Two 12v in series or two 24v in parallel?

[nosys70]

as usual you need to count your battery as load either, because a battery does not fill by itself.
so he did not say how many watts he got available, but he will need to count the battery as a load, split over the time the source (PV ?) are on (7 hours?) minus the time the battery is discharging.
so a 100A 12v batt is 1200W, to be added to the 1500W load, so 2700W load at worst. (if batt is empty and you charge it in one hour)
Since he said the batt will supply power for only 15 min (the batt has 1200w, and 15min of 1500Wh about 400W, this requires only about 30% of battery capacity to restore).
so really we can count the battery as a 500W load, it decrease the total bill to 2000W.
if you split the 500W over 7 hours, it is only 80Wh to add to the 1500Wh, so about 1600Wh.
But if by any chance you empty the battery drastically more than the 30% requested, you could end up with a battery that will never be able to fully charge on one day.
in that case the best would be to put a 300W solar panel just for charging the batt, and leave the other source for the 1500w load.

 

[Max Bender]

However, two 100Ah 12v batteries is 100Ah 24v

Yeah but adding a side of fries to the order will cost more.

 

[Max Bender]

Hey Scott... you still with us? Inquiring minds want to know what you've decided.

 

[scott harris]

Hey, thanks for all of your help. It looks like it would work either way. The load is a level 1 car charger connected to a Chevy Bolt. This will draw either 1500 watts or 1050 watts depending on whether by bolt is set to charge at 800 watts or 1200 watts.

My plan is to start with one 24 volt battery and test the charger at 800 watts (1050 watts draw at the plug) and then expand to two 24 volt batteries in parallel when funds permit.

Thanks again for your assistance,

Scott

 

[scott harris]

BTW, I also reached out to BB and they suggested to not try a 1500 watt load on one battery for even a short time, but that two would handle it just fine.

 

[scott harris]

But then again, they sell batteries.

 

[Max Bender]

BTW, I also reached out to BB and they suggested to not try a 1500 watt load on one battery for even a short time, but that two would handle it just fine.

Cool. Then we're back to square one - two 12v batteries or two 24v ones. Me, I'd go with the 24v ones because like Craig said, if the BMS in one battery in a 2x12v configuration turns the battery off, then.... oops, no 24v power at all.

 

[Max Bender]

But then again, they sell batteries.

That's true. The reason why I figured you could get by with 50 amps was because if 100 amps can handle a 1500 watt load for an hour then 50 amps should be able to handle it for 10-15 minutes.

 

[scott harris]

so two 24 volt 50ah batteries it is.

Thanks for the help

 

[scott harris]

So, I don't really expect this to be a practical solution for charging my Bolt. The plug in my garage does just fine for that. This project is an educational project, a hands on display if you will.

Since retiring last fall I have begun to educate elementary students about where Earth gets its energy and how we can best use it (I used to be a high school physics teacher before I became an optical engineer and I guess the teaching bug has never left) The first slide in my presentation is

this
I then tell the story about two PhD students who planned such a trip 8 years ago. Unfortunately they didn't make it but I think it could be done with today's improved equipment.

The purpose of my project is to bring eight 250w solar panels to the school when I come to do my presentation. I will lay them out on the playground and have them charge my car. During the presentation (after the part that explains (very simply) how solar panels work) I will tell them that I am traveling back home on sunbeams. Of course, they will have already seen the car and the panels, so they should be interested.

In May (when the snow is gone and the days are longer) I plan to take a solar trek from the northern border of my state to the southern boarder traveling only on sunlight (no plugging in along the way, just using the panels in my car to recharge during the day) and stopping at schools to recharge and give the presentation. I plan to average 40 miles a day and take two weeks.

So, even though this is not a practical way to recharge a car, the system has to work well, or I will never get home.

 

[Max Bender]

Lucky you. I can't think of anything that's more fun, more rewarding, and more useful to society than being a teacher.