Connecting unbalanced parallel battery strings to a common DC bus

Question

We are currently researching a system in the design phase which will use 2 parallel 48V lithium battery strings.

• Each string will have a battery management system ensuring the cells are balanced.
• Each string will have individual charging circuits
• The strings will be connected to a common DC bus via a string isolator, probably a high current relay.
• The load can be run off a single string, but system performance would be better if both strings are run at the same time.

The question I have regards connecting to the common DC bus if the strings are not fully equalized. We are considering options such as precharge circuits, connecting the higher string first and then switching the lower string in as they equalize. Would appreciate input from the community on possible methods, which will allow me to research further?

Thanks, Suze

Answer 1

Your statements about "precharge circuits" and "switching...in as they equalize" are pretty vague so I can't comment on those.

However, if you connect two batteries of different voltage together then current will flow from the higher voltage to the lower voltage. If you can limit the current then this is just inefficient but not dangerous. If these are lithium batteries then you must limit the current flowing in this case. The current limit must be set at the lesser of the maximum charging current and the maximum dischaarging current. Also note that while current is flowing from one battery to the other there will be less current available for the load.