Simplest way to limit current?

[RooEUC]

I have a solar charge controller connected to a 12 battery bank.  When the bank gets low and there's no sun I add some power into the system with some large lipo packs that I keep 80% charged for spare energy for the system when I need it.  The problem is that the lipo packs deliver a lot more current than I want them to.  They deliver 30-40 amps and that's too much, creates too much heat and overloads my charge controller.  I would like to limit the current to 10 amps maximum without more than a 5 volt voltage drop.  What's the simplest way to do that?  I know there are buck converters available but I'm hoping for a simpler option.  Can I use a resistor?

[esaj]

29 minutes ago, RooMiniPro said:

I have a solar charge controller connected to a 12 battery bank.  When the bank gets low and there's no sun I add some power into the system with some large lipo packs that I keep 80% charged for spare energy for the system when I need it.  The problem is that the lipo packs deliver a lot more current than I want them to.  They deliver 30-40 amps and that's too much, creates too much heat and overloads my charge controller.  I would like to limit the current to 10 amps maximum without more than a 5 volt voltage drop.  What's the simplest way to do that?  I know there are buck converters available but I'm hoping for a simpler option.  Can I use a resistor?

A simple resistor could work, but you have to take into account how much power it's dissipating (ie. how high wattage resistor you need and if it needs to be heatsinked), plus you need to determine the value at the maximum voltage difference (I guess that's the 5V drop), the current will be lower once the difference gets smaller, so it won't adjust to automatically. For a more complicated setup, you could use a constant current source / sink that adjusts a power mosfet (or multiple parallel mosfets) to keep the current limited at the same amount regardless of voltage (as long as there's enough voltage difference to produce the maximum current, otherwise it will be as much as the voltage difference and the circuit resistance allows). Constant current source/sink isn't really complicated to make, but I don't know if you're looking into making a circuit of your own or buying something ready-made. Buck converter would likely waste much less power as heat, but I have very little knowledge of buck-converter design. Likely there are lots of ready-made buck-converter with constant current / constant voltage -modes that could handle this.

For a resistor dropping 5V and 10A of current, the power dissipation is 50 watts, which is quite a lot for relatively small mass/component size. To cause a current of 10A with 5V dropping over the resistor, the resistor value is simply 5V / 10A = 0.5 ohms. There are power resistors that are rated for 50W, 100W or even more (maximums), but you should take into account that they likely need to be attached to a heatsink for that much continuous dissipation, and likely the sink will get hot (or not, if it's big enough and the thermal connection between the resistor and the sink is good). Since you're looking at 50W of dissipation at maximum, I'd go with double the maximum wattage in case it might go above that at some point (more than 5V difference between the LiPos and the battery bank) and so that it stays cooler.

 

 

There are of course other manufacturers, Arcol was just pretty much the first that came up in Google search. The datasheet ( http://www.arcolresistors.com/wp-content/uploads/2014/03/HS-Datasheet.pdf  ) says that for a 100W power resistor, using a "standard" heatsink of 995 square centimeters (about one foot by one foot) and thickness of 3mm (about 0.12"), at 50W dissipation the surface temperature will reach about 50 Celcius (122 Fahrenheit) above the ambient (surrounding temperature). Without a heatsink, a 100W rated resistor should only be used up to 30W:

 

 

[Chriull]

52 minutes ago, RooMiniPro said:

.. .  The problem is that the lipo packs deliver a lot more current than I want them to.  They deliver 30-40  amps and that's too much, creates too much heat and overloads my charge controller.

Then i assume you operate your charge controller out of his specifications? To much voltage from the lipo pack?

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  I would like to limit the current to 10 amps maximum without more than a 5 volt voltage drop. 

Voltage drop and amps have a fixed relation - you cannot "wish" the values. Depends on your specific sytem whats possible.

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What's the simplest way to do that?  I know there are buck converters available but I'm hoping for a simpler option.  Can I use a resistor?

Dc/dc converters should be the most reliable and simplest solution...

but maybe in your case some cheap halogen bulb(s) could be sufficient? It just gets quite bright and hot around them...