Increasing voltage for battery system

oopsies

Joined Aug 27, 2021
10
Hello,

I started learning about electricity, power, etc. because I want to build an electrical system for a boat. But I'm having some difficulty understanding some of the concepts. I really need help filling in the gaps of my understanding as I'm a complete beginner.

So, let's say I have an electric motor capable of taking 96V and 650 amps. I'm looking at LifePO4 batteries that are 12V with 300AH.

With 2 batteries, I am able to increase the current to 600AH by wiring them in parallel. But how would I get the voltage up to 96V without adding a gazillion batteries? That seems to be very impractical (not to mention expensive and may require a lot of space to store everything). Plus, from my reading, you can only wire most of these types of batteries in series up to a max of 4. Is there some way to increase the voltage "massively"? I read in another thread that if the voltage is increased, the current decreases (or something like that?). But I didn't understand what was going on.

Hoping someone can explain/introduce some concepts I may need to look into to understand this better!

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Papabravo

Joined Feb 24, 2006
17,310
It is possible to adjust either the available voltage or the available current from a DC source. What is NOT possible is to make more power, measured in watts, from less power, also measured in watts. There are several equivalent ways to compute power input and output. One method is to multiply the available voltage by the available current. The immutable rule of DC-DC power conversion is this:

Power out will ALWAYS be less than power in. Under some circumstances it will be a great deal less.
So with this immutable rule in mind we can talk about all manner of conversion schemes.

As an example, let us take your motor which requires 96 VDC @ 650 Amperes. This required power is 62,400 Watts. This is a dangerous power level and the circuitry designed for this power level is not something that the uninitiated should attempt. So assuming we have a conversion scheme that is 80% efficient, the required input power is 78,000 watts. 78,000 watts from a 12 volt source will require 6,500 Amperes. This means your 300 AH battery will be useless after 2.77 minutes, even if it could supply 6,500 Amperes when fully charged.

Note: These numbers are so far from reasonable that I think you should abandon any possibility of fabricating such a system. The wire you would need to carry 6,500 amperes probably does not exist, or if it did would be too expensive for words.

oopsies

Joined Aug 27, 2021
10
Can you show me how to make the calculation for the 2.77 minutes? I think I got the other calcs correctly. Just to double check...

96V x 650A = 62,400W

62,400W / 0.8 = 78,000W

78,000W / 12V = 6,500A

300A / 60 mins = 5A per min? This is where I'm lost for the calculation.

What if I don't use the full power of the motor? Let's say I use a fraction of it? On the water, reasonable cruising speed is about 5-10 knots which wouldn't need to use the full power of this example motor (ignoring weight of the boat, shape of the hull, weather conditions, etc.). How would I go about making such calculations?

Papabravo

Joined Feb 24, 2006
17,310
300 Ampere-Hours / 6,500 Amperes ≈ 0.05 hours
0.05 hours * 60 min/hour ≈ 2.77 minutes

I don't quite understand how you intend to use a fraction of the power that the motor requires. If you do that you will produce a fraction of the torque that is possible and the shaft might not be able to overcome the moment of inertia. In that case it will just sit there and get hot. Possibly hot enough to compromise or destroy itself.

oopsies

Joined Aug 27, 2021
10
Thx for the calc!

The reason I was thinking of an electric conversion is because I saw people doing similar conversions with success and showcasing it on YouTube and other places. One person got their boat to go as fast as if they had a diesel engine and for quite a distance & time. Others with bigger boats were able to use similar motors, but with reduced power (5-10 knots speed). And Greenline is the first to develop a large luxury boat that can do ocean crossings fully on electric - not a drop of diesel! I know the tech isn't quite there yet in terms of battery storage for the average consumer, but it would be nice to slowly transition away from diesel motors. So I was hoping there's a way to do this without breaking the bank. But unfortunately, info on how to make the calculations, etc. is a bit sparse when it comes to the fine details. And I lack the knowledge so I was hoping to learn more.

Papabravo

Joined Feb 24, 2006
17,310
Thx for the calc!

The reason I was thinking of an electric conversion is because I saw people doing similar conversions with success and showcasing it on YouTube and other places. One person got their boat to go as fast as if they had a diesel engine and for quite a distance & time. Others with bigger boats were able to use similar motors, but with reduced power (5-10 knots speed). And Greenline is the first to develop a large luxury boat that can do ocean crossings fully on electric - not a drop of diesel! I know the tech isn't quite there yet in terms of battery storage for the average consumer, but it would be nice to slowly transition away from diesel motors. So I was hoping there's a way to do this without breaking the bank. But unfortunately, info on how to make the calculations, etc. is a bit sparse when it comes to the fine details. And I lack the knowledge so I was hoping to learn more.
As with other electric vehicles a strategy of building arrays of batteries connected in series and parallel will allow you to achieve higher power levels with out the penalty of sacrificing voltage for current or vice versa.

oopsies

Joined Aug 27, 2021
10
Yea, I've watched really good reviews for Lynch motors. I was also looking at HPEVS which seems to have some really good reviews as well.

