Confirm battery wire gauge and fuse

[rmaddy]

I'm working out the details of a solar electrical layout for a trailer conversion.

I'd like some confirmation on some of the needed wire size. I am planning on four 12V 100Ah LiFePO4 batteries in 2S2P to give 24V 200Ah. I am planning on the Victron 24V 2000W inverter/charger. Its manual states that 2AWG wire with a 200A fuse should be used for a 24V system (2/0AWG, 300A for 12V).

Does this mean I should use 2AWG wire for all of the wires connecting all of the batteries together, connecting the batteries to the busbars, and connecting the inverter/charger to the negative busbar and the 200A fuse (which in turn is connected to the positive busbar)?

Am I correct that this choice of wire gauge is independent of the number of batteries? It would be the same whether I had 2, 4, or 6 batteries wired up to be 24V?

 

[Supervstech]

No, the batteries are only capable of 100 amps, so #2 copper should be sufficient between all the batteries. The set is capable of 400 amps, so be certain you have an interrupt set to trip if the calculated load is exceeded.

 

[Supervstech]

A 2000 watt inverter pulling 2000 watts will draw 84 amps from the batteries.

#2 is only good in a very short cable. I would use 1 or 1/0 to the inverter.

 

[Supervstech]

Am I correct that this choice of wire gauge is independent of the number of batteries? It would be the same whether I had 2, 4, or 6 batteries wired up to be 24V?

Yes, the selected wire gauge is dependent on the intended load (the inverter) the batteries are capable of far more than this, so proper fusing is key.

 

[rmaddy]

No, the batteries are only capable of 100 amps, so #2 copper should be sufficient between all the batteries. The set is capable of 400 amps, so be certain you have an interrupt set to trip if the calculated load is exceeded.

I'm not sure which of my specific questions you are saying "no" to here.

A 2000 watt inverter pulling 2000 watts will draw 84 amps from the batteries.

#2 is only good in a very short cable. I would use 1 or 1/0 to the inverter.

Victron's manual states the use of 2AWG (up to 5m in length). And all wire gauge charts I've found show 2AWG (even 4 for 6) is enough for 83A considering the wires will be under a couple feet long at most. Why suggest 1 or 1/0?

Yes, the selected wire gauge is dependent on the intended load (the inverter) the batteries are capable of far more than this, so proper fusing is key.

That leads into my next question. What size fuse would be appropriate for the main fuse between the batteries and the main positive busbar? The recommended fuse size for 2AWG wire (assuming that is in fact the correct size) is 200A so it seems both the main fuse and the inverter/charger fuse should be 200A. Is that correct?

 

[Geriakt]

You may want to read up all the posts in the victron support pages that use BB with the Multiplus 12/2000 and 24/2000 that have undersized their battery cables and the second they switch on the Multiplus the BMS in the BB shuts off. You are better off sizing cable over than under. 1/0 is what should be used to the 24/2000, but you can do your research to save $50 on $5500 worth of equipment.

 

[rmaddy]

You may want to read up all the posts in the victron support pages that use BB with the Multiplus 12/2000 and 24/2000 that have undersized their battery cables and the second they switch on the Multiplus the BMS in the BB shuts off. You are better off sizing cable over than under. 1/0 is what should be used to the 24/2000, but you can do your research to save $50 on $5500 worth of equipment.

I just spent the last hour digging through community.victron.com and I couldn't find any posts related to this. But the search feature isn't very good. I'm all for using the proper wire size. I agree that a few bucks spent on wire is better than killing expensive equipment. But I'd love to understand why the reality of this inverter/battery combination seems to need bigger wire than expected. If you have a link to one of the posts mentioning the need for using bigger wire than stated in the manual for the 24/2000 I'd really appreciate it.

 

[Geriakt]

Looking over your Multiplus 24/200/120 manual Victron calls out for 35mm2 and 200 amp fuse for cable under 5 meters round trip 2AWG is only 33.6 mm2 so you really need 1AWG at 42.4 mm2. 2 AWG is too small. You will not find 1AWG cable and lugs so you will be doing yourself a favor by just using 1/0 AWG at 53.5 mm2. Other factors to consider in addition to cable length is resistance in bus bars, shunts, lugs, and number of connections.
The Multiplus pulls a lot of current to fill the caps when turned on and this over loads the BB battery BMS on start up. This can be controled by using a pre charge resistor before you turn on your master battery switch or contactor. I use a simple 7 watt 120 ohm resistor for 10 seconds before i turn on my master contactor in my system. I have a Multiplus 24/3000 with 500 AH battery (2 Teslas in parallel) and use 1/0 AWG 600 volt welding cable and tined lugs.

Since you are installing BB in a 2S2P why wouldn't you just purchase 4 24 volt 50 AH BB all in parallel? This way if you should have an issue with one battery your other 3 will still power your system?
Go to the Victron website and download the Wiring Unlimited handbook.

Wiring Unlimited - Victron Energy

Wiring Unlimited is now available! Download it for free here: Wiring-Unlimited-EN.pdf What is Wiring Unlimited? A Victron Energy book by Margreet Leeftink, Information Developer for Victron Energy B.V. Wiring Unlimited is all about electrical wiring of systems containing batteries, inverters...

www.victronenergy.com

Tom

 

[rmaddy]

Thanks Tom. I hadn't considered that 2AWG is really a little smaller than 35mm2.

