24V system 100A and Battery wire size.

ddanley

New Member
Joined
Sep 10, 2021
Messages
34
Newbie question, sorry....

I plan on using Growatt 24V SPF 3000TL LVM - 3kW inverted and 2 12V 200AH batteries. Each battery has a 100A BMS, so .. if I connect them in series to get 24 Volts , the max supported draw would be 24V @ 100 Amps. Pleaseeeee correct me if I am wrong.

If the above is true, Would I need 2 guage wire to connect the batteries in series and from the batteries to the inverter?

Thank you!

The Rookie
 

DThames

Photon Sorcerer
Joined
Nov 22, 2019
Messages
1,424
Newbie question, sorry....

I plan on using Growatt 24V SPF 3000TL LVM - 3kW inverted and 2 12V 200AH batteries. Each battery has a 100A BMS, so .. if I connect them in series to get 24 Volts , the max supported draw would be 24V @ 100 Amps. Pleaseeeee correct me if I am wrong.

If the above is true, Would I need 2 guage wire to connect the batteries in series and from the batteries to the inverter?

Thank you!

The Rookie
You are correct about the amps. I used #2 on my similar setup.
 

chrisski

Photon Sorcerer
Joined
Aug 14, 2020
Messages
2,351
Yes that will do, but a couple of things to look at.

-What is the surge rating of the BMS? That can up that amperage for whatever time it allows the surge for.

-Do you really plan on using 100 amps? If not you can make the wiring size smaller and fuse accordingly.

-Is voltage loss a factor? I doubt it, but a longer run may cause voltage loss issues.
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
1,211
3000W / 24V / 0.85 efficiency factor is 150A, not 100A. I’d wire for 150A. Fuse with 190A-200A. 2AWG 1AWG wire should work.

But if your battery BMS only allows 100A continuous discharge then you can’t pull the full 3000W. You might want to fuse lower. Depends on whether you want a fuse to pop or the batteries to over discharge and the BMS to shutdown.

Edit: Should be 1AWG, not 2AWG for 150A
 
Last edited:

chrisski

Photon Sorcerer
Joined
Aug 14, 2020
Messages
2,351
Totally missed the 3000 watt inverter. With that, expect the BMS to shut off before the inverter can pull its max load.
 

ddanley

New Member
Joined
Sep 10, 2021
Messages
34
3000W / 24V / 0.85 efficiency factor is 150A, not 100A. I’d wire for 150A. Fuse with 190A-200A. 2AWG wire should work.

But if your battery BMS only allows 100A continuous discharge then you can’t pull the full 3000W. You might want to fuse lower. Depends on whether you want a fuse to pop or the batteries to over discharge and the BMS to shutdown.

Is it bad to let the BMS shut down as opposed to blowing a fuse? I don't expect to pull this kind of load, but just in case.....
 

Diysolar123

Solar Addict
Joined
Feb 28, 2021
Messages
642
Is it bad to let the BMS shut down as opposed to blowing a fuse? I don't expect to pull this kind of load, but just in case.....
the bms is trying to protect the battery, the fuse is trying to protect the wires(well, and everything else) hehe
I would probably probably prefer in the event of a short/high current drain event to have my fuse blow; its a cheap component to keep a spare of, it is easy to diagnose when it fails, and it does a great job making sure you know there was a fault and before you put in another fuse you should make sure you have figured out what was wrong ;)
 

rickst29

Solar Addict
Joined
Mar 27, 2021
Messages
336
Location
Reno, NV
Don't forget to also add a switched "pre-charge" line, with a moderate-sized resistor, to handle the initial "rush" of current to charge the Inverter's capacitors. Without such a pre-charge circuit, you will probably blow your fuse or exceed your BMS limit.

LFP batteries have extremely low resistance, and the pair of BMS units doesn't add much either. A 3000 watt inverter can pull MUCH more than 150A (maybe as much as 400A?) for a very short period of time, right when you turn it on.

The resistance need not be very high (25 or ohms should be fine). But it does need to have considerable power handling. I use a 50W, 25 ohm wire-wound resistor, which was built into a small heat-dissipating aluminum case. (They cost less than $5). I also have a high-capacity dual battery switch (#1 - Both - #2) to activate the small-wired "pre-charge" circuit first (it's only AWG-10 fused for 30A), for just a second or two. Then I switch through the "Both" switch position, in order to reach the main-line-alone "Battery #2" position before turning on my 120v equipment.

