Li Batteries and inrush current to inverters?

vanski

'05 Snow Camper, '17 170 4x4, Adventure Vissionary
Makes sense… that’s a bit of work just to to turn off/on the batts.

is there anything the battery mfgs could have done in the past year to prevent the fault in the first place? Pretty sure I’m going with battles and he has a magnum inverter.. circa 2016 inverter. Surprised I haven’t heard of this fault occurring all the time; lots of folks with only 1 x 100ah LFP hooked up to their inverter having no issues. Yes, understood it also depends on inverter size.
 

marklg

Well-known member
Makes sense… that’s a bit of work just to to turn off/on the batts.

is there anything the battery mfgs could have done in the past year to prevent the fault in the first place? Pretty sure I’m going with battles and he has a magnum inverter.. circa 2016 inverter. Surprised I haven’t heard of this fault occurring all the time; lots of folks with only 1 x 100ah LFP hooked up to their inverter having no issues. Yes, understood it also depends on inverter size.
Some batteries have higher short time current specs. If they use a relay instead of a FET, the spec can be higher. I don't have a list of such batteries, but you can see it in their datasheets. For example, my 100 Ah Lifeblues are 100A continuous, 300A for 3 seconds. They trip at 420A for 6 seconds.

Regards,

Mark
 

borabora

Well-known member
Makes sense… that’s a bit of work just to to turn off/on the batts.

is there anything the battery mfgs could have done in the past year to prevent the fault in the first place? Pretty sure I’m going with battles and he has a magnum inverter.. circa 2016 inverter. Surprised I haven’t heard of this fault occurring all the time; lots of folks with only 1 x 100ah LFP hooked up to their inverter having no issues. Yes, understood it also depends on inverter size.
I don't really look at it as a battery problem. Allowing 1000+ amps of inrush current is just poor design as far as I am concerned. A lot of AC power tools incorporate inrush current reduction into their on/off switches. Inverters should do the same: include an on/off switch with current management and either tell the user to never power up the unit when it is turned on or automatically turn it off when power is removed. Or make the function completely automatic.
If the owner has an engineering/tinkerer mindset then providing a bypass charge resistor would be fine otherwise it has to be more idiot proof, I think.
Incidentally if you look at Amazon's Lifepo4 battery reviews you will find many frustrated users giving one star for "I connected the new battery to my inverter and the battery immediately died -- sending it back."
 

RVBarry

Well-known member
Are you worried about this because you plan on disconnecting the inverter frequently for many hours or days? Short disconnects shouldn't be an issue because the capacitors won't fully discharge. If you let them discharge then I would use a resistor bypass before closing the switch.
Hi, I may disconnect the house battery when not traveling.

Perhaps a 3-way switch (e.g. a 2-battery disconnect switch) would be prudent for the resistor.
thanks
 

RVBarry

Well-known member
I would suspect that the better inverter manufacturers will soon incorporate inrush current limiting circuits as LIFEPO4 replaces SLA batteries.
I read last night that the very large Victron inverters for houses with solar and battery backup (like a Tesla Wall) do have limiters, but I'm not seeing any under 5kW that do.
And I read the inrush on a 3kVA inverter can be 3000A :(

thanks
 

borabora

Well-known member
Hi, I may disconnect the house battery when not traveling.

Perhaps a 3-way switch (e.g. a 2-battery disconnect switch) would be prudent for the resistor.
thanks
Yes, I described that type of switch several posts up in this thread. Use a light bulb instead of a resistor for user feedback. The problem is that the user still has to understand what's going on and not accidentally leave the switch in the charge setting.
 

autostaretx

Erratic Member
Midwest Drifeter and borabora's suggestion (which i agree with)
1632859087357.png

It could be done with two separate switches, one for the resistor, one for the "On/Off"
(which end of the drawing gets the battery vs the inverter doesn't matter)

Size the resistor (or light bulb) to limit the current to the battery's happy-place.
50 amps (and was it 24 volts?) would be V=A*R, shuffled to R = V/A = 24/50 = a half ohm, capable of handling 50*24= 1200 watts.
Well, that's a bit steep ... using a 24v light bulb would simply extend the time you'd have to wait at "charge" during your switching.

--dick
 
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RVBarry

Well-known member
@autostaretx, thanks for the drawing; meanwhile I was struggling with Visio to produce this one:

1632871588879.png

(one can switch on positive or negative, but there are advantages to using negative in a negative-grounded vehicle)
 
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RVBarry

Well-known member
Size the resistor (or light bulb) to limit the current to the battery's happy-place.
50 amps (and was it 24 volts?) would be V=A*R, shuffled to R = V/A = 24/50 = a half ohm, capable of handling 50*24= 1200 watts.
Well, that's a bit steep ... using a 24v light bulb would simply extend the time you'd have to wait at "charge" during your switching.
I'm having trouble understanding why a (let's say 50 watt) lightbulb would allow more than 50 watts to pass... I'll have to think about that...

