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DIY Wiring Victron Smart Battery Protect and Lithium LIFEPO4 with $60 BMS and 2000W inverter

Danny

Danny

Solar Enthusiast
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Sep 21, 2019
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223
Proposed wiring plan to integrate my inverter onto my Lifepo4 4s Calb 180ah Pack. I am migrating off my lead acid batteries and am going to keep 1 group 27 starting battery and am going to use the Victron SBP to direct connect the Lifepo4 battery to the inverter bypassing the bms. However, I'm going to let the BMS tell the SBP if it should disconnect. Thanks for your review.
 
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Buried in the documentation for the Victron battery protect is a statement that it is not supported for use in-line with an inverter. I believe the reason for this is the large surge current for charging the front-end capacitors of the inverter when you first connect.

@Justin Laureltec might be able to add more info about the Victron Restrictions.
 
Last edited:
FilterGuy said:
Buried in the documentation for the Victron battery protect is a statement that it is not supported for use in-line with an inverter. I believe the reason for this is the large surge current for charging the front-end capacitors of the inverter when you first connect.

@Justin Laureltecmight be able to add more info about the Victron Restrictions.
Click to expand...
Doh! I need to look at this. Is this just Battery Protect or the new Smart Battery Protect? I knew it was too good to be true. I'm thinking about just using a relay / contractor. I can't seem to find the language in the manual for the new Smart Battery Protect. https://www.victronenergy.com/uploa...A-100-A--220-A-EN-NL-FR-SV-DE-PT-ES-IT-TR.pdf
 
FilterGuy said:
Buried in the documentation for the Victron battery protect is a statement that it is not supported for use in-line with an inverter. I believe the reason for this is the large surge current for charging the front-end capacitors of the inverter when you first connect.

@Justin Laureltecmight be able to add more info about the Victron Restrictions.
Click to expand...
Thanks. I sent @Justin a message.
 
Danny said:
Doh! I need to look at this. Is this just Battery Protect or the new Smart Battery Protect? I knew it was too good to be true. I'm thinking about just using a relay / contractor. I can't seem to find the language in the manual for the new Smart Battery Protect. https://www.victronenergy.com/uploa...A-100-A--220-A-EN-NL-FR-SV-DE-PT-ES-IT-TR.pdf
Click to expand...
Hi @Danny, it looks like you sent the wrong Justin a message... no worries though, @FilterGuy's tag worked :)
To answer: This applies to both the BatteryProtect as well the Smart BatteryProtect; the devices cannot be used to directly connect/disconnect the main DC line of an inverter. @FilterGuy is correct: the inrush current of the inverter filling its caps will trigger an E3 short-circuit error code that, repeated enough times, will invariably damage the logic of the BP and lead to malfunction in one way or another... and since it's a non-supported connection, it will also void the warranty.
The only approved method for using a BP to turn an inverter on or off is to use the BP to signal either the remote on/off of the inverter (if so equipped) or to trigger a 3rd-party high-current latching relay that interrupts the main DC feed to the inverter. See the updated (online only) BatteryProtect user manual for the approved wiring diagrams of this.
Sorry for the bad news... but I'm glad we caught it before you damaged your BP and/or caused downstream device failures!
 
@FilterGuy and @Justin Laureltec have sent me back to the drawing board. Thanks so much. Will also told be about how he needed to change one of his designs. I'm now looking at using this relay https://www.electriccarpartscompany...-12-Volt-900-Volt-with-9-Volt-36-Volt-DC-Coil for the disconnect and I"m also thinking about modifying the on/off switch in my low cost inverter so I can use a relay to turn it on/off remotely. I like the idea of turning off my inverter at a more conservative voltage that I have in my cheap bms so I"m looking at integrating one of these modules as my low voltage disconnect driving a relay.
. Perhaps you could review my 2.0 design when I complete it. I'm going to breadboard the design and make a replacement video detailing my discoveries. Thanks for your help.
 
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What about using a thermistor-based current limiter? These initially provide resistance to limit current, but as they heat up, the resistance drops to practically nothing. The disadvantage is they need cool-down time between sessions.
 
Last edited:
markb said:
What about using a thermistor-based current limiter? These initially provide resistance to limit current, but as they heat up, the resistance drops to practically nothing. The disadvantage is they need cool-down time between sessions.
Click to expand...
Do you know of any thermistors that can handle a couple hundred amps?
 
I wonder how much current is required to keep them turned on? If they turn off when the inverter is in an idle state it would lead to some really strange behavior.
 
Well, at 36 A the resistance is 10 mOhm so the Vdrop would be 0.36 V, that's 13 W lost per NTC (and it's 3 times worse at 50 % current with the 60 mOhm figure, also it's almost a 2.2 Vdrop)...

The 78 W total losses and the 224 °C surface temp is a bit high to my taste... ?
 
BiduleOhm said:
Well, at 36 A the resistance is 10 mOhm so the Vdrop would be 0.36 V, that's 13 W lost per NTC (and it's 3 times worse at 50 % current with the 60 mOhm figure, also it's almost a 2.2 Vdrop)...

The 78 W total losses and the 224 °C surface temp is a bit high to my taste...
Click to expand...

Yeah, that's true. 78W is a decent amount of power. What's the Victron's on resistance?
 
Can you charge the capacitors from the lead acid battery and then energize the LFP after via contactor relay? Does this capacitor charging process only happen once when the batteries turn on the first time in a trip? Or each time you cycle the inverter on and off?