1) Figured out why the current drops when the voltage increases when the power stays constant.
2) Figured out that 1 ton of weight on the water requires 1kW to move it at roughly 5 knots. So an 80 ton boat will need an 80kW motor to move it at roughly 5 knots on the water in perfect weather conditions.
3) Discovered that when you convert from DC to AC through the inverter, you lose current.
4) Found out why a lot of LifePO4 batteries cannot be wired to exceed 48V. It's because the BMS shuts down the batteries when it goes above 48V. So if I'm using 12V 300AH batteries, then the most I can do is 4 batteries in series. Each group of 4 needs to connect to an inverter which would crank up the voltage to the desired level while sacrificing current.

So basically, it's as Papabravo said... the problem still remains with energy storage. Which really sucks for the average consumer. If one DIYs the batteries by buying the cells and wiring them together and throwing on your own BMS, you'd save around 30% of the hard cost. But you sacrifice soft costs which would include time and potentially, safety. And you'll still be stuck with a max of 48V.

Any ideas on how to get around this energy storage roadblock?

Juhahoo

Joined Jun 3, 2019
250
4) Found out why a lot of LifePO4 batteries cannot be wired to exceed 48V. It's because the BMS shuts down the batteries when it goes above 48V. So if I'm using 12V 300AH batteries, then the most I can do is 4 batteries in series. Each group of 4 needs to connect to an inverter which would crank up the voltage to the desired level while sacrificing current.
You can connect as many LifePo batteries in series as your BMS allow to. Very typical is to connect 16 cells to reach 48 volts, but thats no way limit. You need to find out how to cascade BMS's to reach beyond limit of single unit.

BobTPH

Joined Jun 5, 2013
4,053
Found out why a lot of LifePO4 batteries cannot be wired to exceed 48V. It's because the BMS shuts down the batteries when it goes above 48V. So if I'm using 12V 300AH batteries, then the most I can do is 4 batteries in series. Each group of 4 needs to connect to an inverter which would crank up the voltage to the desired level while sacrificing current
This is not true. One particular BMS might handle a max of 48V, but it is certainly possible to make one that handles 96 V. The battery in a Tesla model S is 400 V.

Bob

oopsies

Joined Aug 27, 2021
10
Oh, why is it that many standard packs cut it off at 48V though? Is it a heating issue or something?

Ian0

Joined Aug 7, 2020
3,777

Papabravo

Joined Feb 24, 2006
17,310
This is not true. One particular BMS might handle a max of 48V, but it is certainly possible to make one that handles 96 V. The battery in a Tesla model S is 400 V.

Bob
And weighs over half a ton.

BobTPH

Joined Jun 5, 2013
4,053
And weighs over half a ton.
And so will his if he expects to run it for very long!

Edited to add: The Tesla model S battery is 100 KWh. His motor is 62.4 KW. So, at full speed, that half ton of battery would get him 1.6 hours.

Bob

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oopsies

Joined Aug 27, 2021
10
Unfortunately, those have half the power of lithium and are a lot larger for the equivalent energy storage and don't last as many cycles. They are cheaper though, but I imagine cost savings will be made up down the road for battery replacement. And the boat I'm sizing for is about 60-80 tonnes. I'll probably end up having paid as much as getting lithium at the start.

Ian0

Joined Aug 7, 2020
3,777

Ian0

Joined Aug 7, 2020
3,777
Unfortunately, those have half the power of lithium and are a lot larger for the equivalent energy storage and don't last as many cycles. They are cheaper though, but I imagine cost savings will be made up down the road for battery replacement. And the boat I'm sizing for is about 60-80 tonnes. I'll probably end up having paid as much as getting lithium at the start.
Tubular plate lead-acid battery will do 2500 charge-discharge cycles, about the same as lithium, will cost half as much and the scrapman will give you £500 for a dead one, whereas it will cost you £500 to dispose of a lithium battery. Also, it won't catch fire and burn your boat!

BobTPH

Joined Jun 5, 2013
4,053
Four of those would give you 90V and run you motor for 5 minutes. How long do you expect to run it?

Also your motor is 85 HP, what is the rating of its current motor? It sounds like way too little power for an 80 ton boat to me, but I know little about boats.

Bob