Battery Relocation Under Seat

[nhep23]

Hey guys,

 

I’m planning to relocate my battery to under the rear seat. I’ve done quite a bit of research on the forum but I still want to sanity check my plan/thought process and crowdsource some feedback.

Plan:
Relocate battery to under the rear seat using an AGM battery. Removing standard battery and tray from engine bay to free up space.

 

Westco 12V31M Battery:

• 12V
• 31 AH
• 475 CCA - Note: I live in Denver and I plan on driving this in temps as low as 20degF
• 21.7 lb

 

See image of attached diagram.

• Westco battery, mounted to structure using 02 Again battery mount.
• Neg terminal grounded to either a bolt that I can find under the seat, or i will add a new hole/bolt for ground
• 200A marine breaker.
• Quick Car Battery disconnect switch - switch will be accessible behind driver seat.
• 1 gauge wiring going through firewall into a terminal block mounted on the front left corner of the engine bay.
• 12V misc size wires going from terminal block to the starter, alternator, horn relay, ignition switch and fuse box.
• 1 gauge wire grounding the engine block to the engine bay

 

Questions/comments:

• Is the Westco 12V31M sufficient for Denver climate? Notable electrical changes are an electric fuel pump to feed my dual DCOE 40’s. Previous owner installed a stereo system which is currently disconnected but i'll probably hook back up to it. 
  • I'm not really looking for a super lightweight solution so don't really care about lighter batteries. Would be interested in any feedback regarding the 12V31M. The 12V31M is a lot more cost effective ($160) compared to those lighter AGM solutions (Optima red/yellow top ~260$)
• Should the battery cutoff switch be on the pos or neg side? Or does it matter?
• Planning on buying cut to length cables to the local Autozone and crimping with this Hydraulic crimper from Amazon. Has anyone used these?
• Any issue going to the terminal block first, and then the starter? I know all junctions are bad, but if i’m getting pretty low resistance between the terminal on the starter and the pos terminal on the battery does it matter?
• Is 1 AWG sufficient or overkill? I’ve gotten mixed signals, some people use 4 AWG from TEP and some use 1/0 AWG.
• I know a lot of people use the E30 harness/junction box (that was also my original plan) but I know I would have to butcher it up a bit to get the breaker/switch in the circuit either way so that's why I deviated from that path.
• The components coming off of the distribution box (not fused) are the same ones that come off of the pos terminal on the wiring diagram, so the only "fuse" i'm adding it the one right off of battery pos. 
• I selected 1 AWG and 200A breaker based off of the current rating listed on this website. I'm assuming it's 9-10ft from battery to terminal block.

Edited May 11 by nhep23

[NickVyse]

I've had one of those batteries in my NK for a few years, really impressive. When I was in a bind I hooked it up to my old diesel Land Rover Discovery and it even cranked that crazy high compresion engine. It's never let me down even in the middle of a cold winter and it's it's never died on me, even when chasing a start issue that meant a lot of cranking before I'd figured it out. I'd buy another in a heart beat.

[uai]

That's basically my setup but it'll take some days to do some writeup and take photos as I'm a bit busy.

[NYNick]

I did this with my 74tii. Used an Anti-gravity Lithium that weighs less than 3 lbs. I went to an Interstate Battery store/distributor and told him what I was doing. He cut some cable for me (11 feet of each?) and even attached the proper ends to fit the Lithium Battery. I ran it through the A/C holes in the firewall with a left over rubber grommet and then along the firewall to the front of the car. Made direct attachments from there.

[jgerock]

In my 69, I’m using a Big Crank gel battery mounted flat to the right rear under seat area. A home made angle iron base is bolted to the floor. Manual cutoff switch mounted in rear seat bulkhead (easy reach from driver’s seat) using positive side of battery.  Ground using common auto store black negative cable secured to seat belt bolt.  Custom cables made from marine grade copper cables (more flexible than welding cable!). Jegs battery terminal block mounted in right front engine bay. Additional positive cables run to alternator and starter.

[Stevenc22]

1 AWG is overkill I have 2 AWG running from my battery under my back seat.

 

Your wiring diagram is correct. Running a nice bolt through the floor board will act as a good ground. Just make sure you scrape away the undercoating so the bolt can ground out on the metal body.

 

I run dual PC680 batteries under my back seat.

 

 

 

 

Then in the engine bay i use this stud for the positive

 

And this through hole stud for the negative which picks up the ground from being bolted to the wheel well.

