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4000 watt Inverter on 12 V?

The only drawback of using a larger inverter than needed is the cost. There might be a little more power consumed by the inverter at light load, but that difference should be fairly small. A good quality inverter should not be consuming more than about 20 watts when running without a load. When you get to a large low frequency inverter, that might go a little higher, and a cooling fan might pull some as well, but hopefully they use thermal control, so the fan only runs when the unit is getting warm. My cheapo runs the fan all the time, I may put in a thermal switch some day, but it really is just an emergency use unit.

Many years ago, I had a jump starter box with a 17 ah AGM battery. I ended up screwing a 300 watt inverter on the back, and used that thing to run all kinds of stuff for years. About 5 years after I built that, they finally started selling unit like that. Maybe I should have patented my idea back in 1990. When the AMG batter died, it was cheaper to get a whole new jump starter box. And the battery in the new one died in only 2 years. I think I will stuff it full of LFP cells. 4S7P of the 6,000 mah ones they have at Battery Hookup looks like they will fit and push the capacity from 17 AH to 42 AH. The surge current goes to 252 amps. That should start any car I have. And I will change out my ancient 300W modified sine inverter for a 1,000 watt true sine wave one. This is a whole lot smaller scale than you are talking about, but it sure is convenient to have a one hand carry-able power station. Since the AGM battery failed, I have missed having it on hand. Even with my battery backup inverter in the house, I still used that thing often.
 
One reason I ask is because that's what I have already. Also I feel more pressed to use it every day for my home office since Snoobler here on the forums just told me that LFP should not be used as emergency power systems because the batteries are not meant to sit at a high state of charge for long periods of time, they will die out faster. I was originally only going to use this system as a backup power system that may only be used a few times a year based on power outages. I was just going to park it in my home office and keep it on an AC charger and eventually get my 5 315 watt panels out up and hook a couple up to this little backup system. I wasn't aware LFPs are no good for long term storage... I eventually plan on a whole house power system, but that's years down the road.
 
Holding any lithium cell at it's maximum voltage for a long time will slowly reduce the cell capacity. A few days is no problem, a few weeks, might do a little, a month or more is getting bad. They are best stored close to 50% state of charge. Almost no degradation, and also very little self discharge. They can stay at 50% charge for several months without a problem. Maybe cycle them a bit every few months to make sure they stay balanced and end up close to 50% again. If you have a power failure, you have that 50% left to run on until you have the sun on the panels to charge it up again. My home backup battery is cycled every day, only holding at my full charge (about 88%) for an hour or so before the system starts feeding power to my house. It then rests at close to 50% all night (up to 9 hours) until it charges again after the sun is up.
 
GXMnow said:
Holding any lithium cell at it's maximum voltage for a long time will slowly reduce the cell capacity. A few days is no problem, a few weeks, might do a little, a month or more is getting bad. They are best stored close to 50% state of charge. Almost no degradation, and also very little self discharge. They can stay at 50% charge for several months without a problem. Maybe cycle them a bit every few months to make sure they stay balanced and end up close to 50% again. If you have a power failure, you have that 50% left to run on until you have the sun on the panels to charge it up again. My home backup battery is cycled every day, only holding at my full charge (about 88%) for an hour or so before the system starts feeding power to my house. It then rests at close to 50% all night (up to 9 hours) until it charges again after the sun is up.
Click to expand...

Ah, cool, good to know. So is your BMS set to cutoff at 50%?
 
Forbisher said:
No
BMS is set for 10V low voltage disconnect as a last line of defense against over discharging your battery.

Use another method of charging to 50%
Click to expand...

Ah, I didn't know if some of these programmable BMSs can have other cutoff states than the obvious 10V cuttoff.
 
GXMnow said:
Holding any lithium cell at it's maximum voltage for a long time will slowly reduce the cell capacity.
Click to expand...
I think some of that data is old but could still be valid. Anyways we will find out because I am floating my EVE cells at apx. 3.390 volts. I do plan to cycle them at least monthly.

My application is tied to a UPS and I can't adjust the float voltage. I read somewhere on another forum of someone basically doing the same thing and is floating his cells at 3.4 volts. He said he didn't have any appreciable loss in capacity or something like that.
 
Devin82m said:
I wonder how the cell sellers store and or charge the cells...
Click to expand...
The sellers who actually test use test jigs capable of testing many cells at a time. I don't know which sellers have them. Cells are supposed to be stored and shipped with a 50% or less SOC. From what I have seen most here have received their cells with a 40% to 50% SOC, as did I.
 
Gazoo said:
I think some of that data is old but could still be valid. Anyways we will find out because I am floating my EVE cells at apx. 3.390 volts. I do plan to cycle them at least monthly.

My application is tied to a UPS and I can't adjust the float voltage. I read somewhere on another forum of someone basically doing the same thing and is floating his cells at 3.4 volts. He said he didn't have any appreciable loss in capacity or something like that.
Click to expand...

