Battery Charge Rate



I am a Physics student wondering how I can get my eyes on the specs for a car batteries charge rate? Specifically a 2005 Toyota prius. Being a poor student I am designing a method to charge the battery without the gas engine that should be real inexpensive and kind of sexy too!




Automotive batteries are tough because they don’t use the industry standard, amp-hours, to rate their capacity. Instead, they tout CCA, cold cranking amps. Automotive batteries can be charged at a very high rate until their voltage climbs to 13.8-14.2 or so, then the charge rate must be quickly reduced to avoid damaging the battery.

Other battery types, industrial, lift truck, deep-cycle, that are rated by amp-hours of storage capacity, are “Slow Charged” at C-10, 10% of their rated capacity. So a 100 amp/hr battery would be charged at 10 amps for 10 hours or so…C-20 is used for fast charging, 20% of capacity. If a battery is completely discharged, they can be charged at C-50 until the voltage recovers to about 50% charge, then the rate must be reduced to avoid overheating and damage. In the real world, a voltage regulator controls the charge rate according to the batteries state of charge… This applies to lead-acid batteries. The Prius storage battery is NmH, nickel-metal-hydride I think. You can research the properties of these batteries and the charging protocols used to maintain them.


This article at wikipedia has some of the information you need to develope your idea further. You NEED to get the basic chemistry and basic physics, before you can take the next step.


Some newer batteries are rated in a variant of amp hours called reserve minutes. This is based on the average current draw of a vehicle at night with the lights on of about 25A. You can calculate AH from that. Turning off the alternator except at high vacuum would be an interesting idea for normal cars.

Poor students are not supposed to drive a Prius! I don’t think people are getting the idea that you are trying to change the Prius to a purely electric vehicle. Car Talk is the last place I would go if you want good technical information. It is the home of the glorified oil changer.


The first step is to look at what other people have done. This will give you a leg up by leveraging their investment in engineering a solution. You may even discover some design ideas or hazard mitigation that you might not otherwise thought of incorporating. This is especially true if you plan to market your solution to the Prius crowd. what works for you on a sample size of one or in your own garage is not necessarily acceptable as a consumer item. Search on plug in charging Prius to get some idea of what’s already available.

BTW- the Prius battery pack is not your average lead-acid service battery in a conventional car. Charging some of these packs requires multi-mode charging and temperature monitoring to do properly. Good luck!


I wonder just what success a physics student will have that doesn’t know how to use Google. Just a quick look gor me:

“Your 200 volt hybrid battery is rated at 6.5 ampere hours. Fully charged it would have 1300 watt hours of energy or 1.74 horsepower hours. Guesstimate that you need 17 horsepower to move the car, and power the radio and lights.That would let you travel for six minutes, assuming you could drain a fully charged battery flat at 100% efficiency. This is not a big thing. A conventional 12 volt car battery would be rated at about 50 ampere hours, and would store 600 watt hours by comparison. That hybrid battery is not very big. It doesn’t have to be to do its job of mopping up and putting to work the little bits of energy that are routinely wasted in conventional cars.
I think covering the roof of the Prius with solar cells and driving on sunny days would make more sense than trying to plug it in to the mains.”


Cars spend most of their time parked. While the car is parked, it could use several methods of recharging the batteries:

  1. Sun-following solar panels.
    2. Small wind turbines.
    3. Heck! Let’s throw in human powered generator. Turn that energy-wasting-jogging into something useful.


Lets get to the CORE of the problem. Energy Density. Trying to replace the energy contained in gasoline with batteries, solar panels, electric motors, brings you right back to golf carts.

Carefully observe how vehicles are advertised on television…The new 450 hp Benz Burners for instance…Three cars burning rubber and sliding past each other in a 4-wheel drift as smoke boils out of the fender-wells…

Americans will need a BIG attitude adjustment before they will embrace glorified golf carts…


sliding past each other in a 4-wheel drift as smoke boils out of the fender-wells…

That sounds exactly like the carts we drove in our company tournament coming up to the 18th tee. 'Course that was after a few martoonis…


All these posts and NOBODY answered your question (hellokit came close). Have you tried contacting Toyota directly? If you really are a student and doing this for a school project - they may just be interested in helping you!


I tried to answer it in the first post…C-10, C-20 or C-50…Toyota uses a heat sensor to limit charge rate if the battery gets hot…

I doubt Toyota is interested helping tinkerers…Besides, there is a west coast outfit that already retrofits Prius cars with a supplemental battery and a plug-in charger which allows the car to be driven (after the control system is modified) on battery power alone for 20 or 30 miles. Save the planet.


Wow, Thanks for all the replies, they do broaden my vision of the project. My main purpose for figuring out the rate of charge for the battery is so that I can know whether the following will work…

Im going to place 4-6 wind “turbines” in the grill of the car and have it charge the battery. I am calculating whether this will be worth while. The prius has a hidden modification where I can force it to not use gas till over 34 mph and this could potentially make such a feature worth it.

I plan to talk to toyota soon, though they are against the tinkering…


Excuse me, but what your proposing is a ‘perpetual motion’ device that simply is not possible. You cannot get energy for nothing. The motors will use a lot of power to run the car, and your device is going to attempt to recover some of that motion. But, the motors will need to put out even more power (even just a tad more) to overcome the drag from the turbines. By the time you add in the loss through conversion and the loss through the resistance in the wires, you’ll have a net drag on the batteries, and have gained absolutely nothing.

I’m not trying to throw a wet blanket on your creativity, but I had an engineering mentor school me on ideas like this. I’m just trying to pass along the knowledge.


Busted is right… Remember concervation of energy forn high school science class? The energy your turbines will add to the vehicle will be less power than they will use up from the vehicle (friction takes the rest)- at the best possible case you won’t even break even.

Now… if you can use the turbines to assist braking - now you have something - but as you probably already know - hybrids already use braking energy to recharge the batteries…

You need to find a way to recover “wasted energy” braking is one form of wasted energy as it turns momentum into heat.

You do get credit for trying to think outside the box though… keep it up!


I didn’t see anywhere in the OPs posts that they intend to be energy neutral using this approach. What if this method recovers only 20% of the charge lost? Would it be worth the effort? The point is, it doesn’t have to be net zero to be worthwhile. The larger question is how much energy can be recovered from this approach. The first step is to model the proposed generation system and estimate the energy that can be produced. All that can be done in the lab. The method for putting that energy back into the battery packs is way down the list of questions. Perhaps the OP has already done that part?


Think about it this way - If you ONLY run on batteries - the additional energy needed to “push the turbine” into the wind (overcoming the drag caused by turbine veins) will be greater than the energy returned to the batteries - so net negative.

If you run ONLY on the engine - you are using gas to push the turbine - the efficiency of using that for charging the batteries is lilely much lower than turning an alternater or some other mechanical charging device.

If you can turn the veins of the turbine while driving so you can control the drag, then you can turn on the turbine when you are braking and turn it off when you are accellerating - that may result in net positive.