What is the best BMS to use for making a car starting battery?

[n4mwd]

I know there are several manufacturers that make LFP car batteries, but they are always way overpriced. With the price of LFP cells coming down, it seems feasible to make my own since my car doesn't need a huge battery. Since the LFP cells are capable of doing the job, the bottleneck is the BMS.

Some things I am concerned about,

1. LFP cells max out at 3.65V or 14.6V with 4 cells. The car alternator puts out 15V which will trip the BMS. If the BMS constantly disconnects the battery, isn't that going to cause trouble with all the car electronics?

2. The BMS has to allow normal current charging and normal current use, but still allow high current output for starting. I'm guessing it needs about 300A to crank the car.

Does anybody know of a BMS that will work like that?

 

[sunshine_eggo]

I know there are several manufacturers that make LFP car batteries, but they are always way overpriced. With the price of LFP cells coming down, it seems feasible to make my own since my car doesn't need a huge battery. Since the LFP cells are capable of doing the job, the bottleneck is the BMS.

Some things I am concerned about,

1. LFP cells max out at 3.65V or 14.6V with 4 cells. The car alternator puts out 15V which will trip the BMS. If the BMS constantly disconnects the battery, isn't that going to cause trouble with all the car electronics?

Why does it put out 15V? Very few batteries can withstand that.

2. The BMS has to allow normal current charging and normal current use, but still allow high current output for starting. I'm guessing it needs about 300A to crank the car.

You need to look for a non-FET based BMS that has an external electromechanical relay. You should also consider what 300A will do to your cells as most LFP only supports 1C discharge rates.

 

[n4mwd]

Why does it put out 15V? Very few batteries can withstand that.



You need to look for a non-FET based BMS that has an external electromechanical relay. You should also consider what 300A will do to your cells as most LFP only supports 1C discharge rates.

I got that charge number from another site, but 14.7V sounds more reasonable. Higher than normal voltage wont harm a lead acid battery beyond boiling off some of the water.

I would prefer a solid state bms solution that can surge to a high amp output for about 10 seconds that would be needed to start the car. A mechanical relay that could handle that many DC amps would be kinda huge.

 

[sunshine_eggo]

DALY 300A:

https://www.amazon.com/DALY-Programmable-Configurable-Protection-Bluetooth/dp/B08VHYV4S6

10" x 5" x 2"

Spendy and huge.

 

[n4mwd]

DALY 300A:

https://www.amazon.com/DALY-Programmable-Configurable-Protection-Bluetooth/dp/B08VHYV4S6

10" x 5" x 2"

Spendy and huge.

Yes, thanks. I will probably go with a slightly smaller one. That one will handle a 1000A starting load.

The BMS units that I have looked at before would be rated for say 100A and only have a surge of 150A. Those wont work.

I will look into the DALY BMS units.

The LFP cells I'm looking at have a 25C rating on instantaneous surge discharge. So they are more than adequate to start the vehicle.

 

[Sharky722]

Have a look at the smartec 200a with bt.
I've just purchased one for car starting application.
They have huge overcurrent capabilities and I had my hvd set from factory to 3.9v pc to allow for high voltage alternators.
They are a beast of a bms.

 

[Vicriz]

If your car altenator show 15v, I believe your car standard accu is the best choice all the way.

 

[n4mwd]

Have a look at the smartec 200a with bt.
I've just purchased one for car starting application.
They have huge overcurrent capabilities and I had my hvd set from factory to 3.9v pc to allow for high voltage alternators.
They are a beast of a bms.

I couldn't find anything on their website about bms's. The only thing I found were finished batteries, and no prices, which generally means I can't afford them.

 

[Sharky722]

I couldn't find anything on their website about bms's. The only thing I found were finished batteries, and no prices, which generally means I can't afford them.

Products_Products_Shenzhen SmarTEC Technology Co., Ltd

Products-Shenzhen SmarTEC Technology Co., Ltd-产品中心

m.en.szsmartec.com

They are also on alibaba, Mina is the contact, but she prefers whatsapp from comms.