Gazoo said:
The sellers who actually test use test jigs capable of testing many cells at a time. I don't know which sellers have them. Cells are supposed to be stored and shipped with a 50% or less SOC. From what I have seen most here have received their cells with a 40% to 50% SOC, as did I.
Click to expand...

Interesting. Yeah, that's a tough one, I don't have a ton of money to play around to figure it out. I received my at 3.8v, so fully charged from the Electric Car Part Company (Will's primary suggested Fortune cell dealer).
 
Devin82m said:
Interesting. Yeah, that's a tough one, I don't have a ton of money to play around to figure it out. I received my at 3.8v, so fully charged from the Electric Car Part Company (Will's primary suggested Fortune cell dealer).
Click to expand...
3.8V is overcharged.
3.65V is full or 14.6V for a 12.8V battery.
10V is empty.

EDIT. Hopefully you are reading the voltage incorrectly! .
 
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Devin82m said:
I received my at 3.8v, so fully charged from the Electric Car Part Company (Will's primary suggested Fortune cell dealer).
Click to expand...
That's interesting. Did they say anywhere they ship the cells fully charged? And 3.8 volts? What does the spec sheet say about the max charge voltage. I am not familiar with those cells...just know they are available.

EDIT: I saw on the website they are to be charged to 3.65 volts. They should not have been shipped with that high of a SOC and they were overcharged to boot. What are you using to measure the voltage?
 
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Gazoo said:
That's interesting. Did they say anywhere they ship the cells fully charged? And 3.8 volts? What does the spec sheet say about the max charge voltage. I am not familiar with those cells...just know they are available.

EDIT: I saw on the website they are to be charged to 3.65 volts. They should not have been shipped with that high of a SOC and they were overcharged to boot. What are you using to measure the voltage?
Click to expand...
Oh joy. I'm using my cheap Klein multimeter testing each cell individually. I haven't bothered connecting them all together since I don't have my BMS yet. They arrived Friday.
 
I would contact them and ask they why they were overcharged and shipped that way. Test the capacity when you get your BMS. They are probably fine but it's my nature to be skeptical.

Assuming your meter is reasonably accurate I would discharge the cells to a lower voltage. It doesn't take much and the voltage will drop fast.
 
Devin82m said:
Oh joy. I'm using my cheap Klein multimeter testing each cell individually. I haven't bothered connecting them all together since I don't have my BMS yet. They arrived Friday.
Click to expand...
A LFP cell or battery charged to full at 3.65V or 14.6V for four 3.2V nominal cells in series will have the voltage settle to say 3.4V or 13.6V over a few hours so how could your fortune cells be still at 3.8V after days of shipping to you and Friday?

LFP batteries are usually shipped at around 3.2V I think due to safety regulations.
 
Forbisher said:
A LFP cell or battery charged to full at 3.65V or 14.6V for four 3.2V nominal cells in series will have the voltage settle to say 3.4V or 13.6V over a few hours so how could your fortune cells be still at 3.8V after days of shipping to you and Friday?

LFP batteries are usually shipped at around 3.2V I think due to safety regulations.
Click to expand...
Good point. After I parallel top balanced my EVE cells I let them sit for a couple of days. They were all above 3.5 volts when I disconnected them to hook up in series. But still 3.8 volts doesn't sound right to me either. The cells would have to been severely overcharged to settle at 3.8 volts after a few days and I think that's unlikely. I think something else is going on here.
 
Gazoo said:
I would contact them and ask they why they were overcharged and shipped that way. Test the capacity when you get your BMS. They are probably fine but it's my nature to be skeptical.

Assuming your meter is reasonably accurate I would discharge the cells to a lower voltage. It doesn't take much and the voltage will drop fast.
Click to expand...

I looks like I was wrong, think I got the memory confused with my ham radio gear which is 13.8v. I emailed them too, so look more stupid. Nonetheless 3 tested as 3.28 and one as 3.25. I did ask them how they store them in their warehouse and why they ship at full charge instead of at the standard 50%.
 
OK, now that I figured out I don't know anything about LFP batteries, I'm going to go ahead and order an OverKill Solar 12v BMS so I can start using it, I will try to buy another pack in the coming month and a 24v BMS to make a 24v pack. I guess I will just start running it down to 50% then back up to full again for now until I setup the solar panels and start using it actively to power my home office.

So with that said, more questions.

1. Using a 1500 watt inverter, what size cables should I use for a 12V system. Then what should I switch them out for for a 24V system (if needed since I will most likely need the larger gauge for the 12V)?

2. Will my 15 Amp Victron Blue Smart Charger be acceptable for the 12v version of the system until I convert it to 24v?

3. What do you suggest I use to monitor the SOC to know when it hits 50%? I know I could use the BMS with the app, but does anyone suggest a specific easily visible meter or something I can put on it? I know Victron has their smart shunt too.

