high current / starting battery use for offgrid application?

[JohnnyBravo]

Hi,
I have 12V 6000W inverter and 3x 12V LiFePO4 different capacity / brand batteries with their max surge current is 100A or 150A for some seconds (based on their specification).

I have 1000W vacuum cleaner that seems it is using about 2200-2600W initial starting spike therefore none of my batteries will support it, the BMS will detect the overload and cut the power.

Questions are:
1. should I buy a 12V high current / starting battery (still LiFePO4) for such application? besides its high current support, what other features/disadvantage I would have using it?
2. do you have other solution for my case rather than buying a 24V inverter and another same type 12V battery?

thank you

 

[MichaelK]

A 6000W 12V inverter is really a fraudulent product. I would get your money back. I can tell you right now that whatever brand it is, it is NOT CE/UL listed. That inverter can NOT make 6000W. Most likely it is a high-frequency tranformerless model coming out of China. It's real surge capacity is likely to be 8-16 milliseconds. The vacuum needs a surge of at least 500 milliseconds to start.

Sorry, but what you want is a low-frequency transformer-based model with a surge capacity in the >5 second range. I have a Schneider SW4024 that can run a 1.6hp air compressor. A vacuum is easy.

I would say your best option is your #2 suggestion. Buy an additional matching battery, and wire them 2S2P for 24V, and get a quality inverter like my SW4024. Before you buy the battery though, make sure that the manufacturer rates it for serial applications.

Also, it is never a good idea to increase battery capacity without also increasing solar charging capacity. I'd say that at 24V, you want at least 1000W of solar. For my own system, I have 2000W.

 

[Supervstech]

我建议升级到更高电流的磷酸铁锂电池组，这样即使在大电流放电时也能保持稳定的端电压，更安全。

TRANSLATION… I suggest upgrading to a higher current lithium iron phosphate battery pack, which can maintain a stable terminal voltage even when discharging with a high current, which is safer.

 

[JohnnyBravo]

A 6000W 12V inverter is really a fraudulent product. I would get your money back. I can tell you right now that whatever brand it is, it is NOT CE/UL listed. That inverter can NOT make 6000W. Most likely it is a high-frequency tranformerless model coming out of China. It's real surge capacity is likely to be 8-16 milliseconds. The vacuum needs a surge of at least 500 milliseconds to start.

it's rated as 3300W continuous load, 6000W surge.

 

[GXMnow]

3,300 watts on a 12 volt inverter means it needs to pull 300 amps!

A 6,000 watt surge would be close to 600 amps!

I have a real Shop Vac here. Once at speed, it draws about 8 amps at 120 volts. About 960 watts. Is yours similar? I'll bet it has a series wound brush motor like mine. Those look almost like a dead short until they get spinning pretty fast. The surge on my Shop Vac hits about 4,000 watts.

If you parallel all 3 of your batteries, it MIGHT start the vacuum. In most cases, if all of the batteries are LFP 4 cell units, you should be able to safely parallel them. You need to ensure all 3 are charged up to close to the same state. Charging them all to full, using the same charge, would be the best. Then each battery should have it's own 125 amp fuse, and maybe a shut off switch, then to a bus bar. The batteries do not need to be the same capacity. They should share the current based on their true capacity and internal resistance. If one of them pops a 125 amp fuse, you should check all your connections and measure the current on each battery with a DC clamp meter to make sure they are sharing the current. A fuse can take double its trip current for a second or so. In my system, I am now running 4 strings in parallel and they all balance within about 5% on current. The 2 newer strings pull a little more, the two older strings pull a little less.

If you really need 3,000 watts, you should go to 24 volts. If your running power is about 1,500 watts or less, and you just need some surge coverage, then you should be okay.

My basic rule of thumb is to keep the required power limit down to about 100 amps. So 1,000 watts on a 12 volt system, 2,000 watts on a 24 volt system, and 4,000 watts on a 48 volt system. Obviously you can go a bit higher for some time and especially for surges, but for the basic running load, try to stay under those numbers and you will find it works well.

 

[JohnnyBravo]

In concrete I have 2 Liontron batteries that can support 100A and 150A surge and 1 Victron that can support 50A and 100A surge.
Connecting these in parallel why they would trick the BMS from each to not cut the load?

 

[MichaelK]

it's rated as 3300W continuous load, 6000W surge.

A 3300W 12V inverter is really a fraudulent product. I would get your money back. I can tell you right now that whatever brand it is, it is NOT CE/UL listed. That inverter can NOT make 3300W. Most likely it is a high-frequency tranformerless model coming out of China. It's real surge capacity is likely to be 8-16 milliseconds. The vacuum needs a surge of at least 500 milliseconds to start.

Is that better?

Buy a real inverter like I did. Mine (XW+6848) can run the well-pump, a cement mixer, and 7.5" saw, all at the same time. No hick-ups. Even my 24V SW4024 runs the mixer, and a 1.6hp air compressor at the same time. You've hobbled yourself by sticking with 12V, and also by buying a fraudulent product that can't do what they say it can do. But, they already have your money, right?

 

[GXMnow]

In concrete I have 2 Liontron batteries that can support 100A and 150A surge and 1 Victron that can support 50A and 100A surge.
Connecting these in parallel why they would trick the BMS from each to not cut the load?

