# Marine Battery for Inboard / Outboard Electric Motor

Discussion in 'Boat Design' started by Leopard, Jun 20, 2022.

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1. Joined: Nov 2021
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### LeopardJunior Member

Do I need to take into account CCA for electric outboard/inboard motor? Does CCA have any impact for electric motor? I see most of the boats of 12V batteries. However, mine is 350V.

In case of design boats how much space is usually considered for storing batteries? Should I buy one big battery or many batteries?

Could you please show me a sample calculation for - How long my motor will run with my battery?

Say, Battery Specification is
350v, 60amp, Type LFP , Max 4C discharge, Standard Discharge 2 C
Motor Specification is
50kw , 3 phase , 350v , 142 A

2. Joined: Apr 2015
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### HeimfriedSenior Member

In case the "60amp" means 60 Ah (Ampere hours) the max curent draw of your motor means 2,37 C, so it will (in theory) last for 25 minutes until your battery is empty.
In reality things are a bit more complicated and the time will be shorter.

3. Joined: Mar 2013
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CCA means cold cranking ampere and is a measuring unit for lead acid starting batteries. It's completely irrelevant for lithium chemistries.

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4. Joined: Nov 2021
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### LeopardJunior Member

Can this battery run this motor as I see the motor ampere is higher than the battery?

5. Joined: Apr 2015
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### HeimfriedSenior Member

Dealing with a voltage of 350 V on board of a boat is very dangerous.

A battery with 60 Ah capacity and a max. discharge of 4 C would be able to output the max. current for the motor of 142 A. But limited to a very short time, may be only seconds.
BTW: The battery output is DC, a 3 phase motor needs AC, therefore there must be a motor controller in between which transforms the battery current to AC.
As Rumars said: start learning.

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### gonzoSenior Member

Also, to properly calculate the battery capacity, you need to use Peukert's equation. It was originally for lead/acid batteries but applies to Li Ion too. In short, you first need to determine the minimum voltage the equipment will still work at. That is your 0% discharge point. A higher discharge rate will give less capacity. 350V on a small boat is extremely dangerous. Also, converting DC to AC 3 phase will create considerable losses.

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7. Joined: Nov 2021
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### LeopardJunior Member

Could you please tell me how you are calculating these? How you are using the discharge rate ?
I just can calculate the battery can sustain for 25.5 mins with one battery.

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### LeopardJunior Member

The battery has multilayer protection and completely waterproof. Is there any other risk? Is there any disadvantage of using several batteries to sum up the power rather than using one?

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### portacruiseSenior Member

Fire, and some explosions have been reported with lithium batteries despite multi-layer protection (some rare BMS failures), and 350 v electrocution risks, plus perhaps some other trivial risks associated with all batteries.

Several batteries have to be used because there is no single huge battery (or single lithium cell) manufactured that would have the power required. The many batteries that are required would have to be run either in series to get to 350 volts, or in parallel to an inverter that converts low parallel voltage into 350 volts. Either way a DC to three-phase AC converter is required.

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### keroseneSenior Member

I agree with the sentiment that 350V system takes serious knowledge and fair amount learning before getting anywhere near to.

But how to calculate the battery output current (A). The C number tells how many times of the "Capacity" it can provide.
So for example 100Ah battery at 1C is outputting 100A, at 2C it is giving 200A, at 4C 400A and so on.
Most typical cells are rated 1C constant and 2C for some minutes but this of course varies. There are high power cells that are ok to give constant 4C.

To drive a 50kW boat motor needs a monster battery. Unlike cars boats need constantly high power and will eat battery dry very fast. A 200kW car will use less than 15-20kW cruising at decent speed.
A boat with a 50kW motor is probably designed to use 20-30 kw constant duty.

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### keroseneSenior Member

BTW at \$500/kWh the suggested battery is going to cost \$11k

And power the suggested motor for 20 minutes flat out.

Might make sense for a specialty case like a river ferry or whatnot but not fit for many use cases.

12. Joined: Aug 2002
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### gonzoSenior Member

The C number is for charging/discharging For example, C1 means the battery is being charged or discharged at the rated capacity. C3 means that the battery is being charged or discharged at 3X the rated capacity. Higher rates of discharge diminish the total power available from the battery. It may also damage the battery. High rates of charge/discharge generate heat. Large battery banks, like you are suggesting, need a cooling system. They can't simply be stacked together.

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13. Joined: Nov 2021
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### LeopardJunior Member

the battery size is big enough, it is like 900x300x250 mm. One more thing, what is the usual battery storage space for 15 to 20 ft boats, pontoons, fishing trawlers?

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### gonzoSenior Member

A 15 foot boat will probably have on size 24 battery.

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