I want to charge my two lead-acid batteries connected in series to a stair-case lift. I do not have a proper charger (don't have ebay/amazon here) but I do have a 19V, 3A laptop charger (SMPS). I also have to use something else than a typical charger because there is a 2-3V voltage drop from the supply to the batteries (was bigger).

I plan to buy a boost converter (a 600W one because that's what I found in my neighbourhood: youtube link) and charge the batteries with the help of that step-up converter (and the protective diode).

Is the following math right? (without the diode voltage drop)

enter image description here

The batteries are 12V, 7Ah lead-acid and I will limit the current to 10% of the capacity. What do you think?


  • the boost converter has constant voltage regulation and current limiting capabilites
  • the batteries will be continuously standby charged (with float voltage) and will be lightly used
  • 1
    \$\begingroup\$ A proper battery charger costs almost nothing. I understand you don't WANT to buy one, but if you don't charge these right, you'll be buying new batteries - which almost surely will be WAY more cost than a new charger!!!!!! There are dozens of sub $25 24V chargers available on Amazon/Ebay/Etc.... \$\endgroup\$
    – Kyle B
    Jun 21 at 14:52
  • 1
    \$\begingroup\$ Dude.... IMO (and only MY opinion), if this is going is SOMEBODY ELSE'S house, you HAVE to do it right. Even if that means sourcing something that is a pain to source. Leaving a contraption like this in the hands of somebody who doesn't know what they're doing is asking for trouble. You don't wanna be responsible for exploding batteries, fires, electrocution, etc.... Do it with properly engineered and safely designed equipment or don't do it. (IMO again....) \$\endgroup\$
    – Kyle B
    Jun 21 at 15:08
  • 4
    \$\begingroup\$ @KyleB Laptop chargers are not chargers, they are just power supplies. The charging circuitry for batteries is inside the laptop. Just like your mobile charger is just a 5V power supply, the battery charger is inside the phone. As long as you don't exceed the current rating, and stay within reasonable load current changes, the laptop supply can power custom circuitry. \$\endgroup\$
    – Justme
    Jun 21 at 15:11
  • 2
    \$\begingroup\$ @Justme No they're not. Some maybe, but many/most have digital sensing to determine whats at the other end. Try plugging a Dell charger into an HP laptop and see what happens. Manufacturers will say it's for safety and it is, but it's also to ensure you buy THEIR stuff later. \$\endgroup\$
    – Kyle B
    Jun 21 at 15:13
  • 4
    \$\begingroup\$ @KyleB Of course they are manufacturer specific, but what is common is that they just tell the laptop how much power or current is available from the power brick as they sell different supplies. It does not change the fact that they are power supplies and the charger that takes in 19V is inside the laptop. They are definitely not chargers, at least most of them are not. \$\endgroup\$
    – Justme
    Jun 21 at 15:18

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