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Need help with battery charging

Former Member

Hi everyone, I am working on a custom hybrid vehicle and I have designed the motor controller myself. I have a high power MOSFET doing PWM on a large brushed electric scooter motor(24v) with a relay to switch from drive to regenerative modes. This works great as it outputs a power that can charge a capacitor or get dumped into a large resistor. The thing I want to do involves charging a battery ( Its going to be NI-MH or Lith ion or some battery with decent energy weight density) the obvious problem I see with this is designing a high power battery charging circuit something that can take as much as 1kW+ at any given moment. I know its possible to do this because HEVs and EVs do this all the time with their regenerative braking function. It is of the utmost priority to be able to capture as much power during braking as possible, thus I might even add a high value capacitor to act as a buffer. Could anyone point me in the right direction for building a DC to DC battery charger that could safely dump kilowatts of power into a battery. Also I would like some recommendations for what battery type to use ( I would prefer to use a lithium something (for their high energy capacity and small size).

  • 0

    NiCad batteries have some of the fastest charge rates available and they can withstand more charging cycles than most other battery technologies. They are also good at supplying high currents to electric motors and often used in power tools. To absorb more power, spread the current among more batteries.

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    NiCads are great from an electrical standpoint.  The fastest chargers monitor temperature as well as the electrical params.  NiCads need to be disposed of properly, Cd is a toxic heavy metal.  One thing one has to watch out for is they have nearly no Rth, so if nothing limits the current, nothing limits the current.  Also, they are kinda heavy, but not so much as Pb-H2SO4

  • 0

    The problem isn't what batteries to use but how to make a dc battery charger that can handle high amperage. I have not found a dc battery charger that can do this, so if anyone can help me find one or design one. I would prefer to use a lithium battery as I have a limited space to work with and want to get the best capacity available. I know there are a bunch of lithium batteries used in RC hobby applications that can charge at 5 or 6 times their rated capacity. Does anyone know how to make a battery charger that can charger directly from dc and monitor the battery. I would not hesitate to use a micro controller as I am already using one to control the motor and gas engine servos.

  • 0 in reply to Former Member

    The reason why chargers aren't as fast as you'd like are thermal.

  • 0 in reply to D_Hersey

    I guess you would like to explain how the Tesla Model S can charge to half its range in 30 minutes!

    I am aware of thermal problems but would like to be able to capture some energy during braking, This is entirely possible the Prius,Tesla, and just about every modern hybrid and electric vehicle does it.

    If you are just going to disagree with me and say it can't be done I would appreciate you just keep this to yourself because I am trying to build it.

    Besides if a Nicad battery has such a low ESR why would there be thermal problems? ESR is the main limiting factor for how fast a battery can be charged and discharged.

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    Been waiting all of my life to meet a guy whose force of will trumped the laws of physics, met a few claimants, still waiting.  Expects to out-design firms that have been in business for decades for starters.

     

    Why don't you use a capacitor as an intermediate between your regenerative braking and your battery charger?

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    Hi Jack, Part of the problem with capturing short bursts of power with batteries is that they are electrochemical in nature and as such it takes time for chemical reactions to occur over a given reaction area. Doug has correctly advised that one way to inprove the energy capture is to increase the area of the reaction with more batteries in parrallel. Don has also called your attention to the thermal problem. Heat is generated in these chemical reactions whether they are discharging a battery or recharging it. A more efficient means of rapid storage is the capacitor. There are current technology capacitors that are capable of holding multiple watts of power. There are even types with fairly low effective serial resistance that can charge fairly quickly. A bank of capacitors in parrallel with your battery might help with the stress on the battery due to peak demands of starting the vehicle moving as well as pulses from any braking energy recovery. Don't  expect a very high energy recovery from current technology. There are about 350 kW of kinetic energy in a 1 ton vehicle moving at 60 mph. If that vehicle is brought to a stop in 15 seconds I would be surprised if you could recover more than a couple kilo watts with a battery. I believe that your efficiency would improve if the braking could be spread over a longer period. Thanks to Elon Musk's patent generousity at least you can feel free to hack a Tesla and use its technology.

    John

  • 0 in reply to D_Hersey

    Hi Don - I was writing my post at the same time as you so I did not see yours until after mine was posted. I second your capacitor idea. Also I might mention that I at least get some of my best ideas when someone disagrees with me and tells me things I don't want to hear. I never learn anything when my mouth is open and my ears are shut.

    John

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    Yes Originally I had my setup optimized to use ultra caps, As they have an incredibly low ESR and without any electrochemical reactions they can charge and discharge as fast as you can effectively put power in or out of them! The only problem I have had with ultra caps it the cost!!!!! To build a 24V array of ultra caps or any high energy cap, the cost is around $2000usd. That is simply too much for my budget. If you look at my original post I had planned on using ultra caps for their simplicity. Don I would like to apologize especially as a Mechanical and Electrical engineer with a large background in physics, I think you had misunderstood me as being too simple and stubborn. I would like to third the usage of caps! My problem still exists in transferring the energy from the caps to a battery, this still needs to be done rather quickly as the caps(even if they have a high value) can only store a fraction of the energy a battery can hold. So I will still ask for someone who is experienced with battery charge controllers to provide me with a better understanding of how a battery charger works. I would love to buy a commercial product to charge my battery but I simply can't find one that can take 24vDC and charge a 24vDC battery. I would really love to do this project with ultra caps but with their high price I am still looking for a better solution to store that much energy. I will try to get away with a couple cheap Large value caps but I know that the amount of energy that can be stored in these would be minuscule compared to the amount regenerated. I will still do some more calculations to find out. Based on the formulas for kinetic energy and potential energy in a cap.

    Believe me I am all ears for reasonable suggestions and have been engineering almost impossible feats since age 6, So I know the engineering process very well and yes arguing is key to the process! So again If anyone can help guide me to building a battery charger that can charge Lith poly batteries at something other than a snails pace(think near max pace) I would like to hear it.

  • 0 in reply to Former Member

    Hi Jack - I am not an expert on Li batteries but as I understand it the control circuitry is with the battery not the charger. These batteries are sensitive to both over charging and also to being discharged too deeply. As a result the circuit cuts off discharge of the battery before it goes too low and it also allows a high current to flow when the battery begins to charge but then regulates the current down as optimal voltage is reached and finally stops the recharge completely. Not talking about Li batteries but rechargeable batteries in general you can increase charge rate by increasing the voltage that is applied by the charger. If you are talking about recharging a 24 volt battery then this means that putting a reverse polarity 30 volts on it will charge at one rate and 36 volts placed across the same battery will charge more rapidly, as the current flowing back through the internal resistance of the battery will also increase. As the charging voltage increases there are design limits in the battery that will be reached. As you mentioned batteries with low ESR will accept more current without overheating but there are limits. Most of the modern batteries have specific requirements and limits. Since everyone is impatient, the engineers design chargers to match specific battery designs to minimize charge times and still stay within safe limits. You are not likely to get specific advice on charging large rapid charge batteries or modifying chargers to push batteries beyond their design specs from people on a forum like this as no one wants to give advice that deals with conditions that might be dangerous or lethal. Once you settle on the precise battery that you intend to use you may be able to get information from the mfg. data sheet for that battery so that you can minimize the charge time safely. It is also possible that someone on the forum would be able to give specific information if they know the exact battery you will be using.

    John