Can I draw 12 V form a 14,8 V power wall ?

[Lassescout]

Hello guys

I am new to Electronics, but I have been following Wills videos for a while, and also other sources of information. I bougt a small hunting cabin 6 months ago. It has a small 12 V solar panel and 3 lead acid car batteries w. 12 V in parallel. As you can imagine that is not optimal, but I am not allowed to set up more panels, so I have to optimise on that setup.

I am harvesting used 18650 cells from laptop computers and eBike batteries. I now have 200 cells and am aiming for 320. I plan to make a small powerwall with 4S-80P for my hunting cabin. I am going to buy a good charge controller and set it to charge the power wall up to 16,0 Volts. That is 4,0 Volts for each group. Then i plan to discharge down to 3,0 volts for each group which will give med 12 volts out at that point. I assume that the powerwall will give 14,8 volts most of the time, as the cells will giver 3,7 V each most of the time.

I mainly use the electrcity to give me light in the cabin, but there is also a small 12 V water pump that gives med running water from a small container. I find i practical to use 12 V camping gear, and I am a bit concerned that the place might go up in flames if I connect the powerwall directly with up to 16 V. So I thougt I could Mount at DC-DC converter to give me 12 Volts on the load no matter the voltage on the powerwall. I will at no time use more than 25 Amps at a time, so a DC-DC converter that can handle 30 Amps should Work fine.

Is this setup a good idea, or am I missing something here ? Of course I know I would be better off with LiFePo4 cells, but I have no acces to that chemistry for the time beeing.

Regards Lassescout from Denmark.

 

[Stupid Monkey]

"12v" is actually 12.5-14.5v for most applications.

Hence why Will always creams himself at a regulated 13.8v "12v" socket on a solar generator box, as 12.000v is pretty useless / under voltage for most things.

 

[DThames]

I have done a little testing in this area. First question, are you planning to use an inverter? If yes on the inverter, if you go to 24v inverter and 7s on the batteries, the voltage charge cutoff and inverter low voltage are a better match than 3s or 4s for 12v. 115v LED lights give good lumens per watt and I think do better than 12v LEDs but that is more of an opinion than a fact.

If you want to stay with 12v 3s is not that bad. If you want to use 4s with an inverter you need to find the highest volts that the inverter will run at and then stop the battery charge just under that. I have an inverter that will work if I charge to just under 16.0 volts. Again 7s works out better. I recently tested a very cheap 12v inverter that ran on a 3s pack all the way down to 3.25 volts per cell. Discharging below 3.2-3.3 is not ideal for battery life, so I would encourage you to look at the data sheets before you decide 3.0 would be your discharge target.

You might want to build some small packs and do some testing with 3s or 4s. I am working on a similar project for a hunting shack.

 

[Lassescout]

First I live i Denmark, so 110-115 V is not my first choise. We use 220 V AC on the grid here. I do have a 12V DC to 220 V AC inverter of 300 W I can use, but I would prefer to run most of the cabin on 12 V gear, because thats whats in there to start with. I will do some test runs with 3S and 4S but the main question remains: can I regulate the voltage throug a DC-DC converter to get somthing suitable 12 V-13,8 V on the load, or am i stuck with the output from the power wall?

P.S. I was a bit Quick on the 4 to 3 V gap. Of course I will get that absolutely right and safe before I connect the wires.

 

[nibs]

Lived on a sailboat for 8 yrs and in a motor coach for about 18 more.
Since RV gear in the US pretty much all runs on anything up to 16V with no problems, your 12V nominal water pump will like running at 16V, and your lighting wont mind either.
The only concerns I ever had was with overvolting circuit boards in things like fridges and radio's but never had any problems, even when running with no controller on the sailboat or on the RV until 2010.
Cheers.

 

[KBWaldron]

Anything that is voltage sensitive will more than likely include a linear voltage regulator so there should be no issue with a 16 volt battery.

