Dual battery installation question

CharlieRN

I'm in the process of adding a second 100 Ah lithium battery to my T@B 320 and have a question about the appropriate wire size for the jumper cable connecting the two batteries..

I understand that the parallel connection will result in a system with 200 Ah's of capacity but the connection is between two 100 Ah batteries. Does that mean the most amperage that can flow over the jumper is 100 amps, and that is what I should size the wire to? Or should the jumper be able to handle the total 200 amp current of the dual system? I think this basically this boils down to the question of whether I should use 8 or 4 AWG for the jumper.

Horigan

The maximum amperage is not defined by the battery capacity.  It's defined by the downstream component loads.  Think of amp-hour capacity as how many electrons are stored in the battery to provide energy.  Think of amps as how fast those electrons are flowing through the circuit, which is determined by the resistance of the downstream loads.

The biggest draw will be from an inverter, if you have one.  For the battery parallel cables I would match those that go to the inverter.  If you don't have an inverter you could go with 4 or 8 AWG wire.  If it were me, I would match the large wires that attach to the original battery (0 or 00).  Then you have the flexibility of adding an inverter if you don't have one.

Here's a good site for wire gauge vs acceptable current draw.

https://www.powerstream.com/Wire_Size.htm

CharlieRN

Thanks @Horigan , that makes perfect sense.

I don't have an inverter and have no plans right now to add one, but that doesn't mean things won't change at some point. The total theoretical draw of all of the 320's 12 volt devices, including the fan and all lights at their highest setting, is about 16 amps. The factory wiring to the battery looks a lot less robust than 00 or even 0 gauge; I'd guess it's 10 guage (can't see the markings). As you suggest, I may be OK with 8 AWG for the battery jumper, particularly since it's such a short run, but I may go to 4 gauge to be safe.

Dutch061

When I put 2 x BB's in my 400 BDL, I used 2/0 gauge pure copper cable. Way heavier than required but I overdue anything I do.

Brad

AnOldUR

I have a 30A fuse and 12" of 10 gauge between my two 125 Ah batteries. The fuse has never blown which tells me than anything larger than 10 gauge would be a waste. What's wrong with this logic?

Denny16

Whilst using an AWG no 8 wire for a 30-amp circuit will work, the jumper between two batteries (series or parallel) should match the amperage rating of the batteries being used.  So two 100 amp batteries will have an ampacity rating of 125, which requires a AWG no 2 wire for a continuous load, so a AWG no 4 would be the minimum size I would go with to connect two 100 amp batteries.  

(Ampacity rating of wire size must be at least 125% of the continuous current passing through it). 

cheers

CharlieRN

Thanks to all for the comments and advice. It looks like I should be OK with 4 AWG jumpers.

A further question about fusing: is it really necessary? Assuming it's recommended, will the spare 40A breaker I have lying around be OK, given that the ampacity of 4 gauge wire is substantially greater than that?

AnOldUR

Denny16 said:

... the jumper between two batteries (series or parallel) should match the amperage rating of the batteries being used.  So two 100 amp batteries will have an ampacity rating of 125, which requires a AWG no 2 wire for a continuous load ...

So, why does my 30amp fuse on the hot jumper between the batteries not blow?

Horigan

AnOldUR said:

Denny16 said:

... the jumper between two batteries (series or parallel) should match the amperage rating of the batteries being used.  So two 100 amp batteries will have an ampacity rating of 125, which requires a AWG no 2 wire for a continuous load ...

So, why does my 30amp fuse on the hot jumper between the batteries not blow?

Because your DC loads aren't pulling more than 30 amps.  The batteries are capable of providing more than 30 amps, but the actual amps is dependent on what DC loads you have on.  If you had an inverter, that fuse would blow.

ScottG

I'm following this, though more out of curiosity than concern. My T@B has an extra battery harness in the tub that allows the connection of an external battery. Along with the house battery, this is wired through a four position cut-off switch that allows selecting either or both batteries. In the "both" position, I would effectively have a dual-battery set-up, though it's not my main intent to use it in this configuration (nor have I ever done so).

Both batteries are connected the with 10g wire protected by 30A fuses. I too have often wondered why much heavier cables are the convention for dual-battery configurations, even if the total draw will never exceed 30A. I suspect I might be missing something here, but I can't see what.

Denny16

No Scott, a 2015 TaB S is wired for a max 30 amp DC system, so nüCamp matched the wiring for 30 amps.  That said, on most marine hookups I have worked on, battery to battery to battery and battery to battery switch cables are matched to the size of the battery, so AWG 4, 2, or 0 cables are used.  The connection from the switch to the house main DC panel is matched to the max load size on the DC panel, typically AWG 8 for most small boat applications, but can be larger, depending on the system load sizes.
cheers

ScottG

@Denny16, does that configuration on boats have to do with the batteries also serving as starting batteries? I can see in that case where there would be a very large draw on those batteries warranting much heavier wiring. 

Denny16

No and yes, some boats have separate battery banks for engine starting and house power panels, depending on the size and type of bot.  Some smaller boats may have both house and engine starting battery linked on the battery switch, to allow the house battery to be used for emergency engine starting. 

 Most boats I worked on are sail boats, so house and engine batteries are kept separate.  Also engine starting batteries are large cold cranking short cycle starting batteries, while house batteries are deep cycle RV type AGM batteries. 

cheers 

CharlieRN

Denny16 said:

No Scott, a 2015 TaB S is wired for a max 30 amp DC system, so nüCamp matched the wiring for 30 amps.  That said, on most marine hookups I have worked on, battery to battery to battery and battery to battery switch cables are matched to the size of the battery, so AWG 4, 2, or 0 cables are used.  The connection from the switch to the house main DC panel is matched to the max load size on the DC panel, typically AWG 8 for most small boat applications, but can be larger, depending on the system load sizes.
cheers

This makes the issue of fusing the jumper even more confusing for me. What purpose is served if the system downstream is already protected?

Denny16

Charlie, there is no real need to fuse the jumper, if you use the correct size jumper wire.
cheers

Horigan

@CharlieRN, Fuses are generally intended to protect the wiring from overheating from a short circuit to ground.  Very short wire runs with little risk of shorting to ground, like a battery jumper, don't need a fuse.