Question about 12 Volt "starter" Battery (Please move if incorrect)

[DVD3500]

I tired searching the topic but the search kept throwing out the "12" so I was not getting any results.... Please move somewhere else if necessary!

My conversion is a small, light roadster (Triumph Spitfire) so every gram counts as I can`t go over weight too much (Germany=TÜV).
I am putting in lightweight components where I know I can get away with it (billet aluminum flywheel, alloy hubs etc).

One very heavy component is the 12 volt battery.

I know there are new 12 volt Lithium based batteries out there and I know that I need the BMS to talk to it to make sure it doesn't over charge.

I was just curious though as to what solutions everyone else is using?

Still using lead-acid or did you go new Lithium based or did you re-use another battery pack?

Luckily the Spitfire will not have too many demands on the 12 volt system.
Just the usual external lighting, a small courtesy light in the footwell and trunk/boot and the fan for the heater.
There are road legal LED kits for the headlights and I am looking at some electric heating that will likely be in the 1.5 to 3.5 KW range (there is not much space under the dash for heating elements). I may hook the heating up to the high-volt system (likely sub 100 volts) though as that likely makes the most sense...

Not sure any of that is relevant...

I am a bit of caveman with all things electric and electronic so be gentle please.

[muehlpower]

Hello, I have not yet fully understood your configuration. You say you have a flywheel? Is it a hybrid or a combustion engine? Does the 12V battery have to start an engine? There are ready-made lead-acid battery replacement solutions with their own 12V BMS. Nothing needs to be changed on the vehicle. In your case, a battery designed for motorcycles may be sufficient.

[jrbe]

For cabin heating consider the high voltage options, they should be more efficient.

Yes there are lithium ion 12v batteries. They should have their own BMS onboard to handle balancing. Setting the DC-DC voltage to a reasonable level should keep the battery voltage up but also in the lithium ion sweet spot.
The one challenge is temperature. Some of these have heaters to keep them above freezing.
But I don't think there's an established communication method to make sure the battery is up to temp and ready to take a charge. Might be able to use a temp sensor to keep the 12v battery safe - DC-DC won't turn on below freezing or maybe have the logic turn on a heater first. It would be good to have a volt gauge too while working the bugs out. Like is the heater in the battery enough to be useful?..

[johu]

I've been toying the idea to replace my lead battery with 3 NMC cells. They would discharge via a diode (not much load, just car standby) and charged with a 14 to 12 V buck converter.
If you have no current draw at all while the car is off or if you don't mind a manual disconnect switch these can be very small, down to a 12V power tool battery.

Essentially it is just needed for boot strapping, turning the HV relays on. After that the DCDC converter takes over

[barracuda816]

Depends on a lot of factors really.

What temperatures will you see where you live?

I personally think the best all-round option would be lithium titanate, and would be hesitant to use NMC unless you really design for it, make sure the charge current is right and generous bottom and top end limits. LiFePo4 would be a decent choice if you don't see sub circa 3deg C temps.

But for the hassle I would just stick a small 12v AGM motorbike battery in and call it good.

Of course if it is a hybrid then all the above goes out the window.

[DVD3500]

Hello, I have not yet fully understood your configuration. You say you have a flywheel? Is it a hybrid or a combustion engine? Does the 12V battery have to start an engine? There are ready-made lead-acid battery replacement solutions with their own 12V BMS. Nothing needs to be changed on the vehicle. In your case, a battery designed for motorcycles may be sufficient.

I responded to this but the answer never appeared... very odd...
Pure electric.

I need the flywheel because I am keeping the transmission/gearbox for various reasons. Therefore I need the clutch and therefore the flywheel.

