Quick disconnect between two 12v batteries in series

[BroomJM]

Forgive me if this has already been asked and my Search skills just weren't able to locate it.

I have two 100AH LiFePO4 batteries, connected in series for 24v. To do this, I have a 15" length of fairly heavy wire (6 or 8 AWG) running from the positive terminal on one battery to the negative terminal on the other. I have several planned use cases for these batteries and have built them in durable cases, with handles, so they are very easy to move around. I want to place some kind of connector, perhaps an XT90, in that cable run, so that I can leave the leads bolted to the terminals, but disconnect the cable and move each battery independently.

So, I want to cut the single wire, place QD (XT90?) connectors on each end of it, and be able to quickly move the batteries around. My questions are as follows:

1) How much current is flowing between these batteries on the cable that connects them in series?
2) Is the XD90 capable of handling the current between batteries, PRESUMING I DON'T RECONNECT THEM when the SOC is significantly different?

 

[Diysolar123]

would be helpful to have a bit more information as to what is connected to these.
That they are in series does not change the potential current at all (which is still a LOT of current). The voltage does NOT impact the current.
It is the load you have them attached to that decides the current, putting them in series is about the increasing the voltage; when connected in series with NO load, the current is zero.
when you accidentily short out a lifepo4 battery...its much more impressive than a lead acid battery due to the amount of current a lifepo4 can quicky supply!!
XT90 connectors are great for small loads, I would not use them for more than 40amps but I derate all connectors by 50% because I do not like hot connectors hehe
a 6awg will not easily fit an xt90, probably bettery luck with the xt150's.

what load do you plan on putting on the battery is a major factor as to what connector you may use. Transient loads, recharging a cellphone, or jump starting a diesel truck. If you only have 8 or maybe 6awg wire thats not all that heavy of a wire so I hope you are keeping you load under 100amps.
anderson powerpole connectors are great, but expensive; xt150's are much cheaper but you will need a soldering iron in the 60-100w range.

 

[BroomJM]

These batteries will be in my fishing boat 90% of the time. I have a 24v trolling motor that draws 40 amps at maximum output, although inrush current is probably higher for a few seconds at a time, when going from dead stop to max speed. The majority of the time I am using the motor at 1/2 of max or less, which only draws 10-15 amps, per Minn Kota, the manufacturer. So, the current going out of the batteries will vary from 10-40 amps, with occasional surges.

I verified that the trolling motor is run using 8 AWG wire, not 6 gauge. It is my understanding that 8 AWG wire can handle well over 100 amps, over the very short distance (15") between batteries, so the wire connecting the two batteries in series is sized appropriately, as are the wires running to the trolling motor (about 10 feet).

However, it appears that the XT90 connectors typically come wired with no more than 10 gauge wire, which would tend to substantiate your concern about them being perhaps just a little too small for this application. (They might be fine for connecting the wires to the trolling motor, but too small for the series connection between the batteries.)

Since the series connection cable is not running a positive/negative connection, could I put both 10 gauge wires into a single terminal ring? In other words, one XT90 pigtail, with both wires in one terminal ring, connected to each battery? This would be similar to the way many BMS boards are wired, with multiple smaller wires instead of a single large one? The more I think about it, the more I believe that would work quite well.

 

[JoeHam]

8 AWG won’t cut it at 100 amps.

For large connectors look at Anderson Powerpole SB120 or SB175.

They are designed for different big wire sizes also.

Edit:

I make my own but this would be cheaper if you don’t have the proper tools:

Anderson SB175 Copper Battery Cable 4 Gauge AWG with ends lug/terminal/clamp | eBay

You cannot mate a RED and GRAY plug as they are made differently to avoid this. Also, the SB175 will not mate with a larger or smaller plug style such as the SB50 or SB350. The plugs will only mate with their own series (SB175).

www.ebay.com

 

[BroomJM]

8 AWG won’t cut it at 100 amps.

For large connectors look at Anderson Powerpole SB120 or SB175.

They are designed for different big wire sizes also.

