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Cheap system voltage booster

I have been given a 250W solar panel with a maximum output of about 44V. I believe there are two types of controllers (MPPT, PWM) but don't know anything about them and don't want to spend a lot of money on one, as my main aim is main to provide garage lighting.
  • I can make a charger for the 12V lead-acid battery (probably from a circuit on this site :)).
  • I can connect 12V LEDs to the battery.
  • I have an inverter if I want 220V AC for lighting or an electric drill.
If the battery is a bit flat or I want a lot of current, it would be good to be able to boost the battery output voltage with the solar panel. If I get a 48V to 12V DC converter and I'm assuming they have an internal output voltage sensor similar to an LM317, I could connect it as in the circuit below. It sounds alright to me, but I suspect that someone is going to tell me that it's a really bad idea!
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Your LM317 will get very hot then shutdown because the voltage across it times the current in it is the amount of heating. Nothing limits the charging current so the LM317 will deliver its maximum current which is about 2.2A or more.
A 12V lead-acid battery is dead at 12V. It charges with 13.8V to 15V.


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Most Helpful Member
What do you mean by "boosting the battery output voltage"?

A 44V open-circuit panel will be rated at a Vmppt of about 30V, meaning that when the current from the panel is such that the Panel voltage drops to ~30V (~ 8A for a 250W panel), the power out of the panel will be maximum. This would require a switch-mode charge controller that accepts inputs from ~24V to ~48V while delivering ~14V to your single battery.

Doing this brute force using a linear regulator would dissipate lots of power as heat in the regulator (will require a huge heat sink), and you will be wasting more than half of the available panel power.
Your LM317 will get very hot...
LOL - the 33 amp LM317! I wrote "a 48V to 12V DC converter and I'm assuming they have an internal output voltage sensor similar to an LM317"...not literally an LM317...
A 12V lead-acid battery is dead at 12V.
You are right, so aiming at 12V won't work. I think it's the current that needs boosting, but I don't think it's going to be practical to adapt any other sort of voltage/current regulator.

I've had a look online for solar panel charge controller circuits and for cheap ready-made controllers. I conclude that with the complexity of the circuit and the cost of parts, it wouldn't make much sense to make one as I have found several modules on ebay for less than £15.
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