 

 

 

 

 

Edited May 11 by Stevenc22

[Mark92131]

Basically the same setup for my 1970 BMW 1600.  Went with a single PC680, (because it worked well in my 1976) and moved all my relays under the back seat as well. 1/0 AWG to the front distribution box.

 

Mark92131

 

 

[jp5Touring]

2 hours ago, Mark92131 said:

Basically the same setup for my 1970 BMW 1600.  Went with a single PC680, (because it worked well in my 1976) and moved all my relays under the back seat as well. 1/0 AWG to the front distribution box.

 

Mark92131

 

 

Okay thats  pretty cool.

Edited May 12 by jp5Touring

[dlacey]

On 5/11/2022 at 6:12 AM, nhep23 said:

Any issue going to the terminal block first, and then the starter?

I suggest running the heavy cable straight to the starter, thats where the high current is needed...any other junction on the way is just extra resistance. You can then use the starter post as the 'terminal block' to power the loom etc, or run a single lighter cable up to a 30A distribution block.

[nhep23]

10 hours ago, dlacey said:

I suggest running the heavy cable straight to the starter, thats where the high current is needed...any other junction on the way is just extra resistance. You can then use the starter post as the 'terminal block' to power the loom etc, or run a single lighter cable up to a 30A distribution block.

Yep if I were to do that, I would run my 1 AWG straight to the starter and then a 4 AWG to the distribution block on the wall, and then run appropriately sized wires to switches, alternator and fuse box. I initially had a concern over a "bulky" wire going to the engine, but now realizing that's invalid seeing as I'll have a 1 AWG wire going to the engine block for grounding anyway...

[AceAndrew]

Agh!  So many cool ideas.  There's a lot of neat electrical projects happening on the FAQ right now. @Mark92131, I'm going to have to borrow another cool idea from you (communal ignition-switched power bar).  What gauge wire are you running to the bar?

 

@nhep23, My plans are also similar to yours with regard to battery, but with some twists.  Will post the full break down and wiring diagram sometime this summer.  

 

WestCo Miata Battery (under rear seat).

• One power main power wire running to starter (200amp breaker)

• One power wire running to the Power Distribution Module.

Power Distribution Module (under rear seat)

• One power to Audio (fused)

• One power to Bussman Relay/Fuse box (fused)

• One power to buss bar (fused)

 

 

 

 

[jimk]

A power wire from the Alternator to the starter lug instead of connecting it somewhere else can avoid a wire coming off the engine.  That's how I have it.

[Mark92131]

7 minutes ago, AceAndrew said:

What gauge wire are you running to the bar?

 

10 Gauge from the fuse box to the bar.  You can see it snake out from under the fuse box in the last picture.  Right now I am running 4 relays (Fan, Fuel Pump, Microsquirt ECU, Innovate LC-1).  I pulled all fuses except the Fan until I am ready to go wasted spark and fuel injection, (when IE restocks their trigger wheel kit for the M10).  I bought the Bus Bars, Power Distribution and fused Relay Box on Amazon.  Here' some links.

 

Mark92131

 

https://www.amazon.com/gp/product/B07KVVBL4S/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

 

https://www.amazon.com/gp/product/B08PCGFX3L/ref=ppx_yo_dt_b_asin_title_o01_s01?ie=UTF8&psc=1

 

https://www.amazon.com/gp/product/B08KHVHLL5/ref=ppx_yo_dt_b_asin_title_o01_s01?ie=UTF8&psc=1

 

[AustrianVespaGuy]

On 5/11/2022 at 1:12 AM, nhep23 said:

Should the battery cutoff switch be on the pos or neg side? Or does it matter?

It doesn't *really* matter, but I personally prefer the negative side.  Since the whole chassis is ground, if you break the positive side, you still have a small chance of errantly touching whatever is still hot between the switch and the battery terminal to the chassis.  Like say a wrench handle when you're working on the positive battery terminal.  Disconnect the negative however and now the whole chassis is 'safe.'

[jgerock]

1 hour ago, AustrianVespaGuy said:

It doesn't *really* matter, but I personally prefer the negative side.  Since the whole chassis is ground, if you break the positive side, you still have a small chance of errantly touching whatever is still hot between the switch and the battery terminal to the chassis.  Like say a wrench handle when you're working on the positive battery terminal.  Disconnect the negative however and now the whole chassis is 'safe.'

BMW did a neat thing in the e30 with trunk mounted battery. They added a plastic wheel housing cover near the positive battery cable connection. Note my wrench.