Need help with battery charging

Question

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).

jw0752 · Answer

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

jw0752 · Answer

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

Need help with battery charging

Top Replies