[muehlpower]

Your question has also brought the topic back into focus for me. I am planning to use a LiFePo battery for the 12V. Approx. 42Ah, so that I have enough power for the radio etc. when the car is stationary. Batteries designed for storage applications are cheap to buy. These, for example https://www.autobatterienbilliger.de/Ac ... uerbefreit . I think you have to make sure that the charging current is not too high, which can happen with an empty battery and a powerful DCDC converter. A voltage limit of 14V will not be sufficient. My idea is to measure the current flowing into the battery and if necessary regulate the DCDC converter down.

The charging current for this battery is limited to 42A. If you are using a smaller battery, it will certainly be less.

[DVD3500]

TMI warning!
I live in Southwest Germany near Hockenheim. The car will be in the garage when not in use and I am planning on heating it up before I use it if it is a bit cold.
I plan on designing it for daily use, though given we already have an Enyaq and a Jeep Avenger I am not sure how often that will actually happen.
For example I bought a special "targa" hardtop from Körbler that is fiberglass. It is much lighter than the standard hardtop and you can remove only the middle section to make it a targa.
The last two years we have had snaps of 20°C plus weather in early April and late March only for temps to drop into the teens and lower in late April and early May.
I want to be able to switch to the hardtop easily and by myself.

I plan on adding a rear window defroster to the hardtop's rear window (mostly for if the window fogs up from humidity), put in a more powerful fan (turns out the fan motor of a Smartcar/Micra is a 1 to 1 match and quieter while using less power)

A guy 10 km from already converted his Spitfire. He has the original fan motor and two PTC elements that can put out maybe 1 to 1.5 KW of heat. They are hooked up to the high voltage system (I think 96 volts? You can feel it but like a hair dryer if you move 20-30 cm away the effect is greatly reduced. They are basically just on or off.

The heater box of the Spitfire is small. I haven't had a chance to measure it but my guess there is only 10-15 CM left and right of the fan. Here is a picture: https://rimmerbros.com/Item--i-GRID0055 ... dk8dKuOJCV There are baffles left and right to direct the air either to the windshield/windscreen or into the cabin. +There is a tiny bit of space at the bottom but that is wear the wires come in for the motor control. You really can't put any other box in there. It is just too tight.

I will be adding heated seats and have recent seen infra red heating sheets that I could cut to shape and put under the glove shelf (the car does not have a glove box) to make the driver and passenger as comfortable as possible. I also plan on buying heated motorcycle jackets.

I am more worried about keeping the windows fogs free. For example I will be adapting the air circulation from the Triumph GT6 which had added bevels in the dash to direct air at the side windows. The Spitfire, being a convertible never had those...
If I get caught in a downpour I need to see without wiping the windows down as a car this small and low will easily get overlooked by those people in their Beaver Mouth Wide SUVs!

So a bit of a thread drift from the original question but I am seeing a few different options here.

I am also starting to think a motorcycle battery might be a good fit.

My very limited understanding of how the systems work though made me think I should try to get the largest capacity 12Volt in there as well. The thought process was that then the DC/DC converter would not have to work as hard because the battery would not deplete as quickly (for example with all lights on, heater and fan on etc.)
Maybe the advantage is negligible?

In fact, if there is space I might be able to fit another LiFePo cell (main battery) in the same spot with the 12Volt battery (I have to double check the size the friend has in his car but if my eyes are remembering it one or maybe 2 cells might fit...

Or am I barking up the wrong tree?

[snelly]

Hi

I run a 12V Webasto diesel engine block cooling system heater to heat cab and warm battery's and it runs of home heating oil too.

it only use a about 1 Amp when its running

You could use the cabin heater type that is just hot air

my jeep is 750kg and uses the small Japanese car battery "UK 055 Car Battery - 12V 35Ah - HONDA TOYOTA NISSAN SUZUKI" about 6kg

[DVD3500]

The battery sounds interesting!
I have been looking at some sort of fuel-based heating too but am trying to keep the car as emissions free as possible or at the least using some sort of renewable fuel like alcohol.
My other concern is where to keep the fuel. The German TÜV likely has very strong feelings about that. I am pretty sure I read a rule that it cannot be kept in the cabin and cannot be in the extremities of the vehicle.

