An issue with "big" Inverters and LFP batteries. Pre-charge!

[rickst29]

An interesting new hassle occurs between an LFP battery string and a "big Inverter" (1500 continuous watts are higher).

When 120v Inverters are connected to 12v batteries and turned on, they want to "charge up" some big internal capacitors. With a lead-acid battery pack, internal resistance of the batteries themselves will prevent this "pre-charge" current from becoming too high.

But LFP batteries have almost no internal resistance. High current will occur. The connecting DC power leads, if sized for the maximum continuous load, will not be harmed by momentary current mayn times higher than their ampacity ratings - but the high current may either blow a fuse or cause the battery "BMS" current protection circuit to shut down discharge completely. If the BMS shuts down the discharge circuit, the Inverter can't even complete startup.
- - -
This should be prevented by using a 2-way battery selector, the "dial" type which selects from 2 battery power sources. (The 4 dial positions are "off", "battery-1-only" "both", and "battery-2-only".) They have 3 connectors, one for each of the 2 batteries, and the third for the "load" port.

But, in this application, your "battery-2" wire goes to the common bus (12v) for all of your batteries together. That's the huge load-carrying wire. The "battery-1 wire", which need only be AWG-10, goes from the common bus and through a 25-ohm, 100 watt resistor before reaching the selector switch.

Before turning on the Inverter, you switch the "battery selector" from "OFF" to "Battery-1 Only" for several seconds. This allows the Inverter to charge up those capacitors through a high-resistance circuit, slowly and smoothly. Then switch the selector switch through "BOTH", wait another second, and then switch it to "Battery-2-only". This second switch position prevents the Inverter from pulling 12v power through the resistor circuit and wasting energy.

If you have an automatic transfer switch and the Trailer is NOT plugged in on the main power cord, you may then Plug in your 120-VAC appliance(s), and use them. Or you may use your appliances directly on the inverter's own power sockets. Either way, when you are done and want to turn off the Inverter, turn off the Inverter first, and then turn the "battery selector switch" to the OFF position.

[rich2468]

Originally Posted by rickst29

An interesting new hassle occurs between an LFP battery string and a "big Inverter" (1500 continuous watts are higher).



When 120v Inverters are connected to 12v batteries and turned on, they want to "charge up" some big internal capacitors. With a lead-acid battery pack, internal resistance of the batteries themselves will prevent this "pre-charge" current from becoming too high.



But LFP batteries have almost no internal resistance. High current will occur. The connecting DC power leads, if sized for the maximum continuous load, will not be harmed by momentary current mayn times higher than their ampacity ratings - but the high current may either blow a fuse or cause the battery "BMS" current protection circuit to shut down discharge completely. If the BMS shuts down the discharge circuit, the Inverter can't even complete startup.

- - -

This should be prevented by using a 2-way battery selector, the "dial" type which selects from 2 battery power sources. (The 4 dial positions are "off", "battery-1-only" "both", and "battery-2-only".) They have 3 connectors, one for each of the 2 batteries, and the third for the "load" port.



But, in this application, your "battery-2" wire goes to the common bus (12v) for all of your batteries together. That's the huge load-carrying wire. The "battery-1 wire", which need only be AWG-10, goes from the common bus and through a 25-ohm, 100 watt resistor before reaching the selector switch.



Before turning on the Inverter, you switch the "battery selector" from "OFF" to "Battery-1 Only" for several seconds. This allows the Inverter to charge up those capacitors through a high-resistance circuit, slowly and smoothly. Then switch the selector switch through "BOTH", wait another second, and then switch it to "Battery-2-only". This second switch position prevents the Inverter from pulling 12v power through the resistor circuit and wasting energy.



If you have an automatic transfer switch and the Trailer is NOT plugged in on the main power cord, you may then Plug in your 120-VAC appliance(s), and use them. Or you may use your appliances directly on the inverter's own power sockets. Either way, when you are done and want to turn off the Inverter, turn off the Inverter first, and then turn the "battery selector switch" to the OFF position.