This is the first I've heard of a pre-charge resistor. I need to look into that. But can you clarify something? Is the use of 1/0AWG vs 2AWG solving a different issue than adding the pre-charge resistor? Your first post talks about overloading the BMS because 2AWG is too small. Then your second post talks about the BMS issue being resolved by adding the pre-charge resistor. Just trying to understand all of the parts.

The reason I was planning to use 4 12V in 2S2P instead of 4 24V all in parallel is because I may grow to using 6 batteries in the future to have 24V 300Ah and I had seen mention that there can be issues going beyond 3 or 4 batteries/strings in parallel. But if there isn't an issue having 6 batteries in parallel then I would much rather use 24V batteries instead of strings of 12V batteries. It eliminates dealing with balance issues when using series strings.

Thanks for the link to the wiring book, I've actually been making use of it for a while now.

 

[Geriakt]

Thanks Tom. I hadn't considered that 2AWG is really a little smaller than 35mm2.

This is the first I've heard of a pre-charge resistor. I need to look into that. But can you clarify something? Is the use of 1/0AWG vs 2AWG solving a different issue than adding the pre-charge resistor? Your first post talks about overloading the BMS because 2AWG is too small. Then your second post talks about the BMS issue being resolved by adding the pre-charge resistor. Just trying to understand all of the parts.

The reason I was planning to use 4 12V in 2S2P instead of 4 24V all in parallel is because I may grow to using 6 batteries in the future to have 24V 300Ah and I had seen mention that there can be issues going beyond 3 or 4 batteries/strings in parallel. But if there isn't an issue having 6 batteries in parallel then I would much rather use 24V batteries instead of strings of 12V batteries. It eliminates dealing with balance issues when using series strings.

Thanks for the link to the wiring book, I've actually been making use of it for a while now.

It would be best you contact BB about the max parallel string of batteries and yes this seems to be an issue with built in BMS systems that do not communicate with each other. Valence and Victron Lifepo dont have these issues as they are all interconnected in design.
The issue of the overloading of the BB bms with the Victron Multipluses has come up in the Victron Support Forum and i just wanted you to know. One was because the user only has one BB battery connected and was undersized for his inverter. The other was undersized cables when turning on high amperage appliances. The last one could be the need of a pre charger, but has not been confirmed as of yet.
I use the pre charge resistor as it is just easier on the batteries and multiplus in the long run. It really is a low cost solution, but your system may not require it.

The main draw back of drop in Lifepo is the built in bms do not communicate with each other for larger battery banks and thus the need for aftermarket expensive BMS systems like Rec BMS and others.
I use the Electrodacus SBMS0 for my two teslas. The only draw back is the SBMS0 is limited to 8S so i have to use two of them for each battery. Since my batteries are in parallel if one blows a fuse, the other is still connected and running. This is why i think 4 24volt BB in parallel each fused separate to one positive copper bus bar, may be a better choice, however BB would be better to engineer your system.

 

[rmaddy]

Been doing research on the use of a pre-charge resistor. Found some good info, including http://www.diysolarforum.com/threads/will-this-work-for-pre-charging-inverter-capacitors.1526/ so that is now in my plan. I may also add a "post-discharge resistor" if I need to work on the inverter after turning off the master switch.

I just sent off an email to Battle Born. I'll update with further details once I hear back.

 

[stuart]

My thoughts on wire sizing.
Wouldn't the battery interconnect cables current rating be 42a per battery interconnect and inverter cables 84a.(see attachment)
Real cables should be rated at 2X's that due to 2000W inverter max serge of 4000W (84a interconnect and 168a inverter cables)

 

[rmaddy]

Just heard back from Battle Born. I asked about using 12V 100Ah in 2S2P (2S3P) versus using 24V 50Ah in 4S (6S) and whether one was better than the other regarding battery balance with the 12V in series or having too many in parallel with the 24V, etc. The answer I received was:

Most of our customers go with the 100ah 12 volt batteries ran in series because of cost, availability and the internal bms has mosfets that are a bit more robust. Also, if you needed to charge the batteries from any different source then 12 volt chargers are much more readily available.

Some good points but it doesn't really address the comparison I was asking about.

 

[Geriakt]

Just heard back from Battle Born. I asked about using 12V 100Ah in 2S2P (2S3P) versus using 24V 50Ah in 4S (6S) and whether one was better than the other regarding battery balance with the 12V in series or having too many in parallel with the 24V, etc. The answer I received was:



Some good points but it doesn't really address the comparison I was asking about.

That did not answer your question at all since you are using a 24 volt system and not charging from a 12 volt source.

 

[rmaddy]

That did not answer your question at all since you are using a 24 volt system and not charging from a 12 volt source.

Yeah but I think they were referring to some possible need to recharge a single battery away from the solar setup. At least that was my interpretation. But I agree, the answer fell short of what I hoped.

 

[Geriakt]

Yeah but I think they were referring to some possible need to recharge a single battery away from the solar setup. At least that was my interpretation. But I agree, the answer fell short of what I hoped.

i think you should ask how the bms will function in a 2p2s or 4 p system