been doing it (and writing about it) for a long time. \$\endgroup\$ Commented Oct 1, 2016 at 13:14
  • \$\begingroup\$ :) the meter is shown in the video, my business was software and basic computer hardware repairs so my experience in lead acid batteries and making circuits is patchy in comparison to many others, we used to just replace most faulty parts unless it was an easy fix \$\endgroup\$
    – abz
    Commented Oct 1, 2016 at 13:20
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    \$\begingroup\$ If you can only get a 12 volt battery up to 12.3 volts, you probably have a shorted cell in the battery. You should be able to get a 12 volt lead-acid battery over 14 volts while charging. \$\endgroup\$ Commented Oct 1, 2016 at 16:34

You might also consider that a 12 volt battery is getting close to being discharged when its at 12 volts. You need to see around 13.8 volts across it before it can be regarded as fully charged.


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