The other problem is the most common configuration for these "Sports Six" Triumphs (The Spitfire is based off the Herald, which also begat the Vittesse and the GT6 is based on the Spitfire) is you replace the ICE with an electric motor and pile as many HV batteries as you can on top. The hood/bonnet opens like a clamshell including the fenders/wings. That means that there is not a lot of room for other items. The spot behind the seats where the fuel tank was is then filled with chargers and and if you have space and weight allows it; batteries. So not much space there as well...

[johu]

I think the days of Diesel heaters are over with todays battery prices. Who wants that smell back?

These infrared sheets sound great, do you have a link?

The DCDC should be able to supply all loads without the help of the 12V battery

[DVD3500]

I stumbled across them here:


You can find them on the usual online places like eBay etc. I have not purchase any yet though.
Can a DC/DC converter output different voltages?
for example can it feed the 12 volt battery but also output 24 volts or higher?

[muehlpower]

As johu noted, the DCDC converter should be sized to cover the current requirements of all connected devices when they are all running at the same time. In my experience from the 70s, 45A alternators were the lower limit for simple cars. This corresponds to approx. 600W. The DCDC converter that Fellton sells has 1000W, so it is sufficient if there is no 12V/1000W electrical heating. Nevertheless, the 72A can be too much for a LiFePo battery, which is available when no devices are switched on, e.g. daytime, summer, sunshine. How do you deal with this?

[DVD3500]

I understood every word you wrote but still have no idea what you are asking or how it is relevant to my question.
Like I said, I am a caveman when it comes to electric and electronics.
I plug things in and they work or they don’t… that’s about it…

[muehlpower]

If I understood your initial question correctly, the idea is to replace the heavy lead-acid battery with something lighter, e.g. a lithium battery. Lead or lithium must be charged, for this you need a DCDC converter, the size of which is defined by your consumers. With lead it is simple. You take a DCDC converter with e.g. 1000W or 2000W and regulate it to approx. 14V. That's it. Because a lead-acid battery is insensitive when it is charged with a high current. 2000W corresponds to 140A. If a LiFePo is charged with too much current, it can be damaged. And because of the low internal resistance, high currents occur at low cell voltages, up to the limit of the DCDC converter. In my opinion, this must be prevented. Perhaps one of the gurus will comment on the subject

Translated with DeepL.com (free version)

[snelly]

I am using small Lead acid is because its simple and I do not have any car electrics that use much power, lights are all LED, windscreen wipers take the most power and I wonted to save energy in High Voltage battery for drive motor, to keep to a the minimum the energy feed to the DC to DC to the 12v systems.

Things that take a lot of power are power steering, electric windows, electric power brake that needs a 12v compressor and electric heating.

[johu]

Yes I use a large lead acid too, so far. Highest charging current I ever saw was 50 A.
Using LFP in the A2 conversion. Never checked what would happen if the cells are low. It is very unlikely as the car draws near 0 standby current and the battery is kept at 14V during driving and charging. There is no accessory mode in that car.

Like said with NMC I would use a buck converter with current limit. Maybe a power tool battery with low voltage cutoff

[DVD3500]

Isn’t a BMS supposed to take care of all of this…?

[barracuda816]

Isn’t a BMS supposed to take care of all of this…?

Depends on the BMS but for a 4s most do not have current limiting just over current protection. Meaning they would just turn off, then back on, then off. That's not the desired behaviour.

[DVD3500]

What`s a 4s?

[muehlpower]

With a 12V battery, the BMS only distributes the current evenly to the individual cells in the mA range. I have not yet seen a limitation of the total charging current. Even with the HV battery, the BMS does not limit itself, but passes the information on to the charger and opens the contactors in an emergency if the charger is still sending too much.