I had similar architecture in mind on my AGM / LFP mix chemistry system in my RV. I don't want to get rid of the Magnum 2812 or PT100.
I assume the inrush is very momentary (just fraction of 1 second)?
I had planned to just connect the AGM first and then once the inverter was on, turn on the LFP. But in never occurred to me if I turn the inverter on and off on a given trip, the capacitors would need charging again.

Will the inrush also blow a large fuse on the battery (say 300A) like this: https://www.amazon.com/gp/product/B0019ZBTV4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I bought the Gigavac 200GV contactor from EVwest and planned to have the BMS manage it. It looks pretty similar to what you showed from ECPC.

How can you calculate the inrush current?

Doug
 
DW SD said:
Can you charge the capacitors from the lead acid battery and then energize the LFP after via contactor relay? Does this capacitor charging process only happen once when the batteries turn on the first time in a trip? Or each time you cycle the inverter on and off?

I had similar architecture in mind on my AGM / LFP mix chemistry system in my RV. I don't want to get rid of the Magnum 2812 or PT100.
I assume the inrush is very momentary (just fraction of 1 second)?
I had planned to just connect the AGM first and then once the inverter was on, turn on the LFP. But in never occurred to me if I turn the inverter on and off on a given trip, the capacitors would need charging again.

Will the inrush also blow a large fuse on the battery (say 300A) like this: https://www.amazon.com/gp/product/B0019ZBTV4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I bought the Gigavac 200GV contactor from EVwest and planned to have the BMS manage it. It looks pretty similar to what you showed from ECPC.

How can you calculate the inrush current?

Doug
Click to expand...
The inrush current happens when the battery is first connected to the inverter. It will not happen again in 'normal' operation. However, the following are somewhat unusual times it could happen again:

1) You disconnect the battery from the inverter and let the inverter set for a while. When you hook it back up you can get an inrush current.
2) The BMS decides to disconnect for whatever reason (low voltage, low temp, over-current, etc). If the disconnect lasts very long the capacitors will discharge and an inrush current will occur when the BMS turns back on.

The inrush current can be extremely high but it will also be extremely short. It is highly unlikely it would blow a 300 amp fuse. However, your BMS might decide there is a short and disconnect. The other problem is that the high in-rush has the possibility of damaging the MOSFETs in the BMS.
 
DW SD said:
Will the inrush also blow a large fuse on the battery (say 300A) like this: https://www.amazon.com/gp/product/B0019ZBTV4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Doug
Click to expand...

FWIW, two things: 1: the MRBF fuses are okay for a 24v 3kVA unit, but not okay for a 12v 3kVA unit because you'll (depending on your wiring) need a 400A fuse and MRBFs only run up to 300A... but if I'm recalling correctly, you're running at 24v so you should be good to go as long as you size your fuse correctly... which brings me to item 2:
2: Be careful about the listings for these on Amazon, because the one that you linked to does not include the actual fuse... I know, it shows a fuse in the picture, but that listing is only for the MRBF holder, not the holder and fuse - it specifies "fuse sold separately" in the listing detail, but the photo is super misleading. I know because we sell MRBF kits on Amazon with both the holder and fuse and we've gone round and round with people because they point to Amazon's listing (which you linked to) and say "look, Amazon sells these for way cheaper" and we have to come back and point out that, pictures notwithstanding, you're not getting the actual fuse there. So, just... be careful about that sort of thing, Amazon is a bit of a bugger that way.
 
Hi Justin,
Thank you. I did buy the 300A fuse with that holder and caught that.... but I do run a 12V system with 4/0 wire.
Ah.. but you are saying for 2800W ~ 3kVA I could blow the fuse. I see. The current could be higher than 300A.
For me, the BMS would manage a large contactor. Gigavac GV200 - says its rated for 500A continuous.
Admittedly, I sized the fuse based on my use case rather than the Inverter rating. I only run 1 AC appliance at a time and the convection / micro oven is 1500W. Conceivably someone could turn on another appliance (like a hair dryer, though).
HMMM. Thanks for catching the error.

Now... as for the Original thread starter, is it OK for him, it seems, to charge the inverter with the lead acid? Then bring the LFP battery online after a moment? That would be my own plan. And to make sure the lead acid and LFP don't have disparate voltages so one doesn't have a massive discharge event in to the other.

Cheers,
Doug
 
Wow.... we have wandered back into the manual pre-charg switch discussion that has peppered several other threads. Here is what I will be doing in my next install:

1582596541623.png

Theory of operation.
Turn On
  1. Make sure the inverter is off
  2. Turn switch from Off to position one. The resister is now in the circuit and will quickly pre-charge the inverter capacitors.
    Once the capacitors are charged, the current will stop flowing.
    (Ignore the PTC thermistor for a moment. I will come back to that)
  3. After a second or so move the switch to position 2. At this point the inverter is fully connected. The resistor circuit is parallel to the closed switch so it is effectively out of the circuit.
  4. Turn on the inverter.

    Notes
    * The user can turn the switch two position 2, but does not need to.
    * Even if the user does not pause at position 1, the capacitors will probably be sufficiently charged to avoid any problems.
Turn off.
  1. Turn off inverter.
  2. Turn switch through position 1 to the off position. No pause is needed
User errors
It is possible for the user to leave the inverter on when they go to connect the inverter through the switch. In this case the resistors will be handling a large amount of current for potentially an extended time, creating excessive amount of heat and letting all the magic smoke out of the resistors. However, in this case the PTC thermistor will heat up and it's resistance will go way up. This in turn will drop the current to a manageable amount.

The above circuit is for 12 volts. The following circuit is for 24 volts:
1582597460421.png
 
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