capacitor starts

ws9876
ws9876 Solar Expert Posts: 440 ✭✭✭
Maxwell Corp. makes a capacitor starting module for trucks. It replaces 1 battery in a parallel group and gives a big jolt
to the starter and recharges in minutes to do it again. Its a great idea and will not be the only one in the future but for now it is and is overpriced at about 900$.
Anyway I wondered if any off grid folks have made their own capacitor banks and used them to start large power tools so the batteries dont have such a large draw on them at startup .Like a table saw. or drill press.or food disposal or vacuum cleaner..??

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: capacitor starts
    ws9876 wrote: »
    Maxwell Corp. makes a capacitor starting module for trucks. It replaces 1 battery in a parallel group and gives a big jolt
    to the starter and recharges in minutes to do it again. Its a great idea and will not be the only one in the future but for now it is and is overpriced at about 900$.

    I can't imagine why anyone would think this is a good idea, much less advantageous.
    Anyway I wondered if any off grid folks have made their own capacitor banks and used them to start large power tools so the batteries dont have such a large draw on them at startup .Like a table saw. or drill press.or food disposal or vacuum cleaner..??

    The place to put the capacitors here is on the AC motor, if you can. That and only that will help mitigate the demand on the inverter & batteries (by improving the PF of the motor). You would need to do some experimentation with the each motor, and it will only help induction motors; ones with brushes are what they are (and don't have the severe demand of the other type).

    By the way, most power tools are started with no load and as such are not difficult to start. Pumps and compressors that have to start against a load are the ones that will give trouble.
  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    Re: capacitor starts
    ws9876 wrote: »
    Anyway I wondered if any off grid folks have made their own capacitor banks and used them to start large power tools so the batteries dont have such a large draw on them at startup .Like a table saw. or drill press.or food disposal or vacuum cleaner..??

    I am not a certified electrical engineer but I would not think this would work because sure the capacitor would provide the "jolt" instead of it coming from the batteries, but as soon is the jolt is delivered, the capacitors will "jolt" the batteries when they recharge a moment later.
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: capacitor starts
    ws9876 wrote: »
    Anyway I wondered if any off grid folks have made their own capacitor banks and used them to start large power tools so the batteries dont have such a large draw on them at startup .Like a table saw. or drill press.or food disposal or vacuum cleaner..??

    I've used them on small battery systems to reduce voltage sag during starting of compressors. It works reasonably well but:

    1) The time constant is small. With their standard 15V 58 farad module, you only get 4900 watt-seconds at 13V, of which about 1300 is available for voltage support (since you generally can't live with more than about a 2 volt drop.) So you'd get 13kW for only 100 milliseconds. (or 2.6kw for 500 milliseconds etc.) Not a lot but enough to help start a large compressor with a small battery.

    2) You HAVE to precharge. Just connecting them across a battery bank is going to cause dangerous current surges as the (very low ESR) capacitor charges as rapidly as possible. If you connect them via a circuit breaker the breaker will keep blowing until it charges up to a similar voltage. A resistance of a few ohms with a momentary switch to precharge the capacitor before closing the breaker is important. Press the button, wait 10 seconds, then close the breaker.

    3) If you use more than one module (for 24 or 48V systems) balance each module. Simple resistive balancers work OK. There's an application note on the Maxwell site that shows you how to select resistor values.
  • YehoshuaAgapao
    YehoshuaAgapao Solar Expert Posts: 280 ✭✭
    Re: capacitor starts

    Probably better just to have a start capacitor in the offending appliance (air conditioners in particular).
  • ws9876
    ws9876 Solar Expert Posts: 440 ✭✭✭
    Re: capacitor starts

    the caps will charge slower than a big load will draw so I dont think it would hurt the battery when it recharges.
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: capacitor starts
    ws9876 wrote: »
    the caps will charge slower than a big load will draw so I dont think it would hurt the battery when it recharges.

    They will actually charge a bit _faster_ since they are connected in parallel - and will have a much lower ESR than the battery.