4. I would like the charge and 50% cutt-off to be automated if possible so I don't have to mess with the system. Even if that means going with something more like an all-in-one system. Suggestions are welcome.
 
Devin82m said:
So with that said, more questions.

1. Using a 1500 watt inverter, what size cables should I use for a 12V system. Then what should I switch them out for for a 24V system (if needed since I will most likely need the larger gauge for the 12V)?
Click to expand...

1500 watts / .85 conversion efficiency / 12 volts low cutoff = ~147 dc amps
~147 dc amps / .8 fuse headroom = ~184 fuse amps.
2 awg wire rated for 105C will get the job done.
But who wants their wires getting hotter than boiling water.
2/0 awg will stay under 90C.

BTW that is out of spec high for your overkill bms.
Devin82m said:
2. Will my 15 Amp Victron Blue Smart Charger be acceptable for the 12v version of the system until I convert it to 24v?
Click to expand...
How many amp hours is your pack?
Ideally you want to shoot for a charge rate of .2 to .5c.
Charging at 15amps isn't that terrible though.
The other factor is how configurable is the charger?

Devin82m said:
3. What do you suggest I use to monitor the SOC to know when it hits 50%? I know I could use the BMS with the app, but does anyone suggest a specific easily visible meter or something I can put on it? I know Victron has their smart shunt too.
Click to expand...
You overkill bms does current accounting.
If you want a discrete meter get a shunt based battery monitor.
Victron is the Caddilac, Aili is the Chevy.


Devin82m said:
4. I would like the charge and 50% cutt-off to be automated if possible so I don't have to mess with the system. Even if that means going with something more like an all-in-one system. Suggestions are welcome.
Click to expand...
You do this by setting the charge voltage based on observing the shunt and seeing what charge voltage correlates with 50% charge.
Hopefully your charger does sensible charge termination.

To figure out what 50% is you need to figure out what 0% is and what 100% is.
You do this by fully cycling the battery.
You need to top balance first before you can fully cycle your battery.
 
1) 1,500 watts on a 12 volt system is about 150 amps. How long will the wire run be? I would not go smaller than about 2/0 at that current. Windy Nation rates their 2/0 at 325 amps, but that is 105C temp rated, the NEC lists 75C 2/0 cable at 175 amps. But you also need to take the length in account. Losses at 12 volts add up fast. Lose 1.2 volt, and that is 10%.

2) I do not know that Victron unit. Are the voltage settings able to be dialed down for 12 volt? 15 amps will charge slow, as that is under 200 watts, but it will charge.

3) I am happy using the app in my JK BMS, the Overkill looks similar.

4) Setting the charge controller to 3.4 volts per cell on LFP will get you about 95% charged. The discharge side is a bit tougher. LFP cells near 50% do not change voltage much at all. Even after doing a full cycle to know your full capacity, you would need to watch the battery SOC to see when you get close to 50%. Most inverters etc. only go by voltage, and with LFP, you wont se a drop in voltage until down more like to 20%. Are you doing this for the storage of the cells? You could run a heater on a timer, once you know what your capacity is. Dial it in from the SOC on the BMS.
 
smoothJoey said:
1500 watts / .85 conversion efficiency / 12 volts low cutoff = ~147 dc amps
~147 dc amps / .8 fuse headroom = ~184 fuse amps.
2 awg wire rated for 105C will get the job done.
But who wants their wires getting hotter than boiling water.
2/0 awg will stay under 90C.

BTW that is out of spec high for your overkill bms.

How many amp hours is your pack?
Ideally you want to shoot for a charge rate of .2 to .5c.
Charging at 15amps isn't that terrible though.
The other factor is how configurable is the charger?


You overkill bms does current accounting.
If you want a discrete meter get a shunt based battery monitor.
Victron is the Caddilac, Aili is the Chevy.



You do this by setting the charge voltage based on observing the shunt and seeing what charge voltage correlates with 50% charge.
Hopefully your charger does sensible charge termination.

To figure out what 50% is you need to figure out what 0% is and what 100% is.
You do this by fully cycling the battery.
You need to top balance first before you can fully cycle your battery.
Click to expand...

Well, I won't be using the full 1500 watts to power what I need, so does the rule still apply?

My pack is 100 ah Fortune cells.

The charger was suggested by Battleborn (they sell on their site), it has an app via Bluetooth. It has a bunch of settings like: Storage, ABS, Bulk, Normal, and High. It does 4 amp or 15 amp.
 
Devin82m said:
Well, I won't be using the full 1500 watts to power what I need, so does the rule still apply?
Click to expand...
How much will you be using?
Devin82m said:
My pack is 100 ah Fortune cells.

The charger was suggested by Battleborn (they sell on their site), it has an app via Bluetooth. It has a bunch of settings like: Storage, ABS, Bulk, Normal, and High. It does 4 amp or 15 amp.
Click to expand...
Should be fine.
 
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