In general, you don't want EITHER blown fuses or BMS over-current invoked and shutting down. The more likely cause of over-current in your configuration is the capacitor pre-charge and current rush-in - not your 120v load. There are several posts and even a video on this subject - search is your friend, if you need more information.
 

rickst29

Solar Addict
Joined
Mar 27, 2021
Messages
336
Location
Reno, NV
Is it bad to let the BMS shut down as opposed to blowing a fuse? I don't expect to pull this kind of load, but just in case.....
If the "high-current" is so extreme that it fries the BMS internal switch, before the internal detector can open the switch, then it's "new BMS" time. And even for the case of a recoverable event - it is typically more of a hassle to make a BMS 'restart', than it is to replace a fuse. But you should design to avoid both problems.
 

ddanley

New Member
Joined
Sep 10, 2021
Messages
34
Don't forget to also add a switched "pre-charge" line, with a moderate-sized resistor, to handle the initial "rush" of current to charge the Inverter's capacitors. Without such a pre-charge circuit, you will probably blow your fuse or exceed your BMS limit.

LFP batteries have extremely low resistance, and the pair of BMS units doesn't add much either. A 3000 watt inverter can pull MUCH more than 150A (maybe as much as 400A?) for a very short period of time, right when you turn it on.

The resistance need not be very high (25 or ohms should be fine). But it does need to have considerable power handling. I use a 50W, 25 ohm wire-wound resistor, which was built into a small heat-dissipating aluminum case. (They cost less than $5). I also have a high-capacity dual battery switch (#1 - Both - #2) to activate the small-wired "pre-charge" circuit first (it's only AWG-10 fused for 30A), for just a second or two. Then I switch through the "Both" switch position, in order to reach the main-line-alone "Battery #2" position before turning on my 120v equipment.

In general, you don't want EITHER blown fuses or BMS over-current invoked and shutting down. The more likely cause of over-current in your configuration is the capacitor pre-charge and current rush-in - not your 120v load. There are several posts and even a video on this subject - search is your friend, if you need more information.

This post helps me a ton and probably saved me some heartache in my near future.

I now plan on doing what you stated up above. a 1-2 switch where 1 is a pre-charge and 2 is battery connect. I assume I would need to pre-charge each time the batteries are disconnected. Right?

Again... Thanks for the write up!
 

smoothJoey

mumble...
Joined
Nov 30, 2019
Messages
9,278
the bms is trying to protect the battery, the fuse is trying to protect the wires(well, and everything else) hehe
I would probably probably prefer in the event of a short/high current drain event to have my fuse blow; its a cheap component to keep a spare of, it is easy to diagnose when it fails, and it does a great job making sure you know there was a fault and before you put in another fuse you should make sure you have figured out what was wrong ;)
I would add that the all_in_one should protect the bms.
It would do that by having its low voltage disconnect set higher than the bms low voltage disconnect.
 

rickst29

Solar Addict
Joined
Mar 27, 2021
Messages
336
Location
Reno, NV
This post helps me a ton and probably saved me some heartache in my near future.

I now plan on doing what you stated up above. a 1-2 switch where 1 is a pre-charge and 2 is battery connect. I assume I would need to pre-charge each time the batteries are disconnected. Right?

Again... Thanks for the write up!
Pre-charge each time you turn on the Inverter.
 

ddanley

New Member
Joined
Sep 10, 2021
Messages
34
Pre-charge each time you turn on the Inverter.

I read further into the battery specifications and the max discharge rate is 280A for 5 seconds with 100A continuous. Would I still need to pre-charge the capacitors given this scenario?
 

DThames

Photon Sorcerer
Joined
Nov 22, 2019
Messages
1,424
Pre-charge each time you turn on the Inverter.
"Pre-charge" is done before you connect the battery to the inverter. Turning the inverter off and then on, while connected to the battery, requires nothing other than turning the inverter off or on.
 

rickst29

Solar Addict
Joined
Mar 27, 2021
Messages
336
Location
Reno, NV
If the BMS on each battery allows for a 280A short-term discharge (each), then you may likely be able to be able to get by without a pre-charge circuit.

But as far as I know, (DThames post), few if any Inverters will try to maintain high voltage on their capacitors (and consume power) while turned "off" at the main 12v input switch.

You should connect the leads to the Inverter while it is powered off (and no sparks should result!). Then, when you power on the inverter - and before turning on any 120v appliances "downstream" - it will suck in this current, and possibly light up a little display or indicator light. The in-rush has already occurred.

When you turn it off, the capacitor charge slowly dissipates. I don't know of any capacitors which hold their voltage indefinitely when power is "off", and I don't know of any Inverters with "fake" power buttons (power buttons which still leave capacitors connected and drawing power, even when the power button is switched off). UL and others, including me, would probably have a fit if they saw one.
 

