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I am searching about chargers that are specifically for starting forklifts and other things that require, say, 12V-24V and 20A-80A. They have lead-acid batteries.

  • These starters have a switch for 12V or 24V, and another one for maximum or medium current.
  • I suppose how the 12V or 24V is made is with a simple relay in the center tap of the primary so when is closed it gives 24V when it opens 12V.

How is the current controlled? There is no SMPS, only a transformer and I remember when I opened one, I didn't see a PCB to control it.

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Usually, the only limiting factor for current is the transformer leakage inductance and resistance. The battery has ESR too so it won't go to infinity as if you wound imagine putting two stiff voltage sources in parallel. Both are weak in engineering sense.

If it’s purpose made, it’s probably undersized and would overheat if you operated it continuously.

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  • \$\begingroup\$ Clear... If I would have to design one, Lets say that my primary has 100 turns and 1mm. And the Secondary 10 turns and 5mm, this are arbitrary values. To undersize and not spend more than necessary, knowing that it's only used as a starter, I would have to reduce the section of the wire? or reduce symmetrically the turns of both primary and secondary? \$\endgroup\$ Sep 1, 2023 at 0:01
  • \$\begingroup\$ From a known good design made for continuous use, you can’t decrease the number of primary turns as it should be decently close to saturation already. MOT are known to run dangerously close at all times to squeeze out the last %. To save cost, you would use a core size one or two steps smaller and perhaps increase the number of secondary turns to compensate for all the resistive losses during load, but you can only do as much as you can allow the unloaded voltage to go up for your application. To get more into transformer construction, look up the formula Urms = 4.44fNAB. \$\endgroup\$
    – winny
    Sep 1, 2023 at 6:25
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It’s just a transformer designed for small duty cycle, followed by a rectifier, with a tap as you imagine. Nothing fancy to it. The capacitance of the battery acts as a good smoothing filter. Don’t use those with battery disconnected - it can cause interesting problems.

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  • \$\begingroup\$ Yeap, it was what I imagine... What do you think about what i asked in the another comment ( about how i would design a transformer for only a small duty cycle? Thanks! \$\endgroup\$ Sep 1, 2023 at 0:04
  • \$\begingroup\$ basically ignore cooling \$\endgroup\$ Sep 1, 2023 at 6:19

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