Big Ol' Battery Box


Punky Pewster lent a helping hand in building a couple cardboard boxes, one of them being a mock-up of a 12V car battery and the other being the battery box itself.

I needed to build a platform for the 12 volt battery in the electric car project but had no structure onto which to work. Placing the actual battery into position would have meant using supports that would get in the way of building the structure itself because of the weight, so lightweight mock-ups were the way to go!

Step 1. Build a cardboard battery.

Step 2. Build a form around which a tray/platform can be built. This is built a bit on the larger than needed size, knowing it will need to be cut down but still want space all around the battery in case a different size batt is installed in the future.

Step 3. Build a quick lightweight platform in place using cardboard and wrap with carbon fiber.

Step 4. Fit/Clearance check before permanently bonding stuff together.

Step 5. Box/Tray is bonded to the platform and the height trimmed down a bit. A hole is added into the box on the right side which is access to the mounting bolt. Since the platform was cast in place directly on the car’s chassis, it indexes its location automatically and only needs to use a single bolt to hold it in place. This particular bolt was pre-existing and connects the front section of the chassis to the center part. It’s a 5/8" diameter bolt and is way underloaded.


That is definitely thinking outside the box. (Or inside the box. Not sure which way that works best.) :smile:


Why do you need a lead acid battery in an electric car? Seems ironic to make a lightweight carbon fibre box to hold a big lump of lead.


Yes, there are some nice, lightweight, 150 Ah lithium ion batteries out there–problem is they cost 10 to 15 times as much!


Yes but if it is an electric car surely it is packed with lithium batteries. Why do you need a lead acid as well?


Because the high voltage battery is physically disconnected when the car is not running (as a safety measure—e.g. for when a mechanic is working on it). So you need the 12V battery there to power the lights, locks, alarm, user interface, etc. when the car isn’t running. And of course to connect the high voltage battery when you attempt to start the car.


@tim1724 has it.

There is still a 12V system that needs constant power and tapping 12V off a high voltage lithium pack causes balancing issues (particularly when there’s 200 cells to balance).

Lithium 12V batteries (at least the ones of good quality and not prone to fire) are ridiculously over priced, way out of line. The cheaper ones are simply junk.

I have lithium cells for building a 12V battery, same cells that are in the high voltage pack, but for expedience sake I need to get on with the project. I can always swap it out later but right now using something off the shelf to speed up proof of concept eliminates one more question. Besides that, this batt is only 30lbs.

I have probably 200lbs worth of stereo gear in there as it is, and I could stand to lose about 80lbs as well. LOL. Saving 10-15lbs from a battery? Not really worried about it.


Cannot wait to see the car…or from new england…cah.


All the hybrids do the same thing (like the Prius) where they have a regular LA battery in addition to the huge EV pack. (nor do they let you charge the 12V battery from the big pack in the case of a dead battery)


What do all electric cars like Tesla do?

I would have thought a down converter from the main batteries would be the cheapest and lightest solution.


I knew the answer to the lead-acid battery was it needed a separate 12V system, but I didn’t know the details so thanks for that. But even being pretty sure about that, I was wondering why such a large 12V battery since there is no need to move eight cylinders in the dead of a Canadian winter night and…

then all was explained :wink:


I imagine the guys said “there is a warehouse of LA battery solutions hardware already debugged over a century of cars” and said, no reason to go with a new piece of tech.


IIRC my friend’s Nissan Leaf has a separate 12V system. It would seem unlikely there will be a separate lead-acid battery in forty years, but at the moment I can see why the decision was made.


It probably would be. But it’s not necessarily the safest solution.

As far as I know, all hybrids* sold in the US physically disconnect the high voltage battery when the car isn’t running. (On some hybrids you can hear a faint click from the relays when turning the car on and off.) The 12V battery needs to be there to activate the relays (and of course to power the computer that controls those relays). The relays also cut off the high voltage supply in the event of any accident severe enough to trigger the vehicle’s airbags. There’s generally also a manual interlock that can be used to be used to be absolutely certain that the battery is disconnected. (You’re supposed to use this whenever you’re doing maintenance.)

I would certainly assume that all-electric vehicles have the same safety systems (as they all have high voltage batteries) but I’ve never looked into the details.

* At least the hybrids that use high voltage (~150-350V) batteries. A handful of low-end hybrids (such as the short-lived Chevy Malibu hybrid) use medium voltage (36V) which might not need to be disconnected.


My Lexus has both (and an engine too :slight_smile:). The 12V powers stuff like the clock & radio & security system and it fires the relay that connects the big pack to the transmission and starts the car. Even on a 6-figure car that’s a standard lead-acid battery no lithium magic. The gas engine takes over to charge both battery systems as well as picking up when I want more speed than the electric motor provides (the electric motor is inline with the tranny so it’s on all the time).


My Tesla Model S has 12V lead acid battery. (Though I’ve never seen it. Not sure where it’s located actually.). In addition to the reasons given, I think it helps then with various safety approvals since they can use existing parts and systems for that end of it and the regulators already understand it. Then they can focus the new discussions on the parts they have to change.


Oh, and as for the reason the 12V battery is lead acid and not something fancier? Partly because they’re cheap and the charging circuit is trivially simple, but also because lead acid is ubiquitous in the auto industry so when the 12V battery dies and you call roadside assistance the tow truck is likely to be carrying a spare. (Although some hybrids/electrics use smaller size batteries, e.g. Ford hybrids often use a 67R battery, which not every tow truck is going to have.)


You have an access point behind the front “grille” of your S. If you ever happen to park your car and not drive it for a few days (ie: air travel) and you come home to a dead car, you can jump it from another vehicle/AAA service.

There are also inertia switches that can directly control the high current contactor on the high voltage pack.

I put DC conversion in this car but it’s not cheap. High voltage input and high current output plus strong isolation is expensive, my chosen system is at least 10X the cost of a mid-level SLA battery from the local auto parts store.

However, I have built in an “emergency” system where if I ever encounter a dead 12V battery system (which would mean no power to close relays and contactors) I can at least manually bypass it all and power up the DC/DC converter to make the 12V system active and get the car running.

The #26 case size is amongst the smallest automotive battery available, it came in the Geo Metro :smiley: and other tiny tin cans but despite that, it’s also a somewhat rare size. So, no it is not cranking over a V8 in winter cause it can’t do that even if I had a real engine. :smiley: It can handle peak transients in a stereo system though.


Much to the distress of neighboring motorists. :scream:


I didn’t see anything for scale. That makes sense.

Bigger battery, bigger stereo, just sayin.