Has anyone tried removing the 12V battery and replacing it with a second, small DCDC converter that is permanently connected to the HV battery? The converter would only have to be large enough to close the contactors and operate the hazard warning lights, as required by the TÜV. Approx. 120W like this one https://www.fortec-power.de/Mean-Well-W ... AC-12V-10A or this https://www.fortec-power.de/Mean-Well-L ... -12V-12.5A
I know it's for AC, but in my tests they all worked with DC as well.

[barracuda816]

What`s a 4s?

What are refered to as batteries are basically a collection of Cells (for larger batteries, they are made up of modules, these modules are made up of cells) So it goes like this;

Battery -> Module (omitted in small batteries) -> cells

Depending on the characteristics we want from a battery, i.e battery voltage, capacity and C rating (the ability to charge or discharge ( output or accept current) as a function of their capacity. We design the battery with cells in series to sum the individual cell voltage or parallel to sum their capacity rating. So looking at 4 x 3.6v 50ah cells with a 2c discharge rating, in series would be give 14.4v at 50ah (amp hours) so they can draw 100 amperes at 14.4v or we can configure them in parallel where we have 3.6v but we can now draw 400 amperes. Most batteries do both so you may see a 96S 2P for example, that simply means that there are 96 cells in series and there are 2x that amount of cells but the second "set" of 96 cells are wired in parallel with the first, doubling the current capabilities but keeping the voltage as 96 times the cell voltage.

Standard low voltage ranges are 4s 8s 16s so circa 12v, 24v and 48v.

Generally 12v BMSs don't have current limiting (they may be available but I am not aware of any), that functionality is more common on 48v systems.

If you have a charging system that is capable of exceeding the C rating of the cells it can cause damage or depending on the chemistry cause the cells to burn. Lead acid batteries don't have this problem as they generally are more thermally stable.

That's a brief over view, From there it goes a lot deeper.

[barracuda816]

With a 12V battery, the BMS only distributes the current evenly to the individual cells in the mA range. I have not yet seen a limitation of the total charging current. Even with the HV battery, the BMS does not limit itself, but passes the information on to the charger and opens the contactors in an emergency if the charger is still sending too much.

Has anyone tried removing the 12V battery and replacing it with a second, small DCDC converter that is permanently connected to the HV battery? The converter would only have to be large enough to close the contactors and operate the hazard warning lights, as required by the TÜV. Approx. 120W like this one https://www.fortec-power.de/Mean-Well-W ... AC-12V-10A or this https://www.fortec-power.de/Mean-Well-L ... -12V-12.5A
I know it's for AC, but in my tests they all worked with DC as well.

Any decent low voltage BMS will limit the current (charge and discharge) but will not throttle it. So on an over current event, it will "cut the power" like a resettable fuse. But most will reset themselves on a time delay so you will just see the charge mosfets turning off on an overcurrent, then back on where it will immediately over current again, and cycle like this. Obviously not a good situation.

[DVD3500]

My response disappeared again…
Why is this bad?
Is it like marathon and sprinting?
You want marathon (slow and constant) for langevity as opposed to sprinting at high speed for short periods?

[barracuda816]

Dumping too much energy in or out of a cell will cause it to heat up. When you dump more than what they are rated for it can cause them to combust (most likely with NMC chemistry batteries, with LifePo4 they will just die and billow out loads of smoke, and lithium titanate it will just cause a short life, but they are expensive).

Then you have to remember that there are temperature limits for when they will be able to charge and discharge.

What you want to do can be done, and relatively easily, but the system has to be designed for it. Then at the end of it you save maybe a couple of kgs. Really I would recommend just a smaller AGM battery like one out of a motorbike.

Regarding heating, there is some good work being done on HV coolant heaters. That way you can keep the spitfire heater core.

Out of interest what battery, motor and charger set up do you have? You say you kept the flywheel and clutch as you are using the getriebe still. Most people still remove these, so if you are looking at cutting down the weight I would start there. Those old iron flywheels and old style clutches on the spitfire are really heavy. You would also notice an improvement in acceleration too.