A short thesis, which I will describe in the text below: lithium batteries can be connected in parallel only with symmetrical BMS and, preferably, these batteries must be low-voltage (12V for example).

What is a symmetrical BMS and how does it differ from an asymmetric?

  • From the point of view of the user: a symmetric BMS has two contacts, and asymmetric have three (if minus the common) or four.
  • From the point of view of the principle of operation: for a symmetric BMS, the restrictions on the charge current and discharge current are equal to each other, for an asymmetric BMS, usually the discharge current value is several times more than the charge current value. Another difference: asymmetric BMS are not suitable for charging through discharge contacts.

Why is it so important to connect lithium batteries in parallel with a symmetric BMS only? The answer is a very low internal resistance of lithium compared to lead-acid.

As you know from Ohm's law I = U / R

When you connect two batteries with different charge levels, equalizing currents begin to flow between them. When connecting lead-acid batteries, these currents are small, and for lithium they can be more than 100A. For example, the resistance of my 12V lithium batteries in average 0.015 Ohm. To calculate the current flowing between two differently charged batteries, you need to take the voltage difference of both batteries and multiply by the sum of the resistances of both batteries. Suppose that the voltage difference is 3V, then:

I = 3V / (0.015 Ohm + 0.015 Ohm) = 100A

Whether it’s big or small is the thickness of the wires between the batteries, the reliability of the contacts, the ability of the BMS and cells to receive such current for a long time (as the charge is equalized, the current will gradually decrease). For 12V batteries, a 3V charge difference is not such an incredible situation. And if you want to connect not 12V, but 36V or 48V batteries in parallel, then the difference in charge level can be 3-4 times larger and equalizing currents will be as much times larger.

And why can lead batteries be connected in parallel without any problems? The internal resistance of lead batteries is relatively large - of the order of 0.2 Ohms - therefore, the equalizing current is small and its value is within the acceptable range. For lead-acid batteries with R = 0.2 Ohm:

I = 3V / (0.2 Ohm + 0.2 Ohm) = 7.5A

This is a fairly small current compared to the equalizing current between lithium batteries - there is no danger to the connecting wires or the batteries themselves.


Now recall that a boat needs a large supply of energy for an electric motor on a boat, while it is inconvenient to contain the entire amount of energy in one battery, because it will be a large and heavy battery and it will be necessary to place it near the center of mass of the boat. It is still necessary to divide the energy supply into several places, at least into 2-3 parts (batteries). So you can put the batteries under the bunks on the sides of the keel or something else, depending on the configuration of the furniture and bulkheads on your boat. In any case, it is more convenient than one large battery.

On water, for safety reasons, it is advisable to limit the voltage to 48V (the maximum voltage of a 48V full charged battery is 54V). If we cannot connect the batteries in parallel, it remains to connect them in series.

Battery serial connection

Battery serial connection

In order to obtain the required energy reserve on an electric boat, you need to select a sufficient capacity for each battery, so that when several batteries are connected in series, the estimated energy reserve for movement is obtained.

According to the formula: E = U * C,

  • where U is the average battery voltage measured in V,
  • C - battery capacity measured in Ah.

For example, on my boat U = 36V, and C = 180Ah, then the energy reserve of the battery:

E = 36V * 180Ah = 6480 Wh or approximately 6.5 kW ⋅ h My boat has enough of this energy to travel at a speed of 6 km / h for 11 hours.