 

[SolarRat]

You should be fine at 16v, but there's no harm in running a regulator if you can find a low draw one. The problem is finding one that can work with that little overhead. It's easy to find a >18v to 13.8 buck regulator, but 16v-13.8v is more difficult. Most that can do that will be a buck-boost device rather than a just a simple buck converter. This is the closest thing I see with a quick search:

Amazon.com: Stayhome 1PCS DC 12V 24V to DC 13.8V 25A 345W Voltage Regulator 8-36VDC to 13.8VDC Volt Stabilizer Buck Boost Automatic Step up / Step Down : Electronics

Buy Stayhome 1PCS DC 12V 24V to DC 13.8V 25A 345W Voltage Regulator 8-36VDC to 13.8VDC Volt Stabilizer Buck Boost Automatic Step up / Step Down: Connectors & Adapters - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

If you went up to a 7s setup you could run something simpler, like this:

https://www.amazon.com/uxcell-Converter-Regulator-Transformer-Waterproof/dp/B01LY8D7U0/ref=sr_1_6?crid=LP84IZ4CY8G3&keywords=13.8v%2Bregulator&qid=1577978276&sprefix=13.8v%2Bre%2Clawngarden%2C189&sr=8-6&th=1

Either way, you'll want to check the efficiency of the device and decide if it's worth it over just running a 3s or 4s pack straight to your loads.

 

[DThames]

You will need to shop specifically for a charge controller with adjustable voltages if you are going to charge a 4s Li-ion pack.

 

[DThames]

Another question for you, how many cells in parallel do you plan to use? Because of balance concerns I am planning to do many cells in parallel and then series them. Basically for 3s I might have 3 large blocks of cells and then connect them in series with an active balance device attached. I am curious if you have given that any consideration yet.

 

[Lassescout]

I am aiming for groups of 80 cells in parallel and then connecting 4 groups in series. But maybe the voltage I get is a bit off by todays standards, where 12 or 24 volts seem to be more down the middle of the road. So maybe I should choose 40 or 60 cells in parrallel and then connect 7 groups in series. That would give me a straight 24 Volt battery, and it will be easy to find 24 V solarpanel, charge controller and step down DC-DC converter to get 12 V on the load.

I think you Guys are right in that that is a better way to go when i am working with Li-ion chemistry and not LiFePo4.

Of course no matter what I do I will buy a good BMS for balance and safety. I don´t want to see the cabin burning down.

Thank you very much for your advice. It is much appreciated.

 

[DThames]

If you are thinking 24v but want your DC lighting you can put two 12v lights in series and run them directly off of the 24v battery. Then use a 24v to 12v DC-DC converter if you really need 12v for things like the pump.

I purchased a cheap spot welder to build packs with. I have also built some with solder but it is a pain and sort of slow. I am curious on your connection plan in your cell packs.

 

[Lassescout]

My original plan is to solder the connections. I have a Weller soldering station that should be perfect for the job. But I am a lazy guy, so I´ve been considering spot welding too. I have fuse wire so that I can solder the connections from cell to bus-bar with 5 amp fuse wire, which I think is the safest way to connect the cells. But I know of a guy that builds and repairs batteries for others. I might be able to persuade him to spot weld the cells together in the configuration I choose. My concern is, that if I choose spot welding, there won´t be any fuse to each individual cell. But then again, all the eBike batteries I take apart are spot welded, so it can´t be that much of a hazard.

 

[DThames]

I am thinking about using this fuse link strips.

 

[Lassescout]

You will need to shop specifically for a charge controller with adjustable voltages if you are going to charge a 4s Li-ion pack.

Oh yes of course. And I think that the charger can be of a fairly good quality, its not here I am going cheap. Can anyone recommend a fairly good solar charger that woul do the job with Li-Ion batteries to be kept between 13,5 and 16 Volts ? (Probably most of the time I will only charge them to around 15 V for longevity).