[RE-wrenches] Direct PV-EV charging: possible?

Thu Aug 23 13:11:02 PDT 2012

Allan,

Can be done but I don't believe it is economical. You will have more cost in equipment yet your daily PV power production will be less. The reason is that battery charging will not use 100% of PV potential for the entire charge cycle. Every day the power will taper off as the battery gets full, the rest is wasted. This waste will add up to many kWh's. Also, the MPPT conversion efficiency will likely be less directly charging a battery compared to using a grid tie inverter. You can optimize the array voltage for one or the other but probably not both.

Larry Crutcher
Starlight Solar Power Systems
(928) 342-9103

On Aug 23, 2012, at 7:50 AM, Allan Sindelar wrote:

Wrenches,
We have a customer who has built several EVs, and wants to eventually develop a business around them here. A few years ago we installed a GT system on his home. He has now moved and wants to install a system on his new home as well. This time, he'd like to develop a means by which to charge his EV directly from his array, avoiding the inefficiencies of inverting to the grid and then running a grid-based charger. 

I know of no way to do this, but then, I'm a veggiehead (drive an old Mercedes on waste fryer oil), not an EV aficionado. I suspect that there are folks out there trying to do this, as it's a logical objective. Can anyone suggest products, links, or websites for this customer to explore?

Here are his notes about this effort:
...this is essentially the idea. to use the EV as rolling battery backup to dual-purpose the investment in battery. The second plus is avoiding loss from two inversions dc-ac-dc, the third being relatively high-amp charge without buying another charger! and dual purposing the inverter to avoid cost of high-amp inverter only used for emergencies for a backup. 

The tech details of EV battery packs that are going to make this a challenge is the high voltage. The DIY pack running DC are going       to be 120, 144, 156vDC nominal most commonly. Guys running AC motors and OEMs might run much higher.... closer to 300v. 

The charge controller or voltage limit switch needs to be user settable and fairly accurate (+/- 1 volt). The DIY people usually use a 'finish' voltage of 3.65volts / cell being 'done'. So for a 120v nominal system I would want to set the DC charge to come up to 139v and either shut down right then, or if it COULD just hold at 139v until amps drop or a specified time, that would be even better. 

I am planning to do a grid-tie system at my new house, and yes, the intent is to 'stub-in' capability to both quick-charge the pack direct from the DC side of PV if possible, AND use the car pack as night-time/emergency backup just to show its possible.... 

My little car has 12kWhr on board, one I'm working on right now will have 20kWhr onboard.
Thank you in advance for any assistance.
Allan
-- 
Allan Sindelar
Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
Positive Energy, Inc.
3209 Richards Lane (note new address)
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com

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