DThames

Photon Sorcerer
Joined
Nov 22, 2019
Messages
1,424
If the BMS on each battery allows for a 280A short-term discharge (each), then you may likely be able to be able to get by without a pre-charge circuit.

But as far as I know, (DThames post), few if any Inverters will try to maintain high voltage on their capacitors (and consume power) while turned "off" at the main 12v input switch.

You should connect the leads to the Inverter while it is powered off (and no sparks should result!). Then, when you power on the inverter - and before turning on any 120v appliances "downstream" - it will suck in this current, and possibly light up a little display or indicator light. The in-rush has already occurred.

When you turn it off, the capacitor charge slowly dissipates. I don't know of any capacitors which hold their voltage indefinitely when power is "off", and I don't know of any Inverters with "fake" power buttons (power buttons which still leave capacitors connected and drawing power, even when the power button is switched off). UL and others, including me, would probably have a fit if they saw one.
Sorry but "You should connect the leads to the Inverter while it is powered off (and no sparks should result!)." is not correct at all. The power switch does not switch the high current feeding in from the battery. The DC from the battery charges up some large capacitors to the battery voltage. The output from those capacitors is then chopped and fed into step up stage to get your high voltage DC that is used to create the output AC waveform (in HF inverters). You are correct that the step up stage is off and the high voltage caps are likely bleed off over time. There is normally a soft start built inside to allow the step up stage to start up even if there is a load on the output. The pre-charge is to prevent huge current rush to charge the input side of the inverter when the battery is connected. Get a new large inverter (out of the box) and connect it to a hot battery and you will get really big spark and likely knock a divot out of your lug or terminal. Pre-charge is to charge the input. Once charged, as long as you don't disconnect the battery, or stop the BMS output (I did that) the input side of the inverter will be okay.
 

ddanley

New Member
Joined
Sep 10, 2021
Messages
34
Thanks the feedback!

This is my first electrical project so I am being very cautious. I put together a diagram of what I was thinking. I will add a second string of solar panels, but that will be step 2.

The dotted lines represent what I am not sure I need, but sure doesn't cost much at all.

Any inputs are greatly appreciated.
 

Attachments

  • SolarProject_Take1.jpg
    SolarProject_Take1.jpg
    243.4 KB · Views: 13

DThames

Photon Sorcerer
Joined
Nov 22, 2019
Messages
1,424
Thanks the feedback!

This is my first electrical project so I am being very cautious. I put together a diagram of what I was thinking. I will add a second string of solar panels, but that will be step 2.

The dotted lines represent what I am not sure I need, but sure doesn't cost much at all.

Any inputs are greatly appreciated.
Looks good.

You might ask yourself, "How often will I disconnect my battery from my inverter?" I got mine hooked up and have not disconnect it yet. I take that back, I disassembled everything and painted the garage floor, then put it all back. When I connect my battery to the inverter, I connect one side, then before connecting the second terminal (at the battery because connecting at the inverter is a tight fit) I put a resister in series for about 3 seconds...just hold it there with my hand, leads touching (24v battery, maybe insulate with higher voltage), and then connect the second lead. Do this with the inverter off so the inverter will not drain your pre-charge. I see no reason in a hard wired setup to have a pre-charge built in circuit when it will be used almost never. If I have a system where I might swap out one battery while a second battery is charging at another location, or when expecting to do battery experiments, then a pre-charge circuit would be worth the effort. My 2 cents.
 

rickst29

Solar Addict
Joined
Mar 27, 2021
Messages
336
Location
Reno, NV
Sorry but "You should connect the leads to the Inverter while it is powered off (and no sparks should result!)." is not correct at all. The power switch does not switch the high current feeding in from the battery. The DC from the battery charges up some large capacitors to the battery voltage. The output from those capacitors is then chopped and fed into step up stage to get your high voltage DC that is used to create the output AC waveform (in HF inverters). You are correct that the step up stage is off and the high voltage caps are likely bleed off over time. There is normally a soft start built inside to allow the step up stage to start up even if there is a load on the output. The pre-charge is to prevent huge current rush to charge the input side of the inverter when the battery is connected. Get a new large inverter (out of the box) and connect it to a hot battery and you will get really big spark and likely knock a divot out of your lug or terminal. Pre-charge is to charge the input. Once charged, as long as you don't disconnect the battery, or stop the BMS output (I did that) the input side of the inverter will be okay.
Thanks, I didn't know they actually work that way!
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
1,211
3000W / 24V / 0.85 efficiency factor is 150A, not 100A. I’d wire for 150A. Fuse with 190A-200A. 2AWG wire should work.
I just realized I mistakenly suggested 2AWG here for 150A loads. That should be 1AWG for 150A loads. Sorry about that.
 
Top