Notes on batteries for electric motors on a boat
This article will help you figure out which battery is suitable for specific tasks.
About 3 years ago, the choice of battery type was small: AGM or GEL . Both are lead type. The specific capacity per unit weight is extremely low , which means your electric motor won't last long on a battery of reasonable weight for a sailboat.
Now, with the development of lithium technology, a lithium battery has become more profitable in the long run than a lead one .
It makes sense to put lead on your boat as a battery for the engine only if you are satisfied with the extremely short operating time of the electric motor (up to 2-3 hours) from a single battery charge, or you want to install a hybrid installation and the battery will only be needed as a buffer (i.e. .the generator will always run as long as the electric motor is running). In this case, the benefits of quiet running will not be available to you. You will also have to wait a long time for the batteries to be fully charged, because. lead can be charged with currentmore than 0.1C (i.e. one tenth of their capacity, the total charge time is 10 hours ).
Lithium can have various chemistry . For myself, I first of all divide into dangerous and safe chemistry:
- all elements of size 18650
- LiFePO4, LFP
- Li-Ti (Li4Ti5O12, Li2TiO3)
Hazardous chemicals have better capacity per unit weight than safe and cheaper ones. But, as the name implies, if used improperly, it can catch fire and it will be very difficult to extinguish.
On my boat, the service batteries are of the LiFePO4 or lithium ferum type. Each cell of this type has an average voltage of 3.2V, a lower voltage of 2.5V, and an upper voltage of 3.6V-3.65V.
They were assembled for me by a master with the nickname Leon, whom I found on the Electroautos forum
Each battery consists of 4 series-connected ( 4s ) lithium-ferum cells of 100Ah. The result is a battery with an average voltage of 12.8V, which is suitable for on-board electronics. Each battery has a BMS that does the job of looking after the cell voltage and shutting down the discharge or charge when the voltage goes out of the battery's normal operating range to prevent the cells from being destroyed as lithium is very sensitive to overcharging or overdischarging. Also, BMS performs the function of charge balancing between cells.there would be no situation when one cell out of four is fully charged, and the rest are not fully charged.
Using this type of battery for a motor is quite realistic and many do, especially if there is money and a place inside the boat where a battery of sufficient capacity can be placed.
For my boat, I ordered a lipo type battery as a power source for the electric motor . I opted for less cost and less weight over more safety. However, I adhere to the necessary safety rules when working with this type of battery. This includes having a reliable BMS , checking cell voltage from time to time, properly setting chargers, and some tricks to increase safety.
I am convinced that the use of lithium batteries, whether hazardous or non-hazardous, without a BMS or at least active balancers is completely unacceptable .
The voltage of the battery for the engine is better to choose depending on the engine power that you need. If you have a small boat and enough power up to 5 kW, then I recommend choosing a battery voltage of 48V . This is a voltage that is safe enough for a person, more - you already need to take additional precautions when working with cables. For larger boats that require 10 kW or more, the voltage will have to be increased, otherwise the thickness of the cables necessary to pass the required power will be too large. However, I do not recommend using voltages greater than 84V (maximum charged battery voltage) in any case, as then you will be taken to a controller of impressive value. In some cases, I allow the maximum battery voltage - 100V . Anything more is already beyond reasonable from the point of view of the wallet.
48V voltage has a small bonus - there are many power supplies for telecommunications equipment that cost a penny and are just configured to charge 48V batteries, which will allow you to assemble the entire system relatively inexpensively in the first approximation and already walk with a new engine. Later, you can replace the charger with a more reliable one, or simply carry a spare telecommunications unit with you, because. they cost a penny. For example, a Delta charger costs around UAH 500 ($20), its power of 1.5 kW is usually enoughchno. For a different voltage, you will have to order a conversion of such a unit to the desired voltage or purchase an expensive specialized charger (the price of this will be 10-20 times more than the "delta").
I also advise you to set the most simple and indestructible BMS , because. all the bells and whistles like bluetooth, wifi, etc. reduce the reliability of this block, and it is very important.
It is better to place the battery not far from the electric motor controller, but without disturbing the weight distribution of the boat. I placed them near the keel. Naturally, it is better not to conduct cables under the floorboards, because. there might be water. I spent under the side bunks.
How to calculate the required battery capacity
If you have a hybrid engine , i.e. there is a generator - then you do not need to have a large battery capacity on board. Take a battery with a capacity sufficient for one hour of engine operation. Those. if the motor is 3 kW , then you need a 3 kWh battery . If the battery voltage is 48V, then the capacity in Ah will be in this case: 3000 W / 48 V = 62.5 Ah. You need a 48V and 60-70Ah battery.
If you have a purely electric propulsion , then it already depends on how much money you have, because. There is never too much battery capacity . I currently have 7 kWh on board, but I plan to increase the power supply to 12 kWh for the next navigation.
Select the weight of the battery so that you can lift it alone.
See an example of how to make a battery from a Chevrolet Bolt electric car module here .