Rick- thanks for sharing.

I have a 3000 Watt pure sine wave Renogy Inverter powered by at Lithium bank and an auto switch setup to automatically cut my shore power connection when the Inverter kicks in to power my 120 volt needs in the camper.

I've always ensured that I have all 120 volt loads off during the switch, but i`ll need to investigate the need for pre-charge (as well).

[jagizzi]

Thanks Rick, always something to think about ;-)

What is the purpose of the "Both" position in this scenario? Would you not be able to use simply a battery selector switch that does not allow both? A 3 position switch off, 1, 2.

I don't have an inverter in our current plan but it may be added in the future and this would have to be a consideration.

Thanks again.

Jim

[rickst29]

Hi, Rich!

Depending on the size of your LFP battery bank, and how the individual BMS units are tuned, they might not perform that 'temporarily fatal' discharge cut-off while pre-charge happens. It also varies with the Inverter: Some will re-charge the capacitors with each power cycle, while some others pre-charge only once (upon connection to 12v) and pull a small amount of current all the time, to maintain that charge - even when powered off.

If your 12v supply cables are small (less than 4/0, or equivalent "doubled" 2/0 wires on each "12v" and "- ground") the wires themselves become a big factor in suppressing the maximum inrush current. Over at the DIYSolar forum, one member has posted a couple of graphs for current versus time on two different Inverters with "Big" wires. He estimated the maximum inrush demand for the Inverter with "bigger" super-fast capacitors to be almost 20,000 Amps - limited only by supply wires, wire resistance inside the Inverter, and the batteries themselves. With my own "big inverter", I'll be using doubled 2/0 supply wires. The length of the supply wires is also a huge factor.
- - -
My own BMS units are programmed for 250A, 120A, and 100A respectively (as "continuous" current). I don't know the allowed "peak" current of these Daly units, but Daly units have a sort of bad reputation: They "often"
allow the discharge circuit to become "blown" and dead before they successfully detect an excess instantaneous discharge current peak and shut down. Over at DIYSolar, there are many reports of "fried" Daly units, and there's an almost universal preference for JBD/Overkill units. (When I learned of this, I had already purchased by Dalys).

If the Inverter recharges internal capacitors with each "power-on" cycle and the supply wiring is big, the creation of instantaneous high current multiple times certainly causes wear-and-tear on the BMS discharge circuit components, hurting their lifespan. In two cases over at DIYSolar, Daly units handled the startups OK, but failed after a weeks and a few months respectively.
- - -
I don't know the nature of battery bank, and whether your batteries are homebuilt (having somewhat accessible BMS) versus "factory sealed" (often being a BEAR to take apart for BMS replacement and re-buid). But with limited information, I would install a pre-charge circuit if I were you.

Here's a link to a suitable resistor (5 ohms, 100 watts). https://www.ebay.com/p/1200257059. At only 5 ohms, it will warm up pretty hot, burning up to 34 watts at the beginning of the pre-charge. (Mount it into free air, not held against wood via tape.) At roughly 2.6 Amps maximum, this will complete the pre-charge more quickly than the one I currently use. (I give mine about 3 minutes, but it generates less heat.) You can use much smaller wire than I indicated in the first post. (AWG-16 or even a bit smaller might fit the resistor post holes correctly). I had a snarky time soldering my big wires on to the small resistor posts of the "el-cheapo" resistor I used.

[rickst29]

Originally Posted by jagizzi

Thanks Rick, always something to think about ;-)

What is the purpose of the "Both" position in this scenario? Would you not be able to use simply a battery selector switch that does not allow both? A 3 position switch off, 1, 2.

I don't have an inverter in our current plan but it may be added in the future and this would have to be a consideration.

Thanks again.

