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Can I charge and use 18650's simultaneously or better charged first, block from solar, then use with the power inverter? Answered

Hi all,

I have been into DIY Solar / Wind for a while. Everything been working great until I planned to replace my lead acid batteries for the 3.6V LG HG2 3000mAh 18650's. With the lead acid I could power my house anytime during day (using solar panels) or night off-grid with ease. So, I bought a lot of 200 18650's and spot welded them in 4S50P. Balanced them, and all that good stuff (time consuming, and a learning experience). Installed a BMS on it too, have a 30 Amp charge controller for 4x100W solar panels which I reserved for this project. Now, I have been searching for a while if these 18650's can be used anytime (connected to solar panels and with the power inverter feeding the house at the same time) ... with no success.



1 year ago

Im looking at your diagram, and if you have it connected like that it will not work. A good quality BMS should have and input for charging, and an Output for the load. They input should switch the poweroff when full charge is reached, and the output will switch off when the voltage is below a set point to protect the battery pack. Some lead acid solar controllers will be damaged if disconnected from the battery while the solar panel is producing voltage.

Have a look at this, Ive used these BMS and they work well.




Answer 1 year ago

Sorry, I was not very specific with the BMS cabling on the diagram. Yes, my BMS does have a positive input/output (on the same cable), a negative input, and a negative output (separate). Thanks for noticing (that diagram will be edited and replaced soon). I'll take in consideration the BMS that you suggested.


Answer 1 year ago

Do you have some info on your BMS, as it looks to me to your wiring is still not right. The BMS output and input should control relays which turn the charge current and inverter on or off.


Answer 1 year ago

Sorry for the delayed answer (I'm not receiving notifications from this post). The BMS specs are the following:

4S 12V BMS 100A LiFePo4 Lithium Iron

Battery Protection Board w Balance Charging


Short circuit protection

Overcharge protection

Over-discharge protection

Overcurrent protection


Continuous discharge current: 100A

Instantaneous discharge current: 150A


Charging Voltage: 15V

Max charging current: 10A

Overcharge Protection:

Overcharge detection voltage: 3.75±0.025V

Overcharge protection delay: 0.6S

Overcharge release voltage: 3.60±0.05V

Energy Balance:

Energy balance detection voltage: 3.60V

Energy balance release voltage: 3.60V

Energy balance current: 36±5mA

Over-discharge Protection:

Over-discharge detection voltage: 2.0±0.1V

Over-discharge detection delay: 72mS

Over-discharge release voltage: 2.25±0.1V

Overcurrent Protection:

Overcurrent detection voltage: 150mV

Overcurrent detection delay: 10mS

Overcurrent Protection current: 200±10A

Overcurrent Protection release condition: Disconnect the load

Short detection delay: 280μS

Operating temperature: -30℃~80℃

... This is a copy/paste from the vendor's website ...

The 18650's are in 4S50P configuration. I have been monitoring them often for the past 3 weeks, they stay at room temperature. I have one 40" flat screen, a wall mounted fan, and a low power consuming desktop (Alienware Alpha) connected to this configuration. I use it about 3 to 6 hours a day, the maximum voltage floats from 13.72 to 14.11, and my power inverter has a low voltage shut when it reaches 10.5 (which triggered once on a cloudy/stormy day).


Answer 1 year ago

From the info you have given me and the wiring, I can see 2 problems. first, your solar panels can output 25 amps, and the BMS is rated at 10amp. So you're never going to be able to charge at full current.

Next, I would doubt that your inverter will run correctly going through the BMS. I know it's "says" it rates at 100 amps, I find that very difficult to believe.

Any sort of voltage drop will make your inverter play up, or shut down.

I would suggest you use a solid state relay to control the current from your solar panels. Use the BMS to turn the relay off and on.

I would also wire the inverter directly to the batteries to eliminate any voltage drop and use another solid state relay to turn the inverter off or on by wiring it into the inverters on/off switch. Use the BMS to control the relay also.

Now I have not used your particular BMS, so Im, not 100% sure that it will work, but that's the way I wired the last BMS I did and it worked perfectly.

Let me know how you go with it

Screen Shot 2017-07-30 at 4.31.55 pm.pngScreen Shot 2017-07-30 at 4.34.15 pm.png

Answer 1 year ago

Thanks for your response. I ordered the two Solid State Relays, once they arrive I will install them and give you an update.


Answer 1 year ago

Have to agree: Why not?
Keep three things in mind for a big project like this though:
a) You want to make sure the batteries are seperated from the load once their voltage goes too low.
b) You want to use a way of charging that suits your batteries - a standard solar charger is good for lead acid but not really good for Li-Ion.
c) Check if the controller is able to provide the main power directly to to he load and to divert the rest for charging.
Some controllers only allow to get power out through the battery.
Some can be used to feed the load directly while charging the batteries.
The later is much better as it charges the batteries without draining them at the same time.
Only if the sun does not give enough energy the controller will then substitute by draining the batteries.

C is not that important for deep cycle lead acid batteries but can help to get a much longer service life out of your Lipos.
I'll try to explain a bit more since I am bored and still have time for breakfast:
In the ideal world you only use your batteries if you really need them.
That means any power that might be needed during the day should come directly from the solar panels.
Also the batteries should get a full charge with every cycle of the sun.
Sadly reality never really matches our needs LOL
Although Lipos are not really bad when comes to incomplete charging cycles, heir power levels will go down over time.
By making sure they are fully charged whenever possible and NEVER discharged below their ratings they will get many more cycles for you.
If you can help them, they will thank you with a much longer service life.

Lipos are great fire risk once they fail.
For big packs it really pays off to provide extra safety in the form of a dedicated battery box that is fireproof.
Even cement sheeting, a metal box or something padded with a thick layer of rock wool will do.
In a big pack it is enough when one cell has a total failure.
Once on fire this cell will ignite the others and quickly you have a fire that is next to impossible to stop.
I was lucky last year with the battery for a small toy helicopter.
Only the size of a matchbox...
Was left in a drawer and while watching a movie I started to hear a hissing sound.
By the time I found the source there was already thick smoke coming out of the drawer.
Ripped it out and emptied in the garden just to find this tiny battery which right after went up in flames.
Learned the lesson and no longer get any Lipos without checking their safety circuits are not only existing but also working to shut them down on overload.


Answer 1 year ago

Nobody wants to build controllers any more...