My 2x 200a 4s was $560aud delivered air express

 

[mikefitz]

Many vehicles produced in the last 10 years have complex charging systems for their batteries, often AGM. There are also numerous electronic modules in the vehicle that are designed to function with the OEM battery system.
Personally I can see no advantage in using a DIY lithium battery to replace a starter battery. Its a waste of time and money with the possibility of damaging the vehicle, yourself and others.
There are safety implications ranging from possible fire to electrical interference with the vehicle systems.

In Europe any electronic system fitted to the vehicle , this would include a battery BMS, have to comply with certain test standards regarding EMC.

Mike

 

[n4mwd]

Many vehicles produced in the last 10 years have complex charging systems for their batteries, often AGM. There are also numerous electronic modules in the vehicle that are designed to function with the OEM battery system.
Personally I can see no advantage in using a DIY lithium battery to replace a starter battery. Its a waste of time and money with the possibility of damaging the vehicle, yourself and others.
There are safety implications ranging from possible fire to electrical interference with the vehicle systems.

In Europe any electronic system fitted to the vehicle , this would include a battery BMS, have to comply with certain test standards regarding EMC.

Mike

My car is older than 10 years. Actually the first battery from the Toyota factory lasted about 12 years. The second one has been in for just a few years and is already showing signs of weakness. A commercial lithium starting battery typically has only a 5-year warranty. Today, even a high-end lead acid battery only has a 3-year warranty, with some extending it to 5 years with insurance.

The bottom line is that I do believe that I can build a functional DIY lithium starting battery for less than the $400 I would spend for a high quality lead acid battery. Provided I can get the right BMS.

 

[mikefitz]

BMS, here you go,designed for engine start,

3S 12.6V Li ion or 4S 14.4V Lifepo4 emergency car jump starter battery PCB board with 250A discharge current - LLT POWER ELECTRONIC

Application :This is The BMS for 3 series 12.6V (11.1) V Lithium battery pack with 250A constant constant discharge current , Which would be applied for big power rate motorcyle Function :This BMS has such basic functions : over-charge protection ,over-dicharge protection, over-current...

www.lithiumbatterypcb.com

Hope your project goes well.

Mike

 

[n4mwd]

BMS, here you go,designed for engine start,

3S 12.6V Li ion or 4S 14.4V Lifepo4 emergency car jump starter battery PCB board with 250A discharge current - LLT POWER ELECTRONIC

Application :This is The BMS for 3 series 12.6V (11.1) V Lithium battery pack with 250A constant constant discharge current , Which would be applied for big power rate motorcyle Function :This BMS has such basic functions : over-charge protection ,over-dicharge protection, over-current...

www.lithiumbatterypcb.com

Hope your project goes well.

Mike

Thanks, that one looks like a winner. It looks like it was talking about emergency jump starters, but I don't see why it wouldn't work as long as it's common port.

 

[Ampster]

I have no experience with alternators but from what I read an alternator can be damaged if it it disconnected from a load while running. I do not know what changes in wiring would need to be made to prevent that damage.

 

[mikefitz]

Yes, if the load is removed from the alternator the inductive energy stored in the windings has to go somewhere. Unless additional protective circuit units are added, the alternator destroys it's electrics and often part of the vehicles electric modules.

Mike

 

[n4mwd]

Yes, if the load is removed from the alternator the inductive energy stored in the windings has to go somewhere. Unless additional protective circuit units are added, the alternator destroys it's electrics and often part of the vehicles electric modules.

Mike

Car alternators vary the field to vary the inductive current. Also, the battery is not the only load. It needs to be able to handle abrupt changes in load such as turning the headlights on and off.

However, the alternator requires the battery to smooth it's ripple current into pure DC. That said, it would probably make everything work better if there were some super caps across the external battery terminals.