Also, I'd be worried that a 24V bulb might not light up enough to know when it's charging vs done.

thanks
 

autostaretx

Erratic Member
I'm having trouble understanding why a (let's say 50 watt) lightbulb would allow more than 50 watts to pass... I'll have to think about that...
A 50 watt bulb would only "pass" ("consume" actually) 50 watts.
So with a 12v circuit, that's 4 amps.
Also, I'd be worried that a 24V bulb might not light up enough to know when it's charging vs done.
I had mixed up the voltages in *your* system ... you're only looking at 12 volts (if i'm reading the part number correctly).
So my "50 amp" (half of the "C" of a 100 AH battery) using only 12 volts would drop the numbers to:
R = V/A = 12/50 = one quarter ohm.
Power = V*A = 12*50 = 600 watts.

A 12v bulb as the "resistor" would work well ... a headlamp (H7, for example) bulb is 55 watts (and gets REALLY HOT).

--dick
 

rollerbearing

Well-known member
For this type of current limiting I like to use 3156 bulbs. You can straighten the leads enough to screw them down to a lug strip. Will limit the current to less than an amp.

You can also solder to the leads if you wrap halfway up with copper, fold the bulb lead over that, wrap another wrap or two and cover with solder.

As Autostaretx mentioned - the bulbs will get warm with constant current flow - I usually mount in an empty AC junction box.

1632868794973.jpeg
 
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RVBarry

Well-known member
Thanks everyone!

BTW, I updated my diagram above to add Phillips 3156 bulbs.
 

99sport

Well-known member
What is the issue here? Is the current disconnect inside the drop-in battery not able to supply the required current?

My victron system has no disconnect inside the battery (just a giant copper bus to the terminals). I do have a 100 amp disconnect (controlled by the external BMS) for DC loads only. AC loads are handled by the Victron multiplus - it has digital communication to the BMS, but there is no relay or FET between the batteries and the multiplus - just a fuse and fat wires. I do have a bluesea disconnect, which I use all the time, and have no issue disconnecting or reconnecting the inverter. The system was expensive, but has been absolutely bulletproof.

Not all systems need the resistor / lightbulb discussed here. What is driving the need? I assume it is the low current capacity of the internal BMS in a drop-in battery? Seems to me baselining this before verifying your system needs it is premature. RVBarry, I see no mention of which batteries you plan to use. Again, plenty of systems can be connected and disconnected without any problems and no "inverter charging."
 

RVBarry

Well-known member
What is the issue here? Is the current disconnect inside the drop-in battery not able to supply the required current?

My victron system has no disconnect inside the battery (just a giant copper bus to the terminals). I do have a 100 amp disconnect
Hi, many batteries have an internal BMS with 0.5-1.0C limits. But I don't know how quickly they all trigger, and how quickly they could get damaged with a 3000A inrush surge.

Even your external 100A disconnect seems like it could be a problem if it triggers very quickly.

And, battery disconnect switches could get potentially get damaged by arcing at those levels upon connecting.

I'm planning a 12V system with between 400-600Ah of lithium, and I'm not planning to spend $4500+ on the batteries, so that rules out many brands.

thanks
 

RVBarry

Well-known member
What about connecting solar (to the busbar and inverter) before connecting the batteries?
I assume solar chargers are going to be current-limited by the charger (and the panels), so they shouldn't catch fire, right?

Of course this wouldn't help at night or under a pergola.
 

marklg

Well-known member
What about connecting solar (to the busbar and inverter) before connecting the batteries?
I assume solar chargers are going to be current-limited by the charger (and the panels), so they shouldn't catch fire, right?

Of course this wouldn't help at night or under a pergola.
Most solar chargers suggest you connect the batteries first, then the panels. They won't catch fire as they are current limited by the panels. If the battery BMS opens up and the voltage goes up on the battery side from some other charging source, they may trip off or even possibly be damaged.

Looking at the discussion, I would prefer something that limits the surge automatically. Anything that requires a multistep operation in the right order is bound to be done wrong eventually.

Regards,

Mark
 

99sport

Well-known member
Please don't take this as criticism, but I'm still not understanding the need for this resistor to limit current. As I recall, Battleborn's are built with 10 awg wire inside and a small FET to disconnect the load. I could see how they might not be able to supply enough current for a variety of scenarios.

However, If you are going DIY, you'll have an external BMS and can choose any contactor / disconnect you want. Is the issue the FETs / disconnect inside the drop-in batteries? If so, and you go DIY, just choose a contactor with a large enough current supply. RVBarry, are you planing a DIY lithium pack (sounds like drop-ins like Battleborns are not in your price range).

Again, I can say categorically that I have no issues connecting my 3000VA multiplus with a Bluesea disconnect. I have no resistor to the multiplus. Victon has lots of dcumentation on how to wire their inverter chargers, and they never have the resistor discussed in this thread.
 

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