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Charging direct from PV panels without charger?

I've been reading and watching youtube clips a lot for the last two months and I have plans to build a 50S10P 18650 pack. Well, two actually but let's start with one. Now I don't want to go into details here about this pack, nor the controls I have in mind to feed the energy into the house when the sun goes down. My question here is about charging the pack.

Some numbers, first:
- Pack minimum voltage: 3V x 50 = 150V.
- Pack nominal voltage: 3,7V x 50 = 185V.
- Pack maximum voltage: 4,2 x 50 = 210V
- PV panel voltage, I have some in series: 200 - 220V, depening on the weather. Very early in the morning I have seen 193V, but that only occours for like an hour at dawn and dusk, the rest is 200V+. The current produced seems to be of very little influence (none, as far as I can see) on the voltage.
- The PV panels can supply 9,5A current max. I never saw that (we don't have that kind of weather, here), but the manufacturer says in theory they could.

My idea is to put a large two-way contactor in line with the PV panels. With the battery pack below 200V, the contactor directs the power to the pack. The pack sends it to my controller and the controller to the DC-AC converter as need be. When the pack reaches 200V, the contactor switches and the PV panels are connected bypass the pack and controller, and are directly hooked up to the converter. As it is normally, if I wouldn't have a pack. And when the pack is discharged to below, say, 195V, the contactor is switched back. So, my whole 'charger' is a simple volt meter controlling the contactor. I cannot charge at an amperage higher than the pack can handle, as the PV panels cannot product such a current.

There is one thing I don't know, that would make or break the idea. The big question for me is; will  the pack pull down the PV panel voltage? If the PV panels want to give 220V, the pack is at 150, I think the pack will pull the PV panel voltage to below 200V. But if the pack is at, say, 195V, will it still pull the PV panel voltage down to below 200V?

I will put in diodes and fuses here and there to make sure everything goes as it should, but I leave that out of the discussion, now, too.

Please let me know what you think, prefferably from experience.

Its far from ideal. Yes battery will pull down your PV panels

Lets assume your best output is at 220V and your pack is at 180. At 180 you might only get 50% of the power output.. That means you loose 50% by doing that way. Thats why we use MPPT charge controllers that calculates the best voltage to keep the panels at and best output.

PV -> DC/DC that calculates the best voltage to keep the panels at no matter of the sun -> DC/DC to controll charging voltage and current -> Battery bank
This is also the reason Optimizers and stuff like that is added to panels. ITs for getting max from the panels out Smile

So in long run yes you can do it this way redneck style but its better, more efficient and safer with a proper MPPT controller.
Korishan likes this post
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Please put Your project in a box and keep it in a safe place for a while.

What You do right now or what You think of doing is DANGEROUS and may kill You.!!

Then sit down and read the fora here on this Site, and learn why all these users spend thousands of dollars on MPPT Chargers, BMS equipment and Inverters.

Fiddeling around with 200+ volts DC is Extremely Dangerous. Therefore installations using 200 volts are heavily regulated by Law, and is not recommended for home hobbyists..

48 Volts, used by the large systems here on this board, is not for children either, but You will survive if You get Your fingers too deep into Your project.

BMS makes sure that Your battery does not take control, and causes irreversible damage to You or Your property.

Later down the road, Daromers wise suggestions will start to make sense to You.

Then You can open Your box and restart Your project.

Please, I am not trying to patronize You, You can do what You want with Your life, but I am genuinely worried that You may hurt Yourself.


Moderator, If You think I overstep the line, just delete this post, thank You
Korishan likes this post
I agree with the others. Dealing with that high of voltage is dangerous.

The other thing to take into consideration. Your contactor arrangement will burn out quickly. You have a simple, used lightly, volt meter as your trigger. Problem is, what happens when clouds go by? They always effect PV output. So on certain types of cloudy days, you could see 100's of times the contactor is flipped back and forth. This will prematurely wear out the contactor. And it may fail in the "wrong" position.
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I'm guessing your looking at using a 192V UPS (16 x 12V).. that asside... skip to the end for the final answer...

Switching 200V DC under load will give you an arc about half an inch to three quarters of an inch long, so the relay will nedd to be a solid state device if you looking for a cheaper option. Don't even try and AC rated relay as it will just sit, arc and slowly set on fire. I know because when I had 10 panels (300Vmp) disconnected I had a play creating inch long arcs with the wires under full sun, yep, fun and a bit stupid at the same time..

Direct connection of the solar to you pack is ok electrically, don't forget to add a diode in so that the pack does not back feed the solar panels and destroy them.

The pack maximum charge voltage needs to be lower than the Vmp of the solar string, at minimum charge typically you only get around 75% of the solar output due to the lower voltage.

Switching the PV over to direct input to the inverter is not practical because under no or little load the inverter would then see the Voc at the terminals, which will be a lot higher than the unit can cope with. Switch the PV to the battery on/off with solid state, don't redirect. This effectively then is just a PWM style charge controller with a PWM of 1 interval per on/off...

Contact with 200V DC will give 3rd degree burns because it will burn under the skin, so whenever going near the setup, use some good rubber gloves, 2 layers so that the base layer with the sweat in has a barrier for any small holes that are not noticed.

Summary, don't do it. It's possible technically and electrically. I was an electrician for 11yrs and had a fair number of shocks, near misses and scary times and I even abandoned 72V for my first battery setup (28kWh). Learn small, grow big.
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