It's not really tricking anything. The batteries will share the current. If you have a surge of 200 amps, any one of them will shut down. But when you connect all three in parallel, the Liontrons will each only need to supply 75 amps, and the Victron should supply another 50 amps to give you the 200 amps out. That puts all of the batteries at only half of their surge power rating. In the real world the balance might not be that perfect, but you do have a bit of overhead left at each battery to handle some imbalance. Lithium batteries tend to work vey well in parallel as long as they are the same chemistry. Make sure they are all LFP (Lithium Iron Phosphate) and you should be all set. Just be sure they are all charged to the same voltage before connecting them together, AND USE FUSES on each battery. LFP cells can dump huge currents and we don't want anyone getting hurt.

 

[Tomthumb62]

An inverter with a large (heavy) low frequency transformer is an excellent investment if you wish to run devices needing a high surge startup.

I can run my full size Dyson vacuum on a 1000W Victron inverter (heavy low frequency transformer). Vacuum surge at 2400W for about 2 seconds, then settles to 1800W. Try doing that with a cheap 1500W transformer-less inverter.

 

[Hedges]

Midnight Rosie (high frequency 48V) will also start some pretty decent loads.
Surge capability 15kVA or so.

Yes, automotive starting batteries should work great for surge current.
Doesn't mean that paralleling them with lithium works well, but people do parallel.
Also not good for much discharge. But if you have plenty of PV coming in, automotive batteries would take care of surge. Just don't drain deeply.

 

[efficientPV]

I know perfectly well that any solution here isn't acceptable unless it costs thousands, but this is what I did. I was able to run a vacuum cleaner or miter saw from a 400W Onan generator with this and not stall it which it would do without. I took an old 12V UPS and wired it as an auto transformer. This boosts current and lowers the voltage. The two rows of sockets offer different voltages depending on need. A handy little box to have.

 

[Supervstech]

I

I know perfectly well that any solution here isn't acceptable unless it costs thousands, but this is what I did. I was able to run a vacuum cleaner or miter saw from a 400W Onan generator with this and not stall it which it would do without. I took an old 12V UPS and wired it as an auto transformer. This boosts current and lowers the voltage. The two rows of sockets offer different voltages depending on need. A handy little box to have.View attachment 164005View attachment 164006

interesting. Can you post what you built? Wiring, parts list etc? I should move this part of the thread to the danger zone forum I’d think…

 

[Hedges]

240V step-down to 120V to get more current for motors is hardly danger zone.

Derived neutral from auto-transformer might be, depending on how it and ground get wired. Probably easy enough to make safe, unlike with grid connection.

 

[JohnnyBravo]

A 3300W 12V inverter is really a fraudulent product. I would get your money back. I can tell you right now that whatever brand it is, it is NOT CE/UL listed. That inverter can NOT make 3300W. Most likely it is a high-frequency tranformerless model coming out of China. It's real surge capacity is likely to be 8-16 milliseconds. The vacuum needs a surge of at least 500 milliseconds to start.

Is that better?

Buy a real inverter like I did. Mine (XW+6848) can run the well-pump, a cement mixer, and 7.5" saw, all at the same time. No hick-ups. Even my 24V SW4024 runs the mixer, and a 1.6hp air compressor at the same time. You've hobbled yourself by sticking with 12V, and also by buying a fraudulent product that can't do what they say it can do. But, they already have your money, right?

I am using this one:

Its price was about $350.
My 12.8V / 80Ah LiFePO4 Liontron (100A, 150A surge) was not able to run the 1000W (2600W surge) vacuum cleaner, the BMS was cutting the discharge, not the fact that the inverter couldn't sustain it. I can share several videos with that inverter in action with different appliances but that's not the subject of this thread.

 

[efficientPV]

Just noticed OP is in Rumania so this would be 220+ volts. A single UPS with 12V battery would not provide enough drop, two would. 500W 240 to 48V transformer would work, dual 120V or 240V input with dual 24V output is common.

 

[Hedges]

Universal brush-type motors like in a vacuum cleaner will start and run at reduced voltage, lower current. Step-down with a transformer to get started, then switch to full voltage, should work.

A variable speed control (same as light dimmer) should also work. Most dimmers are for lower current. Some tools may have suitable speed controls, but I think those are also lower amperage, and my higher current tools are just on/off.

Possibly a diode to block half the AC phases would work, possibly not. It would make motor run slower, but still have same inrush every other phase. Depends on whether BMS holds of for 8 ~ 10 milliseconds.

A resistor in line should work, like a heater. There are NTC thermisters, e.g. from Antherm, that start out high resistance to limit current, then drop in resistance when hot. They get HOT, so needed to be mounted such that they don't burn anything. Maybe in an extension cord or power strip, with wire cage for airflow.


Would like a BMS that can handle the higher current for a couple seconds.

 

[JohnnyBravo]

With cold motor I managed to see for 1s or less the 2400-2600W surge running it from my EcoFlow Delta 1300. After running it for ~10s, I tried again to take a picture of the starting power, the consumption was at around 1200W then it was more less constant at 900W. Is this normal that cold motors will surge much more current than a warm one?

 

[Hedges]

Might loosen grease on bearings.
Not surprising things get stuck.