Using a high power resistor as a dump load

When choosing a dump load for your charge controller, you need to find heater or heaters that will:

  • Be able to dump the maximum current your combined wind and solar systems will throw at it at once.
  • Not draw more current than the charge controller is able to handle. (Which is the number in the name of the controller – e.g. C40 can handle 40 amps maximum.)

A neat solution to finding a dump load for a charge controller is to use a big wire wound resistor that you can buy from an electronic component supplier.  Look for ones with low resistance and high power rating.  There are not very many to choose from, and their stock is always changing.

The resistor has two important numbers associated with it:  resistance and power rating.  Resistance determines how much current it will dump in your system and the power rating is a guide to the maximum safe wattage it can burn off without over-heating.

When choosing a resistor/heater for a charge controller we need to start by considering the system voltage.  Say it’s a 12 volt system then the heater needs to be safe up to 15 volts.  (Whereas 24 and 48 volt systems can go up to 30 and 60 volts.)

Use Ohm’s Law to find the current the heater will draw at this voltage (if the controller turns it on fully).  For example if the resistance is 1 ohm (written 1R or 1Ω) then Ohm’s Law says:

Current = voltage/resistance = 15/1 = 15 amps.

Next find the power it will have to dissipate (as heat).

Power = voltage x current = 15 x 15 = 225 Watts.

In reality these resistors can take some overload (and the controller is unlikely to need to operate them continuously) so you can get away with a well ventilated 200 watt resistor, although my favourite is 300 watts.  This makes a good building block for a dump load system.  You can add more in parallel to dump more current (up to 3 in parallel for a Tristar 45-amp controller) and you can add more in series to go to higher system voltages.

Above is how they work in parallel and below they are in series.  Each parallel circuit adds to the current that the heaters can draw.  Whereas putting more in series adds to the voltage they can cope with.

All this is talking about a 1 ohm resistor.  Like this one.

If you are working at higher voltages than 12, it can be more sensible to use larger resistors for  simpler and more cost-effective solution.  For example how about 1 ohm/1 kW resistors like this one?  Or here it is at Farnell.  Connect it to a ’24-volt’ battery that is being charged at 30 volts (equalising or in low temperatures maybe) and it will draw 30 amps.  Power is 30 x 30 = 900 watts and so this heater is suitable for the job.  It can dump 28 amps at 28 volts or 26 amps at 26 volts.  (The amount of current it actually dumps depends on how much the controller has to throw its way.)  Don’t use more than one on a Tristar 45 in a 24 volt system or you will overload it.  (You can use two in series on a 48 volt system, and you can use two in parallel on a Tristar 60 safely.)

2.2 ohms 2kW resistors like this make a good load for a ’48-volt’ system where you need to dump 25 amps.  At a nice battery voltage around 56 volts it will do that happily, and it will not reach its maximum power rating until 66 volts (which is probably more action than your battery is likely to want.)

I have often found 3.1 ohm resistors with a 300 watt rating that are good for ’24-volt’ systems, running at roughly 10 amps.  You can use up to 4 of these in parallel (up to 40 amps at 31 volts) on a Tristar 45, and 6 of them in parallel on a Tristar 60.  For a 48 volt system you have to connect them in series pairs, rather like the second diagram above.

Take care when mounting these resistors that they have plenty of room so that air can circulate and cool them.  They do not get red hot but it’s wise to keep them far enough away from flammable surfaces like wood.  You can put them inside a steel enclosure and make them look pretty slick but make sure they are not likely to be overloaded in there.

I like to solder the connections but sometimes it’s simpler to bolt them.  Either way you will need to use high temperature, flexible wires to make these connections.  Equipment wire or ‘tri-rated flex’ is good.

These heaters will make a gentle buzzing noise when the controller is working.  It’s not a loud noise, but it’s definitely there, so if anybody in the building is uncomfortable about buzzing noises then you may have a problem.  Often they end up being mounted in the battery shed.  This won’t protect your batteries from frost (which mostly occurs on calm nights when even the hydro is drying up) but it will safely dump the power that would otherwise potentially damage your battery.

The photo below shows a dump load resistor mounted on a piece of threaded bar (allthread) that is simply screwed into a hole in the woodwork.

There are other kinds of resistor, for example these aluminium housed ones that are not suitable on their own.

This type of resistor needs to be on a heatsink of some sort.  I have clamped them to hot water tanks, and produced some useful hot water that way.  But they do need to be fitted to a surface that will take the heat away.  Without a heatsink their power rating is drastically reduced.

Relay Drivers for load management

 Installing a Tristar controller

Choose a dump load resistor

Charge controllers (relay type and PWM type)

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