Jim

Virtually all '2-way' battery switches are built this way, the concept being that you "make" the second connection before "breaking" the first connection. This switch could be used as "break before make" by switching from battery-1 to battery-2 through the "off" position, but we don't want that. (And other applications, such as yacht "engine start" versus "run", don't want that either - they, like us, want the voltage maintained as you switch to the "only" position for the second battery string).

Another one of these switches can also be used, separately, as a battery shut-off for a TM battery bank of two batteries (via switch, rather than pulling fuses). I have no use for that "prevention of phantom loads", because I leave TM plugged in all the time. But it would allow people to maintain or replace one battery while the other is still connected. Inside the switch, the "12v" lug for the battery which has been shut off would have no connection to the TM or the other battery, it would only have 12v from the battery itself.

[jagizzi]

Blue Seas makes a battery selection switch that is not off-1-2, it is 1-off-2, to eliminate the issue of both being on but I was mostly wondering if the both setting was something that you needed.

I use the switch to select between two battery strings on my TM.

[rickst29]

A 3-position 1-off-2, will have zero Voltage into the Inverter during the switch through the "off" position. But voltage needs to be maintained while switching in the "big" battery connection, so that the Inverter capacitors don't begin to discharge. So my sequence is always:

1. from off position, spin the dial to "bat-1" only. (Precharge with resistor).
2. After a few seconds, turn the dial from "bat-1" to "both" (enabling the big connection).spin the dial from "bat
3. turn the dial again, to the final run position "bat-2" only. This prevents power loss in the resistor while running appliances.
4. Connect and turn on 120v appliances, as needed.
5. Turn off then disconnect 120v appliances. Turn off the Inverter.
6. Turn the battery selector another 1/4 turn, back to "off".

You can always use a 4-way dual battery switch as a 3-way, by switching through the "OFF" position and never using the "BOTH" position. The Blue Sea switches (and similar clones) can be turned in either direction.

[Tymanthius]

So reading thru this, do you turn this on and leave it when you're in place for a few days?

Or is this the procedure you go thru every time you want to make coffee?

[rickst29]

Originally Posted by Tymanthius

So reading thru this, do you turn this on and leave it when you're in place for a few days?

Or is this the procedure you go thru every time you want to make coffee?

No, I will it leave it turned to the running "bat-2" position for extended times, while not actually running 120v appliances. I will be switching through the battery switch "cycle" each time I turn the big Inverter "on" (and later "off"), perhaps 2x or 3x per day.

Leaving the Inverter on all the time consumes a lot of power with its "idle load", and the battery switches are built to handle at least a several hundred switch events (with non-spiking current flow during switch rotation). If a battery selector switch eventually breaks, they don't cost a ton of money to replace.

Some Inverters will want to create the "big bad spark" only once, when they're first connected to 12VDC. With those Inverters, a constant "phantom load" consumes power to keep the capacitors charge up on the DC side, even when the Inverter is turned off. But more typically, the 'off' switch will disconnect the DC12V input from the capacitors, and they will slowly lose charge.

Unless you measure the current in "off" State, you won't know which kind you have. But when I rotate battery selector switch to "off", Inverters of the first type will be forced to run as Inverters of the second type, because there is no "+12V" to supply current to the capacitors - and inverters of the second type will always have battery protection through the switch and pre-load circuit.

[Wavery]

I have a Newpowa, Powall 2000W inverter. It has a remote read out showing all of the input and output loads. When I first turn it on, I can see the voltage ramping up on the 110V volt read-out. I left mine on for 5-days on our recent camp-out (because I was running a 110V fridge) and it seemed to draw so little at idle that the current usage didn't even read out.

However, on previous campouts, I did turn off the inverter when not in use and I noticed that it came on instantly with no "ramp-up" sequence (after the 1st time turning it on).

I wired the output side of my inverter through the buss bars and "Outlets" breaker in the TM converter so I shut off the "Converter/Air Con" breaker when the inverter is being used.

I do intend to buy a 200AH LFP battery soon. I appreciate you explanation about the 3-way switch. I'm just not sure that I understand the physical layout of the wiring of the switch.