Edit:

I make my own but this would be cheaper if you don’t have the proper tools:

Anderson SB175 Copper Battery Cable 4 Gauge AWG with ends lug/terminal/clamp | eBay

You cannot mate a RED and GRAY plug as they are made differently to avoid this. Also, the SB175 will not mate with a larger or smaller plug style such as the SB50 or SB350. The plugs will only mate with their own series (SB175).

www.ebay.com

You're saying the 8 AWG won't cut it when using the batteries for emergency backup power, right? I am pretty sure you're right about that.

The inverter I'm looking at is the MPP 2724, so a 2.7kw inverter. The batteries and BMS can support that because they allow a maximum 2C discharge rate. If the compressor kicks on for both of our refrigerator/freezers at the same time, there will be a surge of somewhere north of 100 amps to support that (2400v from a 24v source).

My dilemma is that I want a quick disconnect option sized adequately to supply a 2700w inverter, while still being able to easily connect in a 1,000 watt electric trolling motor. If I use SB120 connectors, with 4 AWG wires, bolted to each battery, could I then plug into those connectors with identical wire for my inverter, but only 8 AWG wires running to my trolling motor? Do they make pins that small to fit in the SB120 connector? Is the SB120 connector the same physical dimensions as the SB50?

 

[JoeHam]

You're saying the 8 AWG won't cut it when using the batteries for emergency backup power, right? I am pretty sure you're right about that.

The inverter I'm looking at is the MPP 2724, so a 2.7kw inverter. The batteries and BMS can support that because they allow a maximum 2C discharge rate. If the compressor kicks on for both of our refrigerator/freezers at the same time, there will be a surge of somewhere north of 100 amps to support that (2400v from a 24v source).

My dilemma is that I want a quick disconnect option sized adequately to supply a 2700w inverter, while still being able to easily connect in a 1,000 watt electric trolling motor. If I use SB120 connectors, with 4 AWG wires, bolted to each battery, could I then plug into those connectors with identical wire for my inverter, but only 8 AWG wires running to my trolling motor? Do they make pins that small to fit in the SB120 connector? Is the SB120 connector the same physical dimensions as the SB50?

I would have to go to the Anderson Powerpole site to see the pin sizes but they're pretty versatile. A quick visit to Amazon or eBay will give you tons of options. The purpose of the SB50 vs a larger one is so that they cannot be interchanged and overloaded.

A SB50 to SB120 or SB175 adapter can be found on eBay I'll bet but don't use that as an excuse to overload a SB50.

My advice? Stop pussyfootin' around with 8 AWG. Bigger copper only costs a little bit more for 15 inch runs or longer and will only increase efficiency.

Your system, your choice though.

 

[BroomJM]

Answered my own question!

According to the Data sheet on the SB120:

"Wires sizes from 10 to 1 AWG (5.3 to 42.4 mm²) are held in the second smallest SB® housing."

 

[BroomJM]

I would have to go to the Anderson Powerpole site to see the pin sizes but they're pretty versatile. A quick visit to Amazon or eBay will give you tons of options. The purpose of the SB50 vs a larger one is so that they cannot be interchanged and overloaded.

A SB50 to SB120 or SB175 adapter can be found on eBay I'll bet but don't use that as an excuse to overload a SB50.

My advice? Stop pussyfootin' around with 8 AWG. Bigger copper only costs a little bit more for 15 inch runs or longer and will only increase efficiency.

Your system, your choice though.

Well, the wires are already run under the deck of the boat, from a battery compartment just forward of the cabin, to the bow. Since that will never draw more than 40 amps, I really don't see the value in running heavier wire for that application. However, with the SB120 plug and 4 AWG wire on the battery side, connecting to an SB120 plug and 8 AWG on the trolling motor side, I think that solves my problem.

 

[JoeHam]

Well, the wires are already run under the deck of the boat, from a battery compartment just forward of the cabin, to the bow. Since that will never draw more than 40 amps, I really don't see the value in running heavier wire for that application. However, with the SB120 plug and 4 AWG wire on the battery side, connecting to an SB120 plug and 8 AWG on the trolling motor side, I think that solves my problem.

Glad to hear it!

I wasn’t aware the wires were there already. FYI, I have never been known for my tact or subtlety! ?

 

[BroomJM]

Glad to hear it!

I wasn’t aware the wires were there already. FYI, I have never been known for my tact or subtlety! ?

I come from the "stick n' stones" generation...and regularly piss people off online, without meaning to!

No